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.
Which one of given salts was used in Matthew Meselson and Franklin Stahl experiment for DNA separation?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Cesium chloride
Meselson and Stahl separated the heavy and normal DNA by density gradient centrifugation. A solution of cesium chloride (CsCl) is spun at very high speed, and it forms a gradient of density from the top to the bottom of the tube. Each kind of DNA settles at the point where its density equals that of the solution, so heavy, hybrid and light DNA make separate bands.
(1) Cesium fluoride — Incorrect. This salt was not used in the experiment. The chloride of cesium was.
(2) Calcium chloride — Incorrect. Calcium is a different metal. This salt was not used to separate the DNA.
(3) Cesium chloride — Correct. The DNA samples were separated on CsCl gradients.
(4) Cadmium chloride — Incorrect. This salt was not 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:
The Experimental Proof – …
