Roles of helicase and DNA polymeraseSpec D1.1.3
In short
Helicase is the enzyme that unwinds the DNA double helix and breaks the hydrogen bonds between the two strands, separating them so each can act as a template. DNA polymerase then links free DNA nucleotides, matched to the template by complementary base pairing, into a new strand by forming covalent bonds in the sugar–phosphate backbone.
Two enzymes are central to replication.
- Helicase
- Unwinds the double helix and separates the two strands by breaking the hydrogen bonds between complementary bases. Each separated strand can then act as a template.
- DNA polymerase
- Assembles a new strand: it brings free DNA nucleotides into position opposite their complementary bases on the template strand and links them together with covalent (phosphodiester) bonds to form the sugar–phosphate backbone.
- Helicase moves along the DNA, unwinding it and breaking hydrogen bonds between base pairs, so the strands separate.
- Free DNA nucleotides pair with exposed bases on each template strand (A–T, C–G).
- DNA polymerase links the paired nucleotides into a continuous new strand.
- Two DNA molecules result, each with one old and one new strand.
Exam tip:
Say helicase breaks hydrogen bonds between bases, not covalent bonds. DNA polymerase forms covalent bonds between adjacent nucleotides of the new strand.
Written and checked against the IB Biology SL specification · Updated October 2026