Practice questions · Topic 6.1

Unit 6 · Practice for the Topic 6.1 end-of-topic test

You’ve gone through everything in this topic. The summary video below recaps it all, so you’re ready for the questions.

Watch first: DNA and RNA structure, summed up

Video coming soon

Where the DNA sits in the two kinds of cell; plasmids, and viruses whose genetic material is RNA; a purine with a pyrimidine keeps every rung the same width; A–T, A–U and G–C in every organism; the partner strand, the percentages, and why pairing is what makes DNA copyable.

These are practice questions in the shape of the topic test. Work through them before you take the test; every question tells you what it wanted.

Answer every question. For each multiple-choice question, pick one option and press Check; the feedback gives the reasoning. For the free-response questions, write one short sentence for each step of your reasoning, each on its own line, and make every link clear (so, because, therefore). That is what the exam’s ‘paragraph form’ means for you: linked sentences, not bullet points. The first free-response question walks you through one calculation one part at a time, and you can open a hint for each part; the second is at the level of the test. When you finish a question, open the scoring guide and mark your own work against it.

Question 1

Suppose a cell divides into two new cells before it has copied its DNA.

Which statement about the two new cells is correct?

A. Each new cell holds a complete set, because the instructions are in the cell's proteins too B. Each new cell holds a complete set, because RNA carries a second copy of the instructions C. Between them the two new cells hold one set of instructions, so one lacks part of it D. Neither new cell is harmed, because a cell needs its DNA only when it makes gametes

Question 2

A cell is half a millimeter long, hundreds of times the length of a typical bacterium. Its DNA is many copies of one closed circle lying in its cytosol, with few proteins attached.

Which of the following best identifies the cell type and the evidence for it?

A. Prokaryotic: its DNA is a closed circle in the cytosol B. Eukaryotic: it is far larger than a bacterium C. Eukaryotic: it holds many DNA molecules D. Prokaryotic: it is a single cell

Question 3

A round cell drawn with its DNA; structure X is labeled.

The drawing shows a round cell with its DNA. One structure is labeled X.

Which of the following best identifies structure X?

A. A piece broken off the chromosome B. A virus that has entered the cell C. A cluster of histone proteins D. A plasmid

Question 4

A student looks at a particle with no nucleus and says: “It has no nucleus, so it is a virus.”

Which statement about the student's claim is correct?

A. The student is right: every particle without a nucleus is a virus B. The student is wrong: a bacterium has no nucleus and is a cell C. The student is right: a nucleus is what lets a cell reproduce on its own D. The student is wrong: every virus has a nucleus of its own

Question 5

Which statement about the genetic material of viruses is correct?

A. Many viruses carry DNA, and many others carry RNA B. Every virus carries RNA C. Every virus carries DNA, as every cell does D. Every virus carries both DNA and RNA, one inside the other

Question 6

Two bases facing each other across one rung; the backbones are the two rails.

The drawing shows two bases facing each other between the two backbones of a DNA molecule, at one rung.

Which statement about this rung is correct?

A. Both bases are pyrimidines, so the rung is too narrow B. The two bases have different numbers of rings C. Any two bases can share a rung, so this rung is a normal one D. Both bases are purines, so the rung is too wide

Question 7

The RNA strand, 5′-GAUC-3′, drawn flat.

A strand is written from its 5′ end to its 3′ end, and both ends are marked. An RNA strand reads 5′-GAUC-3′, as drawn. It has paired with a DNA strand along its whole length.

Which of the following is the complementary DNA sequence, written 3′ to 5′?

A. 3′-CUAG-5′ B. 3′-GAUC-5′ C. 3′-CTAG-5′ D. 3′-GATC-5′

Question 8

Every organism ever examined pairs adenine with thymine and guanine with cytosine.

Which of the following explains why the pairing rules have stayed the same in every living thing?

A. Every organism lives in conditions that favor these two pairs of bases over any other pairing B. A cell whose pairing rules changed could not copy its DNA and would leave no descendants C. Organisms exchange DNA with one another often enough to keep the rules the same everywhere D. Each organism works out the pairing rules afresh as it develops from a single cell

Question 9

Heat parts the two strands of a DNA molecule, and a new partner grows against each. A student says: “Each old strand needs its lost partner nearby, to show which bases to add.”

Which statement about the student's claim is correct?

A. The student is right: without the lost partner nearby, the new bases are added by chance B. The student is right: the lost partner is copied first and then swapped in C. The student is wrong: the two old strands read each other to rebuild the partners D. The student is wrong: each base has one partner, so an old strand alone fixes its new partner

Question 10

About two meters of DNA fit inside a nucleus about a hundredth of a millimeter across.

Which of the following is the first stage of the packing that makes this possible?

