Question

In a famous experiment, Matthew Meselson and Franklin Stahl grew E. coli cells in a medium containing only the "heavy" isotope of nitrogen, ¹⁵N. These cells were then transferred to a medium with the "light" isotope of nitrogen, ¹⁴N. The results of the experiment supported the hypothesis of semiconservative replication, which was proposed by Watson and Crick.

A) Heavy DNA (¹⁵N DNA), hybrid DNA, and light DNA (¹⁴N DNA) can be separated by centrifugation. If cells containing ¹⁵N DNA are transferred to a medium with only NH₄Cl as a nitrogen source, what percent of daughter molecules are composed of hybrid DNA after 2 generations?
B) Predict what percent of daughter molecules would be composed of hybrid DNA if DNA exhibited conservative replication - that is, if a daughter DNA molecule were composed of newly synthesized DNA only.

Answer 1

The experiment by Meselson and Stahl supported the hypothesis of semiconservative replication. After 2 generations in a medium with only NH₄Cl as a nitrogen source, 50% of daughter molecules would be composed of hybrid DNA. If DNA exhibited conservative replication, 0% of daughter molecules would be composed of hybrid DNA.

Step-by-step explanation:

The experiment conducted by Matthew Meselson and Franklin Stahl involved growing E. coli cells in a medium with ¹⁵N, a heavy isotope of nitrogen, and then transferring them to a medium with ¹⁴N, a light isotope of nitrogen. The results of the experiment supported the hypothesis of semiconservative replication.

If cells containing ¹⁵N DNA are transferred to a medium with only NH₄Cl as a nitrogen source, after 2 generations, the percent of daughter molecules composed of hybrid DNA would be 50%. This is because the DNA would undergo semiconservative replication, with each daughter molecule consisting of one strand of ¹⁵N DNA and one strand of ¹⁴N DNA.

If DNA exhibited conservative replication, where a daughter DNA molecule is composed of newly synthesized DNA only, then 0% of daughter molecules would be composed of hybrid DNA. This is because in conservative replication, the parental DNA remains intact and serves as a template for the synthesis of new DNA strands.