Genetic engineering is in part a biological courier service, aiming to deliver specific genes to a particular destination. Scientists are able to extract a gene with a desired trait from one organism, parcel it up and then send it on its way into another organism to transform it.
There are several different ways of sending foreign genes into a cell. The purpose is to confer some benefit on the target organism. For example, in human genetic engineering (gene therapy) the aim is to send in a fully operational gene to replace a mutated one that's causing a disease. Genetic engineering is used in agriculture to increase crop yield, this can be done by adding a gene that makes a plant able to grow in extreme conditions or confers on it resistance to herbicides. Genes can also be added to plants to increase their nutritional value. Here's our quick guide to some of the most commonly used techniques.
The oldest technique for sending genes into target cells is recombinant DNA where plasmids and viruses are used as the delivery boys and girls, or to use the correct term, vectors. Plasmids are circular pieces of DNA that are usually found in bacteria and can be used to smuggle a gene of interest into other organisms because of their extraordinary ability to cross species boundaries. The plasmid can be cut, and a new gene introduced. Then because the plasmid can cross cellular boundaries it takes this new gene with it, to become incorporated into another genome, typically one belonging to a bacterium. When the bacterium replicates, the new gene, along with the bacterium's own genes, is carried down the generations. So if this gene codes for insulin, a vast amount of bacteria can be used as a sort of mini-factory to create large quantities of the hormone.
Viruses infect cells, and genetic engineering takes advantage of this ability. A desired gene is inserted into viral DNA and when it infects another cell it carries this gene with it. Scientists are able to hijack viral genetic machinery such that a virus cannot redirect the cell to make thousands of copies of itself.
There's a rather more direct delivery route and that's firing the gene of interest into the target cell. This technique labours under several names such as the biolistic method, bioballistics or gene gun technology, and it's used to create transgenic crops. Pellets of metal such as tungsten or silver are coated with DNA and fired at plant cells.
This does what it says on the tin. If the recipient cell is large enough the DNA can be injected into it from a fine-tipped glass needle. And when it works, the gene is somehow able to find its way into the host cell's genome.