Tobacco satellite necrosis virus has such a simple icosahedral structure. However, most quasi-spherical viruses are larger, requiring the assembly of more than three subunits per face of the icosahedron.
These proteins form shells whose subunits are in quasi-equivalent contact. Here, the proteins at the icosahedral vertices remain arranged in a fivefold symmetry, but additional subunits cover the surfaces between in a pattern of sixfold symmetry Figure c.
The atomic structures of a number of icosahedral viruses have been determined by x-ray crystallography Figure a. Interaction of these clefts with cell-surface receptors attaches the virus to a host cell, the first step in viral infection Figure b. Neutralizing antibodies specific for a particular virus also interact with these clefts, thereby inhibiting attachment of the virus to the host cell. Structure of picornaviruses. These icosahedral viruses include poliovirus and the rhinoviruses, which cause the common cold.
This model more In some viruses, the symmetrically arranged nucleocapsid is covered by an external membrane , or envelope, which consists mainly of a phospholipid bilayer but also contains one or two types of virus -encoded glycoproteins Figure The phospholipids in the viral envelope are similar to those in the plasma membrane of an infected host cell.
The viral envelope is, in fact, derived by budding from that membrane, but contains mainly viral glycoproteins. Electron micrograph of a negatively stained influenza virus virion. The virion is surrounded by a phospholipid bilayer; the large spikes protruding outward from the membrane are composed of trimers of hemagglutinin protein and tetramers of neuraminidase more The components of simple viruses such as TMV, which consists of a single RNA molecule and one protein species, undergo self-assembly if they are mixed in solution.
More complex viruses containing a dozen or more protein species do not spontaneously assemble in vitro. The multiple components of such viruses assemble within infected cells in stages, first into subviral particles and then into completed virions.
The genomes of these complex viruses encode proteins that assist in the assembly of the virion , but the assembly proteins are not themselves components of the completed virion.
A virus that infects only bacteria is called a bacteriophage , or simply a phage. Viruses that infect animal or plant cells are referred to generally as animal viruses or plant viruses.
A few viruses can grow in both plants and the insects that feed on them. The highly mobile insects serve as vectors for transferring such viruses between susceptible plant hosts. An example is potato yellow dwarf virus, which can grow in leafhoppers insects that feed on potato plant leaves as well as in potato plants. Wide host ranges are characteristic of some strictly animal viruses, such as vesicular stomatitis virus, which grows in insects and in many different types of mammalian cells. Most animal viruses, however, do not cross phyla, and some e.
The host-cell range of some animal viruses is further restricted to a limited number of cell types because only these cells have appropriate surface receptors to which the virions can attach. The number of infectious viral particles in a sample can be quantified by a plaque assay. This assay is performed by culturing a dilute sample of viral particles on a plate covered with host cells and then counting the number of local lesions, called plaques, that develop Figure A plaque develops on the plate wherever a single virion initially infects a single cell.
The virus replicates in this initial host cell and then lyses the cell, releasing many progeny virions that infect the neighboring cells on the plate.
After a few such cycles of infection, enough cells are lysed to produce a visible plaque in the layer of remaining uninfected cells. Plaque assay for determining number of infectious particles in a viral suspension.
Since all the progeny virions in a plaque are derived from a single parental virus , they constitute a virus clone. This type of plaque assay is in standard use for bacterial and animal viruses. Plant viruses can be assayed similarly by counting local lesions on plant leaves inoculated with viruses.
Analysis of viral mutants, which are commonly isolated by plaque assays, has contributed extensively to current understanding of molecular cellular processes. The surface of viruses includes many copies of one type of protein that binds, or adsorbs, specifically to multiple copies of a receptor protein on a host cell.
This interaction determines the host range of a virus and begins the infection process Figure The entering genetic material may still be accompanied by inner viral proteins, although in the case of many bacteriophages, all capsid proteins remain outside an infected cell.
The genome of most DNA-containing viruses that infect eukaryotic cells is transported with some associated proteins into the cell nucleus , where the cellular DNA is, of course, also found. The viral mRNA that is produced then is translated into viral proteins by host-cell ribosomes, tRNA, and translation factors. Electron micrograph of a T4 bacteriophage adsorbed onto an E. Once viral surface proteins interact with receptors on the host cell, the viral DNA is injected into the cell.
