Question

If replication were conservative (such that the newly replicated strands base pair with each other and the parental strands base pair with each other as DNA replication proceeded) what would chromosomes labeled as in the Meselson Stahl experiment look like after each round of replication? H = heavy nitrogen ¹⁵N, L = light nitrogen ¹⁴N

a) parental generation H-H, first generation all H-L, second generation ½ H-L ½ L-L

b) parental generation H-H first generation ½ H-H ½ H-L, second generation ¼ H-H ¾ H-L

c) parental generation H-H, first generation all H-L, second generation all H-L

d) parental generation H-H, first generation ½ H-H and ½ L-L, second generation ¼ H-H and ¾ L-L

Answer 1

The correct option is Option d) Parental generation H-H, first generation 1/2 H-H and 1/2 L-L, second generation 1/4 H-H and 3/4 L-L.

This is so, as the parental strand is H-H and in the first generation two strands will be generated during replication, and from these two one is of H-H type and the other is of L-L type because DNA replication here is conservative in nature. So 1/2 H-H and 1/2 L-L.

In the second generation four DNA strands will be produced from two DNA strands of first generation. The one H-H DNA of first generation will produce one H-H and one L-L DNA strand. The L-L strand of first generation will produce two L-L DNA strand.

So in total one H-H and three L-L DNA strands will be produced in second generation. i.e., 1/4 H-H and 3/4 L-L in second generation.