Answer :
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.
One can isolate the gene for human insulin from human cells, and put that gene into a plasmid (small circular DNA molecule). That plasmid is then put into bacterial cells, and the bacteria produce insulin as coded for by the human gene. It is necessary to include the necessary bacterial promoters etc, so that the cell can actually read the DNA message, but because the genetic code and the language of codon/anticodon/mRNA/tRNA etc. is universal, a gene in one organism is readable by another organism.