Question

Why is the replication of DNA referred to as a semiconservative process? What is the experimental evidence for the semiconservative nature of the process? What experimental results would you expect if replication of DNA were a conservative process?

Short answer

Replication is semiconservative, as shown by the Meselson-Stahl experiment, which produced intermediate and light DNA bands. Conservative replication would have produced separate heavy and light bands, which did not occur.

Step-by-step solution

Define Semiconservative Replication

In the semiconservative model of DNA replication, each of the two resulting DNA molecules retains one original parental strand and incorporates one new complementary strand. This ensures genetic continuity through generations.

Meselson-Stahl Experiment - Background

The experimental evidence for semiconservative replication is provided by the Meselson-Stahl experiment. They used nitrogen isotopes to label the DNA and distinguish between old and new strands.

Meselson-Stahl Experiment - Procedure

E. coli bacteria were grown in a medium containing \(^{15}N\) (a heavy isotope of nitrogen) and then transferred to a medium with \(^{14}N\) (a lighter isotope). DNA was extracted at various intervals and centrifuged to separate based on density.

Interpretation of Experimental Results

After one generation, the DNA formed a single intermediate band, indicating a mix of \(^{15}N\) and \(^{14}N\). After two generations, DNA formed two bands: one intermediate and one light, consistent with semiconservative replication.

Conservative Replication Hypothesis

If DNA replication were conservative, after one generation, we would expect two distinct bands: one heavy band (\(^{15}N\)) and one light band (\(^{14}N\)). However, this was not observed.

Key concepts

Meselson-Stahl Experiment

The Meselson-Stahl experiment is a cornerstone in molecular biology for demonstrating the semiconservative nature of DNA replication. In this experiment, Matthew Meselson and Franklin Stahl used E. coli bacteria and nitrogen isotopes to track DNA synthesis.
They grew the bacteria in a medium containing a heavy isotope of nitrogen, \(^{15}N\). After several generations, the bacterial DNA incorporated this heavy nitrogen, making it denser. The bacteria were then switched to a medium with the lighter isotope \(^{14}N\) and allowed to replicate.
Centrifugation was employed to separate the DNA strands based on density. After one replication cycle, an intermediate density band was observed, suggesting each DNA molecule contained one heavy and one light strand. This intermediate band matched the predictions for semiconservative replication. After two cycles, two bands appeared: one intermediate and one light. These results strongly supported the semiconservative model of DNA replication.

DNA Replication Models

Before the Meselson-Stahl experiment, three models for DNA replication were proposed:

• Conservative Model: The original DNA molecule stays intact, and a completely new molecule is synthesized.
• Semiconservative Model: Each of the two resulting DNA molecules consists of one parental strand and one new complementary strand.
• Dispersive Model: DNA replication results in two molecules with mixed segments of old and new DNA.

The experimental evidence from the Meselson-Stahl experiment dismissed the conservative and dispersive models. The appearance of intermediate density DNA after the first replication cycle and both the intermediate and light densities after the second cycle aligned only with the semiconservative model. This means DNA unzips, and each strand serves as a template for a new complementary strand, ensuring accurate genetic information inheritance.

Nitrogen Isotopes in DNA

Nitrogen isotopes played a crucial role in distinguishing between old and new strands of DNA in the Meselson-Stahl experiment. Here’s why:

• Isotopes: Atoms with the same number of protons but different numbers of neutrons. For nitrogen, \(^{15}N\) is heavier than \(^{14}N\).
• Incorporation in DNA: When E. coli bacteria were grown in \(^{15}N\) medium, their DNA became denser due to the heavier nitrogen. Switch to \(^{14}N\) medium led to the incorporation of this lighter nitrogen into newly synthesized DNA strands.
• Centrifugation: After DNA replication, the density differences were detected by centrifugation, with heavier DNA settling closer to the bottom and lighter DNA forming a band higher up.

By analyzing the positions of these bands after successive generations, the researchers could determine the replication pattern and confirm the semiconservative model. This innovative use of nitrogen isotopes provided clear, empirical evidence that each new DNA molecule consists of one parental and one new strand.