Ans. Option 3) is correct: Both A) and R) are correct and R) is the correct explanation of A). 

DNA replication is a biological process of formation of two identical copies from the original DNA molecule (chromosomal DNA). Transmission of chromosome DNA from generation to generation is crucial to cell propagation.The M.Meselson and F.Stahl in 1958, performed and gave the experimental evidence to show that DNA replicates semi-conservatively. The following were the experimental steps involved :-
  • They grew E.coli cells in a medium containing ammonium salts (15NH4Cl) prepared with heavy nitrogen until all the cellular DNA was labelled.
  • After the cells were transferred to a medium containing the normal light isotope (14N), samples were then removed and the DNA in each sample were analysed by density gradient centrifugation.
  • The samples were then separated as heavy -heavy (15N-15N), light-light (14N-14N), and heavy-light (15N-14N) duplexes. After one generation of growth, all the extracted DNA had the density of 15N-14N DNA.
  • After two generations, half the DNA had the density of 15N-14N DNA the other half had density of 15N-14N DNA. Shows DNA replicates semi-conservatively.

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