0. Answer the following questions based on Meselson and Stahl’s experiment
1. Why did the scientists synthesize the light and heavy DNA molecules in the organism used in the experiment?

2. How did the scientists make it possible to distinguish the heavy DNA molecule from the light DNA molecule? Explain.

Dear Student,

Ans1. The scientists synthesized DNA, light and heavy both in the organism in order to prove that DNA replicates semi-conservatively. 

Ans2. They distinguished light and heavy DNA molecule by:- 

• E.coli was grown in a medium containing heavy isotope ¹⁵N as the nitrogen source.
• ¹⁵N was incorporated into newly synthesised DNA as well and the DNA became heavy DNA.
• Heavy DNA molecule can be differentiated from normal DNA by density gradient centrifugation using cesium chloride as the gradient.
•  Then, cells were again transferred into a medium with ¹⁴N as nitrogen source. Samples were taken from this media and their DNA was extracted.