Levine, , Viruses, Scientific American Library, p. Most viral protein products fall into one of three categories: special enzymes needed for viral replication; inhibitory factors that stop host-cell DNA , RNA , and protein synthesis; and structural proteins used in the construction of new virions. These last proteins generally are made in much larger amounts than the other two types. After the synthesis of hundreds to thousands of new virions has been completed, most infected bacterial cells and some infected plant and animal cells rupture, or lyse, releasing all the virions at once.
In many plant and animal viral infections, however, no discrete lytic event occurs; rather, the dead host cell releases the virions as it gradually disintegrates. The outcome is the production of a new round of viral particles and death of the cell.
Figure illustrates the lytic cycle for T4 bacteriophage. Adsorption and release of enveloped animal viruses are somewhat more complicated processes. The steps in the lytic replication cycle of a nonenveloped virus are illustrated for E. During adsorption step 1 , viral coat proteins at the tip of the tail in T4 interact with specific more We illustrate the lytic cycle of enveloped viruses with the rabies virus , whose nucleocapsid consists of a single-stranded RNA genome surrounded by multiple copies of nucleocapsid protein Figure , upper left.
Within the nucleocapsid of rabies virions are viral enzymes for synthesizing viral mRNA and replicating the viral genome. The envelope around the nucleocapsid is a phospholipid bilayer containing multiple copies of a viral transmembrane glycoprotein. The internal domain interacts with the viral matrix protein, which functions as a bridge between the transmembrane glycoprotein and nucleocapsid protein.
Figure outlines the events involved in adsorption of a rabies virion , assembly of progeny nucleocapsids, and release of progeny virions by budding from the host-cell plasma membrane. Budding virions are clearly visible in electron micrographs, as illustrated by Figure The steps in the lytic replication cycle of an enveloped virus are illustrated for rabies virus, which has a single-stranded RNA genome.
For example, some viruses called bacteriophages attack bacteria in our bodies and so have a crucial role in regulating our microbiome. Just as an invasive wild animal species can breed out of control when it enters a new area without predators or pathogens think cane toads in Australia, or rats on tropical islands , so too would bacteria override our bodies without these regulating mechanisms.
Viruses also seem to be important in the regulation of our immune system. In humans, the hepatitis G virus can protect against HIV, while in mice herpesvirus is known to reduce autoimmune diseases. These are diseases that are a major factor in many modern illnesses in humans, from asthma to irritable bowel syndrome. Many viruses are clearly very harmful to us and humans have evolved mechanisms to counter their attacks.
Viruses share a deep evolutionary relationship with animals and plants. Every cell in your body is part of an unbroken chain of life that has extended over 3. Viruses have been an important part of that evolutionary waltz from the very start. Getting plenty of rest, staying hydrated, and taking OTC anti-inflammatories to ease pain or headaches can all help. In some cases, antiviral medication may be prescribed.
Polio or severe cases of meningitis or encephalitis may require additional treatment, such as breathing assistance or IV fluids. Practicing good hygiene, avoiding close contact with those who have the virus, and protecting against insect bites can all help to reduce the spread of encephalitis and meningitis. To reduce the risk of spreading rabies, keep your pets vaccinated and avoid approaching wild animals.
There are many viral diseases. Some, such as the common cold or the stomach flu, are minor and go away on their own within a few days. Others, however, are more serious. Instead, treatment usually focuses on managing symptoms and supporting the immune system with plenty of rest and hydration.
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Viral Diseases Medically reviewed by Daniel Murrell, M. Respiratory viral diseases. Gastrointestinal viral diseases. Exanthematous viral disease. Hepatic viral diseases. Cutaneous viral diseases. Hemorrhagic viral diseases. A virus is a non-living submicroscopic pathogen.
It replicates only when it gets attached to a living host. It should be considered as an organism as they are not free-living, cannot reproduce, and carry metabolic processes. In contrast to cells, it is much smaller.
Exist as independent particles called virions. However, the nucleic acid can be single- or double-stranded. The virus is simple as it contains two or three components only.
Unlike Cell, It cannot replicate on its own. It needs a living cell to replicate and the composition is very simple.
They are dependent on the host as they cannot synthesize their protein. Therefore, they lack ribosomes that help in synthesizing proteins. Thus, they depend on the host cells for energy and other metabolic functions. A variety of organisms can be affected by the virus ranging from plants and animals to microorganisms such as bacteria and archaea. Cells and viruses are very different from each other. They have different natures and functionality.
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