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.
Mode of replication of DNA is:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Semiconservative
The passage says that after replication each DNA molecule has one parental strand and one newly synthesised strand. This is called semiconservative replication. Watson and Crick proposed it, and Meselson and Stahl proved it in 1958 using E. coli grown in 15N.
(1) Conservative — Incorrect. In this model both old strands would stay together in one molecule, and the other molecule would be fully new. The Meselson and Stahl experiment ruled it out.
(2) Semiconservative — Correct. Each new DNA molecule keeps one old strand.
(3) Non-conservative — Incorrect. In this model no old strand would be kept whole in either molecule. DNA does not replicate this way.
(4) Discontinuous — Incorrect. This word describes how one of the two new strands is made, in fragments. It is not the mode of replication of the molecule.
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.