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.
Fragments of DNA are joined by the enzyme -
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → DNA Ligase
The passage says that the discontinuously synthesised fragments are later joined by the enzyme DNA ligase. DNA polymerase works only in the 5′ → 3′ direction. So on one template strand the new DNA is made as short pieces. DNA ligase seals these pieces into one continuous strand.
(1) DNA Polymerase — Incorrect. It adds nucleotides to make the new strand, but it does not join the separate fragments.
(2) Topoisomerase — Incorrect. It relieves the twisting of the DNA ahead of the replication fork. It does not join fragments.
(3) DNA Ligase — Correct. It joins the DNA fragments.
(4) RNA polymerase — Incorrect. It makes RNA from a DNA template during transcription.
Note 1: 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.
Note 2: A similar question was also asked in CUET (UG) 2026 on 15-05-2026 (Slot 1).