Classification of Viruses

Classification of viruses

Structure and Function

The obligate intracellular viruses are tiny parasites that contain either an DNA or RNA genome that is covered by a protective coated protein that is coded by viruses. They can be described as biological elements that are mobile, and are likely of cellular origin and distinguished by a lengthy co-evolution between host and virus. To propagate, they depend on specific host cells providing the complicated biosynthetic and metabolic machinery of prokaryotic and eukaryotic cells. A full virus particle is referred to as virion. The principal function of the virion is to transfer its DNA genome or its RNA genome to the host cell so that the genome is expression (transcribed or transformed) through its host cell. The genome of the virus, usually together with basic proteins, is contained in the capsule-like protein that is symmetrical. The nucleic acid-associated proteins, known as nucleoprotein, in conjunction with the genome, form the nucleocapsid. In enveloped virus, the nucleocapsid enclosed by a bilayer of lipids that is derived from the altered host cell membrane and studded with an outer layer of glycoproteins that make up the envelope.

Classification of viruses

Morphology: Viruses are classified according to the size of their bodies and shapes, as well as chemical composition, the structure of the genome and the mode of replication. Helical Morphology is evident in the nucleocapsids of a wide variety of filamentous and pleomorphic virus. Helical nucleocapsids are a helical array made up of capsid protein (protomers) which are wrapped within a filament made of nucleic acid. The icosahedral shape is typical of the nucleocapsids found in many "spherical" viral species. The size and configuration of the capssomeres (morphologic parts of the icosahedron) are helpful in identifying and classifying. A lot of viruses also possess the outer shell.

Chemical Composition and Method of Replication The DNA of a virus can comprise RNA or DNA and may include single strands (ss) and double stranded (ds) circular or linear. The entire genome can be or comprise just one nucleic acid molecule (monopartite genome) or multiple nucleic acid segments (multipartite genome). The various types of genome require different replication methods.

Nomenclature

Apart from physical information such as genome structure and mechanism of replication are also factors used in the nomenclature and classification of viruses which includes its chemical structure and structure for DNA nucleic acid, whether the genome is multipartite or monopartite. The genomic RNA strand in single-stranded virus is known as sense (positive sense as well as sense) in the sense that it is able to function as mRNA and also antisense (negative sense, without sense) in the event that the strand that is synthesized through a virus's RNA transcription enzyme is used as the mRNA. Another factor in the classification of viruses is the place of capsid assembly as well as, for enveloped viruses, it is the place of envelope formation.

Structure and Function

They are inert in of the host cell. They are small viruses e.g. Polio, tobacco mosaic virus are even crystallized. The viruses are incapable of producing energy. Since they are obligate intracellular parasites during replication, they totally depend on the intricate biochemical machinery of eukaryotic and prokaryotic cells. The primary function of viruses is to transfer its genome to the host cell to allow its expression (transcription and translation) by the host cell.

A fully constructed infectious virus is referred to as virion. The most basic virions comprise two fundamental elements: nucleic acid (single- or double-stranded RNA , also known as DNA) and a coat of protein known as the capsid. It serves as a shell to shield the genome of the virus from nucleases. During infecting the virus, it attaches to receptors that are specific to the potential host cell. Capsid proteins are encoded in the genome of the virus. Due to its dimension (Table 41-1) the genome encodes only a handful of specific structural proteins (besides non-structural regulator proteins that play a role in the replication of viruses). Capsids form as two or single protein shells. They comprise only one or two structural proteins. So, multiple copies of proteins need to self-assemble into the three-dimensional capsid structure that is continuous. Self-assembled capsids of viruses has two main designs: helical symmetry where proteins and nucleic acid are arranged in the form of a helix. The other is the icosahedral symmetry in which the protein subunits form an symmetric shell which covers that nucleic acid-loaded core.

Chemical and Morphologic Characteristics from Animal Virus Families that are relevant for Human Disease.

Certain virus families come with an additional cover, known as the envelope. It is typically derived by modifying host cell membranes. The envelopes of viral infections comprise an lipid bilayer that covers a shell of virus encoded membrane-associated proteins. The bilayer's surface is dotted with virus-coded glycosylated (trans-) membrane proteins. Thus, enveloped viruses typically display a border of glycoprotein spikes , or knobs that are also referred to as peplomers. When viruses acquire their envelope through the plasma or an intracellular cell membrane The lipid composition of the envelope of the virus closely mirrors the host membrane. The outer capsid and envelope proteins are glycosylated, and essential in determining host's host range as well as the antigenic profile in the virion. In addition to the virus-specific envelope proteinsand budding viruses have as well specific host cell proteins as integral elements of the viral envelope. The envelope of viruses is an additional layer of protection. The larger viruses typically possess a complex structure composed of both isometric and helical Symmetries that are confined to various structural elements. small viruses e.g., hepatitis B virus or Picornavirus and parvovirus family are orders of magnitude stronger than the more complicated viruses e.g. members of the retrovirus or herpes families.

