Virus

Virus Overview

The virus was first thought about in 1883 when A. Mayer was seeking to find the cause of the tobacco mosaic disease. Though he was unable to see them with the microscopy of his day, he postulated that a small agent caused the disease. D. Ivanowsky, later tried tests as well and also concluded that it was a disease caused by something smaller than they could see. The virus was first found and discovered in 1935 by Wendell Stanly. He was able to crystallize the virus, now known as the tobacco mosaic virus.

The smallest viruses are only 20 nm in diameter, smaller even than a ribosome in a cell. Though not a living thing, viruses are infections particles consisting of only the viral genes in closed in a shell of protein. Their genomes consist of double-stranded DNA, double stranded RNA, single-stranded DNA, or single stranded RNA. The name of a virus, DNA or RNA, is based on the type of nucleic acid that composed its genome. The smaller viruses have only a few genes while others have several hundred.

The capsid is a protein shell that enclosed the viral genome and comes in many shapes and sizes. They are composed of many protein subunits called capsomeres, although the actual numbers of different kinds of protein are few. Many viruses have a membranous envelope that covering their capsid. Composed of the host cell's membrane, the membrane helps with the "life" cycle when the virus is taking over the host cell.

The question of whether or not a virus is a living object or a complex protein is often fuzzy. When isolated from a host cell, a virus is unable to replicate, produce energy, or any other activities required for life. This makes them nothing more than a protein coated set of genes in transit from one host to another, a biological gene transport mechanism.

A virus is only able to "infect" a limited range of host cells, called its host range. This process is referred to as the lock and key fit between proteins on the outside of the virus and the receptor molecules on the surface of the cell. Some have a host range that can infect many species while others only one type of bacteria. A virus that is designed to kill bacteria are phages, or bacteriophages.

The hostile take over of a host cell is the Lytic cycle or the lysogenic cycle. The lytic cycle is the process where the virus attaches to the cell, releases its viral genes and takes over the cell. Once control of the cell is achieved, the genes tell the cell to start producing more viral parts and assemble them. Eventually, so many new viruses are produced, the cell swells and eventually bursts, spewing hundreds or even thousands of new viruses.

The lysogenic cycle, is nearly the same as the lytic cycle, though a bit more controlled and hidden. The gene code splices into the cells code and lies dormant, waiting for the time to then begin producing itself. This way, the cell does not die after infection until the genes take over and begin the lytic cycle.

Many variations on the viral infect scheme are among animal viruses. There are two variations, viruses with envelopes and viruses with RNA genomes, though some have both of these features.

Classes of Animal Viruses

dsDNA Papilloma, Herpes, Small Pox

ssDNA Parvovirus

dsRNA Diarrhea viruses

ssRNA + Poliovirus

ssRNA - Rabies, measles

ssRNA DNA leukemia

Viruses with an envelope outside the capsid use the membrane to help enter the host cell. This membrane is generally a lipid bilayer, with glycoproteins protruding from the surface.

The viruses that infect humans are many, and are a constant threat to the health of the human population. Ebola, AIDS, the common cold,