Question

Which of the following enzyme is used to lay the foundation for DNA polymerase to start the addition of dNTPs during DNA replication?

• A. Topoisomerase
• B. Both b and c
• C. Primase
• D. Ligase

Answer 1

The correct option is C Primase

DNA polymerase III, the primary enzyme catalysing the polymerisation of dNTPs in prokaryotes, can add the nucleotides to an already existing polynucleotide chain implying that it is simply an elongation enzyme. This happens only if a free 3’ OH end is available. Although they can conduct polymerization reactions in 5’ $\to$ 3’ direction, they cannot initiate strand synthesis. So for any new synthesis of DNA to take place, there is a need for a preexisting polynucleotide chain. This chain is called the RNA primer, consisting of a few RNA nucleotides. This primer formation is catalysed by an enzyme known as primase which is a DNA dependent RNA polymerase acting on a single DNA template.
The primase enzyme helps to add RNA primers to which DNA polymerase III adds nucleotides.
DNA ligase helps in joining the Okazaki fragments formed by the lagging strand during DNA replication.
Topoisomerase relieves the tension caused by helicase while uncoiling the dsDNA during replication.