Restriction Endonucleases cut DNA at a specific sequence, normally 4, 6 or 8 bases long. They recognise specific palindromic base sequences in DNA and are used to selectively cut DNA into defined fragments at sites known as 'restriction sites'. Different restriction nucleases are obtained and purified from different species of bacteria. These enzymes are made in bacteria to degrade viral DNA. Cutting DNA with restriction enzymes can produce fragments with either blunt or sticky ends. Different restriction enzymes recognise different DNA sequences.
Examples of restriction endonucleases are:
EcoRI- recognines the sequence GAATTC
BamHI- recognises the sequence GGATCC
HaeIII- recognises GGCC
Both EcoRI and BamHI produce fragments with staggered ends, whilst HaeIII produces a DNA fragment with blunt ends i.e. there is no overhang.