Question

In the classic Meselson and Stahl experiment, Coli are first grown in 15N-enriched media (0th generation) and subsequently switched to 14N media. A DNA sample isolated during the first generation of E. Coli grown in 14N media is subjected to a centrifugation step in a dense CsCl gradient. Assuming that replication occurs in a semiconservative fashion, the DNA sample will _______.

Answer 1

Options are not provided inthe question. Th ecomplete question is as following:

In the classic Meselson and Stahl experiment, Coli are first grown in 15N-enriched media (0th generation) and subsequently switched to 14N media. A DNA sample isolated during the first generation of E. Coli grown in 14N media is subjected to a centrifugation step in a dense CsCl gradient. Assuming that replication occurs in a semiconservative fashion, the DNA sample will _______.

1) run as two bands (of equal intensity), one at the same position as a uniformly 15N-labeled DNA, and one at the same position as a uniformly 14N-labeled DNA.

2) run as one band, at the same position as a uniformly 15N-labeled DNA.

3) run as one band, at the same position as a uniformly 14N-labeled DNA. run as one band, at a position expected for a DNA containing a 14N:15N ratio equal to 7.

4) run as one band, at a position expected for a DNA containing a 14N:15N ratio equal to 1.

Answer:

1)

Explanation:

In 1957, Meselson and Stahl gave an experimental evidence for semi-conservative replication  of DNA, means that each DNA strand act as template for synthesis of new DNA.

For the experiment, they use  E. coli grown in heavy isotope of 15N nitrogen and switch to 14N nitrogen undergoing one, two, or three generations and the DNA samples are collected. The DNA samples are then mix with cesium chloride and separated as light and heavy DNA.

As the replication occurs in a semiconservative fashion, the double DNA sample will run as two bands with intermediate density, having mixtures of 15N and 14N DNA, either of which would have appeared as DNA.

Hence, the correct option is (1).