Genomes are the molecular blueprint of every living organism, with the human genome having approximately 3 billion base pairs and 20,000-25,000 protein-coding genes. Parasites have the smallest genomes of any multicellular organism due to genetic rationalization. The smallest genomes ever discovered belong to obligate symbionts, with the smallest free-living organism genome belonging to Mycoplasma genitalium with 582,970 base pairs and 482 protein-coding genes.
Genomes are the code of life, the molecular blueprint that builds every living organism. Found encoded in DNA in the cell nucleus, the human genome has approximately 3 billion base pairs of amino acids and 20,000-25,000 protein-coding genes. This is relatively typical with a metazoan organism. Genome size is not directly related to organism complexity, but does so in an abstract way, especially when comparing genome lengths between multicellular and unicellular organisms. Parasites have the smallest genomes of any multicellular organism, having undergone a process known as genetic rationalization.
Some organisms have really tiny genomes, the smallest genomes ever discovered. As of 2011, the smallest genome reported found outside an organelle was the Tremblaya genome, which has just 121 genes and is found in a bacterium found in mealybugs. Other extremely small genomes include Candidatus Carsonella rudii, also known as C. rudii, at just 159,662 base pairs, with about 182 genes. This bacterium is found in sap-sucking insects called jumping plant lice. An endosymbiont, C. rudii lives in specialized organs within the insect called bacteriomes. They have a mutually beneficial relationship, with the insect body providing the bacteria with nutrients and shelter and the bacteria providing the insect with vitamins and amino acids.
The second smallest non-viral genome is found in Nanoarchaeum equitaans, a thermophilic archaea found among hydrothermal vents on the floor of the Atlantic Ocean, around the Mid-Atlantic Ridge. N. equitaans is a very unusual organism, lacking certain metabolic pathways found in virtually every other living thing, and having its own phylum in the Archaea domain. Its genome is 490,885 base pairs long. This is one of the smallest genomes of any living thing. Like C. rudii, N. equitaans is an obligate symbiont of another organism, in this case the archaeon Ignicoccus, from which it derives many essential biomolecules.
Of all the smaller genomes mentioned earlier, the organism carrying the genome was an obligate symbiont of another. But the smallest genome of any free-living organism is found in the bacterium Mycoplasma genitalium, which is found on the respiratory tract and genitals of primates, such as humans. This bacterium was first isolated in 1980 from urethral samples of two male paints with inflammation of the urethra. Considered to be the first among the smallest genomes discovered by C. rudii in 2002, M. gentalium has only 582,970 base pairs and 482 protein-coding genes. M. genitalium has been used as a model for the Minimal Genome Project, which seeks to create a bacterium with the minimum genome needed to sustain life.
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