The model that correctly describes the process of DNA replication is

Question

BlackTom AI · Accepted Answer

DNA replication is semiconservative: each daughter DNA molecule contains one parental (old) strand and one newly synthesized strand.

1) Relevant knowledge points and concepts
- Watson–Crick base pairing implies each parental strand can serve as a template for a complementary new strand.
- Three proposed replication models: conservative (parental duplex preserved; new duplex entirely new), semiconservative (each daughter duplex has one old and one new strand), dispersive (parental strands broken into fragments and mixed with new DNA in each strand).
- The Meselson–Stahl experiment (15N/14N density labeling) provided the experimental test distinguishing these models.

2) Step-by-step reasoning process
- Start: bacteria grown in heavy 15N medium produce DNA with both strands labeled heavy.
- Transfer to 14N (light) medium and allow one round of replication (generation 1). Predict outcomes:
  - Conservative: parental heavy duplex remains; newly made duplexes are both light → yields two discrete bands (heavy + light).
  - Semiconservative: each daughter duplex consists of one heavy and one light strand → yields a single intermediate-density band (hybrid).
  - Dispersive: newly synthesized DNA interspersed within both strands → yields a single intermediate-density band as well.
- Observation after generation 1: a single hybrid (intermediate) band. This rules out conservative because conservative would give separate heavy and light bands.

- Allow a second round of replication (generation 2):
  - Semiconservative predicts half the molecules will be light-light (completely light) and half hybrid (heavy-light) → two bands: one light and one intermediate, with the light band increasing in intensity.
  - Dispersive predicts all molecules remain hybrid-like but with reduced average density, producing a single band that shifts closer to light but remains continuous (not two distinct bands).
- Observation after generation 2: two distinct bands (one light, one intermediate). This pattern matches semiconservative and contradicts dispersive.

3) Intermediate steps shown
- Gen0: all heavy.
- Gen1: hybrid only (semiconservative/dispersive), not heavy+light (conservative excluded).
- Gen2: mix of light and hybrid (semiconservative); dispersive would not give two distinct bands.

4) Why the correct answer is correct
The experimental banding pattern (one intermediate after one generation; two bands—light and intermediate—after two generations) matches the semiconservative prediction uniquely.

5) Why other options are incorrect
- a. Conservative: excluded because gen1 would show separate heavy and light bands, not a single hybrid.
- c. Dispersive: excluded because gen2 would not produce two distinct bands (only a single band shifting toward light).
- d. All of the above: impossible because the three models make different, mutually exclusive predictions; only semiconservative matches data.

6) Conclusion
Therefore the correct model is b. the semiconservative model. This matches the provided answer.

The model that correctly describes the process of DNA replication is 单项选择题

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