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 above experiment proves that DNA replication is:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Semi-conservative
Meselson and Stahl grew E. coli for many generations in heavy nitrogen (15N), then moved the cells to normal nitrogen (14N). After one generation the DNA had an intermediate (hybrid) density. After the second generation, it was half hybrid and half light. This is explained only if each new DNA molecule keeps one old strand and gets one new strand. This mode is semi-conservative replication.
(1) Conservative — Incorrect. In conservative replication the old double helix would stay whole and a completely new one would form. The first generation would then show one heavy band and one light band, and no band of intermediate density.
(2) Bio-conservative — Incorrect. This is not a mode of DNA replication. The experiment decided between conservative and semi-conservative replication.
(3) Non conservative — Incorrect. This is not a name used for a mode of DNA replication in NCERT.
(4) Semi-conservative — Correct. Each daughter DNA has one parental strand and one newly made strand.
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 – “… (ii) Then they transferred the cells into a medium with normal 14NH4Cl and took samples at various definite time intervals as the cells multiplied, and extracted the DNA that remained as double-stranded helices. The various samples were separated independently on CsCl gradients to measure the densities of DNA (Figure 5.7).
(iii) Thus, the DNA that was extracted from the culture one generation after the transfer from 15N to 14N medium [that is after 20 minutes; E. coli divides in 20 minutes] had a hybrid or intermediate density. DNA extracted from the culture after another generation [that is after 40 minutes, II generation] was composed of equal amounts of this hybrid DNA and of ‘light’ DNA.”
The Experimental Proof – “It is now proven that DNA replicates semiconservatively.”
Figure 5.7 Meselson and Stahl’s Experiment