Distinguishing taxonomic features
Viruses are classified based on their nucleic acid content, their size, the shape of their protein capsid, and the presence of a surrounding lipoprotein envelope.
The main taxonomic division is based on the nucleic acid content into two classes: DNA viruses or RNA viruses. DNA viruses are further divided into viruses containing double-stranded DNA or single-stranded DNA. RNA viruses are also divided into viruses that contain double-stranded RNA and viruses that contain single-stranded RNA. Further subdivision of RNA viruses is based on whether the RNA genome is segmented. When a virus contains single-stranded RNA as its genetic information, it is called a positive-strand virus if the RNA is messenger-sense (translatable directly into protein), or a negative-strand virus if the RNA needs to be transcribed into mRNA by a polymerase. It is classified as .
All viruses that fall into one of these nucleic acid classifications are further classified based on whether their nucleocapsids (protein coats and encapsulated nucleic acids) are rod-shaped or polygonal (usually icosahedral) in shape. It will be subdivided. Icosahedral viruses are further divided into families based on the number of capsomeres that make up the capsid. Finally, all viruses are divided into two classes, depending on whether the nucleocapsid is surrounded by a lipoprotein envelope.
Some virologists insist on classifying viruses as those that infect bacteria, plants, and animals. Although these classifications have some validity, especially for specialized bacterial viruses with tails, there is otherwise too much overlap and host-based classifications do not seem to work. Classification based on the diseases caused by viruses is also not valid because closely related viruses often do not cause the same diseases. Eventually, it is likely that viruses will be classified based on their nucleotide sequences and their mode of replication, rather than their structural components, as is currently the case.
Basic taxonomic groups are called families and are designated by the suffix -viridae. A major taxonomic disagreement among virologists is whether to classify viruses within a family into specific genera and then subdivide them into species names. In the first decade of the 21st century, there was a shift to the use of binomial nomenclature, which divides viruses into italicized genera and species. This move was largely prompted by the International Committee on Taxonomy of Viruses (ICTV), an affiliate of the International Federation of Microbiological Societies. ICTV oversees the ongoing process of devising and maintaining a universal classification scheme for viruses. In the virus taxonomy hierarchy, ICTV recognizes orders, families, subfamilies, genera, and species. The placement of viruses in these groups is based on information provided by research groups made up of experts on specific types of viruses.
In the ICTV system, each virus species is generally recognized as an isolate, that is, a group of viruses with different nucleic acid sequences. Therefore, a single species of virus may contain multiple isolates. Although isolates of a species have unique genetic sequences, they all share certain genetic traits because they are descendants of the same replicating lineage. Additionally, isolates of certain species also share the ability to thrive within specific ecological niches. Virus classification is expected to become increasingly complex as scientists identify new isolates and species. The following scheme shows examples of well-characterized DNA and RNA viruses classified based on the ICTV system.
Robert R. Wagner Robert M. Krug
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