Bacteria typically carry all of their genes on a single circular chromosome. Occasional species of bacteria have two or more different chromosomes and a few bacteria even contain linear chromosomes (see Ch. 18). Higher organisms have linear chromosomes and the number ranges from a handful to over a thousand in a few flowering plants.
The smallest eubacterial genome is that of Mycoplasma genitalium, which has one circular chromosome, consisting of 580,000 base pairs (bp) of DNA. Since the average gene is about 1,000 base pairs long, and there is little non-coding DNA between bacterial genes, M. genitalium should have approximately 500 genes. The precise number is disputed and ranges from 468 to 517. Extensive analysis of mutations suggests that around 300 of these genes are essential for the growth and reproduction of M. genitalium. Comparison with other small bacterial genomes suggests that around 250 genes may be the minimum essential for a living cell. (This estimate also takes into account those pathways needed for synthesis of all vital cell components, some of which are missing in M. genitalium because it is a parasitic bacterium.) The smallest prokaryotic genome belongs to Nanoarchaeum equitans, a marine archaebacterium that was discovered in 2002. N. equitans has about 15% less DNA than M. genitalium and may also be a parasite, as it cannot grow unless attached to the surface of other microorganisms. Despite having less DNA its genes are more closely spaced and in consequence N. equitans actually has more coding sequences than M. genitalium—approximately 550.
Although parasitic bacteria may have less than 1,000 genes, most free-living bacteria have 2,000 to 4,000 genes. Occasional bacteria with complex life cycles, such as Myxococcus, may have 9,000 to 10,000 genes. Free-living eukaryotes typically have from 6,000 to 50,000 genes (see Table 4.01). However, the parasitic eukaryote Encephalitozoon cuniculi (Protozoa, Microspora) has only 2.9 million base pairs (Mbp) of DNA, implying that it possesses no more than 3,000 genes—less than many bacteria.
Some regions of DNA contain useful genetic information, other regions do not.
Non-coding DNA accounts for the majority of the DNA in most higher animals and plants.
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