The complete scheme was not adopted, but animal viruses were soon classified by family, genus, and species. An important part of the scheme proposed by Lwoff and colleagues is that viruses are grouped according to their properties, not the cells they infect. The nucleic acid genome was also recognized as a primary criterion for classification.
Four characteristics were to be used for the classification of all viruses:. Other characteristics which were subsequently added include the type of disease caused, and which animals and tissues are infected. With the development of nucleic acid sequencing technologies in the s, genomics has played an increasingly important role in taxonomy. Today new viruses are assigned to families based on the nucleic acid sequence of their genome. The International Committee on the Taxonomy of Viruses ICTV is charged with the task of developing, refining, and maintaining a universal virus taxonomy.
Although the ICTV nomenclature is used to classify animals viruses, plant virologists do not place their viruses into families and genera, but use group names derived from the prototype virus.
Because the viral genome carries the blueprint for producing new viruses, virologists consider it the most important characteristic for classification. Lwoff, A. Therefore, they lack most of the components of cells, such as organelles, ribosomes, and the plasma membrane.
A virion consists of a nucleic acid core, an outer protein coating or capsid, and sometimes an outer envelope made of protein and phospholipid membranes derived from the host cell. The capsid is made up of protein subunits called capsomeres. Viruses may also contain additional proteins, such as enzymes. The most obvious difference between members of viral families is their morphology, which is quite diverse. An interesting feature of viral complexity is that host and virion complexity are uncorrelated.
Some of the most intricate virion structures are observed in bacteriophages, viruses that infect the simplest living organisms: bacteria. Viruses come in many shapes and sizes, but these are consistent and distinct for each viral family.
In general, the shapes of viruses are classified into four groups: filamentous, isometric or icosahedral , enveloped, and head and tail.
Filamentous viruses are long and cylindrical. Many plant viruses are filamentous, including TMV tobacco mosaic virus. Isometric viruses have shapes that are roughly spherical, such as poliovirus or herpesviruses. Enveloped viruses have membranes surrounding capsids. Animal viruses, such as HIV, are frequently enveloped. Head and tail viruses infect bacteria. They have a head that is similar to icosahedral viruses and a tail shape like filamentous viruses.
Many viruses use some sort of glycoprotein to attach to their host cells via molecules on the cell called viral receptors. For these viruses, attachment is a requirement for later penetration of the cell membrane, allowing them to complete their replication inside the cell. The receptors that viruses use are molecules that are normally found on cell surfaces and have their own physiological functions.
Viruses have simply evolved to make use of these molecules for their own replication. This attachment allows for later penetration of the cell membrane and replication inside the cell. Overall, the shape of the virion and the presence or absence of an envelope tell us little about what disease the virus may cause or what species it might infect, but they are still useful means to begin viral classification. Among the most complex virions known, the T4 bacteriophage, which infects the Escherichia coli bacterium, has a tail structure that the virus uses to attach to host cells and a head structure that houses its DNA.
Adenovirus, a non-enveloped animal virus that causes respiratory illnesses in humans, uses glycoprotein spikes protruding from its capsomeres to attach to host cells. Non-enveloped viruses also include those that cause polio poliovirus , plantar warts papillomavirus , and hepatitis A hepatitis A virus. Examples of virus shapes : Viruses can be either complex in shape or relatively simple. This figure shows three relatively-complex virions: the bacteriophage T4, with its DNA-containing head group and tail fibers that attach to host cells; adenovirus, which uses spikes from its capsid to bind to host cells; and HIV, which uses glycoproteins embedded in its envelope to bind to host cells.
Enveloped virions like HIV consist of nucleic acid and capsid proteins surrounded by a phospholipid bilayer envelope and its associated proteins.
Glycoproteins embedded in the viral envelope are used to attach to host cells. Other envelope proteins include the matrix proteins that stabilize the envelope and often play a role in the assembly of progeny virions. Chicken pox, influenza, and mumps are examples of diseases caused by viruses with envelopes. Viruses are infectious agents typically less than nanometers in diameter that can only reproduce and metabolize from within the cells of living hosts Fig.
See also: Taxonomy ; Virus. The organization of viruses into a taxonomic framework is an important means for investigators to be able to study and understand the biodiversity of viruses. In general, virus classification is concerned with the naming of viruses and the assembly of these names according to a given criterion. In the classification of biological organisms, this criterion is often based on phylogenetic evolutionary relationships.
However, there is no evidence that viruses possess a common ancestor or are in any way phylogenetically related. Nevertheless, classification along the lines of the Linnaean system as proposed by Carolus Linnaeus in the s into families, genera, and species has been partially successful. In addition, based on the organisms that viruses infect, the first broad division used in virus classification schemes involves the separating of viruses into vertebrate viruses, invertebrate viruses, plant viruses, bacterial viruses, and algae, fungi, yeast, and protozoan viruses.
However, viral families may fall into more than one of these classes. Within these classes, other criteria for subdivision are used, including general morphology for example, envelope or the lack of it ; nature of the genome [deoxyribonucleic acid DNA or ribonucleic acid RNA ]; structure of the genome [single-stranded ss or double-stranded ds , linear or circular, or fragmented or nonfragmented]; mechanisms of gene expression and virus replication positive- or negative-sense RNA ; serological relationships; host and tissue susceptibility; and pathology symptoms or type of disease.
Typically, families of viruses are assigned the suffix -viridae as a naming convention; in addition, families are sometimes divided further into subfamilies, which are given the suffix -virinae.
Further subgroups of a family or subfamily are equivalent to the genera of the Linnaean classification scheme. Notable DNA viruses include variola virus the cause of human smallpox , hepatitis B virus, Epstein-Barr virus, cytomegalovirus, human papillomaviruses, varicella zoster virus the cause of chickenpox and shingles , herpes simplex viruses, and simian virus RNA vertebrate viruses Fig. The single-stranded, negative-sense RNA vertebrate viruses are divided into seven families: Orthomyxoviridae, Paramyxoviridae, Filoviridae, Rhabdoviridae, Bornaviridae, Arenaviridae, and Bunyaviridae.
In addition, Retroviridae comprises a family of single-stranded RNA reverse-transcribing viruses. Virus classification is based mainly on characteristics of the viral particles, including the capsid shape, the type of nucleic acid DNA or RNA, double stranded ds or single stranded ss within the capsid, the process of replication, their host organisms, or the type of disease they cause.
The Table below lists characteristics such as capsid shape, presence of an envelope, and the diseases the viruses can cause. How are viruses classified? Classification of Viruses Like the classification systems for cellular organisms, virus classification is the subject of ongoing debate.
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