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.
E. coli divides every 20 minutes. What was found in the DNA extracted 40 minutes after transfer from the 15N to the 14N medium?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Equal amounts of hybrid DNA and light DNA
After the transfer to 14N, each new strand made is light. E. coli divides every 20 minutes. After one generation (20 minutes), every DNA molecule has one heavy parental strand and one light new strand, so all DNA is hybrid. In the second generation (40 minutes), the strands separate again. The two heavy strands each pair with a new light strand and give hybrid DNA. The two light strands each pair with a new light strand and give light DNA.
Option (1) Only hybrid DNA — Incorrect. Only hybrid DNA is the result after 20 minutes, one generation. By 40 minutes, light DNA has also appeared.
Option (2) Only light DNA — Incorrect. Light DNA alone would mean that no heavy parental strand was left. The two heavy strands from the start are still present, each in a hybrid molecule.
Option (3) Equal amounts of hybrid DNA and light DNA — Correct. At 40 minutes there are four DNA molecules for each parent molecule: two hybrid and two light. So hybrid and light DNA are in equal amounts.
Option (4) Equal amounts of heavy DNA and light DNA — Incorrect. Heavy DNA, with both strands heavy, was present only before the transfer. After the first division no molecule has two heavy strands.
NCERT Extract:
The Experimental Proof – “… (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.”
Figure 5.7 Meselson and Stahl’s Experiment