Anticodons are antiparallel: flip and complement them before you touch the codon table.
The ribosome reads codons, not amino acids. It matches each three-base codon to a tRNA's anticodon, and each tRNA brings the matching amino acid. Repeat that codon by codon, and a base sequence becomes a protein.
Key terms
- Codon — three mRNA bases read as one unit. Of the 64 codons, 61 name an amino acid and 3 mean stop.
- tRNA — the adapter. It carries one kind of amino acid, and its anticodon pairs with the matching codon, antiparallel.
- Ribosome — rRNA plus proteins, in a small and a large subunit. Its rRNA catalyzes each peptide bond.
- A, P, E sites — the ribosome's tRNA slots: A admits new tRNAs, P holds the chain's tRNA, E is the exit.
- Start codon — AUG, which also codes for methionine. It sets the reading frame for everything after it.
- Polyribosome — several ribosomes reading one mRNA at once. Each makes its own complete copy of the protein.
How it works
- The small subunit binds the mRNA. In eukaryotes it binds at the 5' cap and scans to the first AUG, where a methionine-carrying tRNA pairs with it.
- The large subunit joins. The methionine tRNA sits in the P site, and the next codon waits in the empty A site.
- A tRNA with the matching anticodon enters the A site. The chain is bonded to the new amino acid, so the whole chain now hangs from that tRNA.
- The ribosome shifts one codon toward the 3' end. The chain's tRNA moves to P; the empty tRNA moves to E and leaves.
- This repeats until a stop codon (UAA, UAG or UGA) reaches the A site. A release factor binds it, the chain is freed, and the ribosome comes apart.
Why it matters
Expect questions on direction and location. Direction: mRNA is read 5'→3', and the protein grows amino end → carboxyl end. Location: bacteria have no nuclear envelope, so ribosomes start on an mRNA still being transcribed. Eukaryotes process each transcript and export it from the nucleus first. Next up is mutations: insert one base, and every codon after it is read in a new frame.
Exam traps
| Trap | Actually |
|---|
| Several ribosomes on one mRNA each build part of the protein. | Each ribosome reads the whole coding sequence and releases a full chain. More ribosomes, more copies at once. |
| New amino acids are added to the chain's amino end. | They're added to the carboxyl end. The amino end is the first amino acid made, coded by the start codon. |
| ATP drives the ribosome along the mRNA. | GTP powers tRNA delivery and each one-codon shift. ATP is spent earlier, loading amino acids onto their tRNAs. |
| An extra upstream AUG is harmless, because ribosomes find the right frame. | The first AUG after the cap sets the frame. An out-of-frame one upstream yields a short, wrong peptide, and the real protein becomes scarce. |
Worked example
A tRNA has the anticodon 5'-CCA-3'. Which amino acid does it carry? Codon table excerpt, codons written 5'→3': ACC = threonine, CCA = proline, GGU = glycine, UGG = tryptophan.
- The table lists codons, so first find the codon this anticodon pairs with.
- Pairing is antiparallel: the anticodon's 3' end meets the codon's 5' end. So read the anticodon 3'→5': A, C, C.
- Complement each base: A→U, C→G, C→G. The codon is 5'-UGG-3'.
- Look up UGG: tryptophan.
Tryptophan. This tRNA pairs with the codon 5'-UGG-3', so it carries the amino acid UGG codes for.
A tRNA carries the amino acid for the codon it pairs with, so you need that codon before the table is any use. Each shortcut lands on a real table entry, which is what makes it tempting: reading CCA straight off gives proline, complementing without flipping gives GGU (glycine), and flipping without complementing gives ACC (threonine).