DNA-DNA hybridisation

DNA-DNA hybridization is a method in genetics to measure the degree of genetic similarity between DNA sequences. The technique is usually used to determine the genetic "distance" between two species. When several species are compared that way, the similarity values allow the species to be arranged in a phylogenetic tree.

Method The DNA from the two species to be compared is extracted, purified and cut into short pieces (e.g., 600-800 base pairs). The DNA double strand is then separated by heating into two single strands. The single-stranded DNA is now allowed to anneal with the DNA pieces of the other species. The more similar the DNA, the more of the pieces will anneal and form a hybrid (thus the name) double strand. Strands with a high degree of similarity will bind more firmly, and require more energy to separate them: i.e. they separate when heated at a higher temperature than dissimilar strands. To assess this "melting temperature" the mixture is heated in small steps. At each step, samples are tested as to the amount of single- and double-stranded DNA. This results in a profile from which the amount of similar DNA, and thus the degree of genetic similarity, can be determined.