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.
The intermediate density band indicates:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → One old and one new strand of DNA
The first generation after the shift to 14N has DNA of intermediate density. This is hybrid DNA. Each molecule has one old, heavy strand (15N) and one new, light strand (14N). A band of intermediate density is therefore a band of molecules with one strand of each kind.
(1) One old and one new strand of DNA — Correct. This is the hybrid DNA, which has an intermediate density.
(2) Two old strand of DNA — Incorrect. Two old strands, both 15N, would form the heavy band.
(3) Two new strand of DNA — Incorrect. Two new strands, both 14N, would form the light band.
(4) One old and one new strand of RNA — Incorrect. The bands are bands of DNA extracted from the cells. RNA is not involved.
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:
The Experimental Proof – …
