Endonucleases are enyzmes (usually derived from bacterial sources) which cleave DNA at defined "restriction" sites.

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.