We have discussed restriction enzymes as a form of defense used by bacteria against viruses. Restriction enzymes are of heightened importance to today's student because of their importance as tools for recombinant DNA experiments in the molecular biology laboratory. DNA methylation is another aspect to the battle between bacteria and the phages that infect them, and methylases are often part of the same enzyme complex. DNA methylation is a process of chemical modification of DNA bases that occurs in this context as a strategy of self-recognition for the bacterium. Phages that evade restriction enzymes can possess modified bases in target sequences as well. The bacterium methylates its own DNA to prevent it from digestion, so methylated viral DNA is also protected (the methylase recognizes the same palindromic sequence as the restriction endonuclease).
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