A. The DNA winds on histone beads B. The chromosome coils into a short thick rod C. The DNA joins its two ends into a circle D. The DNA is cut into 46 short pieces

How to tackle the free-response questions. Read the verb first: describe asks what you see or know; explain asks why or how, so name the mechanism; predict asks what will happen and why; justify asks for the evidence that supports a claim. Each point is earned by one idea, stated in a sentence that names the thing and the mechanism. Extra words earn nothing; a wrong extra can lose the point. If there is a figure or table, use what it shows. When you finish, check the box for each point your answer earns and compare your sentences with the full-credit answer.

Free response 1 · Analyze Data · 5 points

A laboratory reports the base percentages of a double-stranded DNA sample from a soil bacterium. Cytosine is 14 %; no other base is reported.

(a) State the percentage of guanine in the sample. (1 point)

Hint: Look at the pairing rule for cytosine.

Write down the values in the question:

%C = 14 %

Use the pairs: G equals C:

%G = %C = 14 %

A full-credit answer: Guanine is 14 %, because every cytosine is paired with a guanine.

(b) Calculate the percentage left for adenine and thymine together. (1 point)

Hint: Use what the four percentages add up to.

The four bases add to 100 %:

%A + %T = 100 % - %G - %C

Substitute the values into the equation:

%A + %T = 100 % - 14 % - 14 % = 72 %

A full-credit answer: Adenine and thymine together are 72 %.

Accept a value that follows correctly from the student's own answer to (a) (100 % minus twice that answer).

(c) Calculate the percentage of adenine. (1 point)

Hint: Compare the amounts of adenine and thymine in double-stranded DNA.

Use the pairs: A equals T, so each is half of what is left:

%A = \frac{%A + %T}{2}

Substitute the values into the equation:

%A = \frac{72 %}{2} = 36 %

A full-credit answer: Adenine is 36 %.

Accept a value that follows correctly from the student's own answer to (b) (half of that answer).

(d) Predict the percentage of uracil the laboratory would find in the sample, and justify your prediction. (1 point)

Hint: Think about which of the two nucleic acids the sample is.

A full-credit answer: Uracil would be 0 %, because the sample is DNA, and DNA never carries uracil; uracil is found only in RNA.

Check the box for each point your answer earns

0 % AND the reason (DNA never carries uracil; uracil is RNA's base).

(e) A second sample from the same laboratory reads adenine 38 %, thymine 38 %, guanine 12 %, cytosine 12 %. Determine whether this sample is consistent with double-stranded DNA, and state what your decision rests on. (1 point)

Hint: Check each pair of bases, and check which base marks the sample as DNA.

A full-credit answer: The sample is consistent with double-stranded DNA, because adenine equals thymine, guanine equals cytosine, and it carries thymine, not uracil.

Check the box for each point your answer earns

the decision (consistent with double-stranded DNA; accept "is double-stranded DNA") AND what it rests on (A = T and G = C, with no uracil).

Common slip: Deciding from the matching pairs alone. Matching amounts with uracil in place of thymine would be RNA; the thymine is part of the ground.

Free-response score: 0 of 5

Free response 2 · Conceptual Analysis · 4 points

A soil bacterium holds two kinds of DNA molecule. The table shows what a laboratory finds for each.

Two kinds of DNA molecule in one bacterium.

(a) Identify which molecule is the plasmid. (1 point)

A full-credit answer: Molecule 2 is the plasmid.

Check the box for each point your answer earns

molecule 2.

(b) Describe two features in the table that identify it as the plasmid. (1 point)

A full-credit answer: Molecule 2 is far smaller than the chromosome: 8,000 base pairs against 4,200,000.
It is present in about 15 copies while the chromosome is present once, so it is copied on its own.
It carries one gene of its own, not the thousands the chromosome carries.

Check the box for each point your answer earns

two of — much smaller than the chromosome; many copies per cell (copied on its own, apart from the chromosome); carries a gene of its own (one, for resistance) while the chromosome carries the cell's genes.

Common slip: Naming the shape. Both molecules are closed circles, so shape does not separate them.

(c) The bacterium copies its chromosome and then divides, before it makes any more copies of molecule 2. Predict how many copies of molecule 1 and about how many copies of molecule 2 each daughter cell receives. (1 point)

A full-credit answer: Each daughter cell receives one copy of molecule 1 and about half of the 15 copies of molecule 2: about 7 or 8.

Check the box for each point your answer earns

one copy of molecule 1 AND about half of the plasmid copies (about 7 or 8; accept "about half of 15" or "several").

Common slip: Predicting about 15 copies of molecule 2 for each daughter. The stem says no more copies are made before the division, so the 15 copies are shared between the two daughters.

(d) Justify your prediction. (1 point)

A full-credit answer: Molecule 1 is the chromosome: the cell copies it once before dividing and hands one copy to each new cell.
Molecule 2 is a plasmid, copied on its own, apart from the chromosome, to about 15 copies.
No more copies are made before the division, so the two daughters share the 15 copies.
Each daughter therefore receives about half of them.

Check the box for each point your answer earns

the chromosome is copied once and one copy goes to each new cell, while the 15 plasmid copies are not copied again before the division and are shared between the two daughters, so each gets about half.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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