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End-of-topic test: DNA Replication

Unit 6 · Topic 6.2 end-of-topic test

Suggested time: about 47 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. When you have answered every question, press Submit the test; the feedback then gives the reasoning for each. 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 table with two columns, when the drug was given, and the share of cells that divided within a day; three rowswhen the drug was givencells that divided within a daynever95 %during interphase, for one hour24 %during mitosis, for one hour93 %
The share of cells that divided within a day, by when the drug was given.

Cells are grown in dishes in a laboratory. The scientists add a drug that blocks DNA polymerase to some dishes for one hour during interphase and to others for one hour during mitosis, then wash it away. The table shows the share of cells that went on to divide within the next day.

Which conclusion do the results support?

Question 2
Four monospace rows with a note beside each: 3′-GTACGA-5′, the strand placed in tube 1; 5′-CATGCT-3′, the marked strand found in tube 1; 3′-TCGTAC-5′, the strand placed in tube 2; 5′-AGCATG-3′, the marked strand found in tube 23′-GTACGA-5′placed in tube 15′-CATGCT-3′marked strand found in tube 13′-TCGTAC-5′placed in tube 25′-AGCATG-3′marked strand found in tube 2
The strand placed in each tube and the marked strand found in it afterwards, each written from one of its ends.

The two strands of a short DNA molecule are separated. Each strand is placed in its own test tube with DNA polymerase, an RNA primer paired to it and marked free DNA nucleotides. Afterwards a marked strand is found in each tube, as shown.

Which conclusion do the two tubes support?

Question 3

One all-old DNA molecule goes through several rounds of copying. Afterwards 62 of the molecules are built only from new strands.

How many rounds of copying were there?

Question 4
Four upright tubes numbered 1 to 4, the first with the marks light, half-heavy and heavy named at its right; each tube carries one or two dark bars at the marked heights, of different thicknesseslighthalf-heavyheavy1234
Four tubes after three rounds of copying in heavy nitrogen, one of them right.

Bacteria whose DNA started fully light copy it three times 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. A thicker band means a bigger share of the molecules. Four tubes are drawn below, numbered 1 to 4.

Which tube shows the bands after round three?

Question 5

Bacteria whose DNA started fully heavy copy it in light nitrogen, and 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.

Which observation rules out old and new being mixed along every strand?

Question 6

A bacterium carries a mutation that makes one of its copying enzymes stop working at high temperature. At that temperature its DNA stays paired from end to end, and no Y-shaped opening appears anywhere along it.

Which enzyme has stopped working?

Question 7
A Y-shaped drawing of DNA: a paired part on the left with an oval at the point of the Y, two dark arms fanning to the right with their tabs exposed; 3′ and 5′ are written at both ends of each arm; letters J, K, L and M sit beside four parts with short leaders5′3′3′5′JKML
A replication fork with four parts lettered.

A replication fork is drawn below, with helicase at the point of the Y. Four parts are lettered J to M.

Which letter marks the part helicase opens next?

Question 8
A table with three columns, enzyme X, plasmids that finished copying, plasmids whose forks stalled part-way; two rowsenzyme Xfinished copyingforks stalled part-wayworking78 %14 %blocked22 %73 %
Plasmids copied with enzyme X working or blocked.

Plasmids are copied in test tubes with every part of the copying machine present. In some tubes one enzyme, here called enzyme X, is blocked. The table shows the share of plasmids that finished copying and the share whose forks stalled part-way.

Which statement about the tubes with enzyme X blocked do the results support?

Question 9
Two monospace rows with a note beside each: 3′-TTAGCA-5′, the template strand; beneath it, aligned under its first two bases, 5′-AA-3′, the RNA primer3′-TTAGCA-5′template strand5′-AA-3′RNA primer, paired to the template's first two bases
The template strand and the RNA primer paired to its first two bases.

A template strand and the RNA primer paired to its first two bases are drawn. DNA polymerase is about to add the first DNA nucleotide.

Which nucleotide does DNA polymerase add, and to which end of the primer?

