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Inserting genes into other organisms for expression

Cloning the required DNA fragment in plasmids in bacterial cells is fine for producing large quantities of some proteins artificially. The bacteria Escherichia coli is being used in this way to produce vast quantities of human insulin.

However, bacteria are not capable of producing all human and plant proteins. This is because bacteria make proteins in a different way to us. Also many human gene products are altered or changed in the cell after they are synthesised by the addition of carbohydrates. Bacterial cells can not carry out these modifications.

It is far better and efficient to produce some gene products in animal and plant cells. Neither of these cells have plasmids so other methods have to be used to put the cloned gene into the cells and help them insert themselves into the hosts DNA.

1. Vectors used for Inserting genes into Humans

A. Liposomes

These are little oil droplets that have recombinant plasmids in them. The droplets have sugar particles on their surface, which act as cell receptors and allow the liposomes to enter the human cells. However, very few of the plasmids reach the nucleus so the gene is often not expressed in a large enough quantity.

B. Viruses

Viruses usually cause disease by entering cells and attaching to the chromosomes of the host and taking the cell over. Scientists are able to remove part of the viruses own DNA to make it harmless and insert the cloned gene they want to put into the cells. As the virus enters the cells the recombinant gene combines with the human chromosomes and is expressed.

However, the viruses used so far have made some patients ill and the transferred gene has proved to be unstable and continuous virus treatment is required.

2. Vectors used to Insert genes into Plants

A. The bacteria Agrobacterium tumefaciens is used to transfer recombinant plasmids into plant cells. The plasmids used are usually Ti plasmids. These cause plants to develop growths or tumours on their shoots and are naturally found in Agrobacterium.

When the Agrobacterium infects plant leaf cells grown in culture the recombinant plasmid attaches the new gene to the plant chromosomes. These plasmid genes cause the plant cells to divide rapidly and make a mass of identical cells or a callus. These cells can then be grown into whole new plants that can be tested for the presence of the new gene property.

B. A more commonly used method is Ballistics.

A special gun is used to fire minute gold pellets coated with DNA into the plant tissue/nucleus.

3. Vectors used to Insert genes into animal cells.

A. Much recent genetic research has centred on ensuring the recombinant gene is inserted into every gene of an animal for life. This is done by placing the gene directly in the to the zygote or egg cell using a syringe (a microinjection) and then placing the new recombinant zygote back into the mothers womb. See Transgenic Animals.