Viral classification
Viruses can be classified in several ways, such as by their geometry, by whether they have envelopes, by the identity of the host organism they can infect, by mode of transmission, or by the type of disease they cause. The most useful classification is probably by the type of nucleic acid the virus contains and its mode of expression. This classification was proposed by Nobel-prize winner David Baltimore.
Overview
The various forms of viruses arise because one of the two strands of DNA in which all cellular life forms store their genetic information is redundant, so that viruses can have either single-stranded or double-stranded genomes. Furthermore, some viruses store their genome in RNA rather than in DNA form. RNA arises in cells as an intermediate when genes are translated into proteins. RNA genomes of viruses can be encoded in two different directions: Either the genes are stored in the 5'->3' direction (positive or + polarity), analogous to the direction in which genes are represented in mRNA in cells, or the genes are stored in the opposite direction (negative or - polarity).
The taxonomy of viruses is similar to that of all other forms of life:
- Class (determined by genome type and mode of expression)
- Order (...virales)
- Family (...viridae)
- Subfamily (...virinae)
- Genus (...virus)
- Species (virus)
The division of classes into orders is very recent. To date, only 3 orders have been named, and many families have not yet been classified into orders.
Approximately 80 families and 4000 species of virus are known.
Classification by genome type and mode of expression
Viruses in classes I and II in the following classification are considered to be DNA viruses, all others are considered to be RNA viruses.
Class I double stranded DNA
- Order Caudovirales (bacteriophages with dsDNA). Families:
- Unassigned families:
Class II single stranded DNA
Class III double stranded RNA
Class IV positive single stranded RNA itself acting as mRNA
Class V negative single stranded RNA used as a template for mRNA synthesis
- Order Mononegavirales (non-segmented negative stranded viruses). Families:
- Segmented negative stranded viruses. Families:
Class VI positive single stranded RNA with a DNA intermediate in replication
Class VII double stranded DNA with an RNA intermediate in replication
External links
Referenced By
Coronavirus | DNA virus | Rhabdoviridae | Virus evolution
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