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: how often people differ. Any two people differ at roughly one base in every thousand. Most of these differences are SNPs, and many of them change no amino acid at all.
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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.
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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.
A SNP changes the mRNA codon UCC to UGC. Using the codon table, deduce the effect on the polypeptide.
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