A strand of DNA carries information.

Where in the strand is the information held?

1. A. In which bases are present
   Two strands can hold the same four bases and carry different information.
   The information is in the order of the bases.
2. B. ✓ In the order of its bases
3. C. In how many bases it has
   Two strands can hold the same number of bases and carry different information.
   The information is in the order of the bases.

Why: The same bases in a different order carry a different message.
So the information is in the order of the bases along the strand.

A human body cell holds 46 chromosomes.

What is one chromosome?

1. A. One protein that reads the DNA’s bases
   A chromosome is made of DNA, with proteins wound in it.
   The proteins do not read the DNA.
2. B. One base pair of the DNA
   One base pair is one rung of the DNA ladder.
   A chromosome is a whole DNA molecule, millions of base pairs long, wound on proteins.
3. C. ✓ One very long DNA molecule wound on proteins

Why: A chromosome is one very long DNA molecule packaged with proteins.
A human body cell holds 46 of them.

A human sperm fuses with a human egg at fertilization.

How many chromosomes does each of the two gametes carry?

1. A. ✓ 23
2. B. 46
   46 is the count in a body cell.
   Each gamete carries one complete set, 23 chromosomes, so the zygote has 46.
3. C. 92
   92 would be two body cells’ worth.
   Each gamete carries one complete set, 23 chromosomes.

Why: A gamete is haploid: one complete set.
One human set is 23 chromosomes.
The sperm’s 23 and the egg’s 23 make the zygote’s 46.

Suppose a bacterium divides into two new cells.

Which molecule carries the instructions the old cell hands to each new cell?

1. A. The old cell’s proteins
   Proteins are built by following the instructions.
   The instructions themselves are stored in the DNA, which the old cell copies and hands on.
2. B. ✓ The old cell’s DNA
3. C. The old cell’s RNA
   The instructions are stored in the DNA.
   The old cell copies its DNA and hands one copy to each new cell.

Why: A cell stores its genetic information in its DNA.
Before dividing, the old cell copies its DNA.
Each new cell receives one copy.

A father’s instructions reach his child in a sperm.

In which molecule does the sperm carry them?

1. A. ✓ The sperm’s DNA
2. B. The sperm’s proteins
   The sperm’s DNA holds the father’s instructions.
   The proteins were built from instructions; they do not carry them.

Why: A parent’s genetic information is in its DNA.
The sperm carries 23 chromosomes, each a DNA molecule copied from the father’s.
So the instructions travel in the sperm’s DNA.

A bacterium is a prokaryotic cell.

Where does its DNA lie?

1. A. Inside a nucleus
   A bacterium has no nucleus.
   Its DNA lies in the nucleoid, a region of the cytosol with no membrane around it.
2. B. ✓ In the nucleoid

Why: A bacterium’s DNA lies in the nucleoid.
The nucleoid is the region of the cytosol where the DNA is, with no membrane around it.

An onion skin cell is a eukaryotic cell.

Which part of the cell holds its DNA?

1. A. The cell wall
   The cell wall is the stiff layer outside the cell.
   The DNA sits inside the nucleus.
2. B. A ribosome
   Ribosomes build proteins.
   The DNA sits inside the nucleus.
3. C. ✓ The nucleus

Why: In a eukaryotic cell the DNA sits inside the nucleus, wrapped in the nuclear envelope.

Look at how the cell drawn below holds its DNA.

Which kind of cell is it?

1. A. ✓ Prokaryotic
2. B. Eukaryotic
   The DNA is one closed circle with no nucleus around it.
   One circular chromosome lying free makes the cell prokaryotic.

Why: The cell’s DNA is one closed circle.
No nucleus wraps it.
One circular chromosome in the nucleoid is a prokaryotic cell’s chromosome set.

Look at where the DNA lies in the cell drawn below.

Which kind of cell is it?

1. A. Prokaryotic
   The DNA is in several lines inside a nucleus, not one circle lying free.
   Linear chromosomes inside a nucleus make the cell eukaryotic.
2. B. ✓ Eukaryotic

Why: The compartment with a double edge is a nucleus.
Inside it lie several linear chromosomes.
Linear chromosomes inside a nucleus are a eukaryotic cell’s chromosome set.

The cell drawn below is small and has a thick cell wall. Look at how it holds its DNA.

Which kind of cell is it?

1. A. Prokaryotic
   Small size and a cell wall decide nothing.
   The DNA lies as lines inside a nucleus, so the cell is eukaryotic.
2. B. ✓ Eukaryotic

Why: The compartment with a double edge is a nucleus, and the DNA inside it is linear.
Linear chromosomes inside a nucleus make the cell eukaryotic, whatever its size.

A cell’s DNA is one closed circle with a few proteins attached, lying in its cytosol.

Which kind of cell is it?

1. A. ✓ Prokaryotic
2. B. Eukaryotic
   The DNA is one circle lying in the cytosol, so there is no nucleus.
   A eukaryotic cell keeps linear chromosomes inside a nucleus.

Why: One circular chromosome lying in the cytosol, with few proteins attached, is a prokaryotic cell’s chromosome set.

A cell about two thousandths of a millimeter long holds one DNA molecule whose two ends are joined to each other.

Which kind of cell is it?

1. A. ✓ Prokaryotic
2. B. Eukaryotic
   A DNA molecule whose ends are joined is a closed circle: one circular chromosome.
   That is a prokaryotic cell’s chromosome set.

Why: The two ends of the one DNA molecule are joined, so it is a circular chromosome.
One circular chromosome is a prokaryotic cell’s chromosome set.

A cell’s DNA is in 16 separate molecules, each with two free ends and each wound on proteins, inside a nucleus.

Which kind of cell is it?

1. A. Prokaryotic
   A prokaryotic cell has one circular chromosome and no nucleus.
   Sixteen linear chromosomes inside a nucleus make this cell eukaryotic.
2. B. ✓ Eukaryotic

Why: Each DNA molecule has two free ends, so each is a linear chromosome.
They sit inside a nucleus.
Linear chromosomes inside a nucleus are a eukaryotic cell’s chromosome set.

A cell’s DNA can be joined into a circle or lie in a line.

What is a circular chromosome?

1. A. One DNA molecule with two free ends, wound on proteins
   A DNA molecule with two free ends is a linear chromosome.
2. B. A closed ring of proteins with no DNA wound on it
   A chromosome is made of DNA.
   A circular chromosome is a DNA molecule joined into a closed circle.
3. C. ✓ One DNA molecule whose two ends are joined into a closed circle

Why: A circular chromosome is one DNA molecule whose two ends are joined to each other, so the molecule forms a closed circle.

A cell’s DNA can be joined into a circle or lie in a line.

What is a linear chromosome?

1. A. One DNA molecule whose two ends are joined into a closed circle
   A DNA molecule joined into a closed circle is a circular chromosome.
2. B. ✓ One long DNA molecule with two free ends
3. C. A straight protein fiber with no DNA in it
   A chromosome is made of DNA.
   A linear chromosome is one long DNA molecule with two free ends.

Why: A linear chromosome is one long DNA molecule with two free ends, lying in a line from one end to the other.

E. coli’s one chromosome is a closed loop of DNA.

Which kind of chromosome is it?

1. A. ✓ Circular
2. B. Linear
   A closed loop has no free end.
   A DNA molecule with no free end is a circular chromosome.

Why: The loop is closed, so the DNA molecule has no free end.
A DNA molecule joined into a closed circle is a circular chromosome.

One of a mouse’s chromosomes is a DNA molecule with two free ends.

Which kind of chromosome is it?

1. A. Circular
   A circular chromosome has no free end.
   A DNA molecule with two free ends is a linear chromosome.
2. B. ✓ Linear

Why: The molecule has two free ends, so it lies in a line from one end to the other.
That is a linear chromosome.

One human body cell holds 46 DNA molecules.

Laid end to end, about how long would they be?

1. A. About two millimeters
   Two millimeters is far too short.
   The 46 DNA molecules of one cell would stretch about two meters.
2. B. ✓ About two meters

Why: Laid end to end, the 46 DNA molecules of one human cell would stretch about two meters.

Between one division and the next, a cell reads and copies its DNA.

In which form are its chromosomes during this time?

1. A. ✓ Spread out as chromatin
2. B. Coiled into short thick rods
   Short thick rods are the form for dividing.
   Between divisions each chromosome is spread out as chromatin, so its DNA can be read.

Why: Between divisions each chromosome is spread out as chromatin, a long thin thread of DNA wound on proteins.
In that form the DNA can be read and copied.

The drawing below shows part of one chromosome, with the DNA threading through a row of shapes.

What are the round shapes?

1. A. Genes
   The genes are in the DNA itself, the line that threads through.
   Each round shape is a cluster of histone proteins that the DNA winds around.
2. B. ✓ Clusters of histone proteins
3. C. Molecules of RNA
   No RNA is part of a chromosome’s packing.
   Each round shape is a cluster of histone proteins that the DNA winds around.

Why: The line is the DNA.
It winds around each round shape like thread on a bead.
Each bead is a cluster of histone proteins.

A student looks at the string-of-beads drawing of a chromosome and says: “Each bead is one gene.”

Is the student correct?

1. A. ✓ No: the beads pack the DNA, and the genes are in the DNA
2. B. Yes: each bead is one gene, and the DNA links the genes
   A bead is a cluster of histones, proteins the DNA winds around.
   The genes are in the order of the bases of the DNA itself.

Why: Each bead is a cluster of histone proteins.
The histones only pack the DNA.
The genes are in the DNA that is wound on the beads.

In a liver cell between divisions, a gene is being read.

How is that gene’s DNA wound?

1. A. Tightly, as a condensed chromosome
   Tight coiling is for moving a chromosome during division.
   A gene being read is on loosely wound chromatin.
2. B. ✓ Loosely, as chromatin

Why: Between divisions the DNA is loosely wound as chromatin.
Loosely wound DNA can be read.
So a gene being read is on loosely wound DNA.

A eukaryotic chromosome is DNA wound on proteins.

What is a histone?

1. A. A gene that the cell is currently reading
   A gene is a stretch of the DNA.
   A histone is one of the proteins the DNA winds around.
2. B. The double membrane wrapping the nucleus
   The membrane around the nucleus is the nuclear envelope.
   A histone is one of the proteins the DNA winds around.
3. C. ✓ A protein the DNA winds around, like thread on a bead

Why: A histone is one of the small proteins that a eukaryotic chromosome’s DNA winds around, like thread on a bead.

Between divisions, a chromosome’s DNA loops round small proteins along its whole length.

Which proteins does it loop round?

1. A. Ribosomes
   Ribosomes build proteins and lie outside the nucleus.
   The proteins a chromosome’s DNA loops round are histones.
2. B. ✓ Histones

Why: The proteins a chromosome’s DNA winds around, bead after bead, are histones.

A protein is part of a chromosome, and the DNA is wound around it.

Is that protein a histone?

1. A. ✓ Yes
2. B. No
   A protein that DNA winds around inside a chromosome is a histone.

Why: Histones are the proteins DNA winds around in a chromosome.
So a protein with the DNA wound around it is a histone.
Other proteins attach to a chromosome, but the proteins the DNA winds around are the histones.

