Question

If Meselson and Stahl’s experiment is continued for four generations in bacteria, the ratio of ${}_{}^{15}{N}_{}^{15}N$, ${}_{}^{15}{N}_{}^{14}N$, ${}_{}^{14}{N}_{}^{14}N$ containing DNA in the fourth generation would be?
(a) 1:1:0
(b) 1:4:0
(c) 0:1:3
(d) 0:1:7

Answer 1

Hint: The semi-conservative nature of DNA replication was proposed by Watson and Crick but its experimental verification was done by Meselson and Stahl in Escherichia coli using a heavy isotope of nitrogen ${}_{}^{15}N$ and a centrifuge.

Complete Answer:
Watson and Crick explained that the parental DNA strand would act as a template for the formation of a new strand, thus each new DNA strand would have a parental strand along with a newly synthesized strand. This phenomenon was termed as the semi- conservative nature of DNA replication (as half parental DNA was conserved).

Meselson and Stahl’s experiment proving this nature is as follows:
- E.coli was grown in a ${}_{}^{15}{NH}_{4}Cl$ medium thus allowing ${}_{}^{15}N$ to be incorporated into the DNA strand of this bacteria.
- Now, this bacteria was transferred to a plate containing normal ${}_{}^{14}{NH}_{4}Cl$ where it was allowed to grow.
- Centrifugation after each generation of E.coli showed the presence of ${}_{}^{15}{N}_{}^{14}N$ hybrid DNA which indicated that the ${}_{}^{15}N$ strand of DNA came from the parental strand.
- By the time we reach the fourth generation, there will be no parental ${}_{}^{15}{N}_{}^{15}N$ DNA strands left as they were utilized in making the hybrid strands.
- There will be 2 strands having ${}_{}^{15}{N}_{}^{14}N$ i.e. hybrids and 14 strands having ${}_{}^{14}{N}_{}^{14}N$ i.e. pure strands.
So, the correct option is ‘0:1:7’.

Note:
- Similar experiments were performed by Taylor in faba beans using radioactive thymine in 1958.
- The centrifuge used had a CsCl density gradient.
- The time required for E.coli to produce a new generation is 20 minutes.