A recombinant plasmid containing a full length of HGH cDNA is cleaved with restriction enzymes that release fragments containing the complete HGH coding sequence after codon 24. Several overlapping complimentary oligo nucleotides were joined to form a synthetic strand of a small DNA fragment which contains the coding sequence for the first 24 aminoacids of mature HGH.
The synthetic DNA and cDNA molecules are joined together to produce a new fragment which has the complete coding sequence of HGH. This is then ligated to a restriction site a little below the lac promoter operator gene cloned on a plasmid. These plasmids are cloned in E.coli. Synthesis of HGH is induced by an inducer (IPTG) of lac operon.
The HGH thus obtained is later purified by chemical methods. About 1 lakh molecules of the hormone per cell of E.coli have been produced by genetic engineering techniques.
One of the problems in this is the attachment of an extra methionine to the molecule. This has to be removed to make the molecule biologically active.