A student says: “This cell is eukaryotic, so its DNA is one closed circle lying in its cytosol.”

Is the student correct?

1. A. Yes: a eukaryotic cell’s DNA is one closed circle in its cytosol
   One closed circle lying in the cytosol is a prokaryotic cell’s chromosome set.
   A eukaryotic cell keeps linear chromosomes inside a nucleus.
2. B. ✓ No: a eukaryotic cell’s DNA is linear chromosomes inside a nucleus

Why: A eukaryotic cell keeps its DNA as linear chromosomes inside a nucleus.
One closed circle lying in the cytosol is a prokaryotic cell’s chromosome set.
So the student has described a prokaryotic cell, not a eukaryotic one.

A chromosome in a skin cell is coiled into a short thick rod.

What is the cell about to do?

1. A. ✓ Divide into two cells
2. B. Read a gene on that chromosome
   A gene is read from loosely wound chromatin.
   A chromosome coils into a short thick rod when the cell is about to divide.

Why: A chromosome coils tightly into a short thick rod so that it can be moved without tangling.
The cell moves its chromosomes when it divides.
So the cell is about to divide.

A skin cell is about to divide into two new cells.

Which step gives each new cell a full set of instructions?

1. A. Building a full set of proteins before it divides
   Proteins are built by following the instructions; they do not carry them.
   The cell copies its DNA, and each new cell receives one copy.
2. B. Making RNA before it divides
   RNA is not where a cell stores its instructions.
   The cell copies its DNA, and each new cell receives one copy.
3. C. ✓ Copying its DNA before it divides

Why: A cell stores its instructions in its DNA.
Before it divides, the cell copies its DNA.
Each new cell receives one complete copy, and with it every instruction.

A cell is about to divide. It must move each chromosome to a new cell in one piece.

What makes each chromosome easy to move?

1. A. The DNA has been cut into short pieces
   The DNA is not cut.
   The whole molecule is coiled tightly, so it is short and thick.
2. B. The DNA has been taken off its histones
   The DNA stays wound on its histones.
   The beaded string coils tightly, so the chromosome is short and thick.
3. C. ✓ The DNA is coiled tightly into a short thick rod

Why: The beaded string coils again and again.
So the chromosome becomes a short thick rod.
A short thick rod can be moved without tangling or breaking.

A cell about five thousandths of a millimeter across has a cell wall. Its DNA is linear chromosomes inside a nucleus.

Which kind of cell is it?

1. A. ✓ Eukaryotic
2. B. Prokaryotic
   Size and a cell wall decide nothing.
   Linear chromosomes inside a nucleus make the cell eukaryotic.

Why: The DNA is linear chromosomes inside a nucleus.
That is a eukaryotic cell’s chromosome set, whatever the cell’s size.

Suppose a bacterium divides into two new cells.

What does each new cell receive?

1. A. Half of the old cell’s DNA, uncopied
   Before it divides, the old cell copies its DNA.
   Each new cell receives a complete copy, so each has every instruction.
2. B. The old cell’s proteins and none of its DNA
   The instructions are in the DNA.
   Each new cell receives a complete copy of the old cell’s DNA.
3. C. ✓ A complete copy of the old cell’s DNA

Why: Before dividing, the old cell copies its DNA.
Then it hands one complete copy to each new cell.
So each new cell carries every instruction the old cell had.

A bacterium’s one chromosome is a single DNA molecule.

Which shape is it?

1. A. A line with two free ends
   A DNA molecule with two free ends is a linear chromosome, a eukaryotic cell’s kind.
   A bacterium’s chromosome is one closed circle.
2. B. ✓ A closed circle

Why: The two ends of a bacterium’s DNA molecule are joined to each other.
So the molecule forms a closed circle: a circular chromosome.

The cell drawn below holds two closed loops of DNA.

Which loop is the plasmid?

1. A. ✓ The smaller loop
2. B. The larger loop
   The large loop is the cell’s chromosome.
   The plasmid is the small closed loop lying apart from it.

Why: A plasmid is a small closed circle of DNA lying apart from the chromosome.
The larger loop is the chromosome.
So the smaller loop, lying apart from it, is the plasmid.

The cell drawn below holds three closed loops of DNA.

Which loops are the plasmids?

1. A. The one larger loop
   The one large loop is the cell’s chromosome.
   The plasmids are the small closed loops lying apart from it.
2. B. ✓ The two smaller loops

Why: The larger loop is the chromosome.
Each smaller closed loop lying apart from it is a plasmid.
So the two smaller loops are the plasmids.

Look at the DNA in the round cell drawn below.

How many plasmids does the cell hold?

1. A. 0
   The loop in the middle is the chromosome.
   The much smaller closed loop lying apart from it is a plasmid, so the cell holds one.
2. B. ✓ 1
3. C. 2
   The loop in the middle is the chromosome, not a plasmid.
   Only the much smaller closed loop is a plasmid, so the cell holds one.

Why: The loop in the middle is the chromosome.
The much smaller closed loop lies apart from it, so it is a plasmid.
The cell holds one plasmid.

Look at the DNA in the cell drawn below.

How many plasmids does the cell hold?

1. A. ✓ 0
2. B. 1
   The one large loop is the chromosome, not a plasmid.
   Counting it as a plasmid gives one; the cell holds none.
3. C. 2
   Two plasmids would be two small closed loops apart from the chromosome.
   There is none, so the cell holds no plasmid.

Why: The one large loop is the chromosome.
A plasmid is a small closed circle lying apart from the chromosome, and there is none.
So the cell holds no plasmid.

Look at the DNA in the cell drawn below.

How many plasmids does the cell hold?

1. A. 2
   Two small loops sit near the left end and two near the right end: four in all.
   Each small loop is a plasmid.
2. B. 3
   Two small loops sit near each end, so there are four small loops, not three.
   Each small loop is a plasmid.
3. C. ✓ 4

Why: The large loop is the chromosome.
Each small closed loop lying apart from it is a plasmid.
There are four small loops, so the cell holds four plasmids.

The cell drawn below has a cell wall, and one small closed loop of DNA lies inside it.

Is this cell a eukaryote?

1. A. ✓ Yes
2. B. No
   The DNA lies as linear chromosomes inside a nucleus, so the cell is a eukaryote.
   The small closed loop is a plasmid, which some eukaryotes carry.

Why: The short lines are linear chromosomes, and they lie inside a nucleus.
Linear chromosomes inside a nucleus make the cell a eukaryote.
The small closed loop is a plasmid; some eukaryotes carry plasmids.

A student looks at a bacterium with its chromosome and two plasmids and says: “The plasmids carry all of the cell’s genes.”

Is the student correct?

1. A. Yes: the plasmids carry every gene the cell has
   The chromosome carries thousands of genes.
   Each plasmid carries only a few.
2. B. ✓ No: most of the cell’s genes are on the chromosome

Why: Each plasmid carries a few genes of its own.
The chromosome carries thousands of genes.
So most of the cell’s genes are on the chromosome, not on the plasmids.

A bacterium’s DNA is a chromosome and, often, some extra pieces.

What is a plasmid?

1. A. A protein that DNA winds around, like thread on a bead
   A protein that DNA winds around is a histone, part of a eukaryotic chromosome.
   A plasmid is DNA: a small closed circle lying apart from the chromosome.
2. B. The one big circle of DNA that carries most of the cell’s genes
   The one big circle carrying most of the genes is the chromosome.
   A plasmid is a small closed circle of DNA lying apart from it.
3. C. ✓ A small closed circle of DNA lying apart from the chromosome

Why: A plasmid is a small closed circle of DNA that lies in a cell apart from the chromosome, carrying a few genes of its own.

E. coli’s one big closed circle of DNA, millions of base pairs long, lies in its nucleoid.

Is that circle a plasmid?

1. A. Yes
   The one big circle, millions of base pairs long, is the chromosome.
   A plasmid is a small extra circle lying apart from it.
2. B. ✓ No

Why: The one big circle that carries most of the cell’s genes is the chromosome.
A plasmid is a small closed circle lying apart from the chromosome.
So the big circle is not a plasmid.

A small closed circle of DNA lies in a bacterium’s cytosol, apart from the chromosome, carrying a gene for surviving an antibiotic.

Is that circle a plasmid?

1. A. ✓ Yes
2. B. No
   A small closed circle of DNA lying apart from the chromosome is a plasmid, whatever gene it carries.

Why: The circle is small, closed, and made of DNA.
It lies apart from the chromosome.
So it is a plasmid.

A bacterium builds its own proteins.

Which part of the bacterium builds them?

1. A. ✓ The bacterium’s ribosomes
2. B. The bacterium’s nucleoid
   The nucleoid is the region where the bacterium’s DNA lies.
   Its ribosomes build the proteins.
3. C. The bacterium’s cell wall
   The cell wall is the stiff layer outside the cell.
   Its ribosomes build the proteins.

Why: A ribosome joins amino acids into a protein.
A bacterium’s ribosomes float in its cytosol.
So the ribosomes build the bacterium’s proteins.

A particle with a protein coat sits on a bacterium’s surface. It has no ribosomes, and it reproduces only inside the bacterium.

Is the particle a cell or a virus?

1. A. A cell
   A cell has ribosomes of its own and reproduces on its own.
   No ribosomes and reproducing only inside a cell make the particle a virus.
2. B. ✓ A virus

Why: The particle has no ribosomes.
It reproduces only inside a cell.
Genetic material in a protein coat, with no ribosomes and reproducing only inside a cell, is a virus.

A rod about one thousandth of a millimeter long has ribosomes in its cytosol, and it divides in two on its own.

Is the rod a cell or a virus?

1. A. ✓ A cell
2. B. A virus
   A virus has no ribosomes and reproduces only inside a cell.
   Ribosomes of its own and dividing on its own make the rod a cell.

Why: The rod has ribosomes of its own.
It divides on its own.
Ribosomes of its own and dividing on its own make it a cell.

Suppose a round particle is about one fiftieth of a millimeter across. It holds a nucleus and ribosomes, and it divides on its own.

Is the particle a cell or a virus?

1. A. ✓ A cell
2. B. A virus
   A virus has no ribosomes and no nucleus.
   Ribosomes of its own and dividing on its own make the particle a cell.

Why: The particle has ribosomes of its own.
It divides on its own.
Ribosomes of its own and dividing on its own make it a cell.

Suppose a particle is so large that a light microscope shows it. It has no ribosomes, and it reproduces only inside a cell.

Is the particle a cell or a virus?

1. A. A cell
   Size decides nothing.
   No ribosomes and reproducing only inside a cell make the particle a virus.
2. B. ✓ A virus

Why: The particle has no ribosomes.
It reproduces only inside a cell.
Its size changes neither feature, so it is a virus.

A particle with a protein coat gets a cell to build copies of it. On its own, it makes nothing.

Is the particle a cell or a virus?

1. A. A cell
   A cell builds its own proteins and reproduces on its own.
   A particle that reproduces only by getting a cell to copy it is a virus.
2. B. ✓ A virus

Why: The particle makes nothing on its own.
It reproduces only inside a cell.
Genetic material in a protein coat, with no ribosomes and reproducing only inside a cell, is a virus.

Suppose a particle is about 20 % of a bacterium’s length. It has ribosomes of its own, and it divides in two on its own.

