Question

You decide to repeat the Meselson-Stahl experiment, except this time you plan to grow the E coli cells on light 14N medium for many generations and then transfer them to heavy 15N medium and allow them to grow for 2 additional generations (2 rounds of DNA replication). If the semiconservative model of DNA replication is correct, what is the expected distribution of DNA in the density gradient after one round of replication?1.A single band of intermediate density.2.A single band of light density.3.A single band of heavy density. 4.One band of light density and one band of heavy density. 5.One band of light density and one band of intermediate density.

Answer 1

Answer:

1.A single band of intermediate density.

Explanation:

According to semiconservative model of DNA replication, the two strands of one DNA molecule separate at the time of replication and each strand acts a template for a new strand. So, the resulting new DNA molecule has one old strand and one new strand.

Here, the E coli cells were grown originally in 14N medium. So all the DNA molecules had 14N strands. When they were transferred to a 15N medium, only 15N was available then for further replications. In the first round of replication in new medium, all the old 14N strands (14N14N) separated and formed a new strand each using 15N. Hence all the resulting DNA molecules had one 14N strand and one 15N strand (14N15N).

Since 14N is of lighter density and 15N is of heavier density, 14N15N strands will give band of intermediate density. Since after one round of replication all DNA molecules will be 14N15N, there will be only one band of intermediate density.