End-of-topic test: DNA and RNA Structure

Unit 6 · Topic 6.1 end-of-topic test

Suggested time: about 46 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.

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. Then open the scoring guide and mark your own work against it.

Question 1

A mother's skin cell, one of her egg cells and a skin cell of her child all carry the same order of bases in one gene.

Which of the following explains why the three cells carry the same order?

A. The child's cells rebuilt the gene's bases from the proteins the egg carried B. The mother's cells copied their DNA, and the egg cell carried a copy to the child C. The egg cell carried the gene as RNA, and the child's cells turned the RNA back into DNA D. The same order of bases arises on its own in every human cell

Question 2

A cell with a thick cell wall, drawn with its DNA.

The drawing shows a cell with a thick cell wall.

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

A. Eukaryotic: it has a cell wall B. Eukaryotic: it holds more than one chromosome C. Prokaryotic: it has a cell wall D. Prokaryotic: its DNA is one closed circle in the cytosol

Question 3

A eukaryotic chromosome's DNA is packed in stages.

Which of the following lists the stages in order, from the bare double helix to the condensed chromosome?

A. Double helix → wound on histone beads → beaded string coiled → condensed chromosome B. Double helix → beaded string coiled → wound on histone beads → condensed chromosome C. Double helix → wound on histone beads → condensed chromosome → beaded string coiled D. Wound on histone beads → double helix → condensed chromosome → beaded string coiled

Question 4

A bacterium's plasmid carries a gene for resistance to an antibiotic. When the bacterium divides, one daughter cell receives no plasmid.

Which statement about that daughter cell is correct?

A. It has lost most of its genes B. It cannot divide again C. It keeps every chromosome gene but is no longer resistant to the antibiotic D. It is still resistant, because a plasmid's genes are also on the chromosome

Question 5

Pond water holds both cells and virus particles.

Which of the following observations would show that a particle from it is a virus rather than a cell?

A. It is made partly of protein B. It is smaller than a bacterium C. It reproduces only inside a cell D. It contains a nucleic acid

Question 6

Cells and viruses both carry genetic material.

Which of the following is the genetic material of some viruses and of no cell?

A. A single strand built on ribose, with the bases adenine, uracil, guanine and cytosine B. A double strand built on deoxyribose, with the bases adenine, thymine, guanine and cytosine C. One closed circle of DNA lying free in the cytosol D. Several linear molecules of DNA inside a nucleus

Question 7

A double-stranded DNA molecule is 3,000 base pairs long, so it holds 6,000 bases.

What percentage of its 6,000 bases are pyrimidines?

A. 0 % B. 25 % C. 50 % D. It depends on the order of bases in the molecule

Question 8

In every DNA molecule, each rung is one purine paired with one pyrimidine.

Which of the following follows from this rule?

A. Each strand carries equal numbers of the four bases B. The two backbones stay the same distance apart along the molecule C. The two strands carry the same order of bases D. Every rung is held by the same number of hydrogen bonds

Question 9

An RNA strand lies against a DNA strand, base to base.

Which of the following pairs could form between them?

A. Adenine on the DNA strand with uracil on the RNA strand B. Thymine on the DNA strand with uracil on the RNA strand C. Guanine on the DNA strand with adenine on the RNA strand D. Cytosine on the DNA strand with uracil on the RNA strand

Question 10

Biologists describe base pairing as conserved through evolution.

Which of the following findings would contradict that description?

A. A fungus whose DNA is 40 % adenine while an insect's is 25 % B. A fungus whose DNA pairs adenine with guanine C. A bacterium whose chromosome is a closed circle while an insect's are lines D. An insect whose DNA has more guanine–cytosine rungs than a bacterium's

Question 11

The given DNA strand, 5′-GTACCG-3′, drawn flat.

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

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

A. 5′-CAUGGC-3′ B. 5′-CGGTAC-3′ C. 5′-CGGUAC-3′ D. 5′-GCCAUG-3′

Question 12

The given DNA strand, 5′-AGGCTT-3′, drawn flat.

