Definition
A genome is the complete set of genetic material of an organism, encompassing all of its chromosomes and any organellar DNA.
Explanation
The genome contains all hereditary information encoded in the DNA—or RNA in RNA viruses—of an organism. In eukaryotes, the nuclear genome is organized into multiple linear chromosomes, while mitochondria and plastids maintain their own circular genomes derived from ancestral symbiotic bacteria. Prokaryotic genomes usually consist of a single circular chromosome and may include smaller plasmids that carry accessory genes. Genome size varies enormously: some RNA viruses have fewer than 10 kilobases, the bacterium Escherichia coli has about 4.6 megabases, and the human haploid genome contains roughly 3.1 billion base pairs distributed across 23 chromosomes. Plants such as wheat are polyploid and possess genomes with tens of billions of bases. The genome encompasses protein-coding genes as well as non‑coding sequences, including introns, regulatory elements, repetitive DNA and transposable elements. In sexually reproducing organisms, gametes carry haploid genomes, while somatic cells are typically diploid or polyploid. Genome sequencing projects have produced reference genomes for hundreds of species, enabling comparative genomics and identification of disease-associated variants. Genomic knowledge underpins fields such as molecular evolution, population genetics, forensic science and personalized medicine. Variation within and among genomes generates the diversity on which natural selection acts.
Examples and important facts
The human genome project produced a draft sequence of the human nuclear genome, revealing about 20,000 protein-coding genes. The mitochondrial genome is a circular DNA molecule of about 16.5 kilobases encoding 37 genes important for oxidative phosphorylation. The laboratory strain E. coli K‑12 has a genome of 4.6 megabases with approximately 4,400 genes. The minimal genome of the bacterium Mycoplasma genitalium comprises about 580 kilobases and 525 genes, making it a model for understanding the essential gene set. Giant genomes occur in some amphibians and plants; the axolotl (Ambystoma mexicanum) genome is around 32 gigabases. RNA viruses such as influenza possess segmented genomes and rely on error-prone polymerases, resulting in rapid evolution.
The genome is the blueprint of an organism. Decoding its structure and variation provides insight into evolution, development, disease and the potential for genetic innovation.
Related Terms: DNA, Chromosome, Genome sequencing, Genomics, Mitochondrial DNA