Which of the following enzymes can polymerise the deoxyribonucleotides in 5'→3' direction during DNA replication?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → DNA polymerase
In replication, a new DNA strand is built on an old strand used as a template. The main enzyme is DNA-dependent DNA polymerase. It joins deoxyribonucleotides one after another, and it can add them in one direction only, 5′→3′. So one new strand is made continuously and the other in small pieces.
Option (1) Restriction endonuclease — Incorrect. Restriction endonucleases are “molecular scissors”. They cut DNA at specific base sequences. They do not build DNA.
Option (2) DNA polymerase — Correct. It uses a DNA template to polymerise deoxynucleotides, fast and with high accuracy, always in the 5′→3′ direction.
Option (3) Topoisomerase — Incorrect. Topoisomerase relieves the twisting (strain) in the DNA helix as it opens during replication. It does not add nucleotides.
Option (4) Ligase — Incorrect. DNA ligase joins the short, discontinuously made fragments into one strand. It seals the gaps but does not polymerise new nucleotides.
NCERT Extract:
The Machinery and the Enzymes – “In living cells, such as E. coli, the process of replication requires a set of catalysts (enzymes). The main enzyme is referred to as DNA-dependent DNA polymerase, since it uses a DNA template to catalyse the polymerisation of deoxynucleotides. … 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.”