Question

Bacteria grown in a medium containing 15N H4CL for a number of generations so that all of the DNA is made up of fully "heavy" DNA. The bacteria are moved to a new medium and grown in 14N H4CL so that all new DNA will be "light". If replication were conservative, what would the DNA look like after one generation time?

a. all of the DNA is made up of 2 "light" strands
b. all of the DNA is made up of 2 "heavy" strands
c. all of the DNA is made up of 1 "heavy" strand and 1 "light" strand
d. each strand is made up of a mixture of "heavy" and "light" DNA
e. half of the DNA is made up of 2 "light" strands and half of the DNA is made up of 2 "heavy" strands

Answer 1

Meselson and Stahl's experiment demonstrates that after one generation of replication in a conservative model, DNA would consist of a mix of "heavy" and "light" strands, supporting the semi-conservative model of replication.

Step-by-step explanation:

The question revolves around a classic experiment by Matthew Meselson and Franklin Stahl that helped establish the semi-conservative model of DNA replication. According to this experiment, if replication were conservative, one would expect to see DNA comprised of either all "heavy" or all "light" DNA after one generation.

However, Meselson and Stahl's experiment showed that this was not the case, and instead, DNA sedimented at a density intermediate between the "heavy" and "light" DNA, indicating the presence of hybrid DNA composed of one "heavy" and one "light" strand. This finding ruled out the conservative model of replication. Therefore, the correct answer is (c), as half of the DNA will consist of one "heavy" strand and one "light" strand after one generation in a conservative replication model.