Translation starts at the first AUG
- The idea
- Predict
- See it
- Why
- Check
The idea
“The ribosome starts reading at the first base of the mRNA, where the cap is.”
The ribosome really does land near the 5' cap, so reading seems to start there. It slides along to AUG first.
The short version
The mistake: The ribosome starts reading at the first base of the mRNA, where the cap is.
What's actually true: The ribosome starts at the first AUG it reaches, and that start codon sets the reading frame for the rest of the message.
- The small ribosomal subunit binds near the 5' cap with the initiator tRNA, which carries methionine. Then it scans along the mRNA.
- At the first AUG, the large subunit joins. That AUG codes for methionine and sets the reading frame, the grouping of bases into triplets.
- Bases before the AUG are never translated. Bacterial mRNA has no cap, but reading still starts at the start codon.
On the exam: The ribosome binds at the cap but starts reading at the first AUG. Count codons from there.