Is the particle a cell or a virus?

1. A. ✓ A cell
2. B. A virus
   Being small decides nothing.
   Ribosomes of its own and dividing on its own make the particle a cell.

Why: The particle has ribosomes of its own.
It divides on its own.
Its small size changes neither feature, so it is a cell.

A student says: “A virus is just a very small cell.”

Is the student correct?

1. A. ✓ No: a virus has no ribosomes and reproduces only inside a cell
2. B. Yes: a virus is a cell too small to hold ribosomes
   Every cell, however small, has ribosomes of its own and reproduces on its own.
   A virus has no ribosomes and reproduces only inside a cell.

Why: A cell has ribosomes of its own and reproduces on its own.
A virus has no ribosomes.
A virus reproduces only inside a cell.
So a virus is not a cell, however small a cell can be.

Viruses and cells are both found in a drop of pond water.

What is a virus?

1. A. A very small cell with ribosomes of its own but no nucleus around its DNA
   A small cell with ribosomes but no nucleus is a bacterium.
   A virus has no ribosomes.
2. B. A protein that a cell builds inside itself and then sends out
   A virus carries genetic material, not only protein.
   It is genetic material inside a protein coat.
3. C. ✓ Genetic material in a protein coat that reproduces only inside a cell

Why: A virus is genetic material inside a protein coat.
It has no ribosomes.
It reproduces only inside a cell.

A particle in the pond water has ribosomes of its own.

Could the particle be a virus?

1. A. Yes
   A virus has no ribosomes.
   A particle with ribosomes of its own is a cell.
2. B. ✓ No

Why: A virus has no ribosomes.
So a particle with ribosomes of its own is not a virus.

A particle in the pond water has no ribosomes, and it reproduces only inside a cell.

Could the particle be a virus?

1. A. ✓ Yes
2. B. No
   No ribosomes and reproducing only inside a cell are a virus’s two features.
   The particle could be a virus.

Why: A virus has no ribosomes and reproduces only inside a cell.
The particle has both features.
So the particle could be a virus.

DNA and RNA each use four bases.

Which base does RNA carry in place of thymine?

1. A. Adenine
   Adenine is in both DNA and RNA.
   RNA carries uracil in place of thymine.
2. B. ✓ Uracil
3. C. Guanine
   Guanine is in both DNA and RNA.
   RNA carries uracil in place of thymine.

Why: DNA’s bases are adenine, thymine, guanine and cytosine.
RNA has no thymine.
In its place RNA carries uracil.

The influenza virus’s genetic material contains uracil and no thymine.

Which molecule is its genetic material?

1. A. DNA
   DNA contains thymine and never uracil.
   Genetic material that contains uracil is RNA.
2. B. ✓ RNA

Why: The genetic material contains uracil.
Uracil is RNA’s base in place of thymine.
So the influenza virus’s genetic material is RNA.

The genetic material of a virus that infects bacteria contains thymine.

Which molecule is its genetic material?

1. A. ✓ DNA
2. B. RNA
   RNA carries uracil in place of thymine and never contains thymine.
   Genetic material that contains thymine is DNA.

Why: The genetic material contains thymine.
Thymine is a base of DNA and never of RNA.
So the virus’s genetic material is DNA.

Suppose a newly found virus carries genetic material whose bases are adenine, guanine, cytosine and thymine.

Which molecule is its genetic material?

1. A. ✓ DNA
2. B. RNA
   RNA never contains thymine.
   Genetic material with thymine among its bases is DNA.

Why: The bases include thymine.
Thymine is a base of DNA and never of RNA.
So the genetic material is DNA.

The sugar in HIV’s genetic material is ribose.

Which molecule is its genetic material?

1. A. DNA
   DNA’s sugar is deoxyribose.
   Genetic material whose sugar is ribose is RNA.
2. B. ✓ RNA

Why: The sugar is ribose.
Ribose is RNA’s sugar; DNA’s sugar is deoxyribose.
So HIV’s genetic material is RNA.

Suppose a virus’s genetic material has the bases adenine, guanine, cytosine and uracil.

Which molecule is its genetic material?

1. A. DNA
   DNA never contains uracil.
   Genetic material with uracil among its bases is RNA.
2. B. ✓ RNA

Why: The bases include uracil.
Uracil is a base of RNA and never of DNA.
So the genetic material is RNA.

Suppose a virus’s genetic material is built on the sugar deoxyribose.

Which molecule is its genetic material?

1. A. ✓ DNA
2. B. RNA
   RNA’s sugar is ribose.
   Genetic material built on deoxyribose is DNA.

Why: The sugar is deoxyribose.
Deoxyribose is DNA’s sugar; RNA’s sugar is ribose.
So the genetic material is DNA.

DNA and RNA are built from five kinds of nitrogenous base.

Which base does RNA carry in place of thymine?

1. A. Adenine
   Adenine is found in both DNA and RNA.
2. B. Cytosine
   Cytosine is found in both DNA and RNA.
3. C. ✓ Uracil

Why: RNA has no thymine.
In its place RNA carries uracil, so RNA’s four bases are adenine, uracil, guanine and cytosine.

The base drawn below is one of the five bases.

Which kind of base is it?

1. A. ✓ Purine
2. B. Pyrimidine
   The drawn base has two rings that share one edge.
   A two-ring base is a purine.

Why: The drawn base has two rings.
A base with two rings is a purine.

The base drawn below is one of the five bases, drawn turned round.

Which kind of base is it?

1. A. ✓ Purine
2. B. Pyrimidine
   Turning the drawing round changes nothing.
   The base has two rings, so it is a purine.

Why: The drawn base has two rings, whichever way round it is drawn.
A base with two rings is a purine.

The base drawn below is one of the five bases.

Which kind of base is it?

1. A. Purine
   The drawn base has one ring.
   A one-ring base is a pyrimidine.
2. B. ✓ Pyrimidine

Why: The drawn base has one ring.
A base with one ring is a pyrimidine.

Thymine is one of the five bases.

Which kind of base is thymine?

1. A. Purine
   Thymine is made of one ring.
   A one-ring base is a pyrimidine.
2. B. ✓ Pyrimidine

Why: Thymine is made of one ring.
A base with one ring is a pyrimidine.

Adenine is one of the five bases.

Which kind of base is adenine?

1. A. ✓ Purine
2. B. Pyrimidine
   Adenine is made of two rings that share one edge.
   A two-ring base is a purine.

Why: Adenine is made of two rings.
A base with two rings is a purine.

Uracil is one of the five bases.

Which kind of base is uracil?

1. A. Purine
   Uracil is made of one ring.
   A one-ring base is a pyrimidine.
2. B. ✓ Pyrimidine

Why: Uracil is made of one ring.
A base with one ring is a pyrimidine.

Cytosine is one of the five bases.

Which kind of base is cytosine?

1. A. Purine
   Cytosine is made of one ring.
   A one-ring base is a pyrimidine.
2. B. ✓ Pyrimidine

Why: Cytosine is made of one ring.
A base with one ring is a pyrimidine.

Guanine is one of the five bases.

Which kind of base is guanine?

1. A. ✓ Purine
2. B. Pyrimidine
   Guanine is made of two rings that share one edge.
   A two-ring base is a purine.

Why: Guanine is made of two rings.
A base with two rings is a purine.

An RNA strand from a virus carries four kinds of base: adenine, uracil, guanine and cytosine.

How many of the four are pyrimidines?

1. A. One
   Uracil is one ring and cytosine is one ring.
   Uracil and cytosine are both pyrimidines.
2. B. ✓ Two
3. C. Three
   Adenine and guanine are two rings each, so they are purines.
   Only uracil and cytosine are pyrimidines.

Why: Uracil is one ring and cytosine is one ring: two pyrimidines.
Adenine and guanine are two rings each: two purines.

The five bases fall into two groups by their rings.

What is a purine?

1. A. ✓ A base made of two rings
2. B. A base made of one ring
   A base made of one ring is a pyrimidine.
3. C. A sugar with five carbons
   A purine is a base, not a sugar.
   The five-carbon sugars are deoxyribose and ribose.

Why: A purine is a nitrogenous base made of two rings that share one edge: adenine or guanine.

The five bases fall into two groups by their rings.

What is a pyrimidine?

1. A. A base made of two rings
   A base made of two rings is a purine.
2. B. A phosphate group
   A pyrimidine is a base, not a phosphate group.
   The phosphate group is the part of a nucleotide drawn as a circle marked P.
3. C. ✓ A base made of one ring

Why: A pyrimidine is a nitrogenous base made of one ring: cytosine, thymine or uracil.

A base drawn in a textbook has two rings that share one edge.

Which kind of base is it?

1. A. ✓ Purine
2. B. Pyrimidine
   Two rings that share one edge make a purine.
   A pyrimidine is one ring.

Why: A base made of two rings is a purine.

A student says: “Guanine is a pyrimidine.”

Is the student correct?

1. A. Yes: guanine is made of one ring
   Guanine is made of two rings that share one edge.
   A two-ring base is a purine.
2. B. ✓ No: guanine is made of two rings, so it is a purine

Why: Guanine is made of two rings.
A base with two rings is a purine, not a pyrimidine.

In a cell, the two strands of DNA twist around each other into a double helix.

Where do the two sugar-phosphate backbones lie?

1. A. ✓ On the outside, with the paired bases inside
2. B. On the inside, with the bases outside
   The bases point inward and pair across the middle.
   The two backbones lie on the outside.

Why: In the double helix the two sugar-phosphate backbones lie on the outside.
The paired bases lie inside, between them.

The two backbones of DNA lie a fixed distance apart.

Which pairing fills a rung between them exactly?

1. A. A purine with a purine
   Two purines are four rings across the rung.
   Four rings are too wide for the space between the backbones.
2. B. ✓ A purine with a pyrimidine
3. C. A pyrimidine with a pyrimidine
   Two pyrimidines are two rings across the rung.
   Two rings are too narrow to reach across the space.

Why: A purine is two rings and a pyrimidine is one ring.
Together they are three rings across the rung.
Three rings fill the space between the backbones exactly.

A student says: “Adenine pairs with thymine because the two bases are the same size.”

Is the student correct?

1. A. ✓ No: a two-ring base pairs with a one-ring base
2. B. Yes: paired bases are always the same size
   Adenine is two rings and thymine is one ring.
   A rung is one two-ring base with one one-ring base.

Why: Adenine is a purine with two rings.
Thymine is a pyrimidine with one ring.
Two rings plus one ring fill the rung, so the paired bases are not the same size.

Suppose a newly described base is made of two rings.

Which kind of partner must it have to fill a rung?

1. A. ✓ A one-ring base
2. B. A two-ring base
   Two rings plus two rings are four rings, too wide for the rung.
   A two-ring base needs a one-ring partner.

Why: The new base is two rings.
A rung is three rings across.
So its partner must be one ring.

One strand of a DNA molecule carries a guanine.

Which base lies opposite it on the other strand?

1. A. Adenine
   Adenine pairs with thymine.
2. B. ✓ Cytosine
3. C. Thymine
   Thymine pairs with adenine.

Why: In DNA guanine pairs with cytosine.
So opposite a guanine lies a cytosine.

