The main steps of genetic engineering:
This diagram shows how the genetic engineering of insulin works. Insulin can be produced by bacteria and then purified and used to regulate type 1 diabetes.
Not all of the host cell bacteria will take up the plasmid again once it has had the desired gene added to it. It is important that scientists can select the transgenic bacteria.
The plasmids often have antibiotic resistance genes in them, if a bacterial cell does take up the plasmid again, then antibiotics cannot kill it. If a bacterial cell does not take up the plasmid then it does not have the antibiotic resistance genes and it will die.
Scientists can treat the bacterial population with an antibiotic. They know that the ones that survive are the transgenic ones. This is called selection, as you are selecting for the bacteria that have the plasmid with the desired gene in it.
Restriction enzymes do not cut directly across the double strand of DNA because this would involve cutting any section of DNA into many different pieces and it would not be easy to remove an entire gene.
Instead, restriction enzymes cut across the double strands at two different places. The place where they cut across the DNA is called a sticky end. Restriction enzymes can be used to cut out specific genes, and also cut open places in the plasmid DNA where the genes will fit exactly.