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DNA Structure and Replication - Medical Science

Q1:

Taylor, Woods and Hughes labeled Vicia DNA by allowing new DNA synthesis in the presence of radioactive thymine. After DNA replication (S phase of the cell cycle), it was observed that

A only one chromatid of a chromosome was labeled

B both chromatids of a chromosome were labeled

C both (a) and (b)

D neither chromatid was labeled

ANS:B - both chromatids of a chromosome were labeled

See. DNA in the chromosome was allowed for replication in the presence of radioactive thymine means one strand of DNA will be labelled. After replication also, that strand only having labelled thymine with association of newly formed strand in chromosome. This labelled portion may be present in either of any chromatids. So how it possible to have both chromatids that were labeled ?