Question

In the Meselson-Stahl experiment, the conservative model of DNA replication was ruled out
by which of the following observations?

Answer 1

After the invention and interpretation of DNA structure several hints were identified for the study of way in which DNA is replicated.The complementary base pairing ,double strands were some of those hints.

For understanding DNA replication 3 models were proposed ;

• Conservation mode - where replicated DNA results in one old double stranded DNA and one fully new double Stranded DNA.
• Dispersive - newly replicated DNA will have segments of the parent DNA after replication.
• Semi conservative - Newly replicated DNA will be consisting of one parent strand and one newly synthesized strand equally.

  

One of the most important experiments in studying the model of DNA replication were proposed by Messelson and Stahl;known as the Messelson and Stahl experiment.

• They used bacteria grown in heavy isotope of nitrogen for their studies.

• N- 14 is the most abundant nitrogen and N - 15 is the stable isotope of N -!4,but the latter is little more heavier than the former.
• Choosing nitrogen was due to the presence of nitrogen in the DNA bases.
• Following is the illustration of the experiments ;
• 
• Step 1 - grow bacteria in N -15 media for several generations and bacterial DNA thus had N- 15
• step 2 - Transfer some N-15 containing bacteria to N- 14 containing media;result was the presence of a hybrid DNA,containing both N-14 and N- 15.
• step - 3 - bacterias were continued to grow in N- 14 media,thus resulted DNA had N - 14 in their DNA
• samples from the growing bacteria at different time were taken and their density was analyzed.
• The idea was to mark the old DNA and study the changes in the newly synthesized DNA.

Following results were obtained ;

Such a result is only possible if they follow semi conservative mode of replication ;

Thus it was proved that DNA followed Semi conservative mode of replication.

Result would have been the following if the DNA replication followed Conservative/Dispersive mode ;