Mention the significance of taking E.coli samples at definite time intervals for observation in meslson and stahl’s experiment
Answers
Meselson and Stahls worked with E. coli using a heavy isotope of Nitrogen, 15N E. coli was grown with 15N for several generation till the bacteria become completely labelled with 15N. They were then shifted to medium with normal 14N nitrogen. Samples were taken after every genereation and tested for the presence of 15N and 14N using density gradiant centrifugation. The first generation was found to be hybrid between 15N and 14N second generation contain two types of DNA, 50% light (14N) and 50% intermediate. This is only possible if two strands separate during replication and act as a template for synthesis of new complementary strands, thus proving a semiconservative mode of replication. Since E. coli divides every 20 minutes, the DNA extracted after 20 minutes experiment had a hybrid density. The DNA extracted after 40 minutes had equal amounts of hybrid and light densities.