The big idea: DNA polymerase III checks each nucleotide it adds. If the base is mismatched, it removes that nucleotide and puts in the correct one: this is proofreading.
The points to remember
- Now and then a nucleotide with the wrong base is added: a mismatch (for example G opposite T).
- DNA polymerase III detects the mismatch at the 3' end of the new strand.
- It removes that nucleotide from the 3' end (it works backwards, 3' to 5', for this step).
- It then adds a nucleotide with the correctly matched base and carries on.
Remember it as: Add, check, remove, replace.
Real example: E. coli. Without proofreading, DNA polymerase III would leave about one wrong base in every 100,000. Proofreading cuts this roughly a hundredfold, to about one in 10 million.
Name the enzyme: Proofreading is done by DNA polymerase III, not by ligase or helicase. Say it removes the nucleotide from the 3' end.
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Three ways copying stays accurate: A 'how are mutations avoided' answer uses all three.
Base pairing
- A pairs only with T, C only with G.
- A wrong base rarely fits.
A template
- Semi-conservative copying.
- Each new strand is built on an old one.
Proofreading
- DNA polymerase III spots a mismatch.
- It removes and replaces the nucleotide.
Put together: Pairing and the template make most copies right; proofreading fixes most of the rest.
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How it is asked: A 1A item naming the proofreading enzyme, or a Paper 2 Explain on how mutations are avoided in replication.
IB-style questionExplain[4 marks]
Explain how mutations are avoided when DNA is replicated in a dividing bacterium.
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See the answer broken into clear points, with guidance on how each point earns marks.