Question

Bacteria were grown in a medium containing a heavy isotope of nitrogen (N15) for many generations and all their DNA contained much heavy nitrogen only. A bacterium of this type was transferred to a normal medium and allowed to duplicate. After two divisions of heavy DNA is likely to be that
(a) Only one daughter cell will have heavy DNA
(b) Two daughter cells have normal DNA and the other two
(c) All daughter cells heavy DNA
(d) Half daughter cells have heavy DNA and the other half have normal DNA.

Answer 1

Hint: In eukaryotic cells, DNA divides semi conservatively i.e, one of the daughter DNA has one strand from the parent DNA and another strand is newly formed.

Complete answer:
Duplication of DNA is by the semi-conservative method that was first shown in escherichia coli and later it was observed in higher organisms such as plants and animals. When a DNA with heavy nitrogen is transferred into a normal medium the first generation contains two daughter cells with half heavy nitrogen and half normal DNA. In the second generation, two daughter cells contain normal DNA and the other half contains both normal DNA and heavy DNA because the DNA divides semi conservatively.

Additional Information: - The two scientists named Matthew Meselson and Franklin Stahl performed experiments on E.Coli by placing it in the medium containing ${ NH }_{ 4 }Cl$ that contains ${ N }^{ 15 }$ isotope.
- ${ N }^{ 15 }$ was transferred into newly synthesized DNA. By centrifugation in cesium chloride, this heavy DNA can be distinguished from the normal DNA.
- The DNA extracted from the first generation after the transfer from ${ N }^{ 15 }$ medium to normal medium had a hybrid and or intermediate only and the DNA extracted from the second generation has equal amounts of hybrid DNA and normal DNA.
So, the correct answer is ‘ half daughter cells have heavy DNA and the other half have normal DNA’.

Note: Time taken by the E.Coli to divide is 20minutes. The DNA collected from the culture is extracted after 20min and the second generation DNA is extracted after 80min.
 half daughter cells have heavy DNA and the other half have normal DNA.
${ N }^{ 15 }$ is the heavy isotope of DNA but it is not a radioactive isotope.