Analyze a model or visual representation
A model of a replication fork
- Unit 6
- 4 points
- About 10 minutes
You can use a calculator on this question, just like on exam day.
A short question built around a model or diagram of a biological process. You describe what the model shows, explain how its parts relate, add to or use the model, and connect it to a bigger biological idea. On the exam: Question 5 of 6 in Section II; about 10 minutes suggested.
The question and its sources
Figure 1 is a model of one replication fork in a molecule of DNA that is being copied.
Figure 1 (described)
On the right side of the model is the parental DNA double helix that has not yet been copied. The replication fork moves to the right. At the fork is helicase. Ahead of the fork, topoisomerase is bound to the parental helix.
To the left of the fork, the two parental strands are separated. The top parental strand runs 3′ to 5′ from left to right. The bottom parental strand runs 5′ to 3′ from left to right.
New strand A is paired with the top parental strand. It is one long, continuous piece with an RNA primer at its left end, and DNA polymerase is adding nucleotides at its right end, next to the fork.
New strand B is paired with the bottom parental strand. It is made of three short pieces, labeled fragment 1 (farthest from the fork), fragment 2 and fragment 3 (closest to the fork). Each piece began at an RNA primer. A DNA polymerase is adding nucleotides to fragment 3 at its left end, moving away from the fork. Between fragments 1 and 2 is a small gap labeled site X.
Source: Simplified model
Suggested time: 10 minutes
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Part (a)
1 pointDescribe the role of helicase shown in Figure 1.
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Part (b)
1 pointUsing Figure 1, explain why new strand A is made as one continuous piece while new strand B is made in fragments.
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Part (c)
1 pointA cell has a mutation that makes its DNA ligase nonfunctional. Using Figure 1, describe what new strand B would look like at the end of replication in this cell.
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Part (d)
1 pointExplain how the semiconservative replication shown in Figure 1 results in two DNA molecules with the same base sequence as the original molecule.
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