The Molecular Basis of Heredity

DNA is a double helix built from four bases: A, T, C, and G. Those four letters, read in groups of three, are enough to specify every protein an organism needs, the same way a short alphabet can spell endless words. To copy itself, the helix unzips down the middle, breaking the weak bonds between paired bases. Each separated strand then acts as a template: an enzyme reads it and builds a new partner strand by matching bases (A with T, C with G). The result is two helices, each with one original strand and one freshly built one. That pattern is called semiconservative replication, and it is the mechanism behind every cell division.

Making a protein takes two more steps. Transcription copies a stretch of DNA into messenger RNA, swapping out the base T for U. Translation then sends that RNA to a ribosome, which reads it three bases at a time. Each three-base codon calls for one specific amino acid, except for a stop codon, which ends the chain. String the amino acids together in order and you get a finished protein, folded and ready to work. A coding region of 50 codons, with the last one a stop signal, produces a protein 49 amino acids long, since the stop itself adds nothing to the chain.

This chain from DNA to RNA to protein is why a mutation is not automatically harmful. The genetic code is redundant: most amino acids are specified by more than one codon, so a change in DNA can leave the protein completely unchanged. Other mutations land in parts of the protein that do not affect its shape or function. Only a smaller set of changes actually breaks or alters what the protein does, and occasionally one even improves it.