File Name: viral structure and replication .zip
Viruses are not plants, animals, or bacteria, but they are the quintessential parasites of the living kingdoms.
Molecular Virology pp Cite as. Viruses are infectious units with diameters of about 16 nm circoviruses to over nm poxviruses; Table 2. Their small size makes them ultrafilterable, i. Viruses have evolved over millions of years, and have adapted to specific organisms or their cells.
The infectious virus particles, or virions, are composed of proteins and are surrounded in some species of viruses by a lipid membrane, which is referred to as an envelope; the particles contain only one kind of nucleic acid, either DNA or RNA. Viruses do not reproduce by division, such as bacteria, yeasts or other cells, but they replicate in the living cells that they infect.
In them, they develop their genomic activity and produce the components from which they are made. They encode neither their own protein synthesis machinery ribosomes nor energy-generating metabolic pathways. Therefore, viruses are intracellular parasites. They are able to re-route and modify the course of cellular processes for the optimal execution of their own reproduction. Besides the genetic information encoding their structural components, they additionally possess genes that code for several regulatory active proteins such as transactivators and enzymes e.
Molecular biological characteristics of the different virus families, including some typical prototypes. Yellow fever virus, dengue virus, West Nile virus, tick-borne encephalitis virus. Human coronaviruses E and NL63, feline coronavirus, porcine transmissible gastroenteritis virus. Infectious haematopoietic necrosis virus, viral haemorrhagic septicaemia virus.
Herpes simplex viruses, varicella-zoster virus, bovine, equine, porcine, canine, feline and gallid herpesviruses. Epstein-Barr virus, human herpesvirus 8, alcelaphine herpesvirus 1 bovine malignant catarrhal fever virus. It survives, but continuously produces small numbers of viruses and is chronically persistently infected.
It survives and the viral genome remains in a latent state without producing infectious particles. It is immortalized, thus gaining the capability of unlimited cell division, a process that can be associated with malignant transformation into a tumour cell.
Symmetry forms of viral capsids. Picornaviruses, parvoviruses and adenoviruses are examples of viruses with such capsid forms. The three-dimensional structures of the particles of a number of viruses have been resolved by X-ray structural analyses.
Prerequisite is knowledge of the basic composition of the virus, i. In addition, purification of virus particles must be possible and these must be available as a stable highly concentrated virus suspension on the order of several milligrams per millilitre.
Finally, the purified virions or, alternatively, viral capsids, which are produced in cell culture or by genetic engineering, must be able to crystallize.
In some virus types, the capsids are surrounded by a lipid bilayer envelope, which is derived from cellular membrane systems.
Viral and cellular proteins are embedded in the envelope, and are frequently modified into glycoproteins by sugar groups. If such a membrane envelope is present, it renders the virus sensitive to inactivation by solvents and detergents. A tegument layer can be situated between the membrane and the capsid herpesviruses , and contains additional viral protein components.
The exposed proteins and protein domains on the surface of the virus — either in the envelope or in the capsid — are subject to selection pressure by the immune system. Therefore, viruses change by mutation and selection preferentially the amino acid sequences of antibody-binding regions or epitopes, which are responsible for binding neutralizing immunoglobulins.
In some species of viruses, this variability of the surface regions leads to the formation new subtypes. In addition to this continuous change of the surface of exposed regions that is determined by mutation and selection, in some virus types another source of variability is possible by genetic recombination, by which even large nucleic acid regions can be exchanged between different viruses. This can lead to substantial changes in the viruses involved and to the generation of new viral species.
Satellite viruses, or virusoids, are small RNA or DNA molecules that code for one or two proteins with which they are associated. Their replication and spread is dependent on the presence of another virus. Viroids are plant pathogens and consist of a circular RNA about — nucleotides that does not code for proteins and exhibits a complex two-dimensional structure. A central sequence motif is highly conserved and essential for replication of these nucleic acid molecules.
Other regions are variable and may be responsible for virulence. They have RNase activity, and autocatalytically cleave the concatemeric RNA strands that result after replication. Mimiviruses are a family of very large DNA viruses which were discovered by Didier Raoult in the amoeba Acanthamoeba polyphaga only in The DNA genome of mimiviruses comprises 1. Even larger mamaviruses have been discovered in amoebae, which can be infected by parasitic viruses. These significantly smaller viruses sputnikvirus , also known as virophages, can multiply in amoebae only if they are concurrently infected by mamaviruses.
However, sputniks do not use mamaviruses only as a helper virus, but also inhibit their proliferation and morphogenesis, thus making them virtually sick.
In animals and humans, prions always cause fatal neurodegenerative disorders. After its synthesis, the cellular protein PrP C arrives in the cytoplasmic membrane. PrP C is active at the cell surface only for a limited time, and is subsequently degraded in the endosomes.
