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A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. [1] Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. [2] [3] Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity.
- Virus
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- Overview
- Definition
- Host range and distribution
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A virus is an infectious agent of small size and simple composition that can multiply only in living cells of animals, plants, or bacteria.
What are viruses made of?
A virus particle is made up of genetic material housed inside a protein shell, or capsid. The genetic material, or genome, of a virus may consist of single-stranded or double-stranded DNA or RNA and may be linear or circular in form.
What size are viruses?
Most viruses vary in diameter from 20 nanometres (nm; 0.0000008 inch) to 250–400 nm. The largest viruses measure about 500 nm in diameter and are about 700–1,000 nm in length.
Are all viruses spherical in shape?
Viruses occupy a special taxonomic position: they are not plants, animals, or prokaryotic bacteria (single-cell organisms without defined nuclei), and they are generally placed in their own kingdom. In fact, viruses should not even be considered organisms, in the strictest sense, because they are not free-living—i.e., they cannot reproduce and carry on metabolic processes without a host cell.
All true viruses contain nucleic acid—either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid)—and protein. The nucleic acid encodes the genetic information unique for each virus. The infective, extracellular (outside the cell) form of a virus is called the virion. It contains at least one unique protein synthesized by specific genes in the nucleic acid of that virus. In virtually all viruses, at least one of these proteins forms a shell (called a capsid) around the nucleic acid. Certain viruses also have other proteins internal to the capsid; some of these proteins act as enzymes, often during the synthesis of viral nucleic acids. Viroids (meaning “viruslike”) are disease-causing organisms that contain only nucleic acid and have no structural proteins. Other viruslike particles called prions are composed primarily of a protein tightly complexed with a small nucleic acid molecule. Prions are very resistant to inactivation and appear to cause degenerative brain disease in mammals, including humans.
Viruses are quintessential parasites; they depend on the host cell for almost all of their life-sustaining functions. Unlike true organisms, viruses cannot synthesize proteins, because they lack ribosomes (cell organelles) for the translation of viral messenger RNA (mRNA; a complementary copy of the nucleic acid of the nucleus that associates with ribosomes and directs protein synthesis) into proteins. Viruses must use the ribosomes of their host cells to translate viral mRNA into viral proteins.
Viruses are also energy parasites; unlike cells, they cannot generate or store energy in the form of adenosine triphosphate (ATP). The virus derives energy, as well as all other metabolic functions, from the host cell. The invading virus uses the nucleotides and amino acids of the host cell to synthesize its nucleic acids and proteins, respectively. Some viruses use the lipids and sugar chains of the host cell to form their membranes and glycoproteins (proteins linked to short polymers consisting of several sugars).
The true infectious part of any virus is its nucleic acid, either DNA or RNA but never both. In many viruses, but not all, the nucleic acid alone, stripped of its capsid, can infect (transfect) cells, although considerably less efficiently than can the intact virions.
The virion capsid has three functions: (1) to protect the viral nucleic acid from digestion by certain enzymes (nucleases), (2) to furnish sites on its surface that recognize and attach (adsorb) the virion to receptors on the surface of the host cell, and, in some viruses, (3) to provide proteins that form part of a specialized component that enables the virion to penetrate through the cell surface membrane or, in special cases, to inject the infectious nucleic acid into the interior of the host cell.
Logic originally dictated that viruses be identified on the basis of the host they infect. This is justified in many cases but not in others, and the host range and distribution of viruses are only one criterion for their classification. It is still traditional to divide viruses into three categories: those that infect animals, plants, or bacteria.
Virtually all plant viruses are transmitted by insects or other organisms (vectors) that feed on plants. The hosts of animal viruses vary from protozoans (single-celled animal organisms) to humans. Many viruses infect either invertebrate animals or vertebrates, and some infect both. Certain viruses that cause serious diseases of animals and humans are carried by arthropods. These vector-borne viruses multiply in both the invertebrate vector and the vertebrate host.
Learn about the history, classification, and characteristics of viruses, the smallest infectious agents that can replicate only in living cells. Explore how viruses cause diseases, how they are studied, and how they may have ecological significance.
Mar 29, 2023 · Learn what viruses are, how they infect hosts and cause diseases, and what types of viruses exist. Find out how viruses reproduce, how they are classified and how they can be treated.
- DictionaryVi·rus/ˈvīrəs/
noun
- 1. an infective agent that typically consists of a nucleic acid molecule in a protein coat, is too small to be seen by light microscopy, and is able to multiply only within the living cells of a host: "the hepatitis B virus"
- 2. a piece of code that is capable of copying itself and typically has a detrimental effect, such as corrupting the system or destroying data.
- Dave Mac. 8 years ago. Posted 8 years ago. Direct link to Dave Mac's post “Will science/medicine eve...” Will science/medicine ever find a way to make us immune to every virus and bacteria?
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- Vivin Kannan. 6 years ago. Posted 6 years ago. Direct link to Vivin Kannan's post “How did Viruses develop i...” How did Viruses develop in the first place?
- Don Rowan. 4 years ago. Posted 4 years ago. Direct link to Don Rowan's post “If a surface, e.g., a tab...” If a surface, e.g., a table, is contaminated with a virus, e.g., COVID 19, is the virus unit attached to some carrier unit such as inside a water vapor droplet or attached to dust, etc.
Learn what viruses are, how they reproduce, and how they can cause illness in humans and other living organisms. Find out about different types of viruses, such as coronaviruses, and how they spread and change over time.
Coronavirus disease (COVID-19) is an infectious disease caused by the SARS-CoV-2 virus. Most people infected with the virus will experience mild to moderate respiratory illness and recover without requiring special treatment. However, some will become seriously ill and require medical attention. Older people and those with underlying medical ...
Oct 19, 2023 · Learn about viruses, their structure, replication, and effects on living hosts. Find out how viruses can be used for gene therapy and how they differ from bacteria.
