Although they may seem to behave like living things, viruses are actually nonliving infectious agents. Viruses are not considered to be living organisms because they are incapable of carrying out all life processes. The factors that distinguish them from living things are as follows.
Viruses...
* are not made of cells
* cannot reproduce on their own
* do not grow or undergo division
* do not transform energy (metabolize food)
* lack machinery for protein synthesis
* are so small that they can only be seen with an electron microscope
* What Are Viruses Made Of? *
Viruses are composed of genetic material, proteins and, in some cases, lipids as well.
1. Viral Genetic Material: The genetic material of a virus is nucleic acid. But unlike the genome of living organisms, which is always DNA (deoxyribonucleic acid), viral genetic material can be either DNA or RNA (ribonucleic acid). The nucleic acid of a virus contains the genetic information required for the infected host cell to make more copies of the virus.
2. Viral Capsid: The viral nucleic acid is surrounded by a protective protein coat, called a capsid. In addition to protecting the nucleic acid, the capsid enables the virus to attach to host cells. Capsids are composed of protein subunits called capsomeres. Some capsids are made of a single type of capsomere, while others include a combination of multiple types.
3. Viral Envelope: Some animal viruses are surrounded by an outer membranous layer, called an envelope. The viral envelope is made of lipid and protein, and is obtained from the host cell's plasma membrane when the virus exits the host..
* Classification of Viruses *
There are several different ways that virologists (scientists who study viruses) categorize or classify these acellular infectious agents. Viruses can be classified based on their genetic material, capsid structure, envelope, type of cells they attack and by their size.
Genetic Material: A virus can either have DNA (deoxyribonucleic acid) or RNA (ribonucleic acid) as its genetic material, but never both.
Capsid Structure: Viral capsids come in the following shapes:
* Helical: capsomeres bound together in a spiral shape
* Spherical: round viruses
* Polyhedral: capsomeres in a spherical-like shape comprised of small, flattened plates.
* Complex: any other different shape that is not polyhedral, spherical or helical
Envelope: Viruses can be categorized based on the presence or absence of a membranous envelope surrounding the capsid.
Type of Cells They Attack: Viruses are very specific with respect to the type of cells that they invade. Bacteriophages are viruses that only attack bacteria. Some viruses are specific to animals, while others only infect plants.
Size of Virus: Although all viruses are minute, and only visible with the aid of a powerful electron microscope, they do come in a range of sizes. For example, the smallpox virus that attacks human cells is up to six times larger than the T4 bacteriophage virus that attacks bacterial cells.
* Intracellular and Extracellular Viral Form *
Viruses exist in one of two states; extracellular and intracellular.
Extracellular State: Before a virus invades a host cell, it is in what's considered to be the extracellular state. An extracellular virus is called a virion (vie-ree-on). In this form, the virus consists of a protein coat (capsid) surrounding nucleic acid, and the envelope (if it has one).
Intracellular State: Once the virus invades a host cell it is in an intracellular state. In this state, the capsid is removed and the virus exists only as naked nucleic acid (genetic material).
* Sources *
Bauman, R. (2005) Microbiology. Pearson Benjamin Cumings.
Park Talaro, K. (2008) Foundations in Microbiology. McGraw Hill.