Practice questions · Topic 6.2

Unit 6 · Practice for the Topic 6.2 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 replication, summed up

Video coming soon

One copying before division; half old, half new; the bands in the tube; the fork, helicase and topoisomerase; the primer and DNA polymerase adding only at the 3′ end; leading and lagging; ligase; what breaks when one part is removed.

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 experiment 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

A student says: “A cell copies its DNA while it divides, so each daughter cell receives half of the copies.”

Which statement about the student's claim is correct?

A. The student is right: the cell copies its DNA and divides at the same time, sharing the copies out B. The student is wrong: the copying comes first, in S phase, and each daughter cell gets a complete set C. The student is wrong: each daughter cell copies its own DNA after the division has finished D. The student is right: each daughter cell needs only half of the parent's DNA to work

Question 2

The template strand, written from its 5′ end.

A strand is written from its 5′ end to its 3′ end, and both ends are marked. One strand of a DNA molecule reads 5′-CTTGAC-3′, as drawn. During copying it serves as a template strand.

Written under it with its ends marked, what does the new strand built against it read?

A. 3′-GAACTG-5′ B. 3′-CTTGAC-5′ C. 5′-GAACTG-3′ D. 3′-GAACUG-5′

Question 3

One all-old DNA molecule goes through 4 rounds of copying.

How many of the molecules made are built only from new strands?

A. 2 B. 14 C. 16 D. 30

Question 4

Suppose the old strands stayed together whenever DNA is copied. Bacteria whose DNA started fully heavy copy it three times 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. A thicker band means a bigger share of the molecules.

Which bands would the tube show?

A. One half-heavy band B. A half-heavy band and a light band, equally thick C. A thin heavy band and a thick light band D. A thick heavy band and a thin half-heavy band

Question 5

Bacteria whose DNA started fully heavy copy it once in light nitrogen, and the spun DNA shows one half-heavy band. A student claims that this tube alone proves each old strand took a new partner.

Which further observation would show whether each old strand took a new partner?

A. The bands after a second round of copying in light nitrogen B. The thickness of the band after round one C. The bands before any copying, in heavy nitrogen D. The number of molecules after round one

Question 6

Helicase and topoisomerase both act on a DNA molecule that is being copied.

Which statement compares what the two enzymes do?

A. Helicase cuts one strand ahead of the fork and seals it; topoisomerase breaks the hydrogen bonds at the fork B. The two enzymes both break the hydrogen bonds between paired bases, helicase at the fork and topoisomerase ahead of it C. The two enzymes both cut one strand and seal it again, helicase at the fork and topoisomerase ahead of it D. Helicase breaks the hydrogen bonds at the fork; topoisomerase cuts one strand ahead of it and seals it

Question 7

A replication fork moving to the left, both templates' ends marked.

A replication fork is drawn below, moving to the left. Both templates' ends are marked. DNA polymerase builds a new strand against the lower template.

Toward which marked end does DNA polymerase move?

A. The lower template's 3′ end, at the fork B. The lower template's 5′ end, at its far end C. The upper template's 5′ end, at the fork D. The upper template's 3′ end, at its far end

Question 8

The template strand, written from its 3′ end.

Both ends of each strand are marked. The template is written 3′ to 5′, so that the new strand built against it reads 5′ to 3′ beneath it. A template strand reads 3′-TAGCCA-5′, as drawn. An RNA primer is paired to it just before its first written base, to the left.

Written with both ends marked, what does the finished new strand read?

A. 3′-ATCGGT-5′ B. 5′-TAGCCA-3′ C. 5′-ATCGGT-3′ D. 5′-AUCGGU-3′

Question 9

A student says: “The leading strand is built against the template whose 5′ end faces the fork.”

Which statement about the student's claim is correct?

A. The student is right: the leading strand's own 3′ end is at the fork, so the template it is built against has its 3′ end there too B. The student is wrong: the leading strand is built against the template whose 3′ end faces the fork, not the one whose 5′ end does C. The student is right: DNA polymerase reads a template 3′ to 5′, so it reads toward the fork only when the template's 5′ end is there D. The student is wrong: the leading strand is built against whichever template is drawn on top of the fork, whatever ends are written on it

Question 10

A cell copies its DNA with one part of the copying machine blocked. Forks open, but each stops after moving a short way, and the DNA ahead of each fork is twisted tighter than usual.

Which part is blocked?

