Restriction enzymes are nucleases and as they cut at an internal position of DNA (and not an end) they are known as endonucleases. They are also known as molecular knives, molecular scissors, restriction enzymes or molecular scalpels.
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The sequence recognized by the restriction enzyme to cut the DNA is called restriction site, recognition site or restriction endonuclease site. The recognition site consists of 4-8 base pairs. 


Types of restriction enzymes : The restriction endonucleases are grouped into three types.
  1. Type I Restriction Endonuclease:
  2. Type II Restriction Endonuclease
  3. Type III Restriction Endonuclease
Type I Restriction Endonuclease vs Type II Restriction Endonuclease

Type I Restriction  Endonuclease Type II Restriction Endonuclease
These are most complex. The enzyme is made up of three non identical subunits. These are simplest.The enzyme is made up of two identical sub units.
Molecular weight is 4,00,000 daltons. Molecular weight ranges from 20,000 to 1,00,000  daltons.
The site of cutting  is 1,000 nucleotides away from the recognition site. The site of cutting is in the same recognition site.
The sequence of cutting is non specific. The sequence of cutting is specific.
restriction endonuclease
The enzymes protect DNAs by methylation. No methylation activity.
ATP, Mg2+ and adenosyl methionine are required for activation. Mg2+ alone is required for activated
The enzyme has both endonuclease activity and methylase activity. Restriction activity alone.
Examples: EcoK, EcoB Examples: Hind II, EcoRI, 

Type II restriction endonucleases are most important tools in gene cloning.
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