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Enzymes that cut a DNA double helix at specific locations on that DNA.
Restriction endonucleases, also known as restriction enzymes, play two roles in biology. First is their role as bacteria-associated enzymes that either serve to protect bacteria from invading foreign DNA or instead which serve as the toxin component of toxin-antitoxin, selfishly protecting the continuation of their existence in those same bacteria.
Second is their role in biotechnology as a means of very precisely cutting DNA – in the course of restriction digests – either for the sake of DNA characterization or in the course of gene cloning. Genetic engineering basically would not exist, or at least would not exist in the same manner, were it not for restriction endonucleases.
To function within organisms, restriction endonucleases must be coupled with corresponding modification enzymes which protect restriction enzyme recognition sequences in non-foreign DNA, that is, the DNA associated with the bacterial genome (including plasmids that are already present within a cell). In this way, restriction enzymes can serve as a general pattern recognition system that nevertheless fails to recognize similar patterns associated with self.
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