You can already read codons with the table (Reading the Genetic Code). Here is how a ribosome does it. It reads a mature mRNA, from RNA processing, 5′ → 3′, three bases at a time, using tRNAs as adapters.
- tRNA: each carries one amino acid at one end and an anticodon at the other. Enzymes called aminoacyl-tRNA synthetases attach the right amino acid to each tRNA. The anticodon pairs with the codon, antiparallel.
- Initiation: the small ribosomal subunit binds the mRNA and finds the start codon AUG. The initiator tRNA, carrying methionine, pairs with it; the large subunit joins. The initiator tRNA sits in the P site.
- Elongation: a tRNA matching the next codon enters the A site. The ribosome joins the growing chain to the new amino acid (a peptide bond). The ribosome shifts one codon; the empty tRNA moves to the E site and leaves.
- Termination: a stop codon (UAA, UAG or UGA) reaches the A site. No tRNA matches it; a release factor binds, the chain is released, and the ribosome comes apart.
Key idea
New tRNAs enter the A site; the tRNA holding the growing chain sits in the P site; empty tRNAs leave from the E site. Release factors, not tRNAs, recognize stop codons.
Why it works
Why three sites? The ribosome must hold the chain steady (P) while testing whether a new tRNA fits (A). Only a tRNA whose anticodon pairs with the codon stays long enough for the bond to form. After the bond forms, the chain hangs on the A-site tRNA, so the ribosome steps along, putting that tRNA in the P site and freeing the A site for the next one. The bond itself is made by RNA in the large subunit, so the ribosome is a ribozyme, an RNA enzyme.
Worked examples
Worked example: Codons and anticodons
What anticodon pairs with the codon 5′-UUC-3′, and which amino acid does that tRNA carry?
- Pair each base: U→A, U→A, C→G. Under the codon, the anticodon reads 3′-AAG-5′.
- The tRNA carries the amino acid the codon calls for: UUC is Phe (phenylalanine).
Answer: Anticodon 3′-AAG-5′; it carries Phe.
Worked example: Counting steps
An mRNA reads 5′-GCAUGUUCGGAUAAGC-3′. Which peptide is made, and how many tRNAs enter the A site?
- Start at the first AUG: AUG UUC GGA UAA.
- AUG = Met, UUC = Phe, GGA = Gly, UAA = stop.
- The initiator Met-tRNA starts in the P site; Phe- and Gly-tRNAs enter the A site; the release factor takes the A site at UAA.
Answer: Met-Phe-Gly; 2 tRNAs enter the A site.
Common mistake
Putting the growing chain on the A-site tRNA from the start, or thinking a tRNA reads stop codons. The chain rests on the P-site tRNA until the new amino acid arrives in the A site; then it moves over. At a stop codon, no tRNA pairs: a release factor does. Check: the number of tRNAs that enter the A site is the number of amino acids minus one.