What is a Virus

All viruses contain a discrete amount of DNA or RNA (their genome) either as a single strand or a pair of strands, together with a few non-struc-tural proteins (enzymes) packaged within a protein coat called the capsid. In some viruses, this unit is in turn enclosed within a lipoprotein outer coat. These particles function as a living organism in the sense that they have the capability to reproduce, but unlike the bacteria, they cannot do this without eliciting the assistance of the cells that they invade. They are thus parasitic on their host cell. 

The complete, mature virus particle is known as a virion and usually has a regular shape. Many virions are icosahedral, that is, the capsid is formed from identical protein subunits (capsomeres) that combine to produce a solid with twenty faces, each of which is an equilateral triangle. The herpes viruses are of this type, as are the picomaviruses of which the polio viruses and rhinoviruses (cold viruses) are the bestknown members. The other common regular shape is that of a helix, and the tobacco mosaic virus is of this type. Its single helical strand of RNA is enclosed within a hollow tube, which comprises 2130 protein subunits arranged in a helix. Other viruses with a similar structure are the 

A brief comment about size is in order, since viruses range from the minute, like polio viruses, rhinoviruses and hepatitis A (all around 25 nm), to the comparatively massive pox viruses, which are 10 times larger and just about visible under the light microscope. Whatever their size and shape, the one common feature of all viruses is their use of host-cell biochemistry to help with their reproductive cycle - their replication. It is usual to identify six stages in the life cycle of viruses  

An understanding of the biochemical basis of these various processes should facilitate the design of drugs that interfere with them. 

Once recognition and subsequent tight binding have occurred, the virus must penetrate the cell membrane of the host cell. Those viruses that have an outer envelope become engulfed by the cell membrane in a process called pinocytosis. The viral envelope becomes fused with the cell membrane and ultimately this breaks open to release the virion into the cytoplasm of the host cell. Its coat protein is then digested and the genome (DNA or RNA) is revealed. Viruses that do not possess an outer envelope also associate with the host cell membrane but in this case a pore opens up and the viral particle passes through it into the cytoplasm (endocytosis), where it is uncoated. 

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