Two water molecules sit side by side. The hydrogen on one carries a small positive charge, written δ+. The oxygen on the other carries a small negative charge, written δ−. The hydrogen is attracted to that oxygen.

What is this attraction called?

1. A. ✓ A hydrogen bond
2. B. A covalent bond
   A covalent bond is a shared pair of electrons inside a molecule.
   The attraction between the two molecules is a hydrogen bond.

Why: A hydrogen with a small positive charge (δ+) is attracted to an oxygen or nitrogen with a small negative charge (δ−) on a neighboring molecule.
That attraction is a hydrogen bond.

The base on a DNA strand is adenine. The partner strand is DNA.

Which base sits opposite the adenine?

1. A. Cytosine
   Cytosine pairs with guanine.
2. B. ✓ Thymine
3. C. Uracil
   Uracil is RNA’s base.
   The partner strand here is DNA, so it carries thymine.

Why: Adenine pairs with thymine on a DNA strand.
So the DNA partner carries thymine opposite the adenine.

The base on a DNA strand is adenine. The partner strand is RNA.

Which base sits opposite the adenine?

1. A. Cytosine
   Cytosine pairs with guanine.
2. B. Guanine
   Guanine pairs with cytosine.
3. C. ✓ Uracil

Why: The partner strand is RNA, which carries uracil in place of thymine.
Adenine pairs with uracil on an RNA strand.

The base on a DNA strand is guanine. The partner strand is RNA.

Which base sits opposite the guanine?

1. A. Adenine
   Adenine pairs with thymine or with uracil.
2. B. ✓ Cytosine
3. C. Uracil
   Uracil pairs with adenine.

Why: Guanine pairs with cytosine on a DNA strand and on an RNA strand.
So the RNA partner carries cytosine opposite the guanine.

The base on a DNA strand is thymine. The partner strand is DNA.

Which base sits opposite the thymine?

1. A. ✓ Adenine
2. B. Cytosine
   Cytosine pairs with guanine.
3. C. Guanine
   Guanine pairs with cytosine.

Why: Thymine pairs with adenine.
So the DNA partner carries adenine opposite the thymine.

The base on an RNA strand is uracil. The partner strand is DNA.

Which base sits opposite the uracil?

1. A. ✓ Adenine
2. B. Guanine
   Guanine pairs with cytosine.
3. C. Thymine
   Thymine and uracil never pair with each other.
   Each of them pairs with adenine.

Why: Uracil pairs with adenine.
So the DNA partner carries adenine opposite the uracil.

The base on a DNA strand is cytosine. The partner strand is RNA.

Which base sits opposite the cytosine?

1. A. Adenine
   Adenine pairs with thymine or with uracil.
2. B. ✓ Guanine
3. C. Uracil
   Uracil pairs with adenine.

Why: Cytosine pairs with guanine on a DNA strand and on an RNA strand.
So the RNA partner carries guanine opposite the cytosine.

A nucleic-acid strand carries thymine.

Could the strand be RNA?

1. A. Yes
   An RNA strand carries uracil in place of thymine, never thymine.
2. B. ✓ No

Why: RNA has no thymine.
A strand that carries thymine is DNA.

Every known cell, from bacteria to humans, builds its ribosomes the same way.

What does a feature shared by all living things show?

1. A. ✓ All living things descend from shared ancestors
2. B. Bacteria descend from humans
   Bacteria were on Earth billions of years before humans.
3. C. Each kind of living thing arose on its own
   A feature shared by every living thing was inherited from ancestors they share.

Why: A feature shared by all living things is evidence that all living things descend from shared ancestors: their common ancestry.

Base pairing is conserved through evolution.

Which of the following does conserved mean here?

1. A. The pairing rules have changed a little in every generation since the first cells
   A conserved feature has stayed the same in every descendant of the ancestor that first had it.
2. B. ✓ The pairing rules have been kept the same in every living thing since the first cells
3. C. Each kind of living thing has worked out its own pairing rules
   Every kind of living thing examined pairs its bases the same way.

Why: Conserved means kept the same.
Base pairing has been kept the same in every living thing since the first cells.

A student says: “If one kind of organism changed its base-pairing rules, its descendants would simply inherit the new rules.”

Is the student correct?

1. A. ✓ No: a cell whose pairing rules changed could not copy its DNA, so it would leave no descendants
2. B. Yes: descendants inherit whatever pairing rules their ancestor had, changed or not
   One partner for each base is what lets one strand fix the other.
   A cell whose pairing changed could not copy its DNA, so it would leave no descendants.

Why: Base pairing is what lets one strand fix the other, so the DNA can be copied.
A cell whose pairing rules changed could not copy its DNA.
So it would leave no descendants, and the change would die with it.

A microbe is newly found in a hot spring.

Predict which base pairs with adenine in the microbe’s DNA.

1. A. Cytosine
   Cytosine pairs with guanine in every organism.
2. B. Guanine
   Guanine pairs with cytosine in every organism.
3. C. ✓ Thymine

Why: Base pairing is conserved through evolution.
Every organism pairs adenine with thymine.
So the microbe’s adenine pairs with thymine.

Biologists describe some features as conserved through evolution.

What does it mean that a feature is conserved through evolution?

1. A. ✓ The feature has been kept the same in every descendant of the ancestor that first had it
2. B. The feature is found only in the oldest living things, and the newest kinds have lost it
   A conserved feature is found in every descendant of the ancestor that first had it, the newest kinds as well as the oldest.
3. C. The feature has changed a little with every generation since the ancestor that first had it
   A conserved feature has stayed the same in every descendant of the ancestor that first had it.

Why: Conserved means kept.
A feature conserved through evolution has been kept the same in every descendant of the ancestor that first had it.
Base pairing has been kept in every living thing since the first cells.

Adenine pairs with thymine in a bacterium, in an oak tree and in a human.

Is base pairing conserved through evolution?

1. A. ✓ Yes
2. B. No
   The pairing is the same in every living thing, so it has been kept the same since the first cells.
   That is what conserved through evolution means.

Why: The pairing is the same in a bacterium, an oak tree and a human.
A feature kept the same in every living thing is conserved through evolution.

Bird beaks differ in shape from one kind of bird to another.

Is beak shape conserved through evolution?

1. A. Yes
   A conserved feature is kept the same in every descendant of the ancestor that first had it.
   Beak shape differs from one kind of bird to another.
2. B. ✓ No

Why: Beak shape differs from one kind of bird to another.
A feature that differs is not kept the same, so it is not conserved through evolution.

The base on an RNA strand is cytosine. The partner strand is DNA.

Which base sits opposite the cytosine?

1. A. Adenine
   Adenine pairs with thymine or with uracil.
2. B. ✓ Guanine
3. C. Thymine
   Thymine pairs with adenine.

Why: Cytosine pairs with guanine on a DNA strand and on an RNA strand.
So the DNA partner carries guanine opposite the cytosine.

The base drawn below is one of the five bases, drawn on its side.

Which kind of base is it?

1. A. ✓ Purine
2. B. Pyrimidine
   The drawn base has two rings that share one edge.
   A two-ring base is a purine.

Why: The drawn base has two rings.
A base with two rings is a purine.

A fungus is newly found in a cave.

Predict which base pairs with thymine in the fungus’s DNA.

1. A. ✓ Adenine
2. B. Cytosine
   Cytosine pairs with guanine in every organism.
3. C. Guanine
   Guanine pairs with cytosine in every organism.

Why: Base pairing is conserved through evolution.
Every organism pairs thymine with adenine.
So the fungus’s thymine pairs with adenine.

Suppose two one-ring bases faced each other across one rung of DNA.

Which of the following would happen?

1. A. The two bases would fill the rung exactly
   Two one-ring bases are two rings across the rung.
   A rung needs three rings to be filled.
2. B. The two backbones would be pushed apart
   Two rings across the rung are too narrow, not too wide.
   Four rings would push the backbones apart.
3. C. ✓ The two bases could not reach each other to bond

Why: Two one-ring bases are two rings across the rung.
Two rings are narrower than the space between the backbones.
So the two bases could not reach each other to bond.

A DNA strand carries adenine, thymine, guanine and cytosine.

Which of its bases are purines?

1. A. ✓ Adenine and guanine
2. B. Thymine and cytosine
   Thymine and cytosine are one ring each.
   A one-ring base is a pyrimidine.

Why: Adenine and guanine are two rings each.
A base with two rings is a purine.
So the strand’s purines are adenine and guanine.

A nucleic-acid strand carries uracil.

Could the strand be DNA?

1. A. Yes
   A DNA strand carries thymine, never uracil.
2. B. ✓ No

Why: DNA has no uracil.
A strand that carries uracil is RNA.

A student says: “Guanine pairs with cytosine because both bases have two rings.”

Is the student correct?

1. A. ✓ No: cytosine is made of one ring
2. B. Yes: paired bases have the same number of rings
   Cytosine is made of one ring.
   A rung is one two-ring base with one one-ring base.

Why: Guanine is two rings and cytosine is one ring.
Two rings plus one ring fill the rung.
So the paired bases do not have the same number of rings.

A strand of DNA has two different ends.

Which end carries the free phosphate group?

1. A. ✓ The 5′ end
2. B. The 3′ end
   The 3′ end carries a free hydroxyl group on the last sugar’s carbon 3.
   The free phosphate group sits at the 5′ end, on carbon 5.

Why: The last sugar’s carbon 5 carries a phosphate group joined to nothing further.
That free phosphate group marks the 5′ end.

Biologists write a strand of DNA as a row of letters.

From which end do they start?

1. A. ✓ From its 5′ end
2. B. From its 3′ end
   The 3′ end is where a strand is finished, not started.
   A strand is written from its 5′ end.
3. C. From either end
   The two ends differ, so a strand read from the other end gives a different order of bases.
   A strand is written from its 5′ end.

Why: The two ends of a strand differ, so the strand has a direction.
A strand is written from its 5′ end to its 3′ end.

Two paired strands of DNA lie side by side.

Where one strand has its 5′ end, which end does the other strand have?

1. A. Its 5′ end
   Paired strands are antiparallel.
   So where one strand has its 5′ end, the other has its 3′ end.
2. B. ✓ Its 3′ end

Why: Paired strands are antiparallel: they lie side by side and point in opposite directions.
So one strand’s 5′ end lies beside the other strand’s 3′ end.

One strand of DNA carries an adenine. Its partner strand is RNA.

Which base sits opposite the adenine?

1. A. Thymine
   Thymine is a base of DNA.
   An RNA strand carries uracil in its place, and uracil pairs with adenine.
2. B. Guanine
   Guanine pairs with cytosine.
   Opposite adenine on an RNA strand sits uracil.
3. C. ✓ Uracil

Why: RNA carries uracil in place of thymine.
So on an RNA partner strand, uracil sits opposite adenine.

A strand is written from its 5′ end to its 3′ end, and both ends are marked. One strand of DNA reads 5′-GCATTC-3′. Its DNA partner is written under it, with one base missing.

Which base is missing?

1. A. Adenine
   The gap sits under an adenine.
   The partner strand carries adenine’s partner, thymine, not a copy of adenine.
2. B. ✓ Thymine
3. C. Uracil
   Uracil belongs to RNA.
   This partner strand is DNA, so under adenine sits thymine.

