DNA replication
Read the following passage carefully and answer the given questions.
In addition to DNA-dependent DNA polymerases, many additional enzymes are required to complete the process of replication with high degree of accuracy. 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. 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. In eukaryotes replication of DNA takes place at S-Phase of cell cycle.
During DNA replication, polymerisation occurs in the direction -
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 5′ → 3′
The passage says that DNA-dependent DNA polymerases catalyse polymerisation only in one direction, that is 5′ → 3′. New nucleotides can be added only to the 3′ end of the growing strand. For this reason, the strand on the template with polarity 3′ → 5′ is made continuously. The strand on the template with polarity 5′ → 3′ is made in pieces.
(1) 5′ → 3′ — Correct. This is the only direction of polymerisation.
(2) 3′ → 5′ — Incorrect. This is the polarity of the template of the continuous strand, not the direction in which a new strand grows.
(3) in both direction 5′ → 3′ and 3′ → 5′ — Incorrect. The enzyme works in one direction only. That is why one strand has to be made in fragments.
(4) 3′ → 2′ — Incorrect. A DNA strand is built through the 3′ and 5′ carbons. There is no 3′ → 2′ direction.
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.