One swapped base changes only one codon
- The idea
- Predict
- See it
- Why
- Check
The idea
“When one base is swapped, the reading frame shifts and every codon after it changes.”
One changed base can wreck a protein, so it's easy to picture the damage spreading down the gene. A swapped base stays in its own codon.
The short version
The mistake: When one base is swapped, the reading frame shifts and every codon after it changes.
What's actually true: A substitution keeps the number of bases the same, so the frame holds and at most one codon changes.
- The ribosome reads mRNA three bases at a time from the start codon. That grouping is the reading frame.
- A substitution swaps one base for another. The base count stays the same, so only that base's codon changes.
- A substitution can be silent, missense or nonsense. The new codon codes for the same amino acid, a different one, or a stop.
- Only inserting or deleting bases, in a number that isn't a multiple of three, shifts the frame. Then every codon after that point is regrouped.
On the exam: Count the bases. Same count, same frame, so only one codon can change.