E. coli about to replicate was placed in a medium containing radioactive thymidine for 5 minutes. Then it was made to replicate in a normal medium. Which of the following observations will be correct?

Both the strands of DNA will be radioactive

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One strand radioactive

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Each half strand radioactive

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None is radioactive

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Solution

The correct option is B One strand radioactive
When the E. coli is placed in medium containing radioactive thymidine, the bacteria utilises radioactive thymidine for the replication process, resulting in the two strands of DNA containing radioactive thymidines. Consequently, it is placed in a normal medium and allowed to grow. Since DNA replication is semi conservative, the newly synthesised strand of DNA would be normal, while the strand obtained from parent molecule would be radioactive.

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