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.
What does semiconservative replication mean?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Each new DNA molecule contains one parental and one newly synthesised strand
The passage states that “each DNA molecule would have one parental and one newly synthesised strand. This scheme was termed as semiconservative DNA replication”.
During replication the two strands of the parent DNA separate. Each old strand acts as a template for a new complementary strand. So every daughter molecule keeps one old (parental) strand and gets one new strand. Half of the parent is conserved in each daughter, which is why the scheme is called semi-conservative.
Option (1) Each new DNA molecule contains two newly synthesised strands — Incorrect. If both strands were new, no parental strand would be kept in the daughter molecule. That is not what the experiments showed.
Option (2) Each new DNA molecule contains two parental strands — Incorrect. A molecule with two parental strands would be the old DNA itself, with nothing new made. Replication always adds one new strand to each daughter molecule.
Option (3) Each new DNA molecule contains one parental and one newly synthesised strand — Correct. Each daughter DNA has one parental and one newly synthesised strand. This is semiconservative replication.
Option (4) Each new DNA molecule is identical to the original — Incorrect. The daughter molecules have the same base sequence as the parent, but this does not describe how the old and new strands are shared.