Question

E.coli bacteria grow ${}^{15}N{H_4}Cl$ medium for several generations and are allowed to grow in ${}^{14}N{H_4}Cl$ medium. After 2 generations, the bacteria are isolated from the medium and DNA of bacteria centrifuged in CsCl. The result of the density gradient consists of DNA that is
A. Only hybrid DNA
B. Both hybrid and heavy DNA.
C. Both heavy and light DNA.
D. Both hybrid and light DNA.

Answer 1

Above question is based on semi conservative mode of DNA replication. Semiconservative mode of DNA replication was first proposed by Watson and Crick. Later on it was experimentally proved by Meselson and Stahl (1958) in E-coli and Tailor in Vicia faba (1958). To prove this method, Taylor used Radiotracer technique in which radioisotopes(tritiated thymidine) were used. Meselson and Stahl use a heavy isotope of nitrogen (${N^{15}}$).

Complete answer:
(1) They grew E coli in a medium containing ${}^{15}N{H_4}Cl$ as the only nitrogen source for many generations. The result was that ${N^{15}}$ was incorporated into newly synthesized DNA; this heavy DNA molecule could be distinguished from the normal DNA by centrifugation in caesium chloride (CsCl) density gradient.
(2) Then they transferred the cell into a medium with normal NH4Cl and took samples at various definite time intervals as the cell multiplied and extracted the DNA that remained as double-stranded helices.
(3) Thus the DNA that was extracted from the culture one generation after the transfer from ${N^{15}}$ to ${N^{14}}$ medium (that is after 20 minutes) had a hybrid or intermediate density, DNA extracted from the culture after another generation( second generation that is after 40 minutes) was composed of equal amount of this hybrid DNA and of light DNA.

From above information, the right answer is (D).

Note: Watson and Crick had immediately proposed a scheme for DNA replication while proposing the double helical structure of DNA. The scheme suggested that the two strands would separate and act as a template for the synthesis of new complementary strands. After the completion of replication, each DNA molecule would have one parent and one newly synthesised, this scheme was termed as semi conservative DNA replication.
Cairns proved a semiconservative mode of replication in E.coli by using tritiated thymidine in an autoradiography experiment. He proposed the theta model for replication in circular DNA.