Viruses have the following characteristics.
They are the smallest living organisms.
They do not have a cellular structure.
They can only reproduce by invading living cells. Therefore they are all parasitic. They are obligate endoparasites, meaning that they can only live parasitically inside other cells. Most cause disease.
They have a simple structure, consisting of a small piece of nucleic acid, either DNA or RNA, surrounded by a protein or lipoprotein coat.
They are on the boundary between what we regard as living and non-living.
Each type of virus will recognize and infect only certain types of cell. In other words, viruses are highly specific to their hosts.
Viruses are the smallest living organisms, ranging in size from about 20-30nm; on average they are about 50 times smaller than bacteria. They cannot be seen with the light microscope and they pass through filters which retain bacteria.
We can understand viruses much better if we understand their evolutionary origins. It is suspected, though not proven, that viruses are pieces of genetic material that have escaped from prokaryote and eukaryote cells and have the potential to replicate themselves when they get back into a cell environment. A virus survives in a purely inert state outside cells, but has the set of instructions necessary to re-enter a particular type of cell and instruct it to make many identical copies of itself. It is therefore reasonable to suppose that viruses must have evolved after cells evolved.
Viruses have a very simple structure consisting of the following
Core-the genetic material, either DNA or RNA. The DNA or RNA may be single stranded or double stranded.
Capsid- a protective coat of protein surrounding the core.
Nucleocapsid-the combined structure formed by the core and capsid.
Envelope- a few viruses, such as the HIV and influenza viruses, have an additional lipoprotein layer around the capsid derived from the cell surface membrane of the host cell.
Capsomeres-capsids are often built up of identical repeating subunits called capsomeres.
The overall form of the capsid is highly symmetrical and the virus can be crystallized, enabling information about its structure to be obtained by X-ray crystallography as well as electron microscopy. Once the subunits of a virus have been made by the host, they can self-assemble into a virus.
Certain types of symmetry are common among capsids, notably polyhedral and helical symmetry. A polyhedron is a many-sided figure. The most common polyhedral form in viruses is the icosahedron, which has 20 triangular faces with 12 corners and 30 edges.
Helical symmetry is well illustrated by the tobacco mosaic virus, an RNA virus. Here the capsid is made up of 2130 identical protein capsomeres TMV was the first virus to be isolated in a pure state. It causes a mottled yellowing of leaves called leaf mosaic in tobacco, tomato and many other plants. The virus can spread extremely rapidly, either mechanically if infected plants, or plant parts come into contact with healthy plants, or even as airborne particles such as the smoke of cigarettes made from contaminated leaves.
Viruses that attack bacteria form a group called bacteriophages, or simply phages. Some of these have a distinct icosahedral head, with a tail showing helical symmetry.