Base substitutions at Higher Level: The same statement as SL. At HL SNPs return in conserved sequences, which change very little over long periods.
Practise this as you read
- Deduce the effect of a substitution using a codon table.
- Describe the causes and consequences of a substitution mutation.
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The big idea: A base substitution creates a SNP. Because the code is degenerate, it may change one amino acid, or none.
The points to remember
- A single-nucleotide polymorphism (SNP) is a single base that differs between individuals.
- SNPs are made by base substitution mutations.
- The code is degenerate, so the new codon may code for the same amino acid: no change.
- Or it may code for a different amino acid: one amino acid in the polypeptide changes.
- A substitution does not shift the reading frame, so the rest of the polypeptide is unchanged.
Remember it as: Swap one base: same amino acid, or one new one.
Real example: drinking milk as an adult. In many people of European descent, a single C to T SNP keeps the lactase gene switched on in adults. The SNP lies outside the gene's coding sequence, so it changes no amino acid but changes how much lactase is made.
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Working out the effect: Turn both DNA triplets into mRNA codons, look each up, and compare the amino acids.
Three steps
mRNA
Pair each template base: A with U, T with A, C with G, G with C.
Look up
Find the amino acid for the codon before and after.
Compare
Same amino acid: no change. Different: one amino acid changes.
Worked example. A template triplet 3' G G A 5' becomes 3' G G G 5'. The mRNA codon changes from CCU to CCC. Both code for proline, so the polypeptide is unchanged.
Not a frameshift: A substitution keeps the number of bases the same. Only one codon can change, never the ones after it.
How it is asked: A 1A item with a codon table and a SNP, or a longer answer on the causes and consequences of a substitution.
Explain why a base substitution may have no effect on a polypeptide.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.