Questions: Leading and Lagging Strand Synthesis

Question 1

At a replication fork moving rightward, one parental template strand runs 3'→5' in the rightward direction and the other runs 5'→3' rightward. Which strand can be synthesized continuously, and why?

• A. Neither — both strands require Okazaki fragments because the fork moves in only one direction
• B. The strand templated by the 5'→3' template, since synthesis will run toward the advancing fork
• C. The strand templated by the 3'→5' template, since DNA polymerase synthesizes 5'→3' and can continuously add nucleotides as the fork exposes new template
• D. Both strands — given sufficient primase, continuous synthesis is possible in both directions

Answer: (C)

DNA polymerase synthesizes 5'→3' — it adds to the 3'-OH end of the growing strand. To do so continuously, it needs the template to run 3'→5' in the direction the fork is traveling, so that new template is always being exposed in the correct orientation. The template strand running 3'→5' toward the right satisfies this: polymerase rides the template continuously as the fork opens. The other template (5'→3' rightward) would require synthesis running 3'→5', which DNA polymerase cannot do — hence Okazaki fragments must be synthesized in short 5'→3' bursts running away from the fork.

Question 2

A mutation inactivates DNA ligase in a cell. What would be the most direct consequence for DNA replication?

• A. The lagging strand would be synthesized continuously since ligase normally fragments it into Okazaki pieces
• B. Okazaki fragments would not be synthesized, since ligase initiates fragment production
• C. Okazaki fragments would be produced and their RNA primers would be removed, but the resulting DNA fragments could not be joined into a continuous lagging strand
• D. The leading strand would become discontinuous since it requires ligase for elongation

Answer: (C)

Ligase seals the nick between adjacent Okazaki fragments after RNA primer removal and gap-filling — it is the final step, not an initiating step. Without ligase, all the earlier steps proceed normally: the fork opens, primase lays down RNA primers, DNA polymerase synthesizes Okazaki fragments, and RNase H/DNA polymerase I remove the RNA primers and fill the gaps. But the resulting nicked lagging strand — with correctly synthesized DNA pieces sitting adjacent to each other with gaps between their 3'-OH and 5'-phosphate ends — cannot be sealed. The leading strand is unaffected because it is synthesized as one continuous molecule requiring no ligation.

Question 3

The lagging strand is synthesized in the 3'→5' direction so that the polymerase can follow the replication fork as it opens.

• T. True
• F. False

Answer: False

DNA polymerase always synthesizes 5'→3' — this is the fundamental constraint that creates the lagging strand problem in the first place. The lagging strand is synthesized 5'→3', but in short Okazaki fragments that run away from the fork rather than toward it. Each fragment is initiated by a new RNA primer laid down by primase on the newly exposed template, and polymerase extends it 5'→3' in the direction opposite to the fork's movement. The direction of synthesis never changes — only the strategy for accommodating the antiparallel geometry changes.

Question 4

Okazaki fragments are not a design flaw but an unavoidable solution to the problem of replicating a template strand that runs antiparallel to the direction of fork movement.

• T. True
• F. False

Answer: True

This is a common misconception to correct: students sometimes see the lagging strand's complexity — repeated priming, fragment synthesis, primer removal, gap-filling, ligation — as evidence that it is somehow inferior or error-prone. It is neither. Okazaki fragments are the cell's elegant solution to a geometric constraint: since DNA polymerase can only synthesize 5'→3' and the two template strands run in opposite directions, one strand simply cannot be synthesized continuously. The cell cannot change the directionality of polymerase; Okazaki fragments are what results from working within that constraint.

Question 5

Why does the lagging strand require Okazaki fragments? What two properties of DNA replication make continuous lagging-strand synthesis impossible?

Model answer: Two constraints together make continuous lagging-strand synthesis impossible. First, DNA polymerase can only synthesize new DNA in the 5'→3' direction — it adds nucleotides to the 3'-OH end of the growing strand and cannot work backward. Second, the DNA double helix is antiparallel: the two template strands run in opposite directions. As the replication fork advances, the leading strand template runs 3'→5' in the direction of fork movement, so polymerase can follow it continuously. But the lagging strand template runs 5'→3' in the direction of fork movement, which would require synthesis in the 3'→5' direction — impossible. Instead, as the fork opens a stretch of this template, polymerase synthesizes a short fragment (5'→3') running away from the fork, then stops, waits for more template to be exposed, and repeats. The result is a series of Okazaki fragments that must be joined by ligase after primer removal.

This is a case where understanding two independent constraints (polymerase directionality + antiparallel geometry) is essential. Students who know only one constraint cannot explain why the asymmetry exists. The logical chain is: antiparallel + 5'→3' only = one strand runs the wrong way for continuous synthesis = Okazaki fragments are the only solution.