2. In evaluating the following scenario, assume a DNA molecule consists of two s
ID: 62969 • Letter: 2
Question
2. In evaluating the following scenario, assume a DNA molecule consists of two strands. A strain of bacteria is grown in a heavy nitrogen (^15 N) environment for many generations. A sample of these bacteria is shifted to an environment with only light nitrogen (^14N) and allowed to grow for one generation. It is then shifted to an environment with light nitrogen and radioactive phosphate (^32P) and allowed to grow two more generations. A. What proportion of the DNA strands are labeled with radioactive phosphate? B. What proportion of the DNA molecules are labeled with both heavy nitrogen and radioactive phosphate? (2 points)Explanation / Answer
A.
In the given case, the unlabeled DNA is grown for two generations in a radioactive phosphate medium, and then the proportion of DNA strands that are labelled after two generations is 75 %. This is because after one generation 50 % of DNA strands will be labelled, which are further grown on radioactive medium, thus in the 2nd generation 75 % will be labelled.
Hence, the 75% are radiolabeled after 2 generations.
B.
In the given case, the unlabeled heavy DNA is grown for one generation in a light medium. Thus, 50% of DNA strands will be light. The 50% light DNA containing strands are further grown on light and radiolabeled medium. Thus, after two generations 75% DNA strands will be light and 25% DNA strands will be heavy. As the DNA is grown for only one generation in radioactive phosphate medium, thus the proportion of DNA strands that are labelled after one generation is 50%.
Hence, the 25% have heavy DNA strands after 2 generations and 50% radiolabeled DNA strands.
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