2.Structure and Classification of Viruses.pptx
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Structure and Classification of Viruses DR GM ADEWUYI Viruses are small obligate intracellular parasites, contain either a RNA or DNA genome surrounded by a protective, virus-coded protein coat. Viruses may be viewed as mobile genetic elements, most probably of cellular origin and characterize...
Structure and Classification of Viruses DR GM ADEWUYI Viruses are small obligate intracellular parasites, contain either a RNA or DNA genome surrounded by a protective, virus-coded protein coat. Viruses may be viewed as mobile genetic elements, most probably of cellular origin and characterized by a long co-evolution of virus and host. Viruses are inert outside the host cell. Small viruses, e.g., polio and tobacco mosaic virus, can even be crystallized. Viruses are unable to generate energy. As obligate intracellular parasites, during replication, they fully depend on the complicated biochemical machinery of eukaryotic or prokaryotic cells. For propagation viruses depend on specialized host cells supplying the complex metabolic and biosynthetic machinery of eukaryotic or prokaryotic cells. A complete virus particle is called a virion. The main function of the virion is to deliver its DNA or RNA genome into the host cell so that the genome can be expressed (transcribed and translated) by the host cell. The viral genome is packaged inside a symmetric protein capsid. The nucleic acid-associated protein, called nucleoprotein, together with the genome, forms the nucleocapsid. In enveloped viruses, the nucleocapsid is surrounded by a lipid bilayer derived from the modified host cell membrane and studded with an outer layer of virus envelope glycoproteins. the genome codes for only a few structural proteins (besides non-structural regulatory proteins involved in virus replication). Capsids are formed as single or double protein shells and consist of only one or a few structural protein species. Therefore, multiple protein copies must self assemble to form the continuous three-dimensional capsid structure. Self assembly of virus capsids follows two basic patterns: ◦helical symmetry, ◦icosahedral symmetry, Viral envelopes consist of a lipid bilayer that closely surrounds a shell of virus-encoded membraneassociated proteins. The exterior of the bilayer is studded with viruscoded, glycosylated (trans-) membrane proteins. Therefore, enveloped viruses often exhibit a fringe of glycoprotein spikes or knobs, also called peplomers. In viruses that acquire their envelope by budding through the plasma or another intracellular cell membrane, the lipid composition of the viral envelope closely reflects that of the particular host membrane. • Enveloped viruses obtain their envelope by budding through a host cell membrane •In some cases, the virus buds through the plasma membrane but in other cases the envelope may be derived from internal cell membranes such as those of the Golgi body or the nucleus Classification of Viruses Morphology: Viruses are grouped on the basis of ◦ size and shape, ◦ chemical composition ◦ structure of the genome, and ◦ mode of replication. Helical morphology is seen in nucleocapsids of many filamentous and pleomorphic viruses. Helical nucleocapsids consist of a helical array of capsid proteins (protomers) wrapped around a helical filament of nucleic acid. Classification of Viruses Icosahedral morphology is characteristic of the nucleocapsids of many “spherical” viruses. The number and arrangement of the capsomeres (morphologic subunits of the icosahedron) are useful in identification and classification. Many viruses also have an outer envelope. Classification of Viruses Chemical Composition and Mode of Replication: The genome of a virus may consist of DNA or RNA, ◦ may be single stranded (ss) or double stranded (ds), ◦ linear or circular. The entire genome may occupy either one nucleic acid molecule (monopartite genome) or several nucleic acid segments (multipartite genome). The different types