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.
During DNA replication, in which direction is the new DNA strand polymerised?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 5′→3′ only
Each strand of DNA has a direction, from its 5′ end to its 3′ end. A DNA polymerase adds each new nucleotide to the 3′ end of the growing strand, so it can build only in one direction. At the replication fork the two template strands run in opposite directions. The enzyme keeps its single direction on both, and this is why one new strand is made in pieces.
Option (1) 5′→3′ only — Correct. The polymerase always builds the new strand in the 5′→3′ direction and never the other way.
Option (2) 3′→5′ only — Incorrect. 3′→5′ is the polarity of one template strand, not the direction in which the enzyme builds. Polymerisation itself is never 3′→5′.
Option (3) Both 5′→3′ and 3′→5′ on the same strand — Incorrect. The enzyme works in one direction only. It cannot build both ways on the same strand.
Option (4) 3′→5′ on the continuously synthesised strand and 5′→3′ on the discontinuously synthesised strand — Incorrect. Both new strands are built 5′→3′. Synthesis is continuous on one strand and discontinuous on the other, but the direction does not change.
NCERT Extract:
The Machinery and the Enzymes – “… For long DNA molecules, since the two strands of DNA cannot be separated in its entire length (due to very high energy requirement), the replication occur within a small opening of the DNA helix, referred to as replication fork. The DNA-dependent DNA polymerases catalyse polymerisation only in one direction, that is 5′→3′. This creates some additional complications at the replicating fork. Consequently, on one strand (the template with polarity 3′→5′), the replication is continuous, while on the other (the template with polarity 5′→3′), it is discontinuous. The discontinuously synthesised fragments are later joined by the enzyme DNA ligase (Figure 5.8).”
Figure 5.8 Replicating Fork