Our Genomes

Abstract The genome is composed of all of the nucleotides in the 46 chromosomes present in each cell (except for mature gametes—sperm and eggs that contain 23) and the DNA present in each mitochondrion. Each human cell contains 6.2 billion nucleotides. The genome is organized into genes, which encode proteins, and a variety of RNA molecules: Messenger RNAs are translated into proteins, ribosomal RNAs form the scaffold upon which proteins are synthesized, and a variety of RNAs control the transcription of genes and other functions. The Human Genome Project began in 1990 and was an international collaboration to sequence all human chromosomes. A sequence of about 95 percent was completed in 2003, and the complete sequence was finally achieved in 2024. Each of us has hundreds of thousands of nucleotide sequence variations from the “reference genome.” Comparing genomes among a population and between populations shows that there is more genetic variation within groups. This confirms that the concept of race does not have a biologic basis. Some individual DNA variants account for human traits such as eye color and blood types. Others are harmful and account for more than 5000 rare conditions. Some variants have effects that predispose to disease and require the impact of other genetic variants and nonbiologic factors to produce disease.