Not long ago it was thought that all prokaryotic genomes (both Bacteria and Archae) were much smaller than eukaryotic genomes. However, the application of new techniques for constructing physical maps and whole genome sequencing has demonstrated that there is tremendous diversity in the size and organization of prokaryotic genomes. The following figure shows some examples of genome sizes of Bacteria and Archae. The size of Bacterial chromosomes ranges from 0.6 Mbp to over 10 Mbp, and the size of Archael chromosomes range from 0.5 Mbp to 5.8 Mbp. (For comparison, Eukaryotic chromosomes range from 2.9 Mbp (Microsporidia) to well over 4,000 Mbp, although the largest genomes are littered with a tremendous amount of repetitive "junk" DNA.)
The smallest Archae genome identified thus far is from Nanoarchaeum equtans, a tiny obligate symbiont with a genome size of 0.491 Mbp (491 Kbp). This organism lacks genes required for synthesis of lipids, amino acids, nucleotides, and vitamins, and hence must grow in close association with another organism which provides these nutrients.
The smallest Bacterial genome identified thus far is from Mycoplasma genitalium, an obligate intracellular pathogen with a genome size of 0.58 Mbp (580 Kbp). M. genitalium is restricted to the intracellular niche because it lacks genes encoding enzymes required for amino acid biosynthesis and the peptidoglycan cell wall, genes encoding TCA cycle enzymes, and many other biosynthetic genes.
In contrast to such obligate intracellular bacteria, free-living bacteria must dedicate many genes toward the biosynthesis and transport of nutrients and building blocks. The smallest free-living organisms have a genome size over 1 Mbp.
A feature often used to distinguish prokaryotic and eukaryotic genomes is the amount on "junk" DNA (noncoding DNA that is probably largely remnants of genes that have been lost over the course of evolution). As a general rule, prokaryotes tend to have very little junk DNA (typically less than 15% of the genome) and eukaryotes have substantial amounts of junk DNA. However, genome sequences of certian bacteria with very limited ecological niches (e.g., the human pathogen Mycobacterium leprae and the Aphid endosymbiont Buchnera) indicates that, like in eukaryotes, a substantial amount of bacterial genomes can consist of junk DNA.
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Last modified July 5, 2004