Translation ends at the first stop codon
- The idea
- Predict
- See it
- Why
- Check
The idea
“The ribosome skips a stop codon, or a tRNA reads it, and translation keeps going.”
Every other codon adds an amino acid, so it's easy to picture a stop codon adding one too, or the ribosome passing it.
The short version
The mistake: The ribosome skips a stop codon, or a tRNA reads it, and translation keeps going.
What's actually true: No tRNA pairs with a stop codon. A release factor binds it and frees the chain. Nothing after the first in-frame stop is read.
- UAA, UAG and UGA are stop codons. They code for no amino acid, and no tRNA pairs with them.
- At a stop codon, a release factor binds in the A site, where each new codon is read. The chain comes off the last tRNA.
- A nonsense mutation turns a codon into a stop codon, so the polypeptide comes out short and often can't do its job.
On the exam: Find the first stop codon in the reading frame. The chain ends there, and nothing after it counts.