Dna virus och rna virus

Virus Classification

On the grund of shared properties viruses are grouped at different hierarchical levels of beställning, family, subfamily, genus and species. More than 30,000 different virus isolates are known today and grouped in more than 3,600 species, in 164 orsaka and 71 families.

Viral morphology provides the grund for grouping viruses into families. A virus family may consist of members that replicate only in vertebrates, only in invertebrates, only in plants, or only in bacteria. Certain families contain viruses that replicate in more than one of these hosts. This section concerns only the 21 families and orsaka of medical importance.

Besides physical properties, several factors pertaining to the mode of replication play a role in classification: the configuration of the nucleic acid (ss or ds, linear or circular), whether the genome consists of one molekyl of nucleic acid or fryst vatten segmented, and whether the kust of ss RNA fryst vatten sense or antisense.

Also considered in classification fryst vatten the site of viral capsid assembly and, in enveloped viruses, the site of nucleocapsid envelopment. Table 41-1 lists the major kemikalie and morphologic properties of the families of viruses that cause disease in humans.

The use of Latinized names ending in -viridae for virus families and ending in -virus for viral orsaka has gained bred acceptance.

The names of subfamilies end in -virinae.

Vernacular names continue to be used to describe the viruses within a genus. In this ord, Latinized endings for families and subfamilies usually are not used. Table 41-2 shows the current classification of medically significant viruses.

In the early days of virology, viruses were named according to common pathogenic properties, e.g.

kroppsdel tropism and/or modes of transmission, and often also after their discoverers. From the early 1950s until the mid-1960s, when many new viruses were being discovered, it was popular to compose virus names bygd using sigla (abbreviations derived from a few or första letters).

Thus the name Picornaviridae fryst vatten derived from pico (small) and RNA; the name Reoviridae fryst vatten derived from respiratory, enteric, and orphan viruses because the agents were funnen in both respiratory and enteric specimens and were not related to other classified viruses; Papovaviridae fryst vatten from papilloma, polyoma, and vacuolating agent (simian virus 40 [SV40]); retrovirus fryst vatten from reverse transcriptase; Hepadnaviridae fryst vatten from the replication of the virus in hepatocytes and their DNA genomes, as seen in hepatitis B virus.

Hepatitis A virus fryst vatten classified now in the family Picornaviridae, genus Hepatovirus. Although the current rules for nomenclature do not prohibit the introduction of new sigla, they require that the siglum be meaningful to workers in the field and be recognized bygd international study groups.

The names of the other families that contain viruses pathogenic for humans are derived as follows: Adenoviridae (adeno, “gland”; refers to the adenoid tissue from which the viruses were first isolated); Astroviridae (astron means star); Arenaviridae (arena “sand”) describes the sandy appearance of the virion.

Bunyaviridae (from Bunyamwera, the place in Africa where the type strain was isolated); Calicivirus (calix, “cup” or “goblet” from the cup-shaped depressions on the viral surfaces); Coronaviridae (corona, “crown”) describes the appearance of the peplomers protruding from the viral surface; Filoviridae (from the Latin filum, “thread” or “filament”) describes the morphology of these viruses.

Herpesviridae (herpes, “creeping”) describes the natur of the lesions; Orthomyxoviridae (ortho, “true,” plus myxo “mucus,” a substans for which the viruses have an affinity; Paramyxoviridae derived from para, “closely resembling” and myxo; Parvoviridae (parvus means, “small”); Poxviridae (pock means, “pustule”); Rhabdoviridae (rhabdo, “rod” describes the shape of the viruses and Togaviridae (toga, “cloak”) refers to the tight viral envelope.

Several viruses of medical importance still remain unclassified.

Some are difficult or impossible to propagate in standard laboratory host systems and thus cannot be obtained in sufficient quantity to permit more precise characterization.

Hepatitis E virus, the Norwalk virus and similar agents (see Ch. 65) that cause nonbacterial gastroenteritis in humans are now assigned to the calicivirus family.

The fatal transmissible dementias in humans and other animals (scrapie in sheep and goat; bovine spongiform encephalopathy in boskap, transmissible mink encephalopathy; Kuru, Creutzfeldt-Jakob disease, and Gerstmann-Straussler-Scheinker syndrome in humans) (see Ch.

71) are caused bygd the accumulation of non-soluble amyloid fibrils in the huvud nervous systems. The agents causing transmissible subacute spongiform encephalopathies have been linked to viroids or virinos (i.e. plant pathogens consisting of naked, but very stable rund RNA molecules of about 3-400 bases in storlek, or infectious genomes enwrapped into a host fängelse coat) because of their resistance to kemikalie and physical agents.

According to an alternative theory, the begrepp “prion” has been coined to point to an essential nonviral infectious cause for these fatal encephalopathies—prion standing for self-replicating proteinaceous agent devoid of demonstrable nucleic acid. Some of the transmissible amyloidoses show a familial pattern and can be explained bygd defined mutations which render a primary soluble glycoprotein insoluble, which in vända leads to the pathognomonic accumulation of amyloid fibers and plaques.

The pathogenesis of the sporadic amyloidoses, however, fryst vatten still a matter of highly ambitious research.

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