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Bacterial genomes are generally smaller and less variant in size between species when compared with genomes of animals and single cell eukaryotes. Bacterial genomes can range in size anywhere from about 130 kbp to over 14 Mbp. Recent advances in sequencing technology led to the discovery of a high correlation between the number of genes and the genome size of bacteria, suggesting that bacteria have relatively small amounts of noncoding DNA. A striking discovery by Cole et al. described massive amounts of gene decay when comparing Leprosy bacillus to ancestral bacteria. Studies have since shown that a large number of bacterial species have undergone genomic degradation resulting in a decrease in genome size from their ancestral state. Over the years, researchers have proposed several theories to explain the general trend of bacterial genome decay and the relatively small size of bacterial genomes. Compelling evidence indicates that the apparent degradation of bacterial genomes is owed to a deletional bias. ==Bacterial genomes== Bacteria possess a compact genome architecture distinct from eukaryotes in two important ways: bacteria show a strong correlation between genome size and number of functional genes in a genome, and those genes are structured into operons. The main reason for the relative density of bacterial genomes compared to eukaryotic genomes (especially multicellular eukaryotes) is the presence of noncoding DNA in the form of intergenic regions and introns.〔 Some notable exceptions include recently formed pathogenic bacteria. This was initially described in a study by Cole ''et al''. in which ''Mycobacterium leprae'' was discovered to have a significantly higher percentage of pseudogenes to functional genes (~40%) than its free-living ancestors.〔 Furthermore, amongst species of bacteria, there is relatively little variation in genome size when compared with the genome sizes of other major groups of life.〔 Genome size is of little relevance when considering the number of functional genes in eukaryotic species. In bacteria however, the strong correlation between the number of genes and the genome size makes the size of bacterial genomes an interesting topic for research and discussion. The general trends of bacterial evolution indicate that bacteria started as free-living organisms. Evolutionary paths led some bacteria to become pathogens and symbionts. The lifestyles of bacteria play an integral role in their respective genome sizes. Free-living bacteria have the largest genomes out of the three types of bacteria; however, they have fewer pseudogenes than bacteria that have recently acquired pathogenicity. Facultative and recently evolved pathogenic bacteria exhibit a smaller genome size than free-living bacteria, yet they have more pseudogenes than any other form of bacteria. Obligate bacterial symbionts or pathogens have the smallest genomes and the fewest pseudogenes of the three groups. The relationship between life-styles of bacteria and genome size raises questions as to the mechanisms of bacterial genome evolution. Researchers have developed several theories to explain the patterns of genome size evolution amongst bacteria. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Bacterial genome size」の詳細全文を読む スポンサード リンク
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