DNA Replication Models and the Meselson–Stahl Isotope Test

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What is a DNA template strand?

An existing DNA strand whose base sequence guides the formation of a new complementary strand.

How do conservative, semi-conservative, and dispersive replication differ?

Conservative replication keeps the parental strands together; semi-conservative replication puts one parental strand in each daughter molecule; dispersive replication intersperses old and new segments in both strands.

How did nitrogen isotopes help Meselson and Stahl distinguish DNA molecules?

They labeled DNA with heavy $^{15}\mathrm{N}$, then grew bacteria in light $^{14}\mathrm{N}$. The isotope composition affected DNA buoyant density, allowing DNA to be distinguished by its position in a density gradient.

What did DNA look like after one generation in $^{14}\mathrm{N}$?

It formed a single intermediate-density band, consistent with both semi-conservative and dispersive replication.

What pattern after two generations supported semi-conservative replication?

An intermediate-density band and a light-density band appeared, as expected when hybrid DNA produces hybrid and light DNA.

Quiz questions

1. What does it mean for a DNA strand to serve as a template during replication?Answer: Its base sequence guides the assembly of a new complementary strand.
2. Which description defines semi-conservative DNA replication?Answer: Each daughter DNA molecule contains one parental strand and one new strand.
3. Why did Meselson and Stahl first grow bacteria in a medium containing $^{15}\mathrm{N}$?Answer: To label existing DNA with heavy nitrogen for density comparison.
4. After one generation in $^{14}\mathrm{N}$, what would conservative replication predict for DNA bands?Answer: Separate heavy-density and light-density bands.
5. Why did two bands after two generations in $^{14}\mathrm{N}$ support semi-conservative replication?Answer: They represented hybrid DNA and DNA made with two light strands.