Question 10

In a cell, every new DNA strand begins with a short stretch of RNA, which is later replaced with DNA.

Which statement explains why the strand begins with RNA?

Question 11
One monospace row: 3′-CCGATT-5′, with the note template strand3′-CCGATT-5′template strand
The template strand, written from its 3′ end.

A template strand reads 3′-CCGATT-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?

Question 12
A Y-shaped drawing of DNA: two dark arms on the left with their tabs exposed, a paired part on the right, an oval at the point of the Y. The upper arm is marked 5′ at its far left end and 3′ at the fork; the lower arm 3′ at its far left end and 5′ at the fork5′3′3′5′
A replication fork moving to the right, both templates' ends marked.

A replication fork is drawn below, moving to the right. Both templates' ends are marked, and no new strand has been built yet.

Which statement describes the new strand DNA polymerase builds against the upper template?

Question 13

Suppose a cell's DNA polymerase could join nucleotides to either end of a growing strand, the 5′ end as well as the 3′ end.

Which change at the replication fork would follow?

Question 14

A lagging strand is built from 12 short pieces. Every RNA primer on it has been replaced with DNA.

At how many places between neighboring pieces does ligase join the backbone?

Question 15
A Y-shaped drawing of DNA with its paired part on the left: a solid-outlined oval at the point of the Y and a dashed oval further left on the paired part; two dark arms fan to the right, their ends marked 3′ and 5′; along one arm a long light strand with an arrowhead toward the fork and a dark block at its far end; along the other arm three short light strands with arrowheads pointing away from the fork: the two farther from the fork lie end to end with a small gap between them, a small thick-outlined hollow oval spanning the gap between those two finished pieces, and the third has a dark block at its fork-side end and a gap before it; the middle strand also has a dark block at its fork-side end; a rounded box at the tip of the long light strand. Nine letters J to R sit beside parts with short leaders5′3′3′5′5′3′LMJKPQNRO
A complete replication fork moving to the left, nine parts lettered.

A complete replication fork is drawn below, moving to the left, with nine parts lettered J to R.

Which lettered part joins nucleotides to the 3′ end of a growing new strand?

Question 16

A cell copies its DNA with one part of the copying machine blocked. Forks open along the DNA, and RNA primers lie paired to the exposed templates, but no DNA nucleotide joins any primer.

Which part is blocked?

Question 17

A student says: “A cell with no working ligase makes no new DNA at all.”

Which statement about the student's claim is correct?

Question 18
A Y-shaped drawing of DNA: a paired part on the left with an oval at the point of the Y, two dark arms fanning to the right; 3′ and 5′ are written at both ends of each arm; along the upper arm a long light strand with a dark block at its far end and an arrowhead pointing toward the fork; along the lower arm one short light strand near the far end, with a dark block at its fork-side end and an arrowhead pointing away from the fork; 5′ and 3′ are written at the far ends of the two light strands; letters L and M sit beside the two dark arms with short leaders3′5′5′3′5′3′LM
A replication fork moving to the left, its two templates lettered and both new strands started.

A replication fork is drawn below, moving to the left. Its two template strands are lettered L and M, and the 3′ and 5′ ends of each are written at the fork and at the far end. Both new strands have already been started.

Where is the next RNA primer laid down as the fork exposes more of each template?

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 lake are grown for many generations in heavy nitrogen, so every strand of their DNA is heavy. They are moved to light nitrogen and copy their DNA once, twice and three times. 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. A thicker band means a bigger share of the molecules. The three tubes are drawn below.
Three upright tubes captioned after round one, after round two and after round three; the first has the marks light, half-heavy and heavy named at its right; each tube carries one or two dark bars at the marked heightslighthalf-heavyheavyafter round oneafter round twoafter round three
The DNA after each round of copying in light nitrogen.

(a) Identify the way of copying that the round-one tube alone rules out. (1 point)

A full-credit answer: The round-one tube rules out the old strands staying together.

