Unit 9 - Virology PDF

Unit 9: Virology  Study of viruses, complexes of nucleic acids and proteins that have the capacity for replication in animal, plant and bacterial cells To replicate themselves, viruses usurp functions of the host cells on which they are parasites All viruses are...

Unit 9: Virology  Study of viruses, complexes of nucleic acids and proteins that have the capacity for replication in animal, plant and bacterial cells To replicate themselves, viruses usurp functions of the host cells on which they are parasites All viruses are obligate intracellular parasites  Replication inside a host cell leads to the release of virus particles These particles (called virions) have no metabolism and no organelles They carry all the genetic information necessary for the generation of more virus particles after infection of a new host cell Virus sizes range from 20 to 450 nm  Chemical Composition of Viruses Virus is a minute nucleoprotien particle consist of a central core of nucleic acid (DNA or RNA either linear or circular in shape) and protienaceous coat which surround the nucleic acid Protien coat or capsid is made up of many structural identical units called capsomeres Composition, number and structure of viruses are different from each other  Virus Structure Some viruses are also enclosed by an envelope of fat and protein molecules In its infective form, outside the cell, a virus particle is called a virion Each virion contains at least one unique protein synthesized by specific genes in its nucleic acid  Viroids (meaning "viruslike") are disease- causing organisms that contain only nucleic acid and have no structural proteins Other virus like particles called prions are composed primarily of a protein tightly integrated with a small nucleic acid molecule Virus has a "tail" which it attaches to the bacterium surface by means of proteinaceous "pins" Tail contracts and the tail plug penetrates the cell wall and underlying membrane, injecting the viral nucleic acids into the cell  Capsid is the protein shell that encloses the nucleic acid, called the nucleocapsid This shell is composed of protein organized in subunits known as capsomers  Capsid has three functions – It protects the nucleic acid from digestion by enzymes It contains special sites on its surface that allow the virion to attach to a host cell It provides proteins that enable the virion to penetrate the host cell membrane and, in some cases, to inject the infectious nucleic acid into the cell's cytoplasm  CLASSIFICATION Viruses can be classified according to the host cell they infect – Animal viruses Plant viruses Fungal viruses and Bacteriophages (viruses infecting bacteria, which include the most complex viruses)  Most useful and widely used classification system distinguishes viruses according to the type of nucleic acid they use as genetic material – DNA viruses (divided into double-stranded DNA viruses and single-stranded DNA viruses), RNA viruses (divided into positive-sense single- stranded RNA viruses, negative-sense single- stranded RNA viruses and the much less common double-stranded RNA viruses), Reverse transcribing viruses (double-stranded reverse-transcribing DNA viruses and single-stranded reverse-transcribing RNA viruses including retroviruses)  Baltimore Classification Classified viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), sense, and method of replication Viruses can be placed in one of the seven following groups - I. dsDNA viruses (e.g. Adenoviruses, Herpesviruses, Poxviruses) II. ssDNA viruses (+) sense DNA (e.g. Parvoviruses) III. dsRNA viruses (e.g. Reoviruses) IV. (+)ssRNA viruses (+)sense RNA (e.g. Picornaviruses, Togaviruses) V. (−)ssRNA viruses (−)sense RNA (e.g. Orthomyxoviruses, Rhabdoviruses) VI. ssRNA-RT viruses (+)sense RNA with DNA intermediate in life-cycle (e.g. Retroviruses) VII. dsDNA-RT viruses (e.g. Hepadnaviruses)  DNA Viruses Group I: viruses possess double-stranded DNA Group II: viruses possess single-stranded DNA Examples Virion Capsid Virus Family Nucleic acid type Group (common names) naked/enveloped Symmetry Adenovirus, 1. Adenoviridae Infectious canine Naked Icosahedral ds I hepatitis virus 2. Papillomaviridae Papillomavirus Naked Icosahedral ds circular I Parvovirus B19, 3. Parvoviridae Naked Icosahedral ss II Canine parvovirus Herpes simplex virus, varicella-zoster virus, 4. Herpesviridae Enveloped Icosahedral ds I cytomegalovirus, Epstein-Barr virus Smallpox virus, cow pox virus, sheep pox 5. Poxviridae virus, orf virus, Complex coats Complex ds I monkey pox virus, vaccinia virus 6. Hepadnaviridae Hepatitis B virus Enveloped Icosahedral circular, partially ds VII Polyoma virus; JC virus (progressive 7. Polyomaviridae Naked Icosahedral ds circular I multifocal leukoencephalopathy) 8. Anelloviridae Torque teno virus Naked Icosahedral ss circular II  RNA Viruses Examples (common Capsid Capsid Virus Family Nucleic acid type Group names) naked/enveloped Symmetry 1. Reoviridae Reovirus, Rotavirus Naked Icosahedral ds III Enterovirus, Rhinovirus, Hepatovirus, Cardiovirus, Aphthovirus, Poliovirus, 2. Picornaviridae Naked Icosahedral ss IV Parechovirus, Erbovirus, Kobuvirus, Teschovirus, Coxsackie 3. Caliciviridae Norwalk virus Naked Icosahedral ss IV 4. Togaviridae Rubella virus Enveloped Icosahedral ss IV Lymphocytic 5. Arenaviridae Enveloped Complex ss(-) V choriomeningitis virus Dengue virus, Hepatitis C 6. Flaviviridae Enveloped Icosahedral ss IV virus, Yellow fever virus Influenzavirus A, Influenzavirus B, 7. Orthomyxoviridae Enveloped Helical ss(-) V Influenzavirus C, Isavirus, Thogotovirus Measles virus, Mumps virus, Respiratory 8. Paramyxoviridae syncytial virus, Rinderpest Enveloped Helical ss(-) V virus, Canine distemper virus California encephalitis 9. Bunyaviridae Enveloped Helical ss(-) V virus, Hantavirus 10. Rhabdoviridae Rabies virus Enveloped Helical ss(-) V 11. Filoviridae Ebola virus, Marburg virus Enveloped Helical ss(-) V 12. Coronaviridae Corona virus Enveloped Helical ss IV 13. Astroviridae Astrovirus Naked Icosahedral ss IV 14. Bornaviridae Borna disease virus Enveloped Helical ss(-) V Arterivirus, Equine 15. Arteriviridae Enveloped Icosahedral ss IV Arteritis Virus 16. Hepeviridae Hepatitis E virus Naked Icosahedral ss IV  Reverse Transcribing Viruses Group VI: viruses possess single-stranded RNA genomes and replicate using reverse transcriptase Retroviruses are included in this group, of which HIV is a member Group VII: viruses possess double-stranded DNA genomes and replicate using reverse transcriptase Hepatitis B virus can be found in this group  Structure, Function, and Uses A virus is a small parasite that cannot reproduce by itself Once it infects a susceptible cell, however, a virus can direct the cell machinery to produce more viruses Entire infectious virus particle, called a virion, consists of the nucleic acid and an outer shell of protein  Simplest viruses contain only enough RNA or DNA to encode four proteins Most complex can encode 100 – 200 proteins In many cases, DNA viruses utilize cellular enzymes for synthesis of their DNA genomes and mRNAs All viruses utilize normal cellular ribosomes, tRNAs, and translation factors for synthesis of their proteins  Most viruses utilize the cellular machinery for macromolecular synthesis during the late phase of infection, directing it to synthesize large amounts of a small number of viral mRNAs and proteins instead of the thousands of normal cellular macromolecules  For more details and all examinations, please go through this reference - 1. Medical Microbiology (7th edition) Authors - Murray, Rosenthal, Pfaller Page No.: 393-398 THANK YOU