Deoxyribonucleic acid (DNA) is the molecule that stores genetic information in living organisms. It consists of two complementary strands of nucleotides wound into a double helix, a structure first described by James Watson and Francis Crick in 1953, building on X-ray crystallography by Rosalind Franklin and Maurice Wilkins. Each nucleotide contains one of four bases — adenine (A), thymine (T), guanine (G), cytosine (C) — and the sequence of these bases encodes the instructions for building proteins.
DNA does two things essential to life. First, it replicates: before a cell divides, it copies its DNA so that each daughter cell receives a complete set of instructions. Replication is not perfect — errors (mutations) occur at a low but nonzero rate, and these mutations are the raw material of evolutionary variation. Second, DNA is transcribed into RNA, which is then translated into proteins by ribosomes. Proteins do most of the functional work in cells: catalyzing metabolic reactions, providing structural support, transporting molecules, and mediating communication between cells.
A gene is a segment of DNA that encodes the information needed to produce a functional product — typically a protein, though some genes produce functional RNA molecules. The human genome contains approximately 20,000 protein-coding genes, but these account for less than 2% of the total DNA. The rest includes regulatory sequences, structural elements, transposable elements, and sequences of unknown function.
DNA is not a blueprint that specifies an organism the way an architectural plan specifies a building. The same DNA produces different outcomes depending on which genes are expressed, when, where, and in response to what signals. Gene expression is regulated by an elaborate system of promoters, enhancers, transcription factors, and epigenetic modifications — chemical changes to DNA or its associated proteins that alter gene activity without changing the underlying sequence. The relationship between DNA and phenotype is mediated by development, environment, and chance.
Related terms
- Gene — a functional segment of DNA
- Cell — the structure that houses DNA and the machinery for reading it
- Phenotype — the observable characteristics that DNA helps produce
- Natural selection — the process that acts on variation generated by DNA mutation