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

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Viruses

Intercellular Growth and Replication

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 the tobacco mosaic virus by Martinus Beijerinck in 1898, about 5,000 virus species have been described in detail, although there are millions of different types. A virus consists of two or three parts: (1) a genetic material made from either DNA or RNA, long molecules that carry genetic information; (2) a protein coat, called a capsid, that protects the genetic material; and (3) in some cases an envelope of lipids that surrounds the protein coat when the virus is outside a cell. The making of new viruses may be extremely rapid or take place over a long period of time.

Dennis Howard

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Viruses

Prions

Prions (pronounced /?pra?.?n/ or /?pra?.?n/) are infectious proteins that do not contain nucleic acids. They are responsible for several neurodegenerative diseases in humans and animals, including Creutzfeldt–Jakob disease (CJD), bovine spongiform encephalopathy (BSE; commonly known as "mad cow disease"), scrapie, and Kuru. Prions are also found in some yeast, fungi, and plants. Prions are abnormally folded forms of normal proteins that accumulate in the brains of infected individuals. They are believed to cause damage by disrupting the normal function and morphology of the brain.

Dennis Howard

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Viruses

Virus and Prion Pathogenicity

A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, 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 the tobacco mosaic virus by Martinus Beijerinck in 1898, about 5,000 virus species have been described in detail, although there are millions of different types. Viruses are found in almost every ecosystem on Earth and are the most abundant type of biological entity. The study of viruses is known as virology, a sub-speciality of microbiology. General topics in virology include classification, host-virus interactions, viral ecology, and virus evolution.

Dennis Howard