Class 12th Biology 2016 Set1 Outside Delhi Board Paper Solution
Describe Meselson and Stahl’s experiment that was carried in 1958 on E. coli. Write the conclusion they arrived at after the experiment.
Semi-conservative DNA replication:
The two Strands would separate and act as a template for the synthesis of new complementary strands. After the completion of replication, each DNA molecule would have one parental and one newly sytnthesized strads this scheme was termed as semi-conservative DNA replication.
Who Found:
In 1958 Meselson and Stahl's proved the semi-conservative mode of replication.
Experiment:
They 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 sample were taken after every genereation and tested for the presence of 15N and 14N using density gradiant centrifugation with CsCI the first generation was found be hybrid between 15N and 14N second generation contain tow 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.
Conclusion:
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.
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