Question

In the The Meselson-Stahl experiment, Escherichia coli, grown for a period of time in 15N, was transferred to 14N for one generation of DNA replication, the resulting DNA should have 14N added to all "new" DNA. When DNA was extracted from cells one generation after transfer to light medium and analyzed by analytical ultracntrifugation, this DNA was found:
a. in one band/position in the ultracentrifuge tube, consistent either with semiconservative or dispersive models for replication.
b. in two bands/positions in the ultracentrifuge tube, consistent with conservative replication..
c. in one band/position in the ultracentrifuge tube, consistent only with semiconservative replication.
d. in one band/position in the ultracentrifuge tube, consistent only with dispersive replication.
e. in two bands/positions in the ultracentrifuge tube, consistent either with semiconservative or dispersive models for replication..

Answer 1

Answer:

The correct option among them is option 'C'

The isolated DNA will show one band in the ultracentrifuge tube, which consistent with semiconservative replication.

Explanation:

The Meselson Stahl experiment shows DNA replicates by semiconservative mechanism.E.coli cells initially were grpwn in a medium containing ammonium chloride prepared with heavy isotope of nitrogen (¹⁵N) until all the cellular DNA was labeled. After the cells were transferred to a medium containing the normal light isotope of nitrogen (¹⁴N), samples were removed periodically from the cultures and the DNA in each sample was analyzed by equilibrium density gradient centrifugation. This technique can separate heavy heavy (¹⁵N-¹⁵N) , light light(¹⁴N-¹⁴N) and heavy-light (¹⁵N-¹⁴N) ds DNA into distinct bands.

After one generation of growth, all the extracted DNA had the density of ¹⁵N-¹⁴N DNA. After two generations, approximately half the DNA has density of ¹⁵N-¹⁴N DNA and on the other hand,the density of ¹⁴N-¹⁴N DNA.

With additional generations,a large fraction of the extracted DNA consisted of ¹⁴N-¹⁴N duplexes and ¹⁵N-¹⁵N duplexes never appeared.