New taxonomic methods:DNA/DNA hybridization

This chapter focuses on new taxonomic methods that are DNA/DNA hybridization. The method of DNA/DNA hybridization allows the determination of sequence similarity among genomes of different organisms without the determination of exact nucleotide sequences. It is based on the feature that denatured, single-stranded DNA will renature under suitable conditions. The extent of renaturation is proportional to the complementarity of the DNA strands involved. Furthermore, the stability of the renatured hybrid molecule is proportional to the accuracy of matching between the two strands. Thus, by DNA/DNA hybridization, two parameters for genomic relatedness are obtained. Generally DNA/DNA hybridizations work well for genomic comparisons at the species level. For higher taxons the renaturation level rapidly drops down to background levels; for lower taxons the method is insufficiently sensitive. The method has become particularly popular for genetic studies of prokaryotes, owing to the rather uncomplicated nature of prokaryotic genomes and the subsequent straightforwardness of the method. To carry out DNA/DNA hybridizations, the G + C content of the DNA involved should be known. If the systematic position of the strain involved is rather clear, one might rely on the G + C data of other genus members