Replication enzymes and DNA proofreadingSpec D1.1.8, D1.1.9
In short
In prokaryotic replication, DNA primase makes short RNA primers, DNA polymerase III adds DNA nucleotides to them, DNA polymerase I removes the primers and replaces them with DNA, and DNA ligase seals the remaining gaps in the sugar–phosphate backbone. DNA polymerase III also proofreads, removing a mismatched nucleotide from the 3' end and replacing it.
The functions below are for the prokaryotic system, as the IB requires.
| Enzyme | Function |
|---|---|
| DNA primase | Synthesises a short RNA primer complementary to the template. This provides the 3' end that DNA polymerase III needs to start adding nucleotides. Once on the leading strand; at the start of every Okazaki fragment on the lagging strand. |
| DNA polymerase III | Adds DNA nucleotides to the 3' end of the primer and then of the growing strand, 5' → 3', following complementary base pairing. Makes most of the new DNA. Also proofreads. |
| DNA polymerase I | Removes the RNA primers nucleotide by nucleotide and replaces them with DNA nucleotides. |
| DNA ligase | Seals the nicks between adjacent sections of DNA (for example between Okazaki fragments) by forming a covalent bond in the sugar–phosphate backbone. |
DNA proofreading
Occasionally DNA polymerase III adds a nucleotide with a base that does not pair with the template. A mismatched base pair distorts the double helix at the 3' end of the new strand. DNA polymerase III detects this, removes the nucleotide with the mismatched base from the 3' terminal and then adds a correctly matched nucleotide in its place before continuing. Proofreading greatly reduces the error rate of replication.
In IB answers, proofreading of the new strand is credited to DNA polymerase III. The role to give for DNA polymerase I is removing RNA primers and replacing them with DNA (it can also correct errors, but this is not required).
Quick check
Why is DNA replication described as semi-conservative?
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Each new DNA molecule contains one strand from the original molecule and one newly synthesised strand.
What is the role of helicase?
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It unwinds the double helix and breaks hydrogen bonds between bases so the strands separate.
Why is Taq polymerase used in PCR?
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It comes from a hot-spring bacterium and is not denatured at 95 °C, so it survives the denaturation step of every cycle.
Why do smaller DNA fragments travel further in gel electrophoresis?
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They pass more easily through the mesh of the gel, so they move faster towards the positive electrode.
HL only Why are many RNA primers needed on the lagging strand?
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It is made discontinuously as Okazaki fragments, and each fragment must start from its own primer.
Written and checked against the IB Biology HL specification · Updated October 2026