Question

Meselson and Stahl grew bacteria in the presence of a heavy isotope of nitrogen N${}^{14}$. The DNA was then transferred to a growth medium containing nucleotides with a lighter isotope of nitrogen N${}^{12}$ which then proceeded to replicate.
The DNA was centrifuged to separate it into weight forming bands. After one round of DNA replication, there was one band in the middle of the centrifuge tube. After two rounds of DNA replication in the lighter nitrogen, two separate bands of DNA could be seen. This demonstrated that replication is semi-conservative.
Why do these results support that idea?

• A. After two rounds of replication, only two of the four daughters DNA molecules still contained the heavy nitrogen
• B. After both the first and second round of replication, the ratio of heavy nitrogen versus light nitrogen varied randomly
• C. After two rounds of replication, one molecule of DNA contained the only heavy nitrogen in its structure and the other three contained light nitrogen
• D. After each round of replication, only hybrid N14-N15 DNA molecules formed, creating two bands after two rounds

Answer 1

The correct option is A After two rounds of replication, only two of the four daughters DNA molecules still contained the heavy nitrogen

When a N${}^{14}$ DNA strand was grown in a nutrient medium containing a lighter isotope N${}^{12}$. In the first generation the DNA molecules that were obtained were hybrids containing one strand with light isotope of Nitrogen and the other with regular nitrogen. So, when centrifuged it occupied the center of the tube.

In the next generation two separate bands of DNA were obtained one that occupied the center while the other that occupied a higher place in the tube, as when the hybrid strand replicated in a N${}^{12}$, it formed two hybrid DNA molecules and two pure light DNA molecules.

So, the correct answer is 'After two rounds of replication, only two of the four daughters DNA molecules still contained the heavy nitrogen'