Which of the following statements best explains the mechanism for DNA replication?

Question

BlackTom AI · Accepted Answer

Semi-conservative replication means each parental DNA strand serves as a template and becomes paired with a newly synthesized complementary strand, so each daughter double helix contains one old (parental) strand and one new strand.

1) Relevant knowledge points and concepts
- DNA is double-stranded and complementary; replication requires separation of the two parental strands and synthesis of complementary new strands.
- Three classical models were proposed: conservative (parental duplex stays intact and an entirely new duplex is made), semi-conservative (each daughter helix has one parental and one new strand), and dispersive (parental and new DNA interspersed within both strands).
- The Meselson–Stahl experiment (density-labeling with 15N then 14N and equilibrium density centrifugation) provided experimental tests of these models by predicting distinct banding patterns after successive replications.

2) Step-by-step reasoning process
- If replication were conservative: after one round in 14N, you'd see one heavy band (original 15N/15N) and one light band (new 14N/14N). That was not observed.
- If replication were semi-conservative: after one round, each daughter duplex would be hybrid (15N/14N) and yield a single intermediate-density band. After a second round, half the molecules would be hybrid (15N/14N) and half entirely light (14N/14N), giving two bands (intermediate + light).
- If replication were dispersive: after one round you also get intermediate density, but after subsequent rounds molecules remain mixed and produce a single band whose density gradually shifts toward light, not the two distinct bands seen experimentally.

3) (No numerical calculations required; experimental predictions described above are the “intermediate steps.”)

4) Why the correct answer is correct
- The experimental outcomes match the semi-conservative predictions: one intermediate band after one generation and two bands (intermediate + light) after the second generation. Mechanistically, strand separation and templated synthesis produce daughter helices each containing one parental and one new strand — exactly the semi-conservative description.

5) Why other options are incorrect
- (a) “Reductive” is not a standard model and incorrectly suggests half the DNA is copied without describing strand composition.
- (c) Dispersive predicts interspersed parental/new segments in both strands, which does not produce the two-band pattern after the second replication.
- (d) Conservative predicts the original double helix remains intact and a wholly new duplex is made; this would yield heavy and light bands immediately, contrary to observed intermediate densities.

Conclusion: The correct mechanism is semi-conservative replication; therefore option b is correct.

Which of the following statements best explains the mechanism for DNA replication?单项选择题

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