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

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

A. 3′-AGGCTT-5′ B. 3′-AAGCCT-5′ C. 3′-UCCGAA-5′ D. 3′-TCCGAA-5′

Question 13

A double-stranded DNA sample is 24 % cytosine.

Calculate the percentage of adenine in the sample.

A. 24 % B. 26 % C. 52 % D. 76 %

Question 14

Base percentages of a sample from a leaf and a sample from a virus.

The table shows the base percentages of a sample from a leaf and a sample from a virus.

Which statement about the two samples is correct?

A. Both samples are double-stranded DNA, because in each the paired amounts match B. The virus sample is single-stranded DNA, because it has no thymine C. The leaf sample is single-stranded DNA, because guanine and cytosine are only 15 % each D. The virus sample is RNA, because it contains uracil

Question 15

Base percentages of the student's sample.

A student measures the base percentages of a nucleic acid sample, shown in the table, and says: “The measurement must be wrong, because adenine always equals thymine.”

Which statement about the student's claim is correct?

A. The student is right: every DNA sample has equal adenine and thymine B. The student is wrong: adenine equals thymine only in eukaryotic DNA C. The student is wrong: in a single strand of DNA the amounts need not match D. The student is right: guanine does not equal cytosine either, so the measurement is wrong twice over

Question 16

A bacterium's chromosome and a human chromosome are compared.

Which of the following correctly describes how the two chromosomes are packed?

A. The human chromosome winds on histone beads, and the bacterium's chromosome does not B. Both chromosomes wind on beads of protein, as every chromosome does C. The bacterium's chromosome has no proteins of any kind attached, and the human chromosome has histones D. Histones join the two ends of the bacterium's circle, and the human chromosome's two ends stay free

Question 17

Researchers examining yeast cells find, inside the nucleus, a small closed circle of DNA that lies apart from the chromosomes and is copied on its own.

Is the molecule a plasmid?

A. No: plasmids are found only in bacteria B. Yes: any closed circle of DNA in a cell is a plasmid C. No: a closed circle of DNA lying apart from the chromosomes is a virus that has entered the cell D. Yes: some eukaryotes, yeast among them, carry plasmids

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 · Conceptual Analysis · 4 points

A strand is written from its 5′ end to its 3′ end, and both ends are marked. A virus's genetic material is one single strand of DNA. A short stretch of it reads 5′-GTTACG-3′, as drawn. Inside a cell, loose DNA nucleotides pair onto the strand, and a partner strand made of DNA nucleotides grows against it along its whole length.

The virus's single strand, 5′-GTTACG-3′, drawn flat.

(a) Determine the sequence of the partner strand, written from its own 5′ end. (1 point)

A full-credit answer: Under 5′-GTTACG-3′ the partners are C, A, A, T, G, C, running 3′ to 5′.
Written from its own 5′ end, the partner strand reads 5′-CGTAAC-3′.

Check the box for each point your answer earns

5′-CGTAAC-3′.

Accept 3′-CAATGC-5′ written under the given strand with both ends marked.

Common slip: Writing 5′-CAATGC-3′: the partners in the given strand's order, which run 3′ to 5′, not from the partner's own 5′ end.

(b) Explain why the double-stranded molecule can now be copied exactly, base for base. (1 point)

A full-credit answer: Each base has one partner: adenine with thymine, guanine with cytosine.
So each strand fixes the whole order of bases on the other.
When the strands part, each directs a new partner identical to the one it lost.
So one molecule becomes two molecules with the same order of bases.

Check the box for each point your answer earns

each base has one partner, so each strand fixes its partner base by base and a separated strand rebuilds a partner identical to the one it lost (the copy carries the same order of bases).

Common slip: Saying the two strands are identical. They are partners, not copies; each carries what is needed to rebuild the other.

