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.