Why: The gap sits under the A of 5′-GCATTC-3′.
In DNA, adenine pairs with thymine.
So the missing base is thymine, and the partner reads 3′-CGTAAG-5′.

A strand is written from its 5′ end to its 3′ end, and both ends are marked. One strand of DNA reads 5′-TTGACG-3′.

Written under it, with its ends marked 3′ at the left and 5′ at the right, what does the DNA partner strand read?

1. A. 3′-TTGACG-5′
   Copying the given strand gives the same bases, not each base’s partner.
   Under T sits A, under G sits C.
2. B. 3′-CGTCAA-5′
   These letters are the partner strand reversed.
   Under the given strand the partner reads 3′-AACTGC-5′.
3. C. ✓ 3′-AACTGC-5′
4. D. 3′-AACUGC-5′
   Uracil belongs to RNA.
   This partner strand is DNA, so under A sits T.

Why: Under each base write its partner: T gives A, T gives A, G gives C, A gives T, C gives G, G gives C.
The partner runs the other way, so its ends are 3′ at the left and 5′ at the right.
It reads 3′-AACTGC-5′.

A strand is written from its 5′ end to its 3′ end, and both ends are marked. One strand of DNA reads 5′-CAGTTA-3′.

Written from its own 5′ end, what does the DNA partner strand read?

1. A. ✓ 5′-TAACTG-3′
2. B. 5′-GTCAAT-3′
   These are the partners written left to right under the given strand.
   That starts from the partner’s 3′ end, and a strand is written from its 5′ end.
3. C. 5′-CAGTTA-3′
   Copying the given strand gives the same bases, not each base’s partner.
   Under C sits G, under A sits T.
4. D. 5′-ATTGAC-3′
   Reversing the given strand gives its own bases backward, not each base’s partner.
   Under C sits G, under A sits T.

Why: Under each base write its partner: C gives G, A gives T, G gives C, T gives A, T gives A, A gives T.
Under the given strand that reads 3′-GTCAAT-5′.
The partner’s 5′ end is at the right, so from its own 5′ end it reads 5′-TAACTG-3′.

A strand is written from its 5′ end to its 3′ end, and both ends are marked. One strand of DNA reads 5′-GGATCA-3′.

Written from its own 5′ end, what does the DNA partner strand read?

1. A. 5′-CCTAGT-3′
   These are the partners written left to right under the given strand.
   That starts from the partner’s 3′ end, and a strand is written from its 5′ end.
2. B. 5′-GGATCA-3′
   Copying the given strand gives the same bases, not each base’s partner.
   Under G sits C, under A sits T.
3. C. 5′-ACTAGG-3′
   Reversing the given strand gives its own bases backward, not each base’s partner.
   Under G sits C, under A sits T.
4. D. ✓ 5′-TGATCC-3′

Why: Under each base write its partner: G gives C, G gives C, A gives T, T gives A, C gives G, A gives T.
Under the given strand that reads 3′-CCTAGT-5′.
The partner’s 5′ end is at the right, so from its own 5′ end it reads 5′-TGATCC-3′.

A strand is written from its 5′ end to its 3′ end, and both ends are marked. One strand of DNA reads 5′-TACGGA-3′. An RNA strand pairs with it along its whole length.

Written under the DNA strand, with its ends marked 3′ at the left and 5′ at the right, what does the RNA strand read?

1. A. 3′-ATGCCT-5′
   Thymine belongs to DNA.
   The partner strand is RNA, so under adenine sits uracil.
2. B. ✓ 3′-AUGCCU-5′
3. C. 3′-UACGGA-5′
   Copying the DNA strand with U for T gives the same bases, not each base’s partner.
   Under T sits A, under A sits U.
4. D. 3′-UCCGUA-5′
   These letters are the RNA strand reversed.
   Under the DNA strand the RNA reads 3′-AUGCCU-5′.

Why: The partner is RNA, so under each adenine write uracil.
T gives A, A gives U, C gives G, G gives C, G gives C, A gives U.
Under the DNA strand the RNA reads 3′-AUGCCU-5′.

In every sample of double-stranded DNA, the amount of adenine equals the amount of thymine.

Why does the amount of adenine equal the amount of thymine?

1. A. Adenine and thymine are the same size
   Size has nothing to do with the count.
   Every adenine is paired one-to-one with a thymine, so the two counts match.
2. B. Each strand carries equal numbers of the four bases
   A single strand can carry any mix of bases.
   Every adenine on one strand is paired with a thymine on the other, so across both strands the two counts match.
3. C. ✓ Every adenine is paired with a thymine on the other strand

Why: Every adenine on one strand is paired with a thymine on the other strand.
So there is one thymine for every adenine, and the two amounts are equal.

A nucleic acid sample contains uracil.

Which nucleic acid is it?

1. A. DNA
   DNA’s four bases are adenine, thymine, guanine and cytosine.
   Uracil is found in RNA and never in DNA.
2. B. ✓ RNA

Why: RNA carries uracil in place of thymine.
DNA never contains uracil, so a sample with uracil is RNA.

A nucleic acid is a single strand with no partner strand.

Must its amount of adenine equal its amount of thymine?

1. A. Yes
   Equal amounts come from one-to-one pairing across two strands.
   A single strand has no partner, so its bases are paired with nothing.
2. B. ✓ No

Why: A single strand has no partner strand, so its bases are paired with nothing.
Nothing forces its amount of adenine to match its amount of thymine.

The base percentages of a nucleic acid sample are in the table below.

Which kind of sample is it?

1. A. ✓ Double-stranded DNA
2. B. Single-stranded DNA
   Adenine matches thymine at 22 %.
   Guanine matches cytosine at 28 %.
   Matching amounts are what two paired strands give.
3. C. RNA
   The sample has thymine and no uracil, so it is DNA.

Why: The sample has no uracil, so it is DNA.
Adenine matches thymine at 22 %.
Guanine matches cytosine at 28 %.
Matching amounts come from two paired strands: double-stranded DNA.

The base percentages of a nucleic acid sample are in the table below.

Which kind of sample is it?

1. A. Double-stranded DNA
   Adenine is 27 % but thymine is 21 %.
   In two paired strands every A has a T, so the two would be equal.
2. B. ✓ Single-stranded DNA
3. C. RNA
   The sample has thymine and no uracil, so it is DNA.

Why: The sample has no uracil, so it is DNA.
Adenine is 27 % and thymine is 21 %, so A does not equal T.
Unequal amounts can only come from a single strand: single-stranded DNA.

The base percentages of a nucleic acid sample are in the table below.

Which kind of sample is it?

1. A. Double-stranded DNA
   The sample contains uracil, and DNA never does.
2. B. Single-stranded DNA
   The sample contains uracil, and DNA never does.
3. C. ✓ RNA

Why: The sample contains uracil.
Uracil is found in RNA and never in DNA, so the sample is RNA.

The base percentages of a nucleic acid sample are in the table below.

Which kind of sample is it?

1. A. Double-stranded DNA
   A equals T, but guanine is 19 % and cytosine is 29 %.
   In two paired strands every G has a C, so the two would be equal.
2. B. ✓ Single-stranded DNA
3. C. RNA
   The sample has thymine and no uracil, so it is DNA.

Why: The sample has no uracil, so it is DNA.
Guanine is 19 % and cytosine is 29 %, so G does not equal C.
Unequal amounts can only come from a single strand: single-stranded DNA.

The base percentages of a nucleic acid sample are in the table below.

Which kind of sample is it?

1. A. Double-stranded DNA
   The sample contains uracil, and DNA never does.
   The uracil test comes before the matching test.
2. B. Single-stranded DNA
   The sample contains uracil, and DNA never does.
3. C. ✓ RNA

Why: The sample contains uracil, so it is RNA.
The uracil test comes first: a sample with uracil is RNA, whatever its percentages.

A laboratory measures the base percentages of a virus particle’s genetic material. They are in the table below.

Which kind of sample is it?

1. A. Double-stranded DNA
   The sample contains uracil, and DNA never does.
2. B. Single-stranded DNA
   The sample contains uracil, and DNA never does.
3. C. ✓ RNA

Why: The virus’s genetic material contains uracil.
Uracil is found in RNA and never in DNA, so this virus carries its genetic information as RNA.

The two strands of a DNA molecule are held together along their whole length.

Which bonds hold them together?

1. A. Covalent bonds between the two backbones
   The two backbones never touch.
   The strands hold together only where their bases pair, by hydrogen bonds.
2. B. ✓ Hydrogen bonds between the paired bases

Why: Between the two strands there are only hydrogen bonds, one set at every base pair.
The covalent bonds join the nucleotides along each backbone.

Suppose heat parts the two strands of a DNA molecule. Loose nucleotides then pair onto each strand.

Which of the two strands can direct the building of a new partner strand?

1. A. ✓ Both strands
2. B. Only one of the two strands
   Each base on either strand has one partner.
   So each strand fixes a new partner, base by base.
3. C. Neither strand
   Each base has one partner, so a parted strand fixes what pairs onto it.
   Both strands direct a new partner.

Why: Each base has one partner.
So each parted strand fixes, base by base, the new strand that pairs onto it.
Both strands direct a new partner.

A student looks at the two separated strands and says: “Only one of the two strands holds the information; the other strand is just a copy of it.”

Is the student correct?

1. A. ✓ No: each strand fixes the other, so each strand holds the whole message
2. B. Yes: one strand holds the information and the other is a copy
   The two strands are partners, not copies: each base faces its partner base.
   Either strand fixes the other base by base, so each holds the whole message.

Why: Each base has one partner, so the order of bases on either strand fixes the order on the other.
Either strand alone can direct a new partner.
So each strand holds the whole message.

Suppose, as a new partner strand grows, a cytosine lands opposite an adenine.

How does the wrong base show?

1. A. As a break in the sugar-phosphate backbone
   The backbone is joined by covalent bonds and is not broken by a wrong base.
   A wrong base shows as a pair that does not fit.
2. B. ✓ As a pair whose two bases do not fit each other
3. C. No sign shows; any base pairs with any other base
   Adenine pairs only with thymine, or with uracil on an RNA strand.
   Adenine’s hydrogen bonds do not line up with cytosine’s, so the pair does not fit.

Why: Adenine’s hydrogen bonds line up with thymine’s, not with cytosine’s.
So cytosine opposite adenine does not fit, though the rung is the right width.
The wrong base shows as a mismatched pair.

The base percentages of a nucleic acid sample are in the table below.

Which kind of sample is it?

1. A. ✓ Double-stranded DNA
2. B. Single-stranded DNA
   Adenine matches thymine at 19 %.
   Guanine matches cytosine at 31 %.
   Matching amounts are what two paired strands give.
3. C. RNA
   The sample has thymine and no uracil, so it is DNA.

Why: The sample has no uracil, so it is DNA.
Adenine matches thymine at 19 %.
Guanine matches cytosine at 31 %.
Matching amounts come from two paired strands: double-stranded DNA.

A strand is written from its 5′ end to its 3′ end, and both ends are marked. One strand of DNA reads 5′-GATTC-3′.

Written from its own 5′ end, what does the DNA partner strand read?

