A virus (from the Latin virus meaning toxin or poison) is a sub-microscopic infectious agent that is unable to grow or reproduce outside a host cell. Virus diseases inflict a heavy illness and economic burden on humans and animals and can devastate agricultural crops. Each viral particle, or virion, consists of genetic material, DNA or RNA, within a protective protein coat called a capsid. Their shape varies from simple helical and icosahedral (polyhedral or near-spherical) forms, to more complex structures with tails or an envelope. Viruses infect cellular forms of life and are grouped into animal, plant and bacterial viruses. It has been argued whether viruses are living organisms. Some consider them non-living as they do not meet the criteria of the definition of life. For example, unlike most organisms, viruses do not have cells. However, viruses have genes and evolve by natural selection. They have been described as organisms at the edge of life. Viral infections in human as well as animal hosts, usually result in an immune response and disease. Often, a virus is completely eliminated by the immune system. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat life-threatening infections. Vaccines that produce lifelong immunity, can prevent virus infections.
A complete virus particle, known as a virion, consists, in its simplest form, of nucleic acid surrounded by a protective coat of protein called a capsid. Viruses can have a lipid 'envelope' derived from the host cell membrane. A capsid is composed of proteins encoded by the viral genome and its shape serves as the basis for morphological and antigenic distinction.
Animal DNA viruses, such as herpesviruses, enter the host via endocytosis, the process by which cells take in material from the external environment. Frequently after a chance collision with an appropriate surface receptor on a cell, the virus penetrates the cell, the viral genome is released from the capsid and host polymerases begin transcribing viral mRNA. New virions are assembled and released either by cell lysis or by budding off the cell membrane.
Animal RNA viruses can be placed into about four different groups depending on their modes of replication. The polarity of the RNA largely determines the replicative mechanism, as well as whether the genetic material is single-stranded or double-stranded. Some RNA viruses are actually DNA based but use an RNA-intermediate to replicate. RNA viruses are dependent on virally encoded RNA replicase to create copies of their genomes.
Because viruses use the machinery of a host cell to reproduce and reside within them, they are difficult to eliminate without killing the host cell. The most effective medical approaches to viral diseases so far are vaccinations to provide resistance to infection, and antiviral drugs which treat the symptoms of viral infections.
A complete virus particle, known as a virion, consists, in its simplest form, of nucleic acid surrounded by a protective coat of protein called a capsid. Viruses can have a lipid 'envelope' derived from the host cell membrane. A capsid is composed of proteins encoded by the viral genome and its shape serves as the basis for morphological and antigenic distinction.
Animal DNA viruses, such as herpesviruses, enter the host via endocytosis, the process by which cells take in material from the external environment. Frequently after a chance collision with an appropriate surface receptor on a cell, the virus penetrates the cell, the viral genome is released from the capsid and host polymerases begin transcribing viral mRNA. New virions are assembled and released either by cell lysis or by budding off the cell membrane.
Animal RNA viruses can be placed into about four different groups depending on their modes of replication. The polarity of the RNA largely determines the replicative mechanism, as well as whether the genetic material is single-stranded or double-stranded. Some RNA viruses are actually DNA based but use an RNA-intermediate to replicate. RNA viruses are dependent on virally encoded RNA replicase to create copies of their genomes.
Because viruses use the machinery of a host cell to reproduce and reside within them, they are difficult to eliminate without killing the host cell. The most effective medical approaches to viral diseases so far are vaccinations to provide resistance to infection, and antiviral drugs which treat the symptoms of viral infections.
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