Restriction Endonuclease

Restriction Endonuclease:

Discovered by Werner Arber, Daniel Nathans, and Hamilton Smith.

  • They are known as “molecular scissors” in genetic engineering.
  • They are capable of breaking DNA at specific sites called the restricted sites.

These restricted sites have Palindromic sequences.

Palindromic sequences are sequences that have the same meaning when read from both the sides just like the word ‘MADAM’ & ‘MALAYALAM’.


And sequences like:

5‘  →  3



3‘  →  5

  • They break the phosphodiester bonds.
  • They are found in bacteria & they protect the bacterium from foreign DNA.
  • Due to methylation of its nucleotide, they don’t act on bacteria’s own DNA.
  • The 1st restriction endonuclease isolated was Hind II.
  • More than 900 R.E have been isolated, from 230 strains of bacteria.
  • There are 3 types of R.E.Type I, II, III.

Type III is used in genetic engineering.

Naming an R.E:

• The 1st letter in a capital: it is the genus of the bacteria from which it is isolated (written in italics).
• The next two letters come from the species of the bacteria (written in italics).
• 3rd is the strain from which it has been isolated.
• 4th is the Roman numeral indicating the order of discovery from that particular strain.

EcoRI →
‘E’ represents Escherichia bacteria,
‘co’ represents coli the species,
‘R’ represents the RY13 strain of the bacteria and
‘I’ it is the 1st to be discovered from that strain.
HindII →
‘H’ represents Haemophilus bacteria,
‘in’ represents influenzae species,
‘d’ represents Rd strain and
‘II’ it is 2nd to be discovered from that strain.

 Restricted sites of EcoRI:
5′ GAATTC 3′
3′ CTTAAG 5′

Restricted sites of Hind II:
5′ GTCGAC 3′
3′ CAGCTG 5′

R.E acts in two ways: 

 Sticky ends: Ends produced when the R.E cuts the strand of DNA a little away from the center of the palindromic sequences. It cuts between the same two bases of the two strands, leaving single-stranded parts at the ends. These single-stranded parts are referred as sticky ends.

Ligase enzyme is  used to join DNA fragments with same sticky ends.


 Blunt ends: Ends produced by R.E that cuts the two strands of DNA at the center of the restricted site.

Recombinant Vector:

Same R.E to cut the vector and the foreign DNA  during the creation of recombinant vector. As it will produce same sticky ends that can be ligated by the ligase enzyme.

ALSO WATCH: Restriction Endonuclease


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