Classification of virus classification

Viruses are classified on basis of their morphology and chemical composition, as well as the method of replication. The viruses that attack humans are divided into 21 families, revealing only a tiny fraction of the of viruses that have host ranges that extend from vertebrates through protozoa as well as from fungi and plants bacteria.

Morphology

Helical Symmetry

When viruses replicate that have helical symmetry, the same Protein components (protomers) self-assemble to form a helical structure around that nucleic acid, which follows an identical spiral. Nucleocapsids can form rigid, extremely stretched rods or flexible filaments. In both cases the details of the structure of the capsid can be observed with electron microscope. Alongside classifications as rigid or flexible, and also as enveloped or naked the helical nucleocapsids can be distinguished by the length as well as width, and pitch in the helix and the number of protomers in helical turns. The most researched helical virus is tobacco mosaic virus (Fig. 41-1). A number of important structural characteristics of this plant virus were identified by x-ray diffraction research. Figure 41-2 demonstrates Sendai virus is an enveloped virus sporting an helical nucleocapsid structure, which is one of the family known as paramyxovirus (see Ch. 30).

The shape of the helical structure of the cigar mosaic viral rod. A little over 5 percent of total length is shown. Subunits of protein 17,400-Da (protomers) form the form of a helix that has an Axial repeated that is 6.9 Nm (49 subunits for three turns).

Icosahedral Symmetry

An an icosahedron can be described as a polyhedron that has 20 triangular faces that are equilateral as well as twelve vertices (Fig. 41-3). Lines between opposite vertices are the axes of fivefold symmetry All structural elements that make up the polyhedron repeat 5 times in each 360deg rotation around any of the five axes. Lines running through the middle of the triangular faces opposite to each other make up axes of threefold symmetry. Twofold rotational Axes are created by lines that run through the midpoints of opposite edges. An Icosaheron (polyhedral or spherical) with threefold, fivefold and twofold axes of the symmetry of rotation (Fig. 41-3) is classified to have 532 the symmetry (read in terms of 5,3,2).

Icosahedral models are shown, from from left to right on the fivefold threefold and twofold Axioms that show rotational symmetry. The axes are perpendicular to the surface of the page and traverse the centers of each model. The are polyhedral (upper) as well as Spherical (lower) shapes (more ...)

Viruses were first discovered to possess 532 symmetry through studies of x-ray diffracted diffraction, and then through electron microscopy using negative staining methods. In the majority of icosahedral viruses protomers i.e. structures polypeptide chains are distributed in oligomeric clusters, known as capsomeres. They can be easily identified through negative staining electron microscopy and create the closed capsid shell the arrangement of capssomeres in an icosahedral shell allows the identification of these viruses by their capsomere number and the pattern. This is done by identifying the closest two vertex capssomeres (called penton: the ones that the fivefold symmetry and symmetry axes go) as well as the arrangement of capssomeres within the two.

Adenovirus after negative stain electron microscopy. (A) Capsomeres is the typical isometric sphere made of 20 triangular equilateral faces. There are 252 capssomeres. the 12 pentons and the hollow hexon capsomeres, 240 of which are organized in the T equal 25 degree symmetry (more ...)

In the adenovirus model shown in the figure, one the penton capsomeres has been arbitrarily assigned the index h = 0 K = (0 (origin) in which the indices h and K are the indicated points that form an inclined (60deg) capssomeres' net. These axes of the net are created by lines drawn from the most compact close-packed capsomeres. In adenoviruses the h and k axes are also in alignment to the borders of triangular faces. The second vertex that is adjacent to it is indexed with 5 = h or 5 and k = 0, (or an index of h = 0 or the value of 5). The number of capsomeres (C) can be calculated to be 252 based on the indices h and k and the equation C = 10(h2 + hk +) + 2. This symmetry and the number of capsomeres are typical for any member belonging to the Adenovirus family.

Core Structure of Virus

In addition to helical nucleocapsids there is little information about the arrangement or packaging of the viral genome inside the genome's core. Small virions are basic nucleocapsids with up to two proteins. The larger viruses have in the core of the nucleic acid genome complexed with the basic protein(s) and are protected by a single or double-layered capsid (consisting of multiple type in protein) or an envelope

Two-dimensional diagram of HIV-1 that relates (immunoor) electron microscopic results with the latest nomenclature for the structural components of two-letter codes and the molecular weights of HIV-1 the structural (glycoand) proteins.

