If DNA replication were fully conservative, how many intact parental double helices would have been detected in Meselson and Stahl's experiment after three rounds of replication?
A) 0
B) 1
C) 3
D) 8
The correct option is B - in Meselson and Stahl's experiment DNA labeled with heavy nitrogen isotope(15N) and it act as parental double helix. The heavier DNA then transfer to medium having ordinary nitrogen isotope(14N). The experiment is semi conservative, means each daughter DNA molecule consists of one old strand and one new strand since only one parent is brand is conserved in each daughter molecule. In the first replication we found DNA molecules were hybrid that is all were half heavy(15N-14N). In the second replication two types of DNA molecules from 50 % half heavy(15N-14N) and 50% light (14N-14N). In third replication out of of 8 DNA molecule 6 are light and 2 are half heavy. At last 2 strand of DNA is i.e.. 1 pair is belong to parent DNA.
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