STPM Biology - chapter 3: Revision essay question 15

Describe the experiment that is carried out by Meselson and Stahl to demonstrate that replication of DNA is semi conservative.

Answer

In the 1950s, details about DNA started pouring out of laboratory.  After  Watson and Chick published the DNA double helical structure.  Although they proposed that the DNA replication is semi conservative, but there was no evidence to prove this statement were correct. Therefore scientist Matthew Meselson and Franklin Stahl had designed an experiment to test their model. In Meselson and Stahl experiment, they had devised a strategy to distinguish between old and newly synthesized DNA strand. They labeled them with isotopes to differentiate them. Meselson and Stahl grew Escherichia coli bacteria in the presence of “heavy’’ isotope of nitrogen, N¹⁵ and ‘’light’’ isotope, N¹⁴. After many generations, the bacteria DNA contained either the heavy or light form of nitrogen, but not both. DNA with N¹⁵ is more dense and so under centrifugation settles lower in the test tube than DNA with N¹⁴. They grow the bacteria with N¹⁵for many generations to ensure that all bacteria DNA would be labeled with nitrogen. Then, some of the bacteria was transferred to N¹⁴ ‘’light ‘’medium and allowed to continue to grow, the scientists called their first sample ‘’generation zero’’. After the transfer, a sample was taken every 20 minutes( generation time for bacteria growing at their optimal temperature). The DNA from the sample was prepared for high speed centrifugation. The results was: in generation zero, all the DNA was in the ‘’heavy’’ form; in generation one, the DNA was neither ‘’heavy’’ or ‘’light’’, but an intermediate density. After two generations, half of the DNA was light and half was intermediate. After additional generations, more of the DNA was in the light form and less in intermediate form. This results had proved that the DNA replication was semi conservative which mean when DNA replicates, each DNA strand acts as a template for a new double helix in which each double-stranded molecule is composed of one parental strand and one newly polymerized strand.

Assignment submitted by group 1 2011/2012 (IBM)