Restriction Enzymes

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Restriction enzymes are proteins that cut DNA at specific sequences, used for DNA manipulation in genetic engineering and molecular biology.

  • Restriction enzyme/Restriction endonucleases/Molecular scissors 
  • Produced from bacterial cells.
  • Cleaves DNA at specific site (recognition site or nearby)
  • They have the capacity to recognize specific base sequences on DNA and then cut at the recognition site.

Example: EcoR I, EcoR ll, BamHI, Hind III

Nomenclature:  Each restriction enzyme is named after the bacterium from which it was isolated, using naming system based on bacterial genus, species and strain.

Example: EcoRI

  • E: Escherichia (Genus)
  • co: coli (Species)
  • R: RY13 (strain)
  • I: Order of identification.

Recognition site: Restriction enzymes cut or cleave DNA molecules into fragments at or near a specific base sequence is known as recognition site. 

Restriction site: Restriction enzyme cut are cleave DNA molecule into fragments at a specific recognition site known as restriction site.

  • Cut the DNA molecule by catalysing hydrolysis (splitting of a chemical bond by addition of H₂O) of the bond between adjacent nucleotides.
  • Incisions on each sugar-phosphate backbone of the DNA helix.

Types of Restriction Enzymes

 Three types: Type I, Type II & Type III

Type I :

  1. Most complex type of restriction enzymes.
  2. Composed of 3 different subunits.
  3. Require Mg²+, ATP and S-adenosyl L-methionine to function.
  4. Cleavage site is random upto 100 bp away from the recognition site.
  5. Bifunctional having endonuclease and methylase activity.
  6. Recognizes the nucleotide. sequence of 13-17 bp.

Example: Eco B, Eco K

Type II: 

  1. Simplest for type from all restriction enzymes.
  2. Composed of 2 identical subunits.
  3. Require Mg²+ as cofactor for function.
  4. Cleavage or Restriction site is at the recognition site
  5. Unifunctional endonuclease activity.
  6. Recognizes the 4-8 bp nucleotide sequence.

Example: EcoRI, Alu I, Hind II

Type III : 

  • 1) Complex type of restriction enzyme.
  • Composed of 2 different subunits.
  • Require Mg²+, ATP and S adenosyl-L-methionine as cofactor to function.
  • Cleavage or restriction site is random upto 20-30 bp away from the recognition site.
  • Bifunctional having endonuclease & methylase. activity.
  • Recognizes non-palindromic sequence

Example: Eco PI, Eco P15, Hind III

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