by Keith Hamburger

Created on: November 30, 2008   Last Updated: March 11, 2010

Viruses are very interesting entities. Somewhere between living and not, viruses infect all forms of life from the smallest microbes to the largest plants and animals. The reason they might not be considered living is that they are unable to reproduce on their own. Viruses take over the reproductive functions of cells in order to replicate.

The basic anatomy of a virus comes in two types. All viruses consist of nucleic acid, either DNA or RNA, coated with a layer of proteins, called capsomers, produced by the virus itself. The second type of virus has an additional layer of lipoproteins, or fatty proteins, which are derived from the host cell.

Orders of magnitude smaller than even the smallest bacteria, viruses have a wide variety of shapes. Unable to be seen with a microscope that uses light, electron microscopes are used to see viruses and determine what they look like.

Helical viruses have a shape of a helix with identical capsomers attached to the nucleic acid structure of the virus coiled in a helix. The length of the genetic material determines the length of such a virus while the diameter is determined by the structure of the capsomers surrounding that material. These viruses can be very long and flexible or short and rigid.

Most viruses that infect animals are nearly spherical and are referred to as icosahedral. With a surface made up of identical capsomers, these viruses are of a shape that is optimized for energy and volume.

Viruses that are covered with lipoproteins from the host cell are called enveloped. Not having any regular shapes the coatings are studded with proteins that are coded from a combination of the virus and the host cell. The proteins on the surface of the fatty coating are what the virus uses to attach itself to a host cell and cause infection.

Complex viruses come in many forms. Bacteriophages, or viruses which attack bacteria, are commonly of this structure. Looking like lunar landing craft the virus itself is contained within a "head" which is an icosahedral structure. Attached to that is a shaft with protein filaments attached. When a bacteriophage infects a host bacterium the protein filaments at the base attach themselves to the cellular membrane while the hollow shaft injects the nucleic acid into the cell.

Since they are responsible for many diseases an understanding viruses is very important. In addition, new applications in bioengineering using viruses to transport genetic materials into living cells have resulted in major breakthroughs. For these reasons, and many more, virology, or the study of viruses, is certain to be a growing field for a long time to come.

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