Type I restriction enzymes have both endonuclease (cleaving DNA) and methyl-transferase (modifying DNA by adding a methyl group) activities. They:
• Recognize specific DNA sequences but cleave DNA at random locations far away from the recognition site.
• Methylate specific bases within the recognition sequence to protect the host DNA from cleavage.
This dual functionality is characteristic of Type I restriction enzymes, differentiating them from other types of restriction enzymes that mainly perform cleavage. Type II enzymes cleave DNA at specific site. Type III enzymes cleave DNA at a specific sequence but not at the recognition site.