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Scoca V. Llano M. Ciuffi A. For example, all RNA viruses, with the exception of the Influenza virus. Replication within the host cell's nucleus and cytoplasm. Influenza virus, poxvirus, and other viruses are examples.
Within the nucleus of the host cell, replication takes place. All DNA viruses, with the exception of the pox virus. The virus replicates via a double-stranded DNA intermediate. The virus replicates via a single-stranded RNA intermediate.
There are four main types of viruses based on the type of host:. These viruses infect animals, including humans, by infiltrating their cells. The influenza virus, mumps virus, rabies virus, poliovirus, Herpes virus, and others are all examples of animal viruses. Invasion of plant cells is how these viruses infect plants. Potato virus, tobacco mosaic virus, beet yellow virus, turnip yellow virus, cauliflower mosaic virus, and other plant viruses are well-known examples.
Bacteriophage :Bacteriophage is a type of virus that infects bacterial cells. Insect virus, commonly known as the viral pathogen of insects, is a virus that infects insects. In the present agricultural landscape, these viruses are thought to be a strong biocontrol agent. Insect virus examples include Ascovirus virions and Entomopox virus.
Infections that spread through the air and into the respiratory tract are known as airborne infections. Swine flu and rhinovirus, for example. The virus is transmitted through contaminated water or food via the faecal oral route. Hepatitis A virus, Poliovirus, and Rotavirus, for example. Sexually transmitted diseases STDs - Virus transmission via sexual contact with an infected person. Retroviruses, human papillomaviruses, and other viruses are examples.
Transfusion-transmitted illnesses occur when a virus is delivered during a blood transfusion. Necessary Necessary. Necessary cookies are absolutely essential for the website to function properly. These cookies ensure basic functionalities and security features of the website, anonymously. The cookie is used to store the user consent for the cookies in the category "Analytics".
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If you rotate the icosahedron clockwise, you will find that in degrees you encounter the same arrangement symmetry : a triangle facing up and a triangle facing down. Continuing to rotate the icosahedron brings you back to where you began. This is known as the twofold axis of symmetry, because as you rotate the shape along this axis your pencil , you encounter your starting structure twice in one revolution: once when you begin, and again when rotated degrees.
On the other hand, if you put your pencil axis directly through the center one of the small triangle faces of the icosahedron, you will encounter the initial view two additional times as you rotate the shape, for a total of three times. This is the threefold axis. Similarly, if your pencil axis goes through a vertex or tip of the icosahedron, you will find symmetry five times in one rotation, forming the fivefold axis.
It is for this reason that an icosahedron is known to have 2—3—5 symmetry, because it has twofold, threefold, and fivefold axes of symmetry. This terminology is useful when dealing with an icosahedral virus because it can be used to indicate specific locations on the virus or where the virion has interactions with the cell surface.
For instance, if a virus interacts with a cell surface receptor at the threefold axis, then you know this interaction occurs at one of the faces of the icosahedron. A protein protruding from the capsid at the fivefold axis will be found at one of the vertices tips of the icosahedron. All of the illustrations of viruses in Fig.
How many twofold axes of symmetry are found in one icosahedron? How about the number of threefold or fivefold axes? How many faces, edges, and vertices are found in an icosahedron?
A Icosahedron faces fuchsia triangles , edges red rectangles , and vertices violet pentagons are indicated on the white icosahedron. B The twofold axis of symmetry occurs when the axis is placed through the center of an edge. The threefold axis occurs when the axis is placed in the center of a face C , and the fivefold axis passes through a vertex of the icosahedron D. Viral proteins form each face small triangle of the icosahedral capsid.
Viral proteins are not triangular, however, and so one protein subunit alone is not sufficient to form the entire face. Therefore, a face is formed from at least three viral protein subunits fitted together Fig. These can all be the same protein, or they can be three different proteins. The subunits together form what is called the structural unit. The structural unit repeats to form the capsid of the virion.
A Virus capsids are composed of viral protein subunits that form structural units. The triangulation number T indicates the number of structural units per face of the icosahedron. The red lines outline a triangular face of the icosahedron, while the purple pentagons indicate the vertices fivefold axes of the icosahedron.
