Comprehension:
Meselson and Stahl experiment
Read the following passage carefully and answer the given questions.
While proposing the double helical structure for DNA, Watson and Crick had immediately proposed a scheme for replication of DNA. To quote their original statement that is as follows: “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material” (Watson and Crick, 1953). Matthew Meselson and Franklin Stahl performed the following experiment. They grew E. coli in 15N medium and then shifted to 14N medium. After 20 minutes (One generation), DNA shows intermediate density. After 40 minutes, two bands appeared - One intermediate and one light. The study used density gradient centrifugation to demonstrate the mechanism of 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 by Taylor and colleagues in 1958.
Identify the heavy isotope used in the above experiment:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 15N
Meselson and Stahl grew E. coli in a medium whose only source of nitrogen was 15NH4Cl. 15N is the heavy isotope of nitrogen, and the normal nitrogen is the lighter isotope 14N. The bacteria built 15N into their new DNA, so this DNA became heavy. 15N is not a radioactive isotope. It is separated from 14N only by density.
(1) 15N — Correct. It is the heavy isotope that labelled the DNA.
(2) 14N — Incorrect. This is the normal, lighter nitrogen. The cells were moved to a medium with 14N after they had grown in 15N.
(3) 16N — Incorrect. This is not the isotope used in the experiment.
(4) 13N — Incorrect. This is not the isotope used in the experiment.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
Replication – “… 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 (Figure 5.6).”
The Experimental Proof – “It is now proven that DNA replicates semiconservatively. It was shown first in Escherichia coli and subsequently in higher organisms, such as plants and human cells. Matthew Meselson and Franklin Stahl performed the following experiment in 1958: (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.”
Figure 5.6 Watson-Crick model for semiconservative replication