1. A. 5′-CTAAG-3′
   These are the partners written left to right under the given strand.
   That starts from the partner’s 3′ end, and a strand is written from its 5′ end.
2. B. ✓ 5′-GAATC-3′
3. C. 5′-GATTC-3′
   Copying the given strand gives the same bases, not each base’s partner.
   Under G sits C, under A sits T.

Why: Under each base write its partner: G gives C, A gives T, T gives A, T gives A, C gives G.
Under the given strand that reads 3′-CTAAG-5′.
The partner’s 5′ end is at the right, so from its own 5′ end it reads 5′-GAATC-3′.

A cell copies its DNA before it divides, and each new cell receives an exact copy.

What lets a DNA molecule be copied exactly?

1. A. The two strands are identical to each other
   The two strands are partners, not copies: under A sits T, under G sits C.
   Each strand fixes the other, and that is what lets the molecule be copied.
2. B. The sugar-phosphate backbone carries the information
   The backbone repeats unchanged along every strand.
   The information is in the order of the bases, and each strand’s order fixes its partner’s.
3. C. ✓ Each strand carries everything needed to rebuild the other

Why: Each base has one partner, so each strand fixes the other base by base.
When the strands part, each directs a new partner identical to the one it lost.
So one molecule becomes two exact copies.

A strand is written from its 5′ end to its 3′ end, and both ends are marked. One strand of DNA reads 5′-CCGAT-3′. An RNA strand pairs with it along its whole length.

Written under the DNA strand, with its ends marked 3′ at the left and 5′ at the right, what does the RNA strand read?

1. A. 3′-GGCTA-5′
   Thymine belongs to DNA.
   The partner strand is RNA, so under adenine sits uracil.
2. B. ✓ 3′-GGCUA-5′
3. C. 3′-AUCGG-5′
   These letters are the RNA strand reversed.
   Under the DNA strand the RNA reads 3′-GGCUA-5′.

Why: The partner is RNA, so under each adenine write uracil.
C gives G, C gives G, G gives C, A gives U, T gives A.
Under the DNA strand the RNA reads 3′-GGCUA-5′.

The base percentages of a nucleic acid sample are in the table below.

Which kind of sample is it?

1. A. Double-stranded DNA
   The sample contains uracil, and DNA never does.
2. B. Single-stranded DNA
   The sample contains uracil, and DNA never does.
3. C. ✓ RNA

Why: The sample contains uracil.
Uracil is found in RNA and never in DNA, so the sample is RNA.

A cell is in interphase, the long stretch between one division and the next.

In which stage of interphase does the cell copy its DNA?

1. A. G1
   G1 is the first growth gap.
   The cell copies its DNA in the stage after G1.
2. B. ✓ S phase
3. C. G2
   G2 is the second growth gap.
   G2 begins after the copying is finished.

Why: S phase is the copying stage of interphase.
The S is short for synthesis, the making of new DNA.

A cell holds 12 DNA molecules. In S phase it carries out DNA replication.

How many DNA molecules does the cell hold afterwards?

1. A. 6
   DNA replication adds copies; it does not remove molecules.
   Each of the 12 molecules becomes two, so the cell holds 24.
2. B. 12
   Every one of the 12 molecules is copied.
   Each molecule becomes two, so the cell holds 24.
3. C. ✓ 24

Why: DNA replication copies every DNA molecule.
Each of the 12 molecules becomes two identical molecules.
So the cell holds 24 DNA molecules.

A cell is about to go through meiosis: two divisions in a row.

How many times does it copy its DNA before the two divisions?

1. A. Not at all
   The four cells that meiosis makes each need a complete set of DNA.
   So the cell copies its DNA once, before meiosis I.
2. B. ✓ Once, before the first division
3. C. Twice, once before each division
   Nothing is copied between meiosis I and meiosis II.
   The one copying happens before meiosis I.

Why: A cell entering meiosis copies its DNA once, in S phase, before meiosis I.
Meiosis II follows with no copying between the two divisions.

A cell divides at midnight.

When did the cell copy its DNA?

1. A. ✓ Before midnight, in S phase
2. B. At midnight, while it divided
   During the division the cell shares out DNA that is already copied.
   The copying happened earlier, in S phase.
3. C. After midnight, in each daughter cell
   Each daughter cell needs a complete genome as soon as it forms.
   So the copying happened before the division.

Why: A cell copies its DNA in S phase, before it divides.
So the copying happened before midnight.

Now imagine a cell skips S phase and divides anyway.

Which of the following describes the DNA each daughter cell receives?

1. A. ✓ Only part of one set
2. B. One complete set
   The DNA was not copied, so there is only one set to share between two cells.
   Each daughter cell gets only part of it.
3. C. Two complete sets
   Two complete sets would need the DNA copied twice.
   This cell did not copy its DNA at all.

Why: The cell skipped S phase, so its DNA was not copied.
One set of DNA is shared between two daughter cells.
So each daughter cell receives only part of one set.

A cell copies one of its DNA molecules.

Why can one DNA molecule be copied exactly?

1. A. The two strands are identical copies
   The two strands are partners, not copies: where one has adenine, the other has thymine.
   Each strand can rebuild the other.
2. B. The backbone holds the information
   The information is in the order of the bases, not in the backbone.
   Each strand’s bases can rebuild the other strand.
3. C. ✓ Each strand can rebuild the other

Why: Each base pairs with only one partner base.
So each strand carries everything needed to rebuild the other.
That is why one molecule can be copied exactly.

Two DNA strands pair base to base all the way along.

What are the two strands called?

1. A. Identical
   Identical strands would have the same bases at each position.
   Paired strands have partner bases at each position.
2. B. Parallel
   Paired strands lie side by side, but the word for strands that pair all the way along is complementary.
3. C. ✓ Complementary

Why: Two strands whose bases pair all the way along are complementary.

One DNA molecule with both strands dark is copied once. In the drawings below, dark marks an old strand and light marks a new strand.

Which drawing shows the two molecules the copying gives?

1. A. ✓ Drawing 1
2. B. Drawing 2
   In drawing 2, dark and light pieces alternate along every strand.
   A new strand is built against a whole old strand and stays paired with it.
3. C. Drawing 3
   In drawing 3, one molecule has both dark strands.
   The two old strands come apart, and each one pairs with a new strand.

Why: The two old strands come apart.
A new strand is built against each old strand and stays paired with it.
So each molecule is one dark strand and one light strand: drawing 1.

A student looks at the two molecules made by copying one DNA molecule and says: “One of the two molecules keeps both old strands.”

Is the student correct?

1. A. ✓ No: the two old strands end up in different molecules
2. B. Yes: one molecule keeps both old strands
   The two old strands come apart before the copying.
   Each old strand ends up paired with a new strand, in a different molecule.

Why: The two old strands come apart.
Each old strand serves as a template for a new strand and stays paired with it.
So each molecule keeps one old strand, not both.

One DNA molecule is copied once. Then one of its two daughter molecules is heated until its strands come apart.

How many of its two strands are old?

1. A. None
   A molecule with no old strand would be all new.
   Each molecule made by the copying keeps one old strand.
2. B. ✓ One
3. C. Two
   Two old strands in one molecule would mean the old strands stayed together.
   They came apart, and each took a new partner.

Why: Each molecule made by the copying is one old strand paired with one new strand.
Heating parts the two strands.
So one of the two strands is old.

A DNA molecule is being copied.

What is a template strand?

1. A. The new strand built during the copying
   The new strand is built against the template strand; it is not the template.
2. B. The two paired strands of one molecule
   A template strand is one strand, the one a new strand is built against.
3. C. ✓ A strand whose bases set the new strand’s bases

Why: A template strand is a strand whose sequence fixes the sequence of the new strand built against it.

A DNA molecule is copied.

What is semiconservative replication?

1. A. ✓ Copying that leaves each new molecule half old and half new
2. B. Copying that leaves one molecule all old and one all new
   One molecule all old and one all new would keep the old strands together.
   In semiconservative replication each old strand pairs with a new strand.
3. C. Copying that mixes old and new pieces along every strand
   Old and new pieces along one strand never happen.
   In semiconservative replication each strand is whole: one old, one new.

Why: Semiconservative replication leaves each new molecule half old and half new: one old strand paired with one new strand.

During copying, a new strand is built against an old strand.

Which of the two is the template strand?

1. A. ✓ The old strand
2. B. The new strand
   The new strand’s bases are set by the old strand, not the other way round.
   The old strand is the template strand.

Why: The old strand’s sequence fixes the new strand’s sequence.
So the old strand is the template strand.

One DNA molecule goes through semiconservative replication once, giving two molecules.

How many of the two molecules contain an old strand?

1. A. One of them
   Each old strand ends up in a different molecule, paired with a new strand.
   So both molecules contain an old strand.
2. B. ✓ Both of them

Why: The two old strands come apart and each pairs with a new strand.
So both molecules contain one old strand.

After a copying, one molecule is found to be all new: both of its strands are new.

Could one round of semiconservative replication of one all-old molecule have made this molecule?

1. A. Yes
   One round gives two molecules, each with one old strand.
   A molecule with two new strands has no old strand.
2. B. ✓ No

Why: One round of semiconservative replication gives two molecules.
Each of them is one old strand paired with one new strand.
So neither molecule is all new.

One all-dark DNA molecule goes through two rounds of copying. In the drawings below, dark marks an old strand and light marks a new strand.

Which row shows the four molecules after the second round?

1. A. Row 1
   In row 1 every molecule carries a dark strand.
   There are only two dark strands, so only two of the four molecules can carry one.
2. B. Row 2
   In row 2 one molecule has both dark strands.
   The two dark strands came apart in round one and are in different molecules.
3. C. ✓ Row 3

Why: There are only ever two dark strands, in two different molecules.
The other two molecules are all light.
Row 3 shows two half-dark molecules and two all-light molecules.

One all-dark DNA molecule goes through three rounds of copying. A student says: “Only two of the molecules carry a dark strand.”

Is the student correct?

1. A. ✓ Yes: only the two original strands are dark, one in each of two molecules
2. B. No: each round doubles the dark strands, so more and more molecules carry one
   Every strand the cell builds is new, and new strands are light.
   The two dark strands are the original pair, and each sits in one molecule.

Why: Every new strand is light, so the two dark strands are the only dark strands there are.
Each round, each dark strand is copied and stays in one molecule.
So after three rounds, only two molecules carry a dark strand.

A cell copies its DNA and then divides.

Why does the copying come before the division?

1. A. So the cell has time to grow before it divides
   The cell grows in G1 and G2.
   The copying is there so that each daughter cell can receive a complete set of DNA.
2. B. So the DNA coils tightly before the cell divides
   The DNA coils tightly for the division itself.
   The copying is there so that each daughter cell can receive a complete set of DNA.
3. C. ✓ So each daughter cell receives a complete set of DNA

Why: Each daughter cell needs a complete genome.
Only a copied set can be shared as two complete sets.
So the copying comes before the division.

A strand is written from its 5′ end to its 3′ end, and both ends are marked. A template strand reads 5′-GGTACA-3′.

Written under the template strand, with its ends marked 3′ at the left and 5′ at the right, what does the new strand built against it read?

