3.5 Translation and the Genetic Code

Translation converts the nucleotide language of mRNA into the amino-acid sequence of a polypeptide. A codon is a set of three consecutive mRNA bases read 5′→3′. With four possible bases, there are 64 codons. Sixty-one specify amino acids and three are stop signals. AUG usually establishes the reading frame and codes for methionine at initiation.

The genetic code is degenerate because most amino acids are specified by more than one codon, but it is not ambiguous: in the standard code, a particular codon specifies only one amino acid or a stop signal. Degeneracy can make some substitutions silent, especially at a codon's third position. It does not guarantee that a substitution is harmless, and it offers no comparable protection from a one-base frameshift.

Transfer RNA folds into a compact three-dimensional molecule with an anticodon at one region and an amino-acid attachment site at another. An aminoacyl-tRNA synthetase attaches the correct amino acid to its corresponding tRNA using energy. The ribosome checks codon–anticodon complementarity, but the synthetase is what establishes the link between a tRNA identity and an amino acid. Accurate translation therefore depends on both charging and base pairing.

The anticodon is complementary and antiparallel to the mRNA codon. For an mRNA codon 5′-AUG-3′, a simplified anticodon can be written 3′-UAC-5′. If both are written in the same direction, the anticodon sequence must be reversed. Many errors in sequence questions come from complementing bases without reversing polarity.

A ribosome consists of rRNA and proteins arranged in small and large subunits. It binds mRNA and positions charged tRNAs. During elongation, a peptide bond forms between amino acids held in adjacent sites. The growing chain transfers to the newest tRNA, the ribosome translocates one codon along the mRNA, and the empty tRNA leaves. Translation ends when a stop codon recruits a release factor rather than an amino-acid-carrying tRNA.