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Answer the following questions based on Meselson and Stahl’s experiment: a. Why did the scientists use 15NH4Cl and 14NH4Cl as sources of nitrogen in the culture medium for growing E. coli? b. Name the molecule(s) that 15N got incorporated into. c. How did they distinguish between 15N labelled molecules from 14N ones? d. Mention the significance of taking the E. coli samples at definite time intervals for observations. e. Write the observations made by them from the samples taken at the end of 20 minutes and 40 minutes respectively. f. Write the conclusion drawn by them at the end of their experiment. |
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Answer» a. 15N is the heavy isotope of nitrogen and it can be separated from 14N based on the difference in their densities. b. 15N was incorporated into newly synthesised DNA. c. The two molecules were distinguished by cesium chloride centrifugation in which these two separated into two different bands at different positions based on their densities. d. E. coli culture is taken at equal intervals to know the progress of the experiment as generation time of E. coli is 20 minutes. e. After 20 minutes the culture had an intermediate density showing a band in the middle tube and after 40 minutes, the culture had equal amounts of hybrid DNA and the light DNA showing two bands, one in the centre and one at the bottom. f. They concluded that DNA replicates semi-conservatively. |
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