Question

Did Meselson and Stahl's experiment involve growing E. coli cells in a medium with the heavy nitrogen isotope (15N) for several generations, followed by switching to a medium with the common nitrogen isotope (14N)? How did they use equilibrium density gradient centrifugation to distinguish between DNAs containing only 15N and those with 14N or a mixture of 15N and 14N? What were the observed results after one and two rounds of replication in the 14N medium, and how did these results align with the predictions of semiconservative replication while contradicting conservative and dispersive replication?

Answer 1

Meselson and Stahl's experiment used equilibrium density gradient centrifugation to confirm semiconservative replication of DNA in E. coli. They differentiated DNA with heavy isotope 15N from lighter 14N DNA. The method produced results that matched semiconservative replication predictions after one and two generations.

Step-by-step explanation:

Meselson and Stahl's experiment did involve growing E. coli cells in a medium with the heavy nitrogen isotope (15N) for several generations, followed by switching to a medium with the common nitrogen isotope (14N). They used equilibrium density gradient centrifugation to distinguish between DNA containing only 15N and those with 14N or a mixture of 15N and 14N. After one round of replication in the 14N medium, the DNA sedimented halfway between the levels of 15N and 14N, signifying a mixture of both isotopes consistent with the semiconservative replication model, where new DNA molecules are composed of one old and one new strand.

After two rounds of replication, lighter bands were observed, which represented DNA strands with both strands containing 14N, alongside intermediate bands from the previous generation. This confirmed semiconservative replication and contradicted both conservative replication, which would have resulted in distinct heavy and light bands, and dispersive replication, which would have produced a gradient rather than distinct bands. The results thus align with the predictions of semiconservative replication.