(a) are endonucleases which cleave DNA at specific sites
(b) make DNA complementary to an existing DNA or RNA
(c) cut or join DNA fragments
(d) are required in vectorless direct gene transfer
(a) Restriction enzymes are endonucleases which cleave DNA at specific sites. They are originally extracted from the bacterium E.Coli. Type I restriction enzymes binds to a recognition site of a duplex DNA and cleave one strand only. Type II restriction enzymes are more valuable in gene manipulation and cleave the duplex at specific target sites at or near the binding site.