DNA
DNA (deoxyribonucleic acid) is the double-helix molecule that carries the genetic instructions for building and running every living organism.
Definition
DNA is the molecule inside your cells that holds the instructions for building you; it looks like a twisted ladder, and the order of the rungs spells out the code. Deoxyribonucleic acid stores hereditary information in all living organisms as a double helix of two nucleotide strands, each nucleotide made of a sugar, a phosphate, and one of four nitrogen bases, adenine (A), thymine (T), guanine (G), and cytosine (C), which pair across the strands A with T and G with C, and the base sequence encodes genes, the instructions for making proteins. Chemically it is a polymer of deoxyribonucleotides joined by phosphodiester bonds between the 3' hydroxyl of one sugar and the 5' phosphate of the next, which gives each strand direction, and the two antiparallel strands wind into a right-handed helix held by hydrogen bonds (two for A-T, three for G-C) and base stacking. Semiconservative replication by DNA polymerase runs 5' to 3' from an RNA primer, continuously on the leading strand and in Okazaki fragments on the lagging strand, with proofreading that cuts the error rate to about one in $10^9$ base pairs after repair. Information flows from DNA to RNA by transcription and from RNA to protein by translation.
Example
Every cell in your body, from a hair cell to a heart cell, carries the same DNA, a mix of half from your mother and half from your father. If one strand reads ATGGCT, the other must read TACCGA; human DNA contains about $3.1$ billion base pairs, and each cell keeps a complete copy in its nucleus packaged into $46$ chromosomes. The helix is about $2\ \text{nm}$ in diameter and rises $0.34\ \text{nm}$ per base pair, so $3.1$ billion base pairs stretch to about $1$ meter per copy of the haploid genome, roughly $2$ meters per diploid cell, and X-ray diffraction data from Rosalind Franklin were central to Watson and Crick's 1953 model.
Key Insight
DNA is written in an alphabet of only four letters, A, T, G, and C, and the order of those letters is the difference between a person, a pine tree, and a mushroom. Complementary base pairing is what makes copying possible: split the ladder down the middle and each half is a template for rebuilding the other, which is exactly how cells copy DNA before dividing. DNA stores information digitally in a chemically stable polymer that can be copied with near-perfect fidelity, yet the rare copying errors are the ultimate source of all heritable variation, so the same molecule is both the archive of life and the engine of its change.