| HPV |         |
|
||||||||||||
Virology
Viruses are small, simple micro-organisms. They vary in size and shape, but all viruses are dependent on a host cell for their survival and reproduction (Raven et al 2002).
A single, complete virus particle is known as a virion, and its components can vary depending on the type of virus. All viruses consist of some form of genetic material surrounded by a protein coat, known as a capsid. They may also be surrounded by a protective envelope (Myint 1996; Raven et al 2002).
For its replication, a virus needs to infect a host cell, as it does not contain enough genetic material to direct this process independently. It is this need to replicate inside a host cell that causes some viruses to be pathogenic to the host organism (Abbas et al 1991; Raven et al 2002).
Virus structure
Two components are essential for the structure of any virus: genetic material and an outer shell, or capsid (Myint 1996; Raven et al 2002).
Genetic material
The genetic material of a virus is essential to allow it to replicate and survive; without it, the other components of the virus would not exist (Abbas et al 1991; Raven et al 2002). Viruses can contain DNA or RNA, which can be double-stranded, like the double-stranded DNA in the human genome, or single-stranded (Myint 1996). DNA-based viruses tend to be genetically stable, but RNA-based viruses are prone to mutations and evolve more quickly (Jackson and Charleston 2004). DNA-based viruses and some RNA-based viruses are also capable of integrating their genetic material into the host genome. HPV is a DNA-based virus.
Capsid
The capsid of a virus is its outer protein coat. It protects the genetic material inside and allows the virus to enter and infect host cells. The antigenic structure of the capsid is of particular importance, as it determines which host cells the virus can infect. It is also the capsid that is recognised by antibodies, thereby triggering the host immune response (Raven et al 2002).
RNA-based viruses such as the cold and flu viruses evolve rapidly, and as the shape of the capsid changes, new strains of the virus emerge. DNA-based viruses such as the human papillomavirus (HPV) evolve less rapidly, so their capsid shape does not vary significantly over time (de Villiers et al 2004; Jackson and Charleston 2004).
Viral life cycle
Viruses infect host cells in order to replicate. This involves many steps: replicating their genetic material, their outer capsid, and their envelope, if present (Myint 1996). Viral genomes are small and contain too few genes to independently direct this process, so they rely on recruiting genes and proteins within the host cell to help synthesise these materials (Abbas et al 1991; Raven et al 2002).
It is this need for host cell resources that makes viruses obligate intracellular pathogens. In taking over the host cell, the virus can be toxic (Abbas et al 1991). Alternatively, the virus can override the control mechanisms that regulate DNA replication in the host cell, allowing uncontrolled cell growth and division, which can ultimately lead to the development of cancer (Longworth and Laimins 2004).
