01Decoding
Ribosomes read mRNA in codons (3 bases). Translation starts at AUG (methionine) and ends at a stop codon (UAA, UAG, UGA).
tRNA molecules carry amino acids; each has an anticodon complementary to an mRNA codon. The ribosome's A site accepts incoming tRNA, the P site holds the growing chain, the E site releases empty tRNA.
02The genetic code
64 codons, 20 amino acids: the code is redundant (several codons per amino acid) but unambiguous (each codon means one thing). It is nearly universal across life — evidence for common ancestry and why bacteria can make human insulin.
03Notebook box
Steps: initiation (small subunit + mRNA + initiator tRNA + large subunit) → elongation (peptide bonds) → termination (release factor at stop codon).
Worked example: An mRNA reads 5′-AUG UUU GGC UAA-3′. How many amino acids? → 3 (Met-Phe-Gly), then stop.