Explain the roles of helicase and DNA polymerase during DNA replication.

IB DP Biology Standard Level (2023 syllabus) — D1.1 DNA replication · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Helicase unwinds the double helix by breaking the hydrogen bonds between complementary bases, separating the two strands.
DNA polymerase adds free nucleotides to the 3′ end of the growing strand, forming new complementary strands.
DNA polymerase uses each original strand as a template, joining nucleotides via phosphodiester bonds to produce two identical DNA molecules.

Examiner tips

Use the exact terms ‘unwinds’, ‘breaks hydrogen bonds’, ‘adds nucleotides to the 3′ end’, ‘phosphodiester bonds’
Show the directionality (5′→3′) of DNA polymerase activity
Keep the answer concise – one sentence per point

Common mistakes

Confusing helicase with topoisomerase or DNA ligase
Forgetting to mention the 3′ end or the template strand
Using vague phrases like ‘separates the strands’ without specifying hydrogen bond breaking

Mark scheme (4 marks)

Helicase unwinds / uncoils the double helix
Helicase breaks the hydrogen bonds between complementary bases, separating the two strands
DNA polymerase adds free nucleotides to the 3′ end of the growing strand, forming new complementary strands
DNA polymerase uses each original strand as a template, joining nucleotides via phosphodiester bonds to produce two identical DNA molecules

Key terms in this question

helicase · DNA polymerase