n the Meselson-Stahl DNA replication experiment, if the cells were first grown for many generations in 15N containing media, and then switched to 14N containing media, what percent of the DNA had 1 light strand and 1 heavy strand after 2 generations of growth in 15N growth media?

50%

Explanation:

DNA replication is a semi-conservative process. It means that after DNA replication, the newly formed DNA molecules have one parental strand and one newly formed strand. The cells in the experiment have DNA with 15N. After one round of DNA replication, each DNA molecule would have one parental DNA strand with 15N and one newly formed DNA strand with 14N.

However, after the second round of DNA replication, each hybrid DNA with strand having 15N and the other with 14N would be unwound. The parental heavy DNA strands would be present in the 50% of the newly formed DNA while the rest 50% of DNA molecules would have both strands with 14N (both light strands).