coding DNA

Coding DNA is the part of the genome that actually spells out a protein. If the genome is a long book, coding DNA is the sentences that give direct instructions — the parts read out, letter by letter, to build the working molecules of the cell.

More precisely, coding DNA is sequence that is both transcribed into messenger RNA and then translated into a chain of amino acids. The genetic code reads these stretches in three-letter words called codons, each specifying one amino acid, until a stop signal ends the protein. In organisms whose genes are split by introns, the coding portions are the exons that remain after splicing.

Coding DNA makes up a surprisingly small slice of many genomes. In humans, the protein-coding sequence is only around one to two percent of the total DNA. The rest is non-coding — not junk, but largely doing other jobs such as regulation and structure.

Because coding DNA directly determines protein sequence, mutations there can change a protein and so are often the focus of studies on genetic disease. But a gene's coding portion only tells part of the story; whether and when it is used depends on regulatory sequences outside it.