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Poxviruses -
Poxviruses are enveloped dsDNA viruses that have been found infecting insects, birds and mammals. They are the largest and most complex of the viruses. In contrast to the papovaviruses. adenoviruses and herpesviruses, which are assembled in the nucleus of the host cell, the poxviruses multiply exclusively in the cytoplasm.
They can carry out their life cycle in enucleated cells. Because of their cytoplasmic assembly, the environmental problems of the poxviruses are more like those of RNA viruses than those of the nuclear DNA viruses
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A. Genus Orthopoxvirus
Variola major - causes smallpox in man.
Variola minor (alastrim).
Vaccinia (human) - provides immunity against smallpox.
B. Genus Leporipoxvirus - rabbitpox virus.
C. Genus Avipoxvirrus - fowlpox virus.
D. Genus Capri poxvirus - goat and sheep viruses.
E. Genus Para poxvirus - the orf virus (contagious pustular, dermatitis).
F. Genus Entomopox virus-hosts are arthropods e. g. Coleoptera, Orthoptera, Lepidoptera and Diptera.
G. Ungrouped.
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molluscum contagiosum - benign epidermal tumours in man.Yaba monkeys tumour virus - benign subcutaneous tumours in monkeys and man.
All the poxviruses, except Parapoxvirus, are brick-shaped particles with helical nucleocapsids and dsDNA genomes.Poxviruses fair into two morphological groups, one exemplifed by the vaccinia virus and the other by the orf virus.
The virions of poxviruses are large, oval or brick-shaped bodies. The complex structure consists of a biconcave or dumbell-shaped core, two ellipsoid lateral bodies which fit into concavities, and an envelope. The core consists of a proteinaceous mass which forms a coat enclosing the genome\
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The core wan consists of cylindrical pegs which can be detached from an inner smooth layer. The external surface of the core also has a closely packed array of units. The lateral bodies can be digested with trypsin, releasing the viral core, which is no longer dumbell shaped. A complex envelope surrounds the virus particle.
The external surface of the virion appears beaded in negatively-stained preparations. Randomly-oriented parallel projections composed of 5 non-globular units are found on the surface. These projections give the core layer a mulberry shaped appearance
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The envelope which surrounds the core is a trilaminar membrane, most likely based on a lipid bilayer,
The orf virus is bacillus-shaped, with hemispherical ends, It also has an internal core. The mature virus is enveloped. A long slender strand wound around the virus gives the virus its characteristic criss- cross pattern. Particles showing this pattern appear to be without envelopes.
The dsDN4 genome consists of about 400,000 base pairs (MW over a hundred million) and is the largest of any virus. The large size of the DNA is correlated with the cytoplasmic location of the virus, which makes it necessary to synthesize a number of virus-specific proteins.
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The DNA is sufficient to code for 75 or more proteins. In the vaccinia virus the DNA is circular. Covalent links at either end of double-stranded viral DNA keep both strands together in a circle on denaturation.
About 30 different species of polypeptides have been isolated, of which the core structure contains at least 17. Glycoproteins are part of the internal membrane structure, and not on the outside as in other enveloped viruses. Several enzymes have been demonstrated in the cores. Cores released by disruption with detergents can synthesize mRNAs, and must therefore be containing polymerase and other transcription enzymes.
(1) poly( A) polymerase adds the poly(A) sequence to the 3 end of primer RNA, without the necessity of a template.
(2) mRNA guanylyltransferase and mRNA (guanine-7') methyltrans ferase carry out the coupled reaction of 5' capping and methylation.
(3) mRNA (ribose 2') methyltrans ferase can methylate the purine residue proximal to the cap.
(4) Nucleic-acid-dependent nucleoside triphosphate phosphohydrolases hydrolyse the terminal phosphate from a nucleoside triphosphate. This activity is probably required for the A TP-dependent extrusion of newly made mRNA from the core.
(5) Deoixyribonucleases. Vaccinia virions appear to contain at least three of these enzymes.
(6) Protein kinase can transfer Phosphate residues from A TP to serine threonine residues.
(7) Nicking-closing enzyme removes the strain from the supercoiled closed circular dsDNA by producing and closing nicks in the DNA.
Virus-specified RNA is of two classes, early RNA made before DNA synthesis and late RNA made after. Early RNA specifies early enzymes and also possibly some structural proteins of the virion.The early enzymes include thimidine kinase, a DNA. polymerase, several deoxyribonucleases and polynucleotide ligase.
Some of these enzymes may be 'delayed early'. When viral DNA synthesis is initiated, there is a marked decrease in early enzyme synthesis due to inhibition.
The late phase of the viral growth cycle includes synthesis of viral DNA and of late viral proteins, and maturation of virions.
Late RNAs specify two classes of proteins, the enzymes and the structural proteins of the virions. The half-life period of late mRNAs is very short as compared to that of early mRNAs. DNA synthesis takes place within specific areas, called' factories' or 'viroplasmic matrix’, of the host cell cytoplasm.
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