Read the given passage and answer the following questions:
The Matthew Meselson and Franklin Stahl experiment was to prove that DNA replicates semiconservatively. It was shown first in Escherichia coli and subsequently in higher organisms, such as plants and human cells. 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 parental and one newly synthesised strand. This scheme was termed as semiconservative DNA replication. Very similar experiments involving use of radioactive thymidine to detect distribution of newly synthesised DNA in the chromosomes was performed on Vicia faba (faba beans) by Taylor and colleagues in 1958. The experiments proved that the DNA in chromosomes also replicate semiconservatively.
How was the heavy (15N) DNA distinguished from the normal DNA in the Meselson and Stahl experiment?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Centrifugation in a caesium chloride (CsCl) density gradient
Meselson and Stahl grew E. coli on 15N, the heavy isotope of nitrogen, so its DNA became heavy. 15N is not radioactive, so the heavy DNA could be told apart from normal DNA only by its density. They spun the DNA samples in a caesium chloride (CsCl) density gradient, where denser DNA settles at a different level.
Option (1) Agarose gel electrophoresis — Incorrect. Agarose gel electrophoresis separates DNA fragments by size, not whole DNA molecules by density. It is used to check the results of restriction digestion.
Option (2) Centrifugation in a caesium chloride (CsCl) density gradient — Correct. Heavy (15N) DNA and normal DNA differ only in density. Centrifugation in a CsCl density gradient separates them into different bands.
Option (3) Southern blot hybridisation — Incorrect. Southern blot hybridisation uses a labelled probe to detect particular DNA sequences, as in DNA fingerprinting. It does not separate DNA by density.
Option (4) Polymerase chain reaction — Incorrect. The polymerase chain reaction (PCR) makes many copies of a piece of DNA. It does not separate heavy DNA from light DNA.
NCERT Extract:
The Experimental Proof – “… (i) They grew E. coli in a medium containing 15NH4Cl (15N is the heavy isotope of nitrogen) as the only nitrogen source for many generations. The result was that 15N was incorporated into newly synthesised DNA (as well as other nitrogen containing compounds). This heavy DNA molecule could be distinguished from the normal DNA by centrifugation in a cesium chloride (CsCl) density gradient (Please note that 15N is not a radioactive isotope, and it can be separated from 14N only based on densities).”