A third-base change isn't always silent
- The idea
- Predict
- See it
- Why
- Check
The idea
“The genetic code is redundant, so a change to one base, especially the third, won't change the protein.”
Codons for the same amino acid often differ only in the third base, so a change there sounds harmless. Sometimes it is.
The short version
The mistake: The genetic code is redundant, so a change to one base, especially the third, won't change the protein.
What's actually true: A substitution is silent only if the new codon codes for the same amino acid. You have to check the new codon.
- A change is silent only when the new codon codes for the same amino acid.
- Alanine has four codons, GCU, GCC, GCA and GCG, so any third-base change is silent. Histidine has two, so only one of three is.
- A different amino acid makes it a missense mutation. A stop codon, UAA, UAG or UGA, makes it nonsense, and the polypeptide ends early.
- Position alone doesn't decide it. CUA and UUA differ in the first base, yet both code for leucine.
On the exam: Always look up the new codon. Same amino acid means silent, and anything else changes the protein.