VIRUS STRUCTURE

VIRUS STRUCTURE

Viruses range in size from less than 100 nanometers in diameter to several hundred nanometers in length in the case of the filoviridae (Figure 1 and 2).

All viruses contain a nucleic acid genome (RNA or DNA) and a protective protein coat (called the capsid). The nucleic acid genome plus the protective protein coat is called the nucleocapsid which may have icosahedral, helical or complex symmetry. Viruses may or may not have an envelope. Enveloped viruses obtain their envelope by budding through a host cell membrane. In some cases, the virus buds through the plasma membrane but in other cases the envelope may be derived from other membranes such as those of the Golgi body or the nucleus. Some viruses bud through specialized parts of the plasma membrane of the host cell; for example, Ebola virus associates with lipid rafts that are rich in sphingomyelin, cholesterol and glypiated proteins. Poxviruses are exceptional in that they wrap themselves in host cell membranes using a mechanism that is different from the usual budding process used by other viruses.

Enveloped viruses do not necessarily have to kill their host cell in order to be released, since they can bud out of the cell - a process that is not necessarily lethal to the cell - hence some budding viruses can set up persistent infections.

Enveloped viruses are readily infectious only if the envelope is intact (since the viral attachment proteins which recognize the host cell receptors are in the viral envelope). This means that agents that damage the envelope, such as alcohols and detergents, reduce infectivity