Step 1: The process begins by identifying the part of ​ (a)   – the gene – that codes for the production of a desired ​ (b)   . Enzymes called ​ (c)   are used to cut the DNA on either side of the ​ (d)   . These enzymes cut DNA molecules at particular points. They leave short lengths of unpaired bases at either end of the cut DNA, called ​ (e)   .

Step 2: Next, ​ (a)   are removed from ​ (b)   . The ring of DNA in the plasmid is cut, using the same ​ (c)   that were used for cutting the human DNA. The reason for using the same restriction enzyme is so that the ​ (d)   on the plasmid DNA are complementary to the sticky ends that were left on the ​ (e)   .

Step 3:

The human ​ (a)   and the cut plasmids are now mixed together. The sticky ends on the human DNA pair up with the complementary sticky ends on the plasmid DNA. Another ​ (b)   , called ​ (c)   DNA 连接酶, links the two strands firmly together, closing the ​ (d)   . Now we have bacterial plasmids that contain the human gene. They are called ​ (e)   重组质粒.

Step 4:

Next, these ​ (a)   are added back to the ​ (b)   . These are now genetically modified bacteria because they contain ​ (c)   as well as their own. The GM bacteria are put into ​ (d)   , where they reproduce asexually to form large populations. They express the gene for the human protein – that is, they use the code on the human DNA to make the ​ (e)   , which can be extracted from the fermenters and purified.