A recombinant DNA is artificially constructed DNA molecule with genes of interest that would not be present in the genome otherwise. It is a combination of a vector and a gene of interest. The recombinant DNA is inserted into bacteria E. coli, which, thanks to their reproductive potential, produce multiple copies of this recombinant DNA along with the chromosomal DNA. So, the gene for insulin is inserted int bacteria where is transcribed together with bacterial genes and the great amount of insulin is produced.
