The ribosome reads 5' to 3' and adds to the C end
- The idea
- Predict
- See it
- Why
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The idea
“The ribosome reads mRNA 3' to 5', or adds new amino acids to the chain's first end.”
Polymerases read their template 3' to 5', so it's natural to assume the ribosome does too. And the first amino acid looks like an anchor to build from.
The short version
The mistake: The ribosome reads mRNA 3' to 5', or adds new amino acids to the chain's first end.
What's actually true: The ribosome moves 5' to 3' along the mRNA and adds each new amino acid to the C end, so the N end is made first.
- Translation starts at AUG near the 5' end. The ribosome then moves toward the 3' end, one codon at a time.
- Each amino acid arrives on a tRNA. Its amino group bonds to the carboxyl group at the chain's C end, so the chain grows there.
- The first codon gives the N end, and the last codon before the stop gives the C end.
On the exam: The ribosome reads 5' to 3' and adds to the C end, so the first codon makes the N end.