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Genome Organization: Base Composition

المؤلف:  Katherine Floyd

المصدر:  Core Concepts in Biology: Molecular Biology

الجزء والصفحة:  2nd Edition , p18-19

2026-09-10

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The base composition of DNA in an organism is a fixed value and it is expressed as the % of (G + C) of the total genome. The variation of this value between different prokaryotes is large; this is surprising given that many of the individual proteins produced by the species have similar amino acid sequences. Prokaryotic genomic DNA can have as little as 25 % (G + C) in Mycoplasma genitalium to as high as ~72 % in Micrococcus lysodeikticus. Eukaryotic genomic DNA does not display the same variation between species. The % (G + C) composition of most plant and animal species falls within a narrow range, averaging at 39% with a variation of only ± 6%.

In prokaryotes the bases are distributed evenly throughout the genome with a slightly lower (G + C) content in promoter and intergenic regions; these often have A + T rich segments which melt more readily than G+C rich regions. The relatively constant base distribution within a given bacterial genome suggests that although there may be unequal nucleotide pool sizes inside the cell, the system will have evolved over many generations to be like this, and the rate of DNA replication is constant. The distribution of the (G + C) content throughout each genome in eukaryotes, however, varies significantly, unlike prokaryotes. Whereas the mean variation in % (G + C) content throughout the E. coli genome is only 8.6 % in eukaryotes, this variation is over 30 %. Certain regions of eukaryotic genomic DNA are found to be (A + T) rich, with a % (G + C) content as low as 18 %, while other regions have a (G + C) content as high as 70 %.