(c) Suppose instead that any base could pair with any other base. Predict what would happen to the order of bases when the molecule was copied. (1 point)

A full-credit answer: The copies would not match the original.
The order of bases would change with every copy, so the information would be lost.

Check the box for each point your answer earns

the new strands would differ from the original (the order of bases would change at random / the information would be lost).

Common slip: Predicting that no partner strand would form. Bases would still pair; which base sat where would no longer be fixed.

(d) Justify your prediction. (1 point)

A full-credit answer: Hereditary material must carry the same instructions to each new cell.
With specific pairing, the old strand fixes each new base, so the instructions are copied exactly, and a wrong base shows as a pair that does not fit.
Without specific pairing, nothing fixes the new bases and nothing marks a wrong one.
So the instructions would change with every copy.

Check the box for each point your answer earns

without specific pairing the old strand no longer fixes the new strand's bases (and a wrong base no longer shows as a misfit), so the information cannot be copied exactly and handed on unchanged.

Common slip: Restating the prediction. The point is the mechanism: specific pairing is what makes the old strand fix the new one.

Free-response score: 0 of 4

Free response 2 · Analyze Data · 4 points

A laboratory measures the base percentages of double-stranded DNA from three distantly related organisms: a soil bacterium, a flowering plant and a fish. The table shows the results.

Base percentages of double-stranded DNA from three organisms.

(a) Identify the relationship between the base percentages that holds in each organism. (1 point)

A full-credit answer: In each organism, the percentage of adenine equals the percentage of thymine, and the percentage of guanine equals the percentage of cytosine.

Check the box for each point your answer earns

adenine equals thymine AND guanine equals cytosine in each organism.

(b) Explain how base pairing in double-stranded DNA accounts for this relationship. (1 point)

A full-credit answer: Every adenine on one strand is paired with a thymine on the other, and every guanine with a cytosine.
So across the two strands there is one thymine for every adenine and one cytosine for every guanine.
So the two amounts in each pair are equal.

Check the box for each point your answer earns

each adenine is paired with a thymine and each guanine with a cytosine on the other strand, so the paired bases are present in equal amounts.

Common slip: Saying adenine and thymine are the same size. Size is not the reason; one-to-one pairing is.

(c) Support the claim that the base-pairing rules are conserved through evolution, using the data in the table. (1 point)

A full-credit answer: The three organisms are distantly related, and their overall amounts differ: the bacterium is 32 % adenine and the plant 27 %.
Yet in all three, adenine equals thymine and guanine equals cytosine.
The same rules in organisms so distantly related show that each inherited the rules from their common ancestor and kept them.
So base pairing is conserved through evolution.

Check the box for each point your answer earns

evidence AND reasoning — the same A = T and G = C relationship holds in all three distantly related organisms although their overall amounts differ, so the pairing rules were inherited from a common ancestor and kept the same (conserved).

Common slip: Quoting the equal pairs without the reasoning. Support needs the evidence and what it shows: shared rules across distant lines show inheritance from a common ancestor.

(d) A fourth sample, a nucleic acid sample taken from the fish's cells, reads adenine 33 %, uracil 25 %, guanine 22 %, cytosine 20 %. Explain what this composition shows about the sample. (1 point)

A full-credit answer: The sample has uracil and no thymine, so it is RNA.
Adenine 33 % against uracil 25 %, and guanine 22 % against cytosine 20 %: neither pair matches.
In a double-stranded molecule every base is paired with its partner, so the amounts in each pair would match.
They do not, so the sample is a single strand of RNA.

Check the box for each point your answer earns

the sample is single-stranded RNA — RNA because it carries uracil (and no thymine), and a single strand because adenine does not equal uracil (or guanine does not equal cytosine): in a double strand every base is paired, so the amounts would match.

Common slip: Calling the sample DNA with a faulty measurement. Uracil marks RNA; and in a single strand the amounts need not match, so nothing is wrong with the measurement.

Free-response score: 0 of 4

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Multiple choice checked: 0 of 17 correct.

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