When a DNA solution is boiled, double-stranded DNA denatures. If the solution is then cooled slowly for a long period of time (using the appropriate concentrations of salts), the complementary strands ultimately reassociate, forming proper double-stranded DNA. The process is called renaturation.

a) 15N-labeled DNA from bacteriophage T2 is mixed in equal proportions with T2 DNA of normal density and then heated together and slowly cooled. The resulting DNA is analyzed by centrifiguation in a CsCl density gradient of the type used by Meselson and Stahl. How many bands will be observed and what will their relative proportions be? H = heavy, L = light.

b) The above experiment is repeated, but now 15N-labeled DNA from bacteriophage T2 is mixed with DNA of normal density from a different bacteriophage – T7. The genomes of bacteriophages T2 and T7 have very different DNA sequences. How many bands will be observed and what will their relative proportions be?

A.1 HH: 0 HL : 0 LL

B.0 HH: 1 HL : 0 LL

C.0 HH: 0 HL : 1 LL

D.1 HH: 0 HL : 1 LL

E.1 HH: 1 HL : 1 LL

F.1 HH: 2 HL : 1 L