Question

Escherichia coli fully labelled with ¹⁵N is allowed to grow in ¹⁴N medium. The two strands of DNA molecule of the first generation bacteria have

(a) different density and do not resemble parent DNA

(b) different density but resemble parent DNA

(c) same density and resemble parent DNA

(d) same density but do not resemble parent DNA.

Answer 1

Answer: (a) different density and do not resemble parent DNA

Explanation:

In 1958, Meselson and Stahl devised a model to investigate the model of DNA.

Steps:

They grew E.coli (the experimental organism) in a medium containing ¹⁵NH₄Cl (¹⁵N is the heavy isotope of Nitrogen ¹⁴N). They cultured E.coli for several generations till all the bases in the nucleotides would take up ¹⁵N. This heavy molecule of DNA could be distinguished from the normal DNA containing ¹⁴N by centrifugation in a Cesium Chloride (CsCl) density gradient.

The cells containing DNA labelled ¹⁵N were transferred to a culture medium containing the normal isotope of nitrogen (¹⁴N) and allowed to grow to the generation time for E.coli (50 min at 36 degreeCelsius).

It's not possible to distinguish the two isotopes simply by viewing them, but they can be distinguished based on their weight. N¹⁵ is heavier than N¹⁵ and that's why it looks denser. The separation is done in a centrifuge machine.  

The first generation will have 100% hybrid DNA molecule following the semi-conservative nature of DNA replication.