Meselson and Stahl transferred a few bacteria grown in N15 medium to N14 medium for replicating their DNA. What would be the result after rounds of replication?

Explanation

When Meselson and Stahl conducted their famous experiment, they used bacteria grown in a heavy isotope of nitrogen (N-15) medium, and then they transferred these bacteria to a medium containing the lighter isotope of nitrogen (N-14) for replication. They were investigating the mode of DNA replication, specifically whether it followed a conservative or semi-conservative mechanism. The outcome of the experiment showed that after one round of replication, the DNA consisted of hybrid duplexes, with one strand being the original heavy strand (N-15) and one strand being the newly synthesised light strand (N-14). This result supported the semi-conservative model of DNA replication. After another round of replication, you would expect to see a mixture of DNA molecules. The result would be 50% hybrid duplex (one old heavy strand and one new light strand) and 50% light duplex (both strands containing the lighter isotope, N-14). The heavy duplex (both strands containing the heavier isotope, N-15) would not be present because the bacteria were no longer being cultured in the N-15 medium.