of genome necessitate different replication strategies. Criteria for classification and nomenclature of viruses physical data, genome structure and mode of replication the chemical composition and configuration of the nucleic acid, whether the genome is monopartite or multipartite. The genomic RNA strand of single-stranded RNA viruses is called ◦ sense (positive sense, plus sense) in orientation if it can serve as mRNA, and ◦ antisense (negative sense, minus sense) if a complementary strand synthesized by a viral RNA transcriptase serves as mRNA. the site of capsid assembly and, in enveloped viruses, the site of envelopment. CLASSIFICATION OF VIRUSES The parameters for classification are; Pathogenicity Ecological Physicochemical CLASSIFICATION BASED ON PATHOGENICITY Viruses affecting the same tissue are grouped together eg Viruses affecting the respiratory tract- Influenza, parainfluenza, rhinoviruses Viruses affecting the gastrointestinal tractadenoviruses, rotavirus, norovirus, corona virus Viruses affecting the nervous systemEnteroviruses, mumps, Herpes simplex virus Viruses affecting the genitourinary system CLASSIFICATION BASED ON ECOLOGICAL PROPERTIES Ecological features include involvement of vectors or vertebrate reservoirs in their transmission cycle Arboviruses or athropod borne virusesThese are viruses transmitted by blood sucking athropods and vertebrate hosts Roboviruses or rodent borne virusesViruses with rodent reservoirs e.g the Lassa fever virus CLASSIFICATION BASED ON PHYSICO-CHEMICAL PROPERTIES The most satisfactory criteria. Viruses are classified into families based on their nucleic acid type CLASSIFICATION OF VIRUSES Viruses are separated into major groupings called families on the basis of morphology, genome structure, and replication. Virus families have the suffix –viridae-. Within each family, there are subdivisions, called genera which are usually based on physicochemical or serologic properties. Genus names carry the suffix –virus-. Subfamilies virinae. In 1995, the ICTV had organized more than 4000 animal and plant viruses into 71 families, 11 subfamilies, and 164 genera, with hundreds of viruses still unassigned. Currently 24 families contain viruses that infect humans and animals. CLASSIFICATION OF VIRUSES According to the type of nucleic acid viruses are classified into DNA and RNA viruses. DNA VIRUSES A- Parvoviruses B- Polyomaviuses – Formerly part of Papovaviridae family before it splits into 2 families. C- Papillomaviruses - Formerly part of Papovaviridae family before it splits into 2 families. D- Adenoviruses E- Herpesviruses F- Poxviruses G- Hepadnaviruses RNA VIRUSES Picornaviruses Reoviruses Bornavirus Arboviruses Filoviruses Togaviruses Coronaviruses Bunyaviruses Caliciviruses Orthomyxoviruses Retroviruses Paramyxoviruses Astrovirses Rhabdoviruses Flaviviruses Arenaviruses DNA VIRUSES Family Example Nucleic Envelope Capsid acid Parvoviridae Parvovirus Papoviridae B19 Hepadnavirida symmetry NO Icosahedral ssDNA NO Icosahedral HPV dsDNA YES Icosahedral e Hepatitis B dsDNA NO Icosahedral Adenoviridae Adenovirus dsDNA YES Icosahedral Herpesviridae Herpes dsDNA YES Complex Poxviridae simplex dsDNA RNA VIRUSES FAMILY EXAMPLE NUCLEIC ACID ENVELOPE Capsid symmetry Picornaviridae Polio virus SS (+) RNA NO Icosahedral Caliciviridae Norwalk Virus SS (+) RNA NO Icosahedral Astrovaridae Astrovirus SS (+) RNA NO Icosahedral Togaviridae Rubella SS (+) RNA YES Icosahedral Flaviridae Yellow fever virus SS (+) RNA YES Unknown Coronaviridae Coronavirus SS (+) RNA YES Helical Rhabdoviridae Rhabies Virus SS (-) RNA YES Helical Paramyxovirid Measles virus SS (-) RNA YES Helical NAMING YOUR VIRUSES • Family names end in viridae. • Genus names end in virus • Viral species: A group of viruses sharing the same genetic information and ecological niche (host) • Common names are used for species • Subspecies are designated by a number Example: Herpesviridae ◦ Herpesvirus ◦ Human Herpesvirus Example: Retroviridae ◦ Lentivirus ◦ Human immunodeficiency virus ◦ HIV-1, HIV-2 THANK YOU
