Restriction endonucleases is a bacterial enzymes that can cut double stranded DNA into smaller pieces by recognizing specific nucleotide sequences on DNA. First postulated by W, Arber in late 1960. He found that when viral gene attack on bacterial cell they restricted the entry of viral gen DNA by breaking it into smaller peices do resist viral attach. Hence, they named molecular scissors:
The first restriction enzyme was isolated from bacteriophage in 1970 by Halminth Smith and named - II. Restriction enzymes are belong to the large class of enzymes that is nuclease, they are known to be restriction endonuclease because they act on the nuclic acid of cell.

According the the action on DNA molecules. They are classified in two types.
Exonuclease RE and Endonculease RE.
Restriction Enzymes: 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.For the synthesis of recombinant DNA molecules possible only. When the vector and source DNA is cut by the same restriction enzyme because restriction enzyme are very specific in their action. For the formation of recombinant DNA cut ends of vector DNA should be complementary to the foreign DNA only when the nucleotides will join together by hydrogen bonds. Because the use same endonuclease enzyme are got some free sticky ends on both vector DNA and foreign DNA and the free sticky ends are joined together (end to end) with the help of DNA ligase enzyme. That form the combination of both foreign and vector DNA and so called recombinant DNA. The diagrammatic representation for the formation of recombinant DNA as shown in figure.