1. A. 3′-GGTACA-5′
   These are the template’s own letters, not their partners.
   Under each base sits its partner: G gives C, T gives A, A gives T.
2. B. ✓ 3′-CCATGT-5′
3. C. 3′-TGTACC-5′
   These are the partners read from the wrong end of the template.
   Under the template’s first base, G, sits C.

Why: Each base on the template pairs with only one partner base.
Under each base write its partner: G gives C, G gives C, T gives A, A gives T, C gives G, A gives T.
Under the template that reads 3′-CCATGT-5′.

A cell is in G1, the first growth gap of interphase.

Has the cell copied its DNA yet in this cycle?

1. A. Yes
   G1 comes before S phase.
   The cell copies its DNA in S phase, so in G1 the copying has not happened yet.
2. B. ✓ No

Why: The cell copies its DNA in S phase.
G1 comes before S phase.
So in G1 the cell has not yet copied its DNA.

One all-dark DNA molecule goes through ten rounds of copying, giving 1024 molecules.

How many of the 1024 molecules carry a dark strand?

1. A. ✓ 2
2. B. 10
   The count of dark strands does not grow by one each round.
   The two dark strands are never made again, so two molecules carry one.
3. C. 1024
   Only two of the 1024 molecules contain a dark strand.
   The rest are all light.

Why: The dark strands are never made again.
There are only ever two of them, in two different molecules.
So 2 molecules carry a dark strand, whatever the round.

A DNA molecule goes through semiconservative replication.

What does each old strand do during the copying?

1. A. The old strand pairs again with the other old strand
   The two old strands come apart and do not pair with each other again.
   Each old strand pairs with a new strand.
2. B. ✓ The old strand serves as a template for a new strand
3. C. The old strand breaks into nucleotides that the cell reuses
   The old strand stays whole.
   A new strand is built against it, base by base.

Why: The two old strands come apart.
Each base on an old strand pairs with only one partner base, so the old strand fixes the new strand built against it.
Each old strand is a template strand.

One DNA molecule is copied once by semiconservative replication.

What is each of the two molecules made of?

1. A. ✓ One old strand and one new strand
2. B. Two old strands, or two new strands
   The two old strands come apart, and each pairs with a new strand.
   So no molecule has two old strands.
3. C. Old and new pieces along each strand
   Each new strand is built whole against one old strand.
   No strand is made of old and new pieces.

Why: The two old strands come apart.
Each old strand serves as a template for a new strand and stays paired with it.
So each molecule is one old strand and one new strand.

One all-old DNA molecule goes through two rounds of semiconservative replication, giving four molecules.

How many of the four molecules carry an old strand?

1. A. None
   The two old strands are never destroyed.
   Each old strand sits in one of the four molecules, so two molecules carry an old strand.
2. B. ✓ Two
3. C. Four
   There are only two old strands.
   So only two of the four molecules can carry one; the other two are all new.

Why: There are only ever two old strands.
After round two they sit in two different molecules.
So two of the four molecules carry an old strand, and two are all new.

A scientist builds glucose from a heavier form of carbon, ¹³C, and feeds it to cells.

Which of the following can tell the heavier carbon apart from ordinary carbon?

1. A. ✓ An instrument that measures mass
2. B. The cells’ enzymes
   Enzymes treat the heavier form of an atom exactly like the ordinary form.
   Only an instrument tells the two apart, by mass.
3. C. Neither
   An instrument can tell the two forms apart by their mass.
   That is what makes the heavier form a label.

Why: Enzymes treat ¹³C exactly like ordinary carbon.
An instrument can tell ¹³C apart by its greater mass.
So the heavier carbon is a label the scientist can trace.

Bacteria whose DNA started fully heavy copy it once in light nitrogen by semiconservative replication, giving two molecules.

How many heavy strands does each of the two molecules carry?

1. A. None
   Each new strand pairs with an old strand, and every old strand is heavy.
   So each molecule carries one heavy strand.
2. B. ✓ One
3. C. Two
   The two old strands come apart before the copying.
   So no molecule keeps both heavy strands.

Why: The two heavy strands come apart.
Each heavy strand takes a new, light partner.
So each of the two molecules carries one heavy strand.

Bacteria whose DNA started fully heavy copy it once in light nitrogen by semiconservative replication, and a centrifuge spins the DNA. Heavy DNA settles lower in the tube than light DNA, and half-heavy DNA settles between them. Three tubes are drawn below, numbered 1, 2 and 3.

Which tube shows the bands after round one?

1. A. Tube 1
   A heavy band needs molecules with two heavy strands, and a light band needs molecules with none.
   After round one every molecule has one heavy strand and one light strand.
2. B. Tube 2
   A light band needs molecules with two light strands.
   After round one every molecule still carries one heavy strand.
3. C. ✓ Tube 3

Why: After round one every molecule has one heavy strand and one light strand.
Every molecule is half-heavy DNA, so all of them settle at the half-heavy mark.
Tube 3 shows one half-heavy band.

Bacteria whose DNA started fully heavy copy it twice in light nitrogen by semiconservative replication, giving four molecules.

Which of the following describes the four molecules?

1. A. One half-heavy and three light
   Each of the two heavy strands ends up in its own molecule.
   So two molecules are half-heavy, not one.
2. B. ✓ Two half-heavy and two light
3. C. Four half-heavy
   There are only two heavy strands, so only two molecules can carry one.
   The other two molecules have two light strands each.

Why: There are only ever two heavy strands, and they sit in two different molecules.
Those two molecules are half-heavy.
The other two molecules have two light strands each: light DNA.

Bacteria whose DNA started fully heavy copy it twice in light nitrogen by semiconservative replication, and a centrifuge spins the DNA. Heavy DNA settles lower in the tube than light DNA, and half-heavy DNA settles between them. Three tubes are drawn below, numbered 1, 2 and 3.

Which tube shows the bands after round two?

1. A. ✓ Tube 1
2. B. Tube 2
   After round two, two of the four molecules have two light strands.
   Those two molecules make a light band as well.
3. C. Tube 3
   A heavy band needs molecules with two heavy strands.
   The two heavy strands came apart in round one and never pair again.

Why: After round two, two molecules are half-heavy and two are light.
Half-heavy DNA settles at the half-heavy mark and light DNA at the light mark.
Equal numbers give equally thick bands: tube 1.

Suppose old and new are mixed along every strand. Bacteria whose DNA started fully heavy copy it once in light nitrogen, and a centrifuge spins the DNA.

How many bands does the tube show?

1. A. ✓ One
2. B. Two
   Two bands need two kinds of molecule.
   After round one every molecule is the same: half of every strand old, half new.
3. C. Three
   Three bands need three kinds of molecule.
   After round one every molecule is the same: half heavy.

Why: After round one, half of every strand is old and half is new.
Every molecule is the same, half heavy.
So all the molecules settle at one height: one band.

Suppose old and new are mixed along every strand. Bacteria whose DNA started fully heavy copy it twice in light nitrogen, and a centrifuge spins the DNA.

Does the tube show a light band?

1. A. Yes
   A light band needs molecules with no heavy nitrogen at all.
   Here every strand still carries some old, heavy pieces.
2. B. ✓ No

Why: Every strand is a mixture of old and new pieces.
So every molecule still carries some heavy nitrogen.
No molecule is light DNA, and the tube shows no light band.

Bacteria whose DNA started fully heavy copy it three times in light nitrogen by semiconservative replication, and a centrifuge spins the DNA. Heavy DNA settles lower in the tube than light DNA, and half-heavy DNA settles between them. A thicker band means a bigger share of the molecules. Three tubes are drawn below, numbered 1, 2 and 3.

Which tube shows the bands after round three?

1. A. Tube 1
   A thick half-heavy band needs most molecules to carry a heavy strand.
   Only two of the eight molecules carry one; the other six are light.
2. B. Tube 2
   Equally thick bands need equal numbers of half-heavy and light molecules.
   After round three there are two half-heavy molecules and six light ones.
3. C. ✓ Tube 3

Why: After round three there are 8 molecules.
The two heavy strands sit in 2 half-heavy molecules; the other 6 are light.
The light band holds three times as many molecules, so it is the thicker band: tube 3.

Bacteria whose DNA started fully heavy copy it once in light nitrogen, and a centrifuge spins the DNA. Heavy DNA settles lower in the tube than light DNA, and half-heavy DNA settles between them. The tube is drawn below.

Which way of copying does this tube rule out?

1. A. ✓ Old strands kept together
2. B. Each old strand with a new partner
   Each old strand with a new partner gives molecules with one heavy strand each.
   Those settle at the half-heavy mark, the band the tube shows.
3. C. Old and new mixed along every strand
   Old and new mixed along every strand gives molecules that are half heavy after round one.
   Those settle at the half-heavy mark, the band the tube shows.

Why: Old strands kept together gives one molecule with two heavy strands and one with two light strands.
That is a heavy band and a light band.
The tube shows one half-heavy band instead, so that way of copying is ruled out.

Bacteria whose DNA started fully heavy copy it once in light nitrogen, and the spun DNA shows one half-heavy band. A student says: “After this one round, two ways of copying still fit the tube. A second round is needed to tell them apart.”

Is the student correct?

1. A. ✓ Yes: two ways of copying predict this round-one tube, and only the round-two tube separates them
2. B. No: one half-heavy band after round one fits only one way of copying, so this tube settles it
   Two ways of copying leave every molecule half heavy after round one.
   Both ways predict this tube, so this tube alone cannot settle it.

Why: After round one, each old strand with a new partner gives one half-heavy band.
Old and new mixed along every strand gives one half-heavy band too.
So one tube after round one cannot tell the two apart; round two is needed.

Bacteria whose DNA started fully light copy it once in heavy nitrogen, and a centrifuge spins the DNA. Heavy DNA settles lower in the tube than light DNA, and half-heavy DNA settles between them. Three tubes are drawn below, numbered 1, 2 and 3.

Which tube shows the bands after round one?

1. A. Tube 1
   A light band needs molecules with two light strands.
   After round one every molecule has taken one new, heavy strand.
2. B. ✓ Tube 2
3. C. Tube 3
   A heavy band needs molecules with two heavy strands.
   After round one every molecule still carries its old, light strand.

Why: Each old, light strand takes a new partner built from heavy bases.
So every molecule has one light strand and one heavy strand: half-heavy DNA.
All of them settle at the half-heavy mark: tube 2.

Bacteria whose DNA started fully light copy it twice in heavy nitrogen, and a centrifuge spins the DNA. Heavy DNA settles lower in the tube than light DNA, and half-heavy DNA settles between them.

Which bands does the tube show after round two?

1. A. One heavy band
   One heavy band needs every molecule to have two heavy strands.
   Two of the four molecules still carry an old, light strand.
2. B. ✓ A half-heavy band and a heavy band
3. C. A half-heavy band and a light band
   A light band needs molecules with two light strands.
   The two old, light strands came apart in round one and never pair again.

Why: There are only two old, light strands, and they sit in two different molecules.
Those two molecules are half-heavy.
The other two molecules have two new, heavy strands: heavy DNA.
So the tube shows a half-heavy band and a heavy band.

Suppose the old strands stayed together whenever DNA is copied. Bacteria whose DNA started fully light copy it once in heavy nitrogen, and a centrifuge spins the DNA. Heavy DNA settles lower in the tube than light DNA, and half-heavy DNA settles between them.

