What are the Smallest Genomes?

Genomes are the code of life -- the molecular blueprint that builds every living organism. Found coded in DNA in the nucleus of the cell, the human genome has about 3 billion amino acid base pairs and 20,000-25,000 protein-coding genes. This is relatively typical with a metazoan organism. Genome size does not directly correlate to the complexity of the organism, but does abstractly, 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 streamlining.

Some organisms have truly tiny genomes, the smallest genomes yet discovered. As of 2011, the smallest reported genome found outside an organelle was the Tremblaya genome, which has just 121 genes, and is found in a bacteria found in mealybugs. Other extremely small genomes include Candidatus Carsonella rudii, also known as C. rudii, at just 159,662 base pairs, with approximately 182 genes. This bacteria 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's body providing the bacteria with nutrients and shelter, and the bacteria giving the insect vitamins and amino acids.

The second smallest non-viral genome is found in Nanoarchaeum equitaans, a thermophilic archaea found among hydrothermal vents on the Atlantic Ocean floor, around the Mid-Atlantic Ridge. N. equitaans is a highly unusual organism, lacking certain metabolic pathways found in practically every other living thing, and being given its own phylum in domain Archaea. Its genome is 490,885 base pairs long. This is among the smallest genomes of any living thing. Like C. rudii, N. equitaans is an obligatory symbiont of another organism, in this case, the archeon Ignicoccus, from which it gets many essential biomolecules.

Of all the smallest genomes mentioned previously, the genome-bearing organism was an obligatory symbiont of another. But the smallest genome out of any free-living organism can be found in the bacteria Mycoplasma genitalium, which is found on the respiratory tracts and genitals of primates, such as humans. This bacterium was first isolated in 1980 from urethral specimens of two male paints with inflammation of the urethra. Thought to be first among the smallest genomes under the discovery of 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 necessary to sustain life.