NEET 2001 Biology question

E. Coli about to replicate was placed in a medium containing radio active thymidine for five minutes. Then it was made to replicate in a normal medium. Which of the following observation shall be correct : -

(1)Both the strands of DNA will be radio active
(2)One strand radio activeCorrect
(3)Each strand half radio active
(4)None is radio active

Answer: (2)

Solution

DNA replication is semi-conservative: each daughter DNA molecule has one parental (old) strand and one newly synthesised strand. Thymidine is incorporated only into new strands, so when replication takes place in radioactive thymidine each daughter DNA has one non-radioactive parental strand and one radioactive new

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Question ID VK-088894