A<unk>  (or<unk>  endonuclease) is an<unk>  that cuts double-stranded DNA. The<unk>  makes two incisions, one through each of the sugar-phosphate backbones (i.e., each strand) of the double helix without damaging the nitrogenous bases. They work with cutting up foreign DNA, a process called<unk> .<unk> s therefore are believed to be a mechanism evolved by bacteria to resist attack from viruses called bacteriophages and to help in the removal of viral sequences. They are part of what is called the<unk>  modification system.
The chemical bonds that the<unk> s cleave can be reformed by other<unk> s known as ligases, so that<unk>  fragments carved from different chromosomes or genes can be spliced together, provided their ends are complementary (more below). Many of the procedures of molecular biology and genetic engineering rely on<unk> s.
The 1978 Nobel Prize in Medicine was awarded to the well-known scientist Daniel Nathans, Werner Arber and Hamilton Smith for the discovery of<unk>  endonucleases, leading to the development of recombinant DNA technology. The first practical use of their work was the manipulation of E. coli bacteria to produce human insulin for diabetics.
Restriction enzyme
Ligase
Ribozyme
Deoxyribonuclease
DNA polymerase
RNA polymerase
DNA glycosylase
Enzyme