Question

Meselson and Stahl grew bacteria for many generations in medium containing heavy 15N, then shifted the bacteria into medium containing light 14N for one, two, or three rounds of DNA replication. The DNA was extracted after each round and the density of the DNA molecules was determined by density gradient centrifugation. After the first round of replication, Meselson and Stahl saw only one DNA band of density intermediate to DNA containing only 15N or 14N. After this observation, which hypothesis for DNA replication could be eliminated?

Answer 1

Answer:

The correct answer is conservative model.

Explanation:

There are 3 hypothesized DNA replication models: semi-conservative, conservative and dispersive model (see figure). Heavy isotope of nitrogen makes diference when density gradient centrifugation experiment conducted and two diffent bands contaning different isotopes of nitrogen are observed. With one band only containing 14N means bacteria only copies one strand in one replication which supports semi conservative model.