Viruses | Biology

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Viruses

Is it Alive? A Virus Overview

In biology, a virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants, and the discovery of viruses by Dmitri Ivanovsky in 1898, the classification of viruses has been unclear. Most of the early work on viruses was done by Frederick Twort, Félix d'Herelle, and Ronald Ross. Viruses are mainly classified by the type of nucleic acid they use as a genetic material (either RNA or DNA) and the type of protein capsid they employ. Viruses range in size from about 30 nm to about 450 nm, which means that most of them are smaller than the wavelength of light. With the exception of some large dsDNA viruses, all viruses are composed of either double-stranded RNA or single-stranded DNA, along with a few types of single-stranded RNA viruses. The origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. The most probable ancestral type of today's viruses is thought to be the DNA viruses, because they are the oldest and because they are simpler and smaller than RNA viruses.

Dennis Howard

Is it Alive? A Virus Overview

Intercellular Growth and Replication

Prions

Virus and Prion Pathogenicity