Check the box for each point your answer earns

Common slip: Naming the mixed way. Old and new mixed along every strand also gives one half-heavy band after round one, so round one does not rule it out.

(b) Describe how the bands change from round one to round two. (1 point)

A full-credit answer: After round one there is one band, at the half-heavy mark.
After round two there are two bands of equal thickness, at the half-heavy mark and the light mark.

Check the box for each point your answer earns

(c) Evaluate the claim that old and new are mixed along every strand, using the round-two tube. (1 point)

A full-credit answer: The claim is rejected.
With old and new mixed along every strand, every molecule keeps some heavy DNA, so no molecule is ever all light.
The round-two tube shows a light band, so some molecules are all light.
So old and new are not mixed along every strand.

Check the box for each point your answer earns

Common slip: Accepting the claim because round two shows two bands. The mixed way gives one band, and never a light one.

(d) Explain why the half-heavy band is thinner after round three than after round two. (1 point)

A full-credit answer: Only the two original heavy strands exist, and each sits in one molecule with a light partner.
So the same two molecules are half-heavy in every round.
After round two those two are 2 of 4 molecules; after round three they are 2 of 8, a smaller share.
A band holding a smaller share of the molecules is thinner.

Check the box for each point your answer earns

Common slip: Saying the half-heavy molecules are lost or that their heavy strands are diluted. The two original heavy strands stay whole, each paired with a light partner, in every round; only their share of the molecules falls.

Free-response score: 0 of 4
Free response 2 · Analyze Model · 4 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 Y-shaped drawing of DNA with its paired part on the right: a solid-outlined oval at the point of the Y and a dashed oval further right on the paired part; two dark arms fan to the left, their ends marked 3′ and 5′; along one arm a long light strand with an arrowhead toward the fork and a dark block at its far end; along the other arm three short light strands with arrowheads pointing away from the fork: the two farther from the fork lie end to end with a small gap between them, a small thick-outlined hollow oval spanning the gap between those two finished pieces, and the third has a dark block at its fork-side end and a gap before it; the middle strand also has a dark block at its fork-side end; a rounded box at the tip of the long light strand. Nine letters J to R sit beside parts with short leaders3′5′5′3′5′3′LMJKPQNRO
A complete replication fork moving to the right, nine parts lettered.

(a) Describe the job of the part lettered K at the position where it is drawn. (1 point)

A full-credit answer: K is topoisomerase, on the paired part ahead of the fork.
Helicase pushes the twist into that paired part, so it twists tighter than a double helix normally is.
Topoisomerase cuts one strand there, lets the extra twist unwind and seals the strand, so the fork keeps moving.

Check the box for each point your answer earns

(b) Explain, using the ends written on the two templates, why the strand lettered Q is made in short pieces. (1 point)

A full-credit answer: Q is built against the template whose 3′ end is at the fork.
A new strand pairs the other way round to its template, so Q's growing 3′ end points away from the fork.
DNA polymerase can add only at that 3′ end, so Q grows away from the fork.
Each time the fork exposes more of Q's template, a new piece must start from a new primer.

Check the box for each point your answer earns

Common slip: Saying Q's template has its 5′ end at the fork. That template gives the one-piece strand, P.

(c) Identify the part lettered N. (1 point)

A full-credit answer: N is an RNA primer.

Check the box for each point your answer earns

(d) Explain what the drawing would show at the join between two pieces of Q if the part lettered R were removed and the copying ran to the end. (1 point)

A full-credit answer: The pieces of Q would stay separate, each a finished piece of DNA, with a break in the backbone between one piece and the next.
R is ligase, the enzyme that joins the backbone between two neighboring pieces.
DNA polymerase fills the space where each primer was but forms no join between pieces, so without R the breaks remain.

Check the box for each point your answer earns

Common slip: Saying that Q is not built at all. Every piece is built by DNA polymerase; only the joining is lost.

Free-response score: 0 of 4
Feedback and scoring guides appear after you submit.
Multiple choice checked: 0 of 18 correct.
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