DNA & Gene Expression is a core part of A-Level Biology. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
FidelityAccuracy of DNA replication — ~1 error per billion bases overall
3' to 5' exonucleaseProofreading domain of DNA polymerase — removes mis-paired bases
Mismatch repairPost-replication system that detects and corrects remaining errors
Mutation rateFrequency of permanent base changes per replication
TelomereRepetitive sequence at chromosome ends — shortens each replication
TelomeraseEnzyme extending telomeres — active in germ cells, stem cells, cancer
Replication forkY-shaped region where DNA is being unwound and copied
HelicaseBreaks H-bonds between bases, unwinds the double helix
PrimaseLays down short RNA primers — needed for DNA polymerase to start
DNA polymerase IIIMain replicating enzyme — adds nucleotides 5' to 3'
Leading strandContinuously synthesised towards the fork
Lagging strandSynthesised in Okazaki fragments away from the fork
SemiconservativeEach new DNA molecule contains one parent + one new strand
ConservativeHypothesis: both strands of one daughter molecule are old
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
What does 'semi-conservative' DNA replication mean?✗ One daughter molecule contains both original strands; the other contains both new strands. ✓ Each new DNA molecule contains one original (parental) strand and one newly synthesised strand.
What does a gene code for?✗ A gene codes for a specific physical feature like eye colour directly. ✓ Typically a polypeptide (protein) — through transcription to mRNA and translation by ribosomes.
Which activity of DNA polymerase removes a wrongly added base during replication?✗ 5' to 3' polymerase ✓ 3' to 5' exonuclease
What is the role of telomerase?✗ Joins Okazaki fragments ✓ Extends telomeres at chromosome ends
Where does transcription happen in a eukaryotic cell, and what is the product?✗ Transcription happens at ribosomes, where DNA is copied directly into protein. ✓ In the nucleus — DNA is copied into mRNA, which then travels to the cytoplasm for translation.
Practise DNA & Gene Expression — free games
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