the twenty standard amino acids
Out of the millions of chemical structures imaginable, life builds nearly all of its proteins from a fixed alphabet of just twenty amino acids. Think of them as the twenty letters available to write every word, sentence, and book in the language of proteins. The order in which they are strung together spells out a protein's identity.
These twenty share the same backbone but differ entirely in their side chains. Some side chains are small (glycine has just a hydrogen; alanine a single carbon); others are huge rings (tryptophan); some carry positive or negative charge (lysine, glutamate); some grease-like and water-fearing (leucine, valine); some can form special links (cysteine, with its sulfur). Each one is written as a three-letter abbreviation or a single letter — Gly or G for glycine, Trp or W for tryptophan — and these codes are the working shorthand of molecular biology. The genetic code in DNA is read in three-base words, each specifying one of these twenty.
Why exactly twenty? It is partly a frozen accident of early evolution and partly a sweet spot: enough variety to build subtle, capable machines, but few enough that the cell can make and recognize each one reliably. A useful honesty: 'twenty' is the standard set, but life bends the rule — selenocysteine and pyrrolysine are rare 21st and 22nd genetically encoded amino acids, and many more arise from chemical modification after a protein is built.
The same twenty letters that spell the human enzyme that breaks down alcohol also spell the silk a spider spins. Different orderings of one shared alphabet produce wildly different molecular machines.
A fixed twenty-letter alphabet, ordered differently, writes every protein in nature.
Twenty is the canonical set, not an absolute limit: selenocysteine and pyrrolysine are genuinely encoded extras in some organisms, and dozens more amino acids appear only as later modifications.