Historically, these bacterially synthesized enzymes were called restriction endonucleases because they restricted bacteriophages from infecting through digestion of viral DNA.

There are four types of restriction enzymess:

  • Type I: Can both methylate (SAM dependent) and digest DNA (ATP dependent). Cleave several nucleotides away from the restriction sites.
  • Type II: Cleave at the restriction site. No methyl-transferase activity.
  • Type III: Cleave 20-30 nt away from the restriction sites in ATP dependent way. Have methyl-transferase activity.
  • Type IV: Cleave methylated DNA.
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