Restriction Enzyme: Restriction Enzymes are bacterial enzyme followed by protective machenism to viral DNA and such enzymes are not found in eukaryotic cells. Restriction enzymes was postulated for the first time by W. Arber in late 1960. He noticed that when DNA of bacteriophage virus entered in host bacterium (E. coll) the host DNA cut virus DNA into smaller pieces.

"First restriction endonculease isolated from bacteriophage by Halmilton and named Hind II. According to there functions restriction enzymes are classified into two kinds.

• Exonuclease: Cleave DNA nucleotide from the ends, usually they are non - specific.
• Endonuclease: Cleave DNA between nucleotide at specific sequences. They are highly specific in different species.

Type I endonuclease are first identified but Type II endonuclease are first isolated from E. coli. Restriction enzymes produce both sticky and blunt ends of DNA. They cut same, sequences in both foreign DNA and vector DNA so they can early ligated. Usually they follow palindromic sequences.
Name of enzymes that help in the formation of r - DNA are:

1. Restriction endonuclease
2. Ligases
3. Proteases
4. DNA ploymerase
5. Chitinase
6. Lysozymes
7. Cellulase