Viruses Characteristics

Certain type of virus exists in several forms of stages. The free virus particle, or virion, consists of a molecule of nucleic acid, either DNA or RNA, depending on the specific virus, surrounded by protein and in some viruses, also lipid and carbohydrate. The virion is inert, because its lacks many of the components that are necessary for independent life and reproduction; it does, however, provide for the transfer of the nucleic acid from host cell to host cell. Replication can occur only when the nucleic acid enters a host cell. Within the cell, the nucleic acid functions as genetic material and directs the synthesis of proteins. This results in the production of new virions, which then are released from the cell and can initiate new cycles of infection.

The virions of different viruses vary greatly in size, shape, and complexity, but those of any virus are uniform. The simplest types are rods and regular 20 sided polygons (icosahedrons); the tobacco mosaic virus, for example, consists of a single molecule of a single molecule of RNA surrounded by about 2,200 molecule of a single type of protein arranged in helical fashion to produce a rod 3,000 angstroms (1/100,000 in) long.

Many virions having a simple rod or polygonal shape are composed of several types of proteins, and some of these virions are surrounded by a less regularly shaped envelope, which often includes components of the host-cell membrane. At the other extreme of complexity, bacteriophage T4 has a virion composed of at least 25 different types of protein that make up a hollow head in which a DNA molecule is enclosed, and a tubular tail by which the virion attaches to its host cell and injects its DNA into it.