Restriction maps
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In molecular biology, restriction maps are used to compare relatedness of two different species at the molecular level. "Restriction mapping" is one of the three methods used by scientists, the other two methods being: DNA-DNA hybridization, and DNA or RNA sequence analysis.
In restriction maps, the same restriction enzymes are used as in recombinant DNA technology. Each type of restriction enzyme recognizes a specific sequence of a few nucleotides and cleaves DNA wherever such sequences are found in the genome. The DNA fragments found after the treatment can be separated by electrophoresis and compared to restriction fragments derived from the DNA of another species. Two samples of DNA with similar maps for the locations of restriction sites will produce similar collections of fragments. In contrast, two genomes that have diverged extensively since their last common ancestor will have a very different distribution of restriction sites, and the DNA will not match closely in the sizes of restriction fragments.
Because so many fragments are obtained from the genome of the nucleus, restriction mapping is more practical for comparing smaller segments of DNA, usually a few thousand nucleotides long. Several laboratories have used restriction maps to compare mitochondrial DNA (mtDNA), which is relatively small. There is the added benefit that mtDNA changes by mutation about ten times faster than that does the nuclear genome, which makes it possible to sort out phylogenetic relationships between very closely related species, or even between different populations of the same species.
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