Human DNA

The human genome holds about 3 billion base pairs. The National Institute of General Medical Sciences gives 3.2 billion.

One copy of the genome has about 3 billion nucleotides spread across 23 chromosomes. Most human cells are diploid, with two copies, and most humans have 46 chromosomes per cell.

DNA is built from four bases: adenine (A), guanine (G), cytosine (C) and thymine (T). A pairs with T, and C pairs with G.

Two strands wind around each other in a twisted ladder shape called a double helix. The backbone of each strand alternates sugar and phosphate groups.

The order of the bases along a strand encodes information, such as the instructions for making a protein or an RNA molecule.

Most DNA sits in the cell nucleus, with a small amount in the mitochondria. Mitochondrial DNA carries 37 genes.

The human genome has about 20,000 protein-coding genes. MedlinePlus Genetics gives about 19,900.

Protein-coding genes make up less than 2% of the genome, and over 98% is noncoding DNA. MedlinePlus Genetics puts coding DNA at about 1 percent.

Genes vary greatly in length, from a few hundred DNA base pairs to more than 2 million base pairs.

Humans are 99.9% genetically identical, and the remaining 0.1% accounts for individual differences. MedlinePlus Genetics, more cautiously, says more than 99 percent of bases are the same.

The National Human Genome Research Institute calls 99.9% an over-simplification, because it counts only single-nucleotide differences, which occur about once every 1,300 nucleotides.

Counting every kind of variant, any two people's genomes are on average about 99.6% identical and about 0.4% different.

A difference counts as a SNP, a single-nucleotide polymorphism, only when the single-letter variant is present in at least 1% of the human population.

Compared with the reference genome, a typical genome carries about 5,000,000 single-nucleotide variants, about 600,000 insertion or deletion variants and about 25,000 structural variants. Together they involve roughly 27,000,000 nucleotides, about 0.4% of the genome.

The Human Genome Project ran from 1990 to 2003, two years ahead of its original 2005 target.

The sequence announced in 2003 covered 92% of the genome, with fewer than 400 gaps.

The project's initial cost projection was $3 billion.

On 31 March 2022 the Telomere-to-Telomere consortium filled the remaining gaps and produced the first truly complete human genome sequence.