Chemical Composition and the Mode of Replication

Genomes of RNA Virus

The RNA viruses, which comprise 70 percent of all viruses differ significantly in their DNA structure (Fig. 41-6). Due to the high errors of the enzymes involved in the replication of RNA These viruses typically exhibit higher mutation rates than DNA viruses. Rates of mutation of 10-4 lead to the constant creation of variants of viruses that exhibit a remarkable adaptability to host. The viral RNA could have a single-strand (ss) as well as double-stranded (ds) and the genome could be located on an RNA segment that is only one or be spread across multiple segment (segmented genomes). Additionally the RNA strand in a single-stranded genome could be either an sense strand (plus strand) and can act in the capacity of messenger RNA (mRNA) as well as an antisense or (minus strand) (minus strand) that functions as a complement in function to the sense-strand but is not able to function as an mRNA protein expression (see Ch. 42). The sense viral RNA is the only one that is able to replicate when it is injected into cells, as it functions as mRNA, and also initiate the transcription of viruses-encoded proteins. Antisense RNA, on other hand, is not able to perform a function in translation and does not by itself produce virus-related components.

Schematics of 21 viruses which infect humans and displaying a range of distinct criteria, including the there is an enclosed, or (doubledouble) capsid as well as an the internal nucleic acid genome. + The strand of Sense; - antisense strand--, dsRNA or DNA 0, circular DNA C numbers (more ...)

DsRNA viruses e.g. and members belonging to the reovirus family comprise 10 or 12 distinct genome segments that encode for three enzymes that are involved in the replication of RNA and 3 capsid proteins major and several lesser structural proteins. Each segment is composed of a sense and an antisense strand which is connected to form a linear structure. The replication process of the viruses is a complex process and only the strands of RNA that have a sense can be released by the virus to begin replication.

The retrovirus genome consists of two identical plus-sense ssRNA molecules that are each monomer 7-11 kb in length which are covalently joined over a narrow terminal region. Retroviruses have two envelope proteins encoded through the gene env as well as 4-6 nonglycosylated core proteins, and three non-structural functional proteins (reverse transcriptase (RT), integrase (IN), protease: In, RT, and PR) identified in the gag gene (Fig. 41-5). 41-5). RT converts the viral ssRNA into double-stranded circumferential proviral DNA. The DNA, which is mediated by the viral Integrase is then covalently linked to cells that contain the host cell to make possible the transcription of sense strands, which eventually produce retrovirus progenitors. After budding and assembly retroviruses undergo functional and structural maturation. In immature viruses, the structural core proteins are found in a huge precursor shell of protein. After the proteolytic processing of the viral protease, the components of the virion mature are rearranged to make up the dense, isometric cone-shaped center of mature virion and the particle is then infectious.

DNA Genomes of Viruses

Most DNA viruses possess only a single genome of dsDNA in linear form. The papovaviruses that comprise the papilloma- and polyomaviruses however, possess circular DNA genomes that range between 5.1 to 7.8 kb in size. DsDNA is used as a template self-transcription as well as for mRNA. Three or two structural proteins compose the capsid of papovavirus. In addition five to six nonstructural proteins are encoded and are involved in viral transcription DNA replication, the transformation of cells.

Single-stranded linear DNAof 4-6 km in size, can be present in the individuals of the Parvovirus family which comprises the parvo-, erythro-, and the dependoviruses. The virion is composed of four structural protein species that differ in their origins from the identical genetic product (see Ch. 64). The Adeno-associated virus (AAV is a type of dependovirus) cannot produce progeny virions , except when it is in contact with aider virus (adenovirus as well as herpesvirus). This is why it is believed to be defective in replication.

Circular single-stranded DNA ranging from 1.7 to 2.3 Kb is present in the members of the family of Circoviruses that contain the smallest independently propagated viruses. The isometric capsid is 17 nm in size and is made up of two protein species.

Virus Classification

Based on shared characteristics, viruses are placed in different hierarchical levels of family, order subfamily, genus and species. More than 30,000 distinct virus strains are currently identified and are classified into more than 3600 species, spread across more than 71 families and 164 genera. Viral morphology is the foundation for dividing virus families. A family of viruses may comprise members that reproduce exclusively in vertebrates, in invertebrates, or only in plants, or in bacteria. Certain families have viruses that reproduce in multiple of the hosts. This article focuses on the 21 genera and families that are of medical significance.

