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D1.1.8 (HL)—Functions in replication

Functions in replication explains how DNA is copied, checked, amplified or analysed through specific strand relationships, enzymes and molecular techniques.

Syllabus
First assessment 2025
Objective
D1.1.8
Level
HL

Exam analysis

Chance of appearing12%of analysed past papers
Latest appearanceMay 2024
Most common paperPaper1
Typical marks1–3

Common command terms

  • Identify
  • Explain
  • Outline
  • Describe

Scoring notes

Common mistake
Saying helicase forms new strands instead of unwinding DNA and breaking hydrogen bonds.

Recent exam appearances

May 2024Paper1 ["HL"] · TZ127[ 1 ]D1.1.8 (HL)—Functions in replication
November 2023Paper1 ["HL"] · TZ221[ 1 ]D1.1.8 (HL)—Functions in replication
November 2022Paper1 ["HL"] · TZ027[ 1 ]D1.1.8 (HL)—Functions in replication
May 2022Paper2 ["HL"] · TZ27(b)[ 3 ]D1.1.8 (HL)—Functions in replication
May 2021Paper2 ["HL"] · TZ27(b)[ 3 ]D1.1.8 (HL)—Functions in replication

Coverage 2013–2024 · Updated 16 Jul 2026

Primase Starts; DNA Polymerase III Extends

HL only
1

DNA polymerase III cannot start a strand from unlinked nucleotides; it needs an existing paired chain with a free 3′-OH.

Primase is an RNA polymerase that synthesizes a short RNA primer complementary to the DNA template. It can start without a pre-existing 3′ end.

2

DNA polymerase III binds at the primer's 3′ end and adds complementary DNA nucleotides, extending the new strand 5′→3′.

New strand Primase action DNA polymerase III action
leading makes one initial primer extends continuously
lagging makes a new primer for each fragment extends each Okazaki fragment

DNA Polymerase I Replaces Primers; Ligase Seals Nicks

HL only
1

DNA polymerase I removes each RNA primer and replaces its RNA nucleotides with DNA nucleotides.

After replacement, adjacent DNA sections contain the correct nucleotides but remain separated by a nick: one missing phosphodiester bond in the sugar–phosphate backbone.

2

DNA ligase catalyses formation of the missing phosphodiester bond, joining adjacent DNA fragments into one continuous strand.

Enzyme Changes nucleotide identity? Joins a final backbone gap?
DNA polymerase I yes—RNA is replaced with DNA leaves a nick between sections
DNA ligase no yes—seals the nick

Replication Enzymes Hand the Growing DNA from One Job to the Next

HL only
1

Helicase: separates the parental strands at the replication fork.

2

Primase: makes an RNA primer that supplies a free 3′-OH.

3

DNA polymerase III: extends complementary DNA from the primer.

4

DNA polymerase I: removes the RNA primer and replaces it with DNA.

5

DNA ligase: seals the remaining nick between DNA sections.

Material present Next enzyme needed
paired template but no starting chain primase
RNA primer with free 3′ end DNA polymerase III
RNA segment embedded before DNA DNA polymerase I
adjacent DNA sections with a nick DNA ligase

Solve a Replication Fork from Direction to Fidelity

HL only
Clue at the fork Consequence
polymerase can add only to a 3′ end every new segment grows 5′→3′
synthesis can follow helicase leading strand is continuous
synthesis must restart as template is exposed lagging strand uses repeated primers and Okazaki fragments
an RNA primer remains DNA polymerase I replaces it with DNA
two DNA sections meet at a nick ligase seals the phosphodiester bond
the terminal base is mismatched DNA polymerase III removes and replaces it

helicase opens → primase starts → DNA polymerase III extends → DNA polymerase I replaces RNA → ligase seals → polymerase III proofreading corrects many terminal mismatches

Directionality creates the leading–lagging difference. Enzyme handoffs solve that geometry, while complementary pairing plus proofreading preserves sequence fidelity.

Functions in replication

HL only

[ 3 ]

Describe the function of three named enzymes involved in DNA replication.

Concept essentials

  • Functions in replication depends on matching each strand, enzyme or laboratory step to its exact function.
  • Template strands and complementary base pairing give functions in replication its copying accuracy.
  • Functions in replication is clearer when amplification, separation and profiling are not confused.
  • Molecular evidence for functions in replication must be interpreted from bands, isotopes, primers or enzyme roles.