The genetic code
Three bases each form a codon, and every codon stands for an amino acid or a stop signal. Four bases yield 64 codons that encode 20 amino acids plus start and stop. The code is redundant, so several codons name the same amino acid, and it is the same across almost all living things.
Three bases, one word
The ribosome reads the messenger RNA in fixed steps of three. Each triplet is a codon and sets the next amino acid. Where reading begins is fixed by the start codon AUG, which also stands for methionine. When the ribosome reaches a stop codon, the chain is finished.
Why the code is redundant
64 codons face 20 amino acids, so most amino acids have several codons. Often these differ only in the third base. This redundancy cushions many letter changes: if a change hits the third position, the amino acid often stays the same. Such silent changes are called synonymous.
What this means for variants
A letter change in an exon can do three things. Synonymous: the amino acid stays the same, usually without consequence. Missense: a different amino acid is built in, the effect depends on the location. Nonsense: a codon becomes a stop signal and the protein breaks off too early. Which case applies follows directly from the code.
What Genome measures. Genome shows the letter at a position. Whether a letter change alters the amino acid is decided by the code: some changes are silent, others are not.
Related topics
Sources
- 1Nirenberg & Matthaei, 1961 The dependence of cell-free protein synthesis in E. coli upon naturally occurring or synthetic polyribonucleotides. PNAS 47:1588–1602. doi.org/10.1073/pnas.47.10.1588
- 2Crick et al., 1961 General nature of the genetic code for proteins. Nature 192:1227–1232. doi.org/10.1038/1921227a0