Which bands would the tube show?

1. A. One half-heavy band
   One half-heavy band needs every molecule to carry one light strand and one heavy strand.
   With the old strands kept together, no molecule mixes the two.
2. B. A half-heavy band and a heavy band
   A half-heavy band needs a molecule with one light strand and one heavy strand.
   With the old strands kept together, the two old, light strands stay in one molecule.
3. C. ✓ A light band and a heavy band

Why: With the old strands kept together, the two old, light strands stay in one molecule: light DNA.
The two new strands, built from heavy bases, pair with each other: heavy DNA.
So the tube shows a light band and a heavy band.

Bacteria are grown for many generations in food whose nitrogen is the heavier form, ¹⁵N.

Which part of their DNA carries the heavy nitrogen?

1. A. The sugars
   A sugar is built from carbon, hydrogen and oxygen atoms.
   The nitrogen atoms of DNA are in the bases.
2. B. ✓ The bases
3. C. The phosphate groups
   A phosphate group is built from phosphorus and oxygen atoms.
   The nitrogen atoms of DNA are in the bases.

Why: Every base contains nitrogen atoms; the sugar and the phosphate group contain none.
The bacteria build their bases from the food’s nitrogen.
So the bases carry the heavy nitrogen.

Bacteria whose DNA started fully heavy copy it twice in light nitrogen, and the spun DNA shows one band, between the half-heavy mark and the light mark.

Which way of copying gives this band?

1. A. Old strands kept together
   Old strands kept together gives a heavy band in every round.
   This tube shows no heavy band.
2. B. Each old strand with a new partner
   Each old strand with a new partner gives a half-heavy band and a light band after round two.
   This tube shows one band, and none at the light mark.
3. C. ✓ Old and new mixed along every strand

Why: With old and new mixed along every strand, every molecule is the same after each round.
After round two every molecule is 25 % heavy.
A molecule that is 25 % heavy settles between the half-heavy mark and the light mark: one band there.

Suppose old and new were mixed along every strand whenever DNA is copied. Bacteria whose DNA started fully light copy it once in heavy nitrogen, and a centrifuge spins the DNA. Heavy DNA settles lower in the tube than light DNA, and half-heavy DNA settles between them.

At which mark do the molecules settle after round one?

1. A. ✓ The half-heavy mark
2. B. The heavy mark
   A band at the heavy mark needs molecules with two heavy strands.
   Here every strand is half old, light pieces and half new, heavy pieces.
3. C. The light mark
   A band at the light mark needs molecules with no heavy nitrogen.
   Here every strand has taken new, heavy pieces.

Why: With old and new mixed along every strand, half of every strand is old and light and half is new and heavy after round one.
So every molecule is half heavy.
All the molecules settle at the half-heavy mark: one band there.

Bacteria whose DNA started fully heavy copy it once in light nitrogen, and a centrifuge spins the DNA. Heavy DNA settles lower in the tube than light DNA, and half-heavy DNA settles between them. The tube drawn below shows the result a student predicted.

Which way of copying would give the bands the student predicted?

1. A. ✓ Old strands kept together
2. B. Each old strand with a new partner
   Each old strand with a new partner gives molecules with one heavy strand each after round one.
   Those settle as one half-heavy band only.
3. C. Old and new mixed along every strand
   Old and new mixed along every strand gives half-heavy molecules after round one.
   Those settle as one half-heavy band only.

Why: A heavy band needs a molecule with two heavy strands, and a light band a molecule with two light strands.
Only old strands kept together gives both after round one.
So the student predicted old strands kept together.

Bacteria whose DNA started fully heavy copy it four times in light nitrogen by semiconservative replication, giving 16 molecules.

How many of the 16 molecules are half-heavy?

1. A. 1
   One molecule keeping both heavy strands is old strands kept together.
   Here the two heavy strands sit in two different molecules, each half-heavy.
2. B. ✓ 2
3. C. 4
   The count of heavy strands does not grow with the rounds.
   There are only two heavy strands, so two molecules are half-heavy.

Why: The heavy strands are never made again.
There are only ever two of them, in two different molecules.
So 2 of the 16 molecules are half-heavy, and the rest are light.

In a DNA molecule, two strands lie side by side with their bases facing each other.

What holds the two strands together along their length?

1. A. Covalent bonds along each backbone
   The covalent bonds along a backbone join the nucleotides of one strand.
   Hydrogen bonds between paired bases hold the two strands together.
2. B. Covalent bonds between the paired bases
   Paired bases are held by hydrogen bonds, drawn as dashed lines.
   Covalent bonds join the nucleotides of one strand, not the two strands.
3. C. ✓ Hydrogen bonds between the paired bases

Why: Where two bases face each other, hydrogen bonds hold them together.
There is a base pair at every step along the strands.
So hydrogen bonds hold the two strands together along their length.

One strand of DNA is a chain of nucleotides.

Which kind of bond joins one nucleotide to the next along the strand’s backbone?

1. A. ✓ A covalent bond
2. B. A hydrogen bond
   Hydrogen bonds join paired bases across the molecule.
   Along a backbone, sugar joins phosphate by covalent bonds.

Why: A backbone is sugar, phosphate, sugar, phosphate.
A covalent bond joins each sugar to the next phosphate group.
So covalent bonds join the nucleotides along the backbone.

A DNA molecule holds both hydrogen bonds and covalent bonds.

Which kind of bond takes less energy to break?

1. A. ✓ The hydrogen bonds
2. B. The covalent bonds
   A covalent bond is strong and takes much energy to break.
   A hydrogen bond is weak and takes far less.
3. C. The two kinds take the same energy
   The two kinds differ in strength.
   A hydrogen bond is weak; a covalent bond is strong.

Why: A hydrogen bond is a weak attraction between two molecules or two parts of a molecule.
A covalent bond is a strong bond that joins atoms.
So the hydrogen bond takes less energy to break.

A DNA molecule is being copied.

What does helicase do?

1. A. Joins new nucleotides one at a time to a growing new strand
   Joining nucleotides builds the new strand; that is not helicase’s job.
   Helicase parts the two old strands.
2. B. Pairs each new nucleotide with its partner base on the template
   Pairing nucleotides against the template builds the new strand.
   Helicase parts the two old strands so that a template is exposed.
3. C. ✓ Breaks the hydrogen bonds between paired bases and parts the strands

Why: Helicase breaks the hydrogen bonds between paired bases.
So the two strands part, and each is exposed as a template strand.

A DNA molecule is being copied.

What is the replication fork?

1. A. The far end of a template strand
   The far end of a template is the end away from the fork.
   The fork is the point where the two strands come apart.
2. B. ✓ The Y-shaped point where the two strands come apart
3. C. The part of the molecule that is still paired
   The paired part lies ahead of the fork; the fork is at its edge.
   The fork is the Y-shaped point where the strands come apart.

Why: At one point the paired strands come apart into two arms, and the drawing is shaped like a Y.
That point is the replication fork.

A student says: “Helicase cuts through each strand’s backbone to open the molecule.”

Is the student correct?

1. A. Yes: helicase cuts through each strand’s backbone, so the molecule can open
   Cutting a backbone would break a strand into two pieces.
   Helicase breaks the hydrogen bonds between paired bases, and each strand stays whole.
2. B. ✓ No: helicase breaks only the hydrogen bonds between the paired bases; every backbone stays whole

Why: The two strands are held together by hydrogen bonds between the paired bases.
Helicase breaks those hydrogen bonds.
The covalent bonds of each backbone stay in place, so each strand stays whole.

Imagine a drug blocks helicase in a cell, and the cell reaches S phase.

What happens to the cell’s DNA?

1. A. ✓ No fork opens: no template strand is exposed
2. B. A fork opens but no new strand is built against the templates
   Only helicase parts the two strands.
   With helicase blocked, the strands stay paired and no fork opens.
3. C. A fork opens; both strands are copied as usual
   Helicase is blocked, so no hydrogen bonds between paired bases are broken.
   The strands stay paired and no fork opens.

Why: Helicase breaks the hydrogen bonds between the paired bases and parts the strands.
The drug blocks helicase, so the strands stay paired.
No fork opens, so no template strand is exposed.

Helicase is at work on a DNA molecule with a replication fork.

Where on the molecule does helicase sit?

1. A. ✓ At the point of the Y, where the strands part
2. B. At the far end of a template strand
   The far ends of the templates parted long ago.
   Helicase sits where the strands are parting now: the point of the Y.

Why: Helicase breaks the hydrogen bonds where the strands are parting.
The strands part at the point of the Y, the replication fork.
So helicase sits at the point of the Y.

Behind the replication fork, each old strand is on its own.

What is each single strand behind the fork called?

1. A. A new strand
   A new strand has yet to be built there.
   Each parted old strand is a template strand, the pattern for a new strand.
2. B. ✓ A template strand

Why: Each parted strand is an old strand.
A new strand will be built against it, so it is a template strand.

Helicase is at work on a DNA molecule.

At the replication fork, what are the two strands doing?

1. A. ✓ Coming apart
2. B. Joining together
   Helicase at the fork breaks hydrogen bonds; it joins nothing.
   At the fork the two strands are coming apart.

Why: Helicase sits at the fork and breaks the hydrogen bonds between paired bases.
So at the fork the two strands are coming apart.

One strand of DNA has two ends that differ.

What are the two ends of a strand called?

1. A. ✓ A 5′ end and a 3′ end
2. B. A head end and a tail end
   Head and tail name the ends of an animal, not of a strand.
   A strand’s ends are its 5′ end and its 3′ end.
3. C. A left end and a right end
   Left and right depend on how the strand is drawn.
   A strand’s own ends are its 5′ end and its 3′ end.

Why: A strand’s 5′ end carries a free phosphate group, and its 3′ end carries a free –OH.
So the two ends are the 5′ end and the 3′ end.

Two paired DNA strands lie side by side.

How do the two paired strands lie?

1. A. In the same direction
   Paired bases only fit together when the two strands run opposite ways.
   So one strand’s 5′ end lies beside the other’s 3′ end.
2. B. ✓ In opposite directions

Why: The two strands of DNA are antiparallel.
One strand’s 5′ end lies beside the other strand’s 3′ end.

The drawing below shows a replication fork, with four parts lettered J to M.

Which letter marks the replication fork?

1. A. ✓ J
2. B. K
   K marks a single dark strand with its bases exposed: a template strand.
   The fork is the point where the two strands come apart.
3. C. M
   M marks the part where the two strands are still paired.
   The fork is the point where they come apart.

Why: The two strands come apart at one point, and the drawing forms a Y there.
Letter J marks that point: the replication fork.

The drawing below shows a replication fork, with four parts lettered J to M.

Which lettered parts are the template strands?

1. A. K only
   L is also a single dark strand with its bases exposed.
   K and L are both template strands.
2. B. ✓ K and L
3. C. J and M
   J marks the fork and M the paired part.
   The template strands are the two parted arms, K and L.

Why: Behind the fork each old strand is on its own, its bases exposed.
K and L are those two single strands.
So K and L are the template strands.

The drawing below shows a replication fork, with four parts lettered J to M.