by Erich Rosenberger M.D.

Created on: January 20, 2008

Viruses hold a unique classification in the world of microbiology. They are organisms which are not based on a cellular structure. There is much debate over whether a virus is truly alive or not, however there is no doubt that they are fascinating and tremendously important to modern medical science.

The anatomy of a virus is very different from that of cellular organisms. Viruses are incapable of replicating independently. They are also not able to produce their own energy, nor can they make proteins. Most viruses are we too small to be seen using standard light microscopes. Viruses are typically between 20 and 300 nm in diameter. The largest viruses are still slightly smaller than even the smallest cell. Viruses are often described as having different shapes, such as brick-shaped, rod-shaped, spherical, or bullet like. It is important to note that these descriptions are only approximations, and some artistic license has been taken in naming them thus.

All viruses contain a core which is comprised of either DNA, or RNA, but never both. DNA or RNA found in the center of a virus and have either a single strand, or a double strand. Cellular organisms always contain double-stranded DNA. Therefore only viruses will be found to have genetic material in the form of single stranded DNA or RNA of any type. This is a subtle yet key distinguishing feature that separates cellular organisms from viruses.

The central core of genetic material is then covered up in a protective protein coat. As a protein coat is known as capsid. A capsid contains several subunits which are known as capsomers. The geometrical arrangement of the capsomers is what gives a virus into physical structure and shape.

The capsid proteins serve to protect the genetic material on the inside of the virus. They also serve to control the attachment of the virus to receptors on the cell which the virus will attempt to invade. The capsid proteins are the chemicals which cause reactions by your immune system as well.

Some viruses have another structure surrounding the capsid. This structure is known as an envelope, and is a lipoprotein membrane that is partially derived from the host cell that the virus is invading. Not all viruses have an envelope. As a general rule viruses with an envelope tend to be less stable than ones that do not. This may seem somewhat counterintuitive, as you'd think an envelope would provide extra protection. However the envelope makes the virus susceptible to heat, detergents, and other solvents such as alcohol. Viruses with only capsid are less likely to be damaged by these agents.

Viral anatomy is a fascinating aspect of microbiology. There is significantly more to know about the structure and function of viruses than what can be explained in this short article. Scientists are making new strides every day in working out the structure and function of viruses. The more we know about viral anatomy, the more we are able to develop treatments to protect ourselves from these infective organisms.

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