Question

DNA replication is

• A. Conservative
• B. Dispersive
• C. Semi-conservative
• D. None of these

Hint:

The Meselson-Stahl experiment using heavy nitrogen ($^{15}N$) provided definitive evidence for semi-conservative DNA replication. Understanding this experiment helps solidify the concept.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to describe the mechanism by which DNA replicates, specifically referring to how the new DNA molecules are composed of original and newly synthesized strands.

Step 3: Detailed Explanation:
- Semi-conservative replication: This is the established mechanism of DNA replication. During replication, the two strands of the parent DNA molecule separate. Each original strand then serves as a template for the synthesis of a new complementary strand. The result is two new DNA molecules, each consisting of one original (parental) strand and one newly synthesized strand. This mechanism was famously demonstrated by the Meselson-Stahl experiment.
- Conservative replication: In this hypothetical model, the parent DNA molecule would remain entirely intact, and a completely new daughter DNA molecule would be synthesized.
- Dispersive replication: In this hypothetical model, both original and newly synthesized DNA would be interspersed in both daughter molecules.
Therefore, the semi-conservative model is the correct description.

Step 4: Final Answer:
DNA replication is semi-conservative.

Answer 2

This question is best settled by recalling the classic experiment that directly distinguished between the three possible models of DNA replication.

1. Conservative: This model predicted that the original double helix would stay completely intact, and an entirely new double helix would be built alongside it. In the Meselson-Stahl experiment, bacteria were grown in heavy nitrogen (N-15) medium and then shifted to normal nitrogen (N-14) medium. If replication were conservative, after one round of replication there would be two distinct density bands, one fully heavy (old) and one fully light (new). This is not what was observed, so this model was ruled out.
2. Dispersive: This model predicted that both strands of every daughter molecule would be a patchwork mixture of old and new DNA. If this were true, later generations would show DNA of a continuously shifting, blended density rather than settling into two clean bands. This too did not match what was observed after further generations.
3. Semi-conservative: This model predicted that each daughter DNA molecule would have one strand from the parent and one newly made strand. After one round of replication in the shift experiment, all the DNA showed a single band of intermediate (hybrid) density, exactly matching this prediction. After a second round of replication, two bands appeared, one intermediate and one fully light, again matching the semi-conservative model precisely and ruling out both alternatives.
4. None of these: Since the semi-conservative model was confirmed experimentally, there is a correct option among the first three, so this choice does not apply.

The density-gradient results after successive rounds of replication line up exactly with the semi-conservative model and with no other.

Therefore, the correct answer is Semi-conservative.