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E. coli about to replicate was placed in a medium containing radioactive thymidine for five minutes. Then it was made to replicate in a normal medium. Which of the following observation shall be correct : -
A
Both the strands of DNA will be radioactive
B
One strand radioactive
C
Each strand half radioactive
D
None is radioactive
Detailed Solution
DNA replication is semiconservative: each daughter DNA molecule has one old (parental) strand and one newly synthesised strand.
During the five minutes in radioactive thymidine, the new strands being made take up the label, so the DNA formed has one radioactive (new) strand and one non-radioactive (old) strand.
When this cell then replicates in a normal medium, each labelled strand acts as a template for a new unlabelled strand. So DNA molecules carrying the label have only one radioactive strand.
So, as per the answer key, one strand of the DNA will be radioactive.
During the five minutes in radioactive thymidine, the new strands being made take up the label, so the DNA formed has one radioactive (new) strand and one non-radioactive (old) strand.
When this cell then replicates in a normal medium, each labelled strand acts as a template for a new unlabelled strand. So DNA molecules carrying the label have only one radioactive strand.
So, as per the answer key, one strand of the DNA will be radioactive.