DNA and the genome: bases, genes and chromosomes

DNADNA (deoxyribonucleic acid)The hereditary material in humans and almost all other organisms, written in four chemical bases: adenine (A), guanine (G), cytosine (C) and thymine (T). is the hereditary material in humans and almost all other organisms. Its information is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C) and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people.^[1]

Bases pair up with each other — A with T, and C with G — to form units called base pairsBase pairTwo bases on opposite DNA strands joined together. A pairs with T, and C pairs with G.. Each base is also attached to a sugar molecule and a phosphate molecule; together they make a NucleotideNucleotideOne building block of DNA: a base attached to a sugar molecule and a phosphate molecule.. Nucleotides are arranged in two long strands that form a spiral called a double helix, which is somewhat like a ladder: the base pairs are the rungs and the sugar and phosphate molecules are the sides.^[1]

Most DNA is located in the cell nucleus, where it is called nuclear DNA; a small amount is found in mitochondria, the structures that convert energy from food into a form cells can use.^[1] Scroll through the steps below to zoom in.

A GeneGeneA stretch of DNA, from a few hundred to more than two million bases long, that carries instructions. Most genes code for proteins; humans have about 19,900 protein-coding genes. is the basic physical and functional unit of heredity. Genes range in size from a few hundred DNA bases to more than 2 million bases. The Human Genome Project estimated that humans have between 20,000 and 25,000 protein-coding genes; we now know the genome contains about 19,900.^[3]

Every person has two copies of each gene, one inherited from each parent. Most genes are the same in everyone, but a small number — less than 1 percent of the total — are slightly different between people. These alternative forms are called allelesAlleleOne of the slightly different forms of the same gene. Each person inherits two copies of each gene, one from each parent..^[3]

Inside the nucleus, DNA is packaged into chromosomesChromosomeA thread-like package of DNA. People usually have 23 pairs (46 in total): 22 pairs of autosomes and one pair of sex chromosomes.. People usually have 23 pairs, 46 in total: 22 pairs of autosomes, which look the same in males and females, and one pair of sex chromosomes. Females typically have two X chromosomes and males one X and one Y. A picture of a person’s chromosomes arranged in pairs is called a KaryotypeKaryotypeA picture of a person’s chromosomes, arranged in pairs by size..^[2]

A VariantVariantA permanent change in a DNA sequence. Most variants do not cause disease. Guidelines prefer this neutral word to “mutation” or “polymorphism”. is a permanent change in the DNA sequence. Most variants do not lead to disease, and those that do are uncommon in the general population.^[4] The simplest kind is a single nucleotide polymorphism (SNP)Single nucleotide polymorphism (SNP)A genomic variant at a single base position in the DNA, where one person’s sequence differs from another’s.: a variant at a single base position in the DNA.^[5]

In April 1953, Nature published James Watson and Francis Crick’s proposed double-helix structure for DNA, in the same issue as Rosalind Franklin and Raymond Gosling’s paper on the molecular configuration of DNA.^[6][7] Crick, Watson and Maurice Wilkins received the 1962 Nobel Prize in Physiology or Medicine for their discoveries concerning the molecular structure of nucleic acids.^[8]

The Human Genome Project ran from 1990 to 2003 and produced an essentially complete sequence in April 2003 that covered about 92 percent of the genome. The first truly complete sequence, from the Telomere-to-Telomere consortium, followed on 31 March 2022.^[9][10]