Randomness in mutation at Higher Level: The same statement as SL. At HL it contrasts with gene editing: CRISPR-Cas9 does change a chosen base on purpose, but only in the laboratory.
Practise this as you read
- Identify a general feature of mutations.
- Explain why mutations are described as random.
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The big idea: Mutations are random: they can happen anywhere in the genome, and nothing aims them at a useful trait.
The points to remember
- A mutation can occur anywhere in the base sequence of a genome.
- Where and when a mutation happens cannot be predicted.
- Some bases have a higher probability of mutating than others.
- No natural mechanism makes a deliberate change to a particular base to change a trait.
- Mutagens raise the rate of mutation, but not its direction.
Remember it as: Anywhere, any time, never on purpose.
Real example: replica plating. In 1952 Joshua and Esther Lederberg copied bacterial colonies onto plates with an antibiotic. The resistant colonies were already present on the original plate, before the bacteria ever met the antibiotic: the mutations had happened by chance.
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What 'random' means: Random means unpredictable and not aimed, not 'equally likely everywhere'.
True of mutations
- Can happen anywhere in the genome
- Cannot be predicted
- Some bases mutate more often
- Rate raised by mutagens
Not true of mutations
- Happen only in certain genes
- Happen because the organism needs them
- Are aimed at a useful trait
- Only happen in germ cells
Need does not cause mutation: Bacteria do not mutate because an antibiotic is present. Resistant mutants arise by chance; the antibiotic then selects them.
How it is asked: A 1A item asking for a true general feature of mutations: they occur randomly.
Explain why the appearance of antibiotic resistance in bacteria does not mean that bacteria can change their genes when they need to.
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