Which of the following statements correctly describes the difference between the leading strand and the lagging strand in DNA replication?

Question

BlackTom AI · Accepted Answer

Question restatement: Which of the following statements correctly describes the difference between the leading strand and the lagging strand in DNA replication?

Option 1: The leading strand is synthesized continuously in the 5'→ 3' direction, while the lagging strand is synthesized discontinuously in the 5'→ 3' direction. This is a true description of the core difference: DNA polymerase adds nucleotides in the 5' to 3' direction, and the leading strand is synthesized as a continuous polymer as the fork opens, whereas the lagging strand is formed in short fragments (Okazaki fragments) away from the fork and then joined. This option aligns with standard DNA replication mechanics.

Option 2: The lagging strand requires an RNA primer, whereas the leading strand does not. In fact, both strands require RNA primers to start synthesis. Each Okazaki fragment on the lagging strand begins with an RNA primer, and the leading strand also starts with a primer at the origin or near the fork as needed. Therefore, this statement is incorrect because it falsely claims only the lagging strand needs primers.

Option 3: The leading strand is synthesized by adding nucleotides to the 3' end of the growing strand, and the lagging strand is synthesized by adding nucleotides to the 5' end. DNA polymerases add nucleotides to the 3' end of the growing chain on both strands; synthesis proceeds in the 5'→3' direction. There is no synthesis toward the 5' end on either strand. This option misstates the directionality of synthesis for both strands, making it incorrect.

Option 4: The leading strand is synthesized in the 3'→ 5' direction in a discontinuous fashion, while the lagging strand is synthesized in the 5'→ 3' direction in a continuous fashion. This reverses the actual situation: both strands are synthesized 5'→3', with the leading strand being continuous and the lagging strand discontinuous. The described directions and continuity are opposite to the biology, so this statement is incorrect.

Synthesis of the analysis: The key concept to anchor here is that DNA polymerase can only extend a strand in the 5' to 3' direction, requiring a continuous template read for the leading strand and short fragments for the lagging strand. The leading strand follows the replication fork and grows smoothly, while the lagging strand grows in fragments that are later connected. The RNA primer requirement is shared between both strands, not unique to one. The incorrect options either flip the directionality, deny primer usage where primers are actually required for both strands, or mischaracterize continuity of synthesis.

Which of the following statements correctly describes the difference between the leading strand and the lagging strand in DNA replication?单项选择题

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