The most recent methods of classification are based on the comparison of the nucleotide sequences of the DNA of different organisms. Because the complete genomes of relatively few microorganisms have been determined, other, less precise methods are used to rapidly compare and contrast general similarities. Some of these methods crudely examine total DNA and are used to broadly sort organisms as to their evolutionary related-ness; others more closely inspect specific nucleic acid segments as a mechanism of fine-tuning the evolutionary scale.
DNA Base Ratio (G + C Content)
One way to roughly compare the genomes of different bacteria is to determine their base ratio, which is the relative portion of adenine (A), thymine (T), guanine (G), and cytosine (C). Because of nucleotide base-pairing rules, the number of molecules of G in double-stranded DNA always equals the number of molecules of C. Likewise, the number of molecules of T equals that of A. The base ratio of an organism is usually expressed as the percent of guanine plus cytosine, termed the G + C content, or more simply, the GC content. If an organism has a GC content of 60%, then the remaining 40% must be the AT content.
The GC content is often measured by determining the temperature at which the double-stranded DNA denatures, or melts. Because three hydrogen bonds occur between G-C base pairs, and only two hydrogen bonds between A-T, DNA that has a high GC content melts at a higher temperature. The temperature at which double-stranded DNA melts can readily be determined by monitoring the absorbance of UV light by a solution of DNA as it is heated. The absorbance rapidly increases as double-stranded DNA denatures (figure 10.16).
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