The aim of gene therapy is the successful integration of corrected, therapeutic DNA into the genomes of cells to alter gene expression and, in turn, protein synthesis to correct genetic diseases.

Virus life cycle

Viruses are particles made up of an outer protein coat and an inner core of genetic material in the form of DNA or RNA. Viruses do not possess the components needed to multiply or survive on their own and instead depend on a host to replicate.

Viruses may be air-bourne, passed on through sexual contact, skin-to-skin contact or even through sharing drinks and toothbrushes. An outline of viral infection and replication is given below:

• Adsorption (attachment) of the virus to a host cell membrane
• The virus injects its DNA or RNA into the host cell
• The DNA or RNA from the virus takes over the cell and recruits the host's enzymes and nutrients
• The cellular enzymes then make new virus particles in a process called replication
• The particles of the virus created by the cell unite to form new viruses, a process called assembly
• The newly formed viruses kill the cell and are released to infect other healthy cells

Uses in the laboratory

Viruses can be modified in the laboratory to provide vectors that carry corrected, therapeutic DNA into cells, where it can be integrated into the genome to alter abnormal gene expression and correct genetic disease.

This involves removing the viral DNA present in the virus and replacing it with the therapeutic genes. In this way, the virus becomes merely a "vector" that is capable of transferring the desired gene into cells but not capable of taking over or harming cells.

Some of the viruses currently used in gene therapy include retroviruses, adenoviruses, adeno-associated viruses and the herpes simplex virus.