A 1-base deletion can hurt more than a 3-base one
- The idea
- Predict
- See it
- Why
- Check
The idea
“Bigger deletions change the protein more, and a deletion near the end barely matters.”
It seems a bigger deletion should do more damage, and one near the end less. Ribosomes read in threes, so whether the count divides by 3 often matters more.
The short version
The mistake: Bigger deletions change the protein more, and a deletion near the end barely matters.
What's actually true: First check whether the deleted count is a multiple of 3. If it isn't, every codon after the cut is read in a new frame.
- Nothing marks where one codon ends. The reading frame is set only by counting threes from the start codon.
- Delete a count that isn't a multiple of 3 and every later codon is regrouped. This frameshift usually changes many amino acids and often makes an early stop codon.
- Delete a multiple of 3 and the frame holds. Whole amino acids are lost, and if the cut splits two codons, the joined codon can be new, even a stop.
On the exam: Divide the deleted bases by 3. If there's a remainder, every codon after the cut shifts.