A. Topoisomerase B. Helicase C. DNA polymerase D. Ligase

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 · 4 points

Bacteria from a yogurt culture are grown for many generations in ordinary, light nitrogen, so every strand of their DNA is light. They are moved to heavy nitrogen and copy their DNA once, then twice. After each round a centrifuge spins the DNA. Heavy DNA settles lower in the tube than light DNA, and half-heavy DNA settles between them. The two tubes are drawn below.

The DNA after round one and after round two of copying in heavy nitrogen.

(a) Describe the bands in the tube after round one. (1 point)

Hint: Read the drawn tube against the three marks. Say how many bands, and at which mark.

A full-credit answer: After round one the tube shows one band, at the half-heavy mark.

Check the box for each point your answer earns

one band at the half-heavy mark (no heavy band, no light band).

(b) State how many light strands each molecule carries after round one, using the tube. (1 point)

Hint: Read where the band sits against the three marks. Ask what a molecule at that mark is made of.

A full-credit answer: Each molecule carries one light strand.

Check the box for each point your answer earns

one (the band is at the half-heavy mark, so each molecule is one light strand paired with one heavy strand).

(c) Determine the first round whose tube tells apart each old strand taking a new partner from old and new being mixed along every strand, and justify your answer. (1 point)

Hint: Work out the bands each way of copying predicts after round one, then after round two. Find the first round where the two predictions differ.

A full-credit answer: Round two is the first, because after round one both ways of copying give one half-heavy band.
After round two, each old strand taking a new partner gives a half-heavy band and a heavy band, while old and new mixed along every strand gives one band between the half-heavy and heavy marks.
The round-two tube shows a heavy band, which only each old strand taking a new partner gives.

Check the box for each point your answer earns

round two AND the ground — both ways predict one half-heavy band after round one, and only each old strand taking a new partner predicts a heavy band (two bands) after round two.

(d) For one starting molecule, calculate how many of the molecules after nine rounds carry a light strand. (1 point)

Hint: Only the original strands are light. Count how many original strands there are, and remember that each sits in its own molecule.

molecules

Write down the values in the question:

rounds = 9

molecules = 2^{9} = 512

Light strands = the two original strands, one in each of two molecules:

molecules carrying a light strand = 2

A full-credit answer: After nine rounds, 2 molecules carry a light strand.

Free-response score: 0 of 4

Free response 2 · Analyze Model · 5 points

A complete replication fork is drawn below, moving to the right, with nine parts lettered J to R. The 3′ and 5′ ends of each template are written at the fork and at the far end.

A complete replication fork moving to the right, nine parts lettered.

(a) Identify the part lettered R. (1 point)

A full-credit answer: R is topoisomerase.

Check the box for each point your answer earns

topoisomerase.

(b) State how many RNA primers the strand lettered L and the strand lettered M have each needed to reach the stage drawn, and explain why the two numbers differ. (2 points)

A full-credit answer: L has needed one primer.
M has needed one primer for every piece: two blocks are drawn, and the first piece’s primer is already replaced with DNA, so three in all.
L grows toward the fork in one piece, so it started once.
M grows away from the fork, so each new piece starts when the fork exposes more template.
DNA polymerase adds only to an existing 3′ end, so each piece needs its own primer.

Check the box for each point your answer earns

L one primer; M more than one — two (the primer blocks drawn) or three (counting the joined first piece’s primer, already replaced with DNA) — with the count matching the student’s reasoning. the strand growing away from the fork (M) starts a new piece from its own primer each time more template is exposed, because DNA polymerase adds only to an existing 3′ end; the strand growing toward the fork (L) is one piece started once.

(c) Predict what the drawing would show if the part lettered Q stopped working at the moment drawn. (1 point)

A full-credit answer: Q is helicase, at the point of the Y.
With Q stopped, no more hydrogen bonds are broken, so the fork stops moving and the paired part stays paired.
No new template is exposed.
So the new strands grow only as far as the template already exposed, and then the copying stops.

Check the box for each point your answer earns

the fork stops moving (the paired part stays paired; no new template is exposed), so the new strands stop growing once the exposed template is used.

(d) A student looking at the drawing says: “The part lettered P adds the nucleotides to the new strands.” Evaluate the student’s claim. (1 point)

A full-credit answer: The claim is rejected.
P is ligase, the hollow oval spanning the gap between two pieces of M.
Ligase joins the backbone between two neighboring pieces; it adds no nucleotides.
DNA polymerase, the rounded box lettered O, adds the nucleotides to every new strand.

Check the box for each point your answer earns

the claim is rejected because P (ligase) joins the backbone between neighboring pieces and adds no nucleotides; DNA polymerase (O) adds them.

Free-response score: 0 of 5

Multiple choice checked: 0 of 10 correct.

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