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Ratio of Guanine and Cytosine to Thymine and Adenine as found making up organism genomes.
Organisms have characteristic DNA base compositions that differ, particularly for prokaryotic organisms, as a consequence of biases in tRNA usage. That is, species tend to differ in terms of what synonymous codons they prefer to use for specifying amino acids, resulting across genomes in biases towards the use of some bases versus others.
In addition, organisms that exist at very high temperatures – i.e., thermophiles and hyperthermophiles – tend to have an excess of GC over AT simply because GC base pairs resist denaturation to a larger extent than do AT base pairs.
Similarities in DNA base composition are insufficient evidence that two isolates are the same species, though large differences in DNA base composition do serve as a reasonable evidence that two isolates are not the same species. Notwithstanding such utility, in fact with widespread, inexpensive DNA sequencing, determination solely of DNA base composition as a means of organism classification is becoming somewhat unnecessary.
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