But how can some viruses form very large icosahedral capsids? The answer is repetition. The structural unit can be repeated over and over again to form a larger icosahedron side. The number of structural units that creates each side is called the triangulation number T , because the structural units form the triangle face of the icosahedron.
The geometry and math involved with icosahedral capsid structure can be complex, and only the very basics are described here. In any case, by increasing the number of identical structural units on each face, the icosahedron can become progressively larger without requiring additional novel proteins to be produced. Some viruses have triangulation numbers over 25, even! The proteins that compose the structural unit may form three dimensional structures known as capsomeres that are visible in an electron micrograph.
In icosahedral viruses, capsomeres generally take the form of pentons containing five units or hexons containing six units that form a visible pattern on the surface of the icosahedron See Fig. Capsomeres are morphological units that arise from the interaction of the proteins within the repeated structural units. Why does the icosahedral virus structure appear so often?
Research has shown that proteins forming icosahedral symmetry require lesser amounts of energy, compared to other structures, and so this structure is evolutionarily favored. Many viruses that infect animals are icosahedral, including human papillomavirus, rhinovirus, hepatitis B virus, and herpesviruses Fig.
Like their helical counterparts, icosahedral viruses can be naked or enveloped, as well. Poliovirus A , rotavirus B , varicella—zoster virus C , the virus that causes chickenpox and shingles, and reovirus D. Note that C is enveloped. The majority of viruses can be categorized as having helical or icosahedral structure. A few viruses, however, have a complex architecture that does not strictly conform to a simple helical or icosahedral shape.
Poxviruses, geminiviruses, and many bacteriophages are examples of viruses with complex structure Fig. Poxviruses, including the viruses that cause smallpox or cowpox, are large oval or brick-shaped particles — nm long. The geminiviruses also exhibit complex structure. As their name suggests, these plant-infecting viruses are composed of two icosahedral heads joined together. Bacteriophages , also known as bacterial viruses or prokaryotic viruses , are viruses that infect and replicate within bacteria.
Many bacteriophages also have complex structure, such as bacteriophage P2, which has an icosahedral head, containing the nucleic acid, attached to a cylindrical tail sheath that facilitates binding of the bacteriophage to the bacterial cell. Vaccinia virus A , a virus belonging to the poxvirus family, has a complex capsid architecture with a dumbbell-shaped core. Geminiviruses B have a double-icosahedron capsid.
Bacteriophages, such as P2 C , often have complex capsid structure. The classification of viruses is useful for many reasons.
It allows scientists to contrast viruses and to reveal information on newly discovered viruses by comparing them to similar viruses. It also allows scientists to study the origin of viruses and how they have evolved over time. The classification of viruses is not simple, however—there are currently over different viral species with very different properties! One classification scheme was developed in the s by Nobel laureate David Baltimore.
The Baltimore classification system categorizes viruses based on the type of nucleic acid genome and replication strategy of the virus. As will be further discussed in the next chapter, positive-strand also positive-sense or plus-strand RNA is able to be immediately translated into proteins; as such, messenger RNA mRNA in the cell is positive strand.
Negative-strand also negative-sense or minus-strand RNA is not translatable into proteins; it first has to be transcribed into positive-strand RNA. Baltimore also took into account viruses that are able to reverse transcribe , or create DNA from an RNA template, which is something that cells are not capable of doing. Together, the seven classes are. There are a variety of ways by which viruses could be classified, however, including virion size, capsid structure, type of nucleic acid, physical properties, host species, or disease caused.
Because of this formidable challenge, the International Committee on Taxonomy of Viruses ICTV was formed and has been the sole body charged with classifying viruses since Taxonomy is the science of categorizing and assigning names nomenclature to organisms based on similar characteristics, and the ICTV utilizes the same taxonomical hierarchy that is used to classify living things. It is important to note that viruses, since they are not alive, belong to a completely separate system that does not fall under the tree of life.
Whereas a living organism is classified using domain, kingdom, phylum, class, order, family, genus, and species taxa singular: taxon , or categories, viruses are only classified using order, family, genus, and species Table 2. The ICTV classifies viruses based upon a variety of different characteristics with the intention of categorizing the most similar viruses with each other.
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