Models of DNA Replication

What are the three proposed models of DNA replication

Conservative, semiconservative, and dispersive replication.

What is conservative DNA replication

The parental double helix remains together, while an entirely new double helix is produced.

What is semiconservative DNA replication

Each daughter DNA molecule contains one parental strand and one newly synthesized strand.

What is dispersive DNA replication

Each daughter strand contains interspersed segments of parental and newly synthesized DNA.

What did the Meselson–Stahl experiment investigate

Which model of DNA replication is used by E. coli.

Which organism was used in the Meselson–Stahl experiment

Escherichia coli (E. coli).

Which nitrogen isotopes were used in the Meselson–Stahl experiment

Heavy nitrogen-15 ((^{15}\mathrm N)) and light nitrogen-14 ((^{14}\mathrm N)).

Why was E. coli initially grown in (^{15}\mathrm N)

To label its existing DNA with heavy nitrogen.

Why were the bacteria transferred to (^{14}\mathrm N) medium

Newly synthesized DNA incorporated light nitrogen, allowing parental and new DNA to be distinguished.

What technique separated DNA according to density in the Meselson–Stahl experiment

Cesium chloride (CsCl) density-gradient centrifugation.

What does a heavy DNA band represent

DNA containing (^{15}\mathrm N), which has a higher density.

What does a light DNA band represent

DNA containing (^{14}\mathrm N), which has a lower density.

What does an intermediate-density DNA band represent

DNA with intermediate overall density, such as hybrid DNA containing one heavy and one light strand.

What DNA banding pattern is expected before transfer to (^{14}\mathrm N) medium

One heavy band, because both parental strands contain (^{15}\mathrm N).

What pattern does conservative replication predict after one generation

One heavy band and one light band.

What pattern does semiconservative replication predict after one generation

One intermediate-density band containing hybrid DNA.

What pattern does dispersive replication predict after one generation

One intermediate-density band containing mixed heavy and light DNA segments.

What pattern does conservative replication predict after two generations

One heavy band and one light band, with more DNA in the light band.

What pattern does semiconservative replication predict after two generations

One intermediate band and one light band.

What pattern does dispersive replication predict after two generations

One band that shifts toward the light position but remains intermediate.

What observation after one generation ruled out conservative replication

A single intermediate-density band instead of separate heavy and light bands.

What observation after two generations supported semiconservative replication over dispersive replication

Separate intermediate and light bands appeared.

What is the overall conclusion of the Meselson–Stahl experiment

DNA replication is semiconservative.

If all DNA is initially heavy, what happens after one semiconservative replication in light medium

All DNA molecules become hybrid, each with one heavy and one light strand.

After two semiconservative generations, what fraction of DNA molecules is hybrid

One-half hybrid and one-half light DNA.

After three semiconservative generations, what fraction of DNA molecules is hybrid

One-quarter hybrid and three-quarters light DNA.