Bacteria can produce dnases, which can double cleave the dna at specific sites. Restriction endonucleases or restriction enzymes can cleave the dna at specific sites. The 1st endonuclease to be identified is ecori isolated from e. coli. Many other restriction enzymes have been isolated. Restriction enzymes can provide a defense system for dna against viruses and bacteriophages. Majority of restriction enzymes recognize either 4 or 6 base pairs. Ecori recognizes and cleaves gaattc, haeiii recognizes and cleaves. Restriction maps can be used to determine where the enzymes will cleave. Restriction enzymes play a key role in dna cloning. The basic steps required to clone a gene are: The fragment of dna containing the gene to be cloned is inserted, with restriction enzymes and ligation enzymes into a vector producing recombinant dna. Vector is usually circular but it can also be linear. The vector acts as a vehicle to transport the gene into a host cell, which is often a bacterium.