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

Semiconservative replication results in each DNA molecule having one old and one new strand, as evidenced by the Meselson-Stahl experiment. In conservative replication, old and new strands would be separate.

Step-by-step solution

Understanding Semiconservative Replication

In semiconservative replication, each of the two strands of the DNA molecule serves as a template for creating a new complementary strand. After replication, each DNA molecule consists of one original strand and one newly synthesized strand.

Meselson-Stahl Experiment

The Meselson-Stahl experiment provided evidence for semiconservative replication. They grew E. coli in a medium containing heavy nitrogen (15N) and then switched to medium with light nitrogen (14N). When the DNA was extracted and subjected to density gradient centrifugation, the results supported semiconservative replication.

Expected Results in Conservative Replication

In a conservative model, the original DNA molecule would remain intact, and an entirely new DNA molecule would be synthesized. After the first replication in 14N, two bands would appear: one heavy (15N) and one light (14N), representing the original heavy DNA and the newly synthesized light DNA.

Key concepts

Meselson-Stahl experiment

The Meselson-Stahl experiment is a crucial piece of scientific history that helped confirm the semiconservative nature of DNA replication. In 1958, Matthew Meselson and Franklin Stahl conducted an experiment using the bacterium Escherichia coli (E. coli). They first grew E. coli in a medium containing heavy nitrogen isotope \(^{15}\text{N}\), which incorporated into the bacterial DNA. This made the DNA denser. Later, they transferred these bacteria to a medium with a lighter nitrogen isotope \(^{14}\text{N}\).
During replication, DNA strands in the medium would incorporate the lighter nitrogen. Meselson and Stahl then extracted the DNA at various intervals and used density gradient centrifugation to separate different DNA forms based on density. If DNA replication were semiconservative, after one round of replication, the DNA molecules would consist of one old \(^{15}\text{N}\) strand and one newly synthesized \(^{14}\text{N}\) strand. They observed exactly this: after one generation in light nitrogen, a single intermediate-density band appeared, consistent with semiconservative replication.
After two generations, two bands were found: one of intermediate density and one of light density, confirming that one strand of each DNA molecule acted as a template in replication.

Replication models

Understanding DNA replication requires knowing the three different models that scientists proposed: Conservative, Semiconservative, and Dispersive.
The **Conservative model** suggested that the parental DNA molecule remains intact after replication. Two new strands of DNA would form, leaving the original strands together and producing a wholly new double helix.
The **Semiconservative model** posited that each of the two parental DNA strands would serve as templates for new strands. Thus, each resultant DNA molecule would consist of one old (parental) and one new (daughter) strand. This method ensures genetic consistency across generations.
The **Dispersive model** suggested that all resultant DNA strands would be a mix of old and new DNA. This model hypothesized that the parental DNA was chopped up and reassembled with new DNA into the double helix.
Meselson and Stahl's experiment debunked the conservative and dispersive models, providing evidence for the semiconservative replication model, which is now widely accepted.

DNA synthesis

DNA synthesis is a critical part of cell division and involves several key steps and enzymes. The process starts at specific locations called **origins of replication**. Here, the DNA double helix unwinds with the help of the enzyme **helicase**, and single-strand binding proteins stabilize the separated strands.
The enzyme **primase** synthesizes a short RNA primer to provide a starting point for DNA synthesis. DNA polymerases then extend this primer, adding nucleotides in the 5' to 3' direction to synthesize the new DNA strand.
The **leading strand** is synthesized continuously in the direction of the replication fork, while the **lagging strand** is synthesized discontinuously, creating Okazaki fragments that are later joined together by the enzyme **DNA ligase**.
Importantly, the enzymes involved in DNA replication have proofreading functions to ensure the accuracy of DNA synthesis, maintaining genetic fidelity across cell divisions. This coordinated process allows for efficient and accurate duplication of the genome, ensuring that each daughter cell receives an identical set of genetic instructions.