One extra base changes every codon after it
- The idea
- Predict
- See it
- Why
- Check
The idea
“Adding or deleting one base only changes the codon where it happens.”
One extra base looks as small as a swapped one. The ribosome can't tell it's extra, though. It keeps counting in threes.
The short version
The mistake: Adding or deleting one base only changes the codon where it happens.
What's actually true: Codons are read in threes from the start codon, so one added or lost base regroups every codon after it.
- The ribosome reads mRNA in codons, groups of three bases, counted from the start codon AUG. Nothing marks where a codon ends.
- Insert or delete one base and every triplet after it is regrouped. That's a frameshift. Most amino acids after it change, and a stop codon often appears early.
- A substitution swaps one base and leaves the count alone, so only that one codon changes.
On the exam: One base added or lost regroups every codon after it. A substitution changes just one codon.