a. Plasmid DNA : The construction of the first recombinant DNA molecule emerged from the possibility of linking a gene encoding antibiotic resistance with a native plasmid of Salmonella typhimurium. Stannely Cohen and Herbert Boyer accomplished this in 1972 by isolating the antibiotic resistance gene by cutting out a piece of DNA from a plasmid that was responsible for conferring antibiotic resistance.
b. Recognition sequence : The first restriction endonuclease HindII, whose functioning depended on a specific DNA nucleotide sequence, was isolated and characterized five years late.This specific base sequence is knows as the recognition sequence for HindII.
c. Gel electrophoresis : The cutting of DNA by restriction endonuclease results in the fragments of DNA. These fragments can be separated by a technique knows as gel electrophoresis.