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The main steps of genetic engineering:

• Restriction enzymes are used to isolate the required gene from the chromosome. They cut the DNA at a specific sequence. Restriction enzymes leave sticky ends that are overhangs of DNA.
• A vector is needed to get the gene into the host cell. This is usually a plasmid that is taken from a bacterial cell. The plasmid is cut with the same restriction enzymes so it gets the same sticky ends.
• The sticky ends on the plasmid stick with the ones on the gene.
• The gene and the plasmid are joined together using an enzyme called DNA ligase.
• The vector is transferred back into the bacteria host cells.
• The host cells now have a gene from another organism and so are said to be transgenic.

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.

Selection of transgenic bacteria

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

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.