Manipulation of DNA in genetic engineering became possible due to the discovery of?

Explanation

DNA restriction endonuclease is an important class of restriction exonucleases, class II, which cut double­stranded DNA molecules only at sites characterized by a specific nucleotide sequence. Restriction enzymes are isolated from bacterial cells and are tools for molecular biologists. Several hundred restriction enzymes are now known, each with a specific sequence requirement dictating where it will cut DNA. Some, such as Hind III, makes staggered cuts leaving 'sticky ends', three nucleotides long protruding on one strand from each severed terminus; others make clean cuts in both strands at the same place and thus generate 'blunt ends'. Digesting DNA with a restriction enzyme, therefore, creates a characteristic set of fragments, which can be isolated by electrophoresis and subsequently analysed.