Apart from physical properties, a variety of aspects that relate to the manner of replication play an important role in classification. These include the configuration that is present in the nucleic acid (ss or ds linear or circular) as well as whether the genome is just one single molecule nucleic acid or is segmented in any way, and whether the strand of ssRNA is antisense or sense. Another factor to be considered in the classification process is the location of capsid assembly in viral viruses and for enveloped viruses the nucleocapsid's site of envelope. Table 41-1 lists main characteristics of the morphologic and chemical aspects of the various families of viruses which cause illness in humans.

The usage of Latinized names that begin with the -viridae suffix for viruses and ending in -virus to refer to the viral genera have gained widespread acceptance. Subfamilies' names end in the -virinae. The vernacular names are used to identify the virus within the Genus. In this document, Latinized endings for families and subfamilies are rarely utilized. Table 41-2 outlines the current classification of medically significant viruses.

Current classification of major groups of medically relevant viruses.

In the beginning of virus research, viruses were named in accordance with common characteristics of pathogens, e.g. organ tropism or modes of transmission, and sometimes even after their creators. In the 1950s and into the mid-1960s when numerous new viruses were discovered and discovered, it became popular to make virus names using sigla (abbreviations that were derived from a handful of or the initial letters). This is why names like Picornaviridae is taken from the word pico (small) in addition to RNA. the term Reoviridae comes from enteric, respiratory, and orphan virus because they were present in both enteric and respiratory specimens , and they were not connected to any other viruses classified as such. Papovaviridae is derived from papilloma polyoma, and vacuolating agents (simian virus 40, SV40(simian virus 40 [SV40]) Retrovirus comes an example of reverse transcriptase. Hepadnaviridae originates from its replication inside the hepatocytes as well as their DNA genomes as is evident in the case of hepatitis B virus. Hepatitis A virus has been now classified in the family of Picornaviridae in the in the genus Hepatovirus. While the current nomenclature rules allow the introduction of new siglas however, they do require that the sigla be useful for those working in the field , and also be acknowledged by international research groups.

The names of other families with viruses that can be which are harmful to humans are from the following: Adenoviridae (adeno, "gland" is a reference to the adenoid tissues that the viruses first became isolated); Astroviridae (astron means star); Arenaviridae (arena "sand") refers to the appearance of the virion as sandy. Bunyaviridae (from Bunyamwera, the area in Africa in Africa where the strain of type was first isolated); Calicivirus (calix, "cup" or "goblet" because of the cup-shaped holes on surface of the virus); Coronaviridae (corona, "crown") refers to the look of peplomers sticking out of the surface of the virus; Filoviridae (from"filomidae" which is the Latin"filum, "thread" as well as "filament") refers to the morphology of these virus. Herpesviridae (herpes, "creeping") describes the characteristics of the lesions. Orthomyxoviridae (ortho, "true," and myxo "mucus," a substance with which the viruses have an affinity Paramyxoviridae is derived from para "closely like" and myxo. Parvoviridae (parvus is a Latin word meaning "small"); Poxviridae (pock is a reference to "pustule"); Rhabdoviridae (rhabdo, "rod" describes the shape of the virus as well Togaviridae (toga, "cloak") refers to the tight envelope of the virus.

Many viruses with medical significance are still not classified. Some are extremely difficult or impossible to spread within the standard host system for laboratory research and, consequently, cannot be cultivated enough to allow more precise identification. Hepatitis E virus Norwalk virus and the other substances (see Ch. 65) which cause non-bacterial gastroenteritis in humans have been classified as belonging to the calicivirus family.

The fatal transmissible dementias in humans and other animals (scrapie in sheep and goat; bovine spongiform encephalopathy in cattle, transmissible mink encephalopathy; Kuru, Creutzfeldt-Jakob disease, and Gerstmann-Straussler-Scheinker syndrome in humans) (see Ch. 70) can be caused due to the accumulation of amyloid non-soluble fibrils in the central nerve systems. The agents responsible for transmitsible subacute spongiform and spongiform disorders have been connected to viroids and viruses (i.e. plants that are made up of naked, yet very solid circular RNA molecules that measure around 3-400 bases or infectious genomes wrapped inside the host cell coat) because of their inability to withstand physical and chemical agents. According to an alternative theory, the term "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. Certain amyloidoses that are transmissible exhibit a family-like pattern and are explained through identified mutations that render the glycoprotein that is soluble in its primary form, which then causes the pathognomonic accumulation of amyloid plaques and fibers. The sporadic nature of amyloidoses remains the subject of very scholarly research.