The mRNA from transcription is a message written in bases. A protein is a chain of amino acids (see Proteins). Translation turns the first language into the second.
Key idea
The ribosome reads the mRNA three bases at a time. Each three-base word is a codon. A transfer RNA (tRNA) with the matching anticodon brings the right amino acid. The ribosome links the amino acids in order, starting at the start codon AUG and stopping at a stop codon.
Why it works
Why three bases? With four bases, one-letter words would name only 4 amino acids and two-letter words only 4 × 4 = 16. Three-letter words give 4 × 4 × 4 = 64 codons: enough for 20 amino acids plus start and stop signals. The tRNA is the adapter: one end pairs with the codon, the other end carries the amino acid.
- Start: the small ribosome subunit binds the mRNA and finds AUG. A tRNA carrying methionine pairs with it.
- Elongation: the next tRNA brings its amino acid; the ribosome joins it to the chain and moves one codon along.
- Stop: at UAA, UAG or UGA no tRNA fits; a release factor frees the finished chain, which folds into a protein.
Worked example: Counting amino acids
An mRNA has AUG, then 99 more bases, then the stop codon UAA. How many amino acids are in the protein?
- AUG is a codon and codes for methionine: 1 amino acid.
- 99 more bases make 99 ÷ 3 = 33 codons, each one amino acid.
- The stop codon adds none.
Answer: 1 + 33 = 34 amino acids.
Worked example: Codon and anticodon
A codon reads 5′-UUC-3′. What is the tRNA anticodon, written 3′→5′ under it?
- The anticodon pairs with the codon: U with A, U with A, C with G.
Answer: 3′-AAG-5′
Common mistake
Two misunderstandings are common. One is that each base stands for one amino acid; in fact three bases make one codon. The other is that the tRNA carries a codon. The tRNA carries the anticodon, which pairs with the codon on the mRNA, and it carries the amino acid. Check: the number of amino acids should be the number of codons from AUG up to, not including, the stop.