Question

Through their experiment on DNA replication in E. coli grown in ${}^{15}$NH${}_4$Cl media, Meselson and Stahl were able to prove that each of the DNA molecules formed after replication is composed of

• A. two parental strands
• B. two newly synthesised strands
• C. one parental strand and one newly synthesised strand
• D. two strands with variable proportion of parental and newly synthesised strand

Answer 1

The correct option is C one parental strand and one newly synthesised strand

Meselson and Stahl chose E. coli for their experiments on semi - conservative replication of DNA. They initially grew E. coli which had been in the ${}^{15}$NH${}_4$Cl medium for several generations, thus ${}^{15}$N was incorporated into their genetic material, later they transferred those bacteria to ${}^{14}$NH${}_4$Cl media and allowed them to grow. At regular intervals, the sampling was done and DNA was extracted. Using CsCl density gradient centrifugation, the DNA was separated based on their density.

The heavy DNA (${}^{15}$N) formed a band at the bottom, the lighter DNA with ${}^{14}$N formed a band at the top whereas hybrid DNA was present in between the two. As a conclusion to their experiment, they proved that DNA replication is semi-conservative in nature. It means that both the strands of parental DNA molecules get separated during replication and one new complementary strand is synthesised on each parental strand.

Hence, the DNA molecules formed after replication of one parent molecule have one parental strand and one newly synthesised strand.

1 DNA molecule formed after replication = 1 parental strand + 1 newly synthesised strand