This process is referred to as prion conversion. PrP Sc proteins cannot be efficiently degraded and accumulate in the cells. The function of PrP C has not been completely resolved. Experiments with knockout mice containing a deletion of the PrP coding genome sequences revealed that PrP C appears to be dispensable for development and survival of the mice. However, without PrP C they cannot develop a prion disease. Human prion diseases include Creutzfeldt-Jakob disease, kuru and variant Creutzfeldt-Jakob disease.
In animals, the most famous representatives are scrapie sheep , bovine spongiform encephalopathy cattle and chronic wasting disease deer. The peculiarity of prion diseases is that they appear in three manifestations: acquired infectious exogenous , sporadic endogenous and genetic endogenous.
Inasmuch as prions are restricted to the central nervous system, their infectious transmission is generally limited. The further subdivision into genera and virus types is largely based on serological criteria and the similarity of genome sequences.
Download reference work entry PDF. Viruses have evolved over longtime period, and have adapted to specific organisms or their cells. Table 2.
Viruses exist in different conditions. They can actively replicate in cells, and produce a great number of progeny viruses. This is known as a replicationally active state. After infection, some virus types can transition into a state of latency by integrating their genetic information into the genome of the host cell, or maintain it as an episome in an extrachromosomal status within infected cells.
Certain viral genes can be transcribed during that time, contributing to the maintenance of latency herpesviruses. In other cases, the expression of the viral genome is completely repressed over long periods of time e. In both cases, cellular processes or external influences can reactivate the latent genomes, leading to a new generation of infectious viruses.
Depending on the virus type, the infection can have different consequences for the host cell: 1. It is destroyed and dies. Depending on the virus type, the nucleic acid is single-stranded or double-stranded, linear, circular or segmented. A single-stranded genome that has the same polarity as the messenger RNA is referred to as a positive or plus strand.
The genome forms a nucleocapsid complex with cellular histones polyomaviruses or viral proteins e. This nucleic acid-protein complex can be surrounded by particular protein structures, the capsids in polyomaviruses, papillomaviruses, adenoviruses and herpesviruses. In some cases such as picornaviruses, flaviviruses, togaviruses and parvoviruses , the nucleic acid interacts directly with the capsids.
In viruses containing an envelope, the capsid layer can be absent as in coronaviruses, rhabdoviruses, paramyxoviruses, orthomyxoviruses, bunyaviruses and arenaviruses. Open image in new window. Capsids are rod-shaped or cubic-spherical protein structures. In some virus types, they consist of multimeric units of only one polypeptide, in other cases they are composed of heteromeric complexes.
The capsid protein subunits can aggregate into discrete subunits or even into so-called capsomeres, i. Rod-shaped capsids have a helical symmetry. The two planes of symmetry, i. By contrast, spherical capsids have an icosahedral structure with a rotational symmetry; an icosahedron consists of 20 equilateral triangles and 12 vertices Fig.
The symmetry axes have the same length: the fivefold symmetry axis is located at the vertices of the icosahedron; the threefold axis passes through the centre of a triangle, the twofold axis passes along the edges.
The taxonomic classification of viruses into different families is done by an international commission of virologists and is continuously adapted to current insights. It is based on the following main criteria: 1.
The site of viral replication within the cell cytoplasm or nucleus. VIIIth report of the international committee on taxonomy of viruses.
Academic, San Diego Google Scholar. Fraenkel-Conrat H The viruses. Catalogue, characterization, and classification. Plenum, New York Google Scholar.
Virus , infectious agent of small size and simple composition that can multiply only in living cells of animals , plants , or bacteria. A virus particle is made up of genetic material housed inside a protein shell, or capsid. The genetic material, or genome, of a virus may consist of single-stranded or double-stranded DNA or RNA and may be linear or circular in form. Most viruses vary in diameter from 20 nanometres nm; 0. The largest viruses measure about nm in diameter and are about —1, nm in length. Shapes of viruses are predominantly of two kinds: rods or filaments , so called because of the linear array of the nucleic acid and the protein subunits, and spheres, which are actually sided icosahedral polygons.
Life history theory posits that the sequence and timing of events in an organism's lifespan are fine-tuned by evolution to maximize the production of viable offspring. In a virus, a life history strategy is largely manifested in its replication mode. Here, we develop a stochastic mathematical model to infer the replication mode shaping the structure and mutation distribution of a poliovirus population in an intact single infected cell. We measure production of RNA and poliovirus particles through the infection cycle, and use these data to infer the parameters of our model.
The replication of poliovirus RNA genomes containing amber mutations was studied to test whether viral proteins provided in trans could rescue the replication of an RNA genome that could not be completely translated itself.
Viruses are noncellular genetic elements that use a living cell for their replication and have an extracellular state. Viruses are ultramicroscopic particles containing nucleic acid surrounded by protein, and in some cases, other macromolecular components such as a membranelike envelope. The virion is metabolically inert and does not grow or carry on respiratory or biosynthetic functions.
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Topics. –Structure. –Classification. –Multiplication. –Cultivation and replication. –Nonviral infectious agent. –Teratogenic/Oncogenic. Viruses in Action!! Human.Heliena B. 17.05.2021 at 21:23
PLoS Pathog 13 4 : e