Only one or two percent of human DNA actually spells out proteins
Your genome runs to about 3.1 billion letters per set of chromosomes, yet the parts that code for proteins fill just one to two percent. Half is repeats, a big share comes from ancient viruses and jumping genes, and more than 60 percent of our smell-receptor genes are broken.
The human genome spans 22 paired chromosomes plus the X and Y sex chromosomes, with a small separate genome inside each mitochondrion. The standard reference, a composite built from several people, holds about 3.1 billion base pairs; most cells carry two sets, roughly 6.2 billion. It took decades to finish. Scientists announced 88 percent in 2000, at least 8 percent was still missing in 2020, a complete female genome came in 2021, and the stubborn Y chromosome, all 62,460,029 base pairs of it, was finished in January 2022. A 2023 draft pangenome drew on 47 people of varied ancestry to capture more diversity.
Between 19,000 and 20,000 genes make proteins. Each averages ten introns, stretches that are cut out before the message is used, so genes average about 62,000 base pairs and occupy some 40 percent of the genome even though coding sequence itself is tiny. About 13,000 pseudogenes, disabled copies of old genes, sit alongside them. More than 60 percent of human olfactory receptor genes are pseudogenes, compared with 20 percent in mice, which helps explain our weaker noses.
Repeats make up around half the total. Transposons, sequences that copy and paste themselves around the genome, and their dead relics account for more than half of human DNA; the Alu family alone has about 50,000 active copies. Some are fossil viruses, and envelope genes from two such viruses now help form the placenta. Short tandem repeats vary so much between individuals that they underpin forensic and genealogy tests, and an expanded CAG repeat on chromosome 4 causes Huntington's disease.
How much of all this does anything is disputed. Only about 10 percent appears conserved by selection, and estimates for regulatory DNA range from 8 percent to 20 percent or more. People differ from each other by about 0.1 percent in single letters, while humans and chimpanzees or bonobos differ by about 1.1 percent.
Source: Human genome
