in Principles and process of Biotechnology by

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Restriction enzymes are found in bacteria and archaea and help in defense against phages. These enzymes destroy phage DNA after its entrance. These enzymes recognize restriction sites on foreign DNA thereby acting on it. Cells protect their own DNA by methylating nucleotide in the recognition sites. These enzymes can’t recognize methylated nucleotide. Foreign or phage DNA is not methylated and is cleaved by restriction enzymes.