Virology Lecture 1 PDF

Lecture 1 Introduction of virology  Virology is the 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 use up functions of the host cells on which they are parasites. The...

Lecture 1 Introduction of virology  Virology is the 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 use up functions of the host cells on which they are parasites. The viral parasite causes changes in the cell, particularly its antigenicity; moreover, directing the host cell's metabolism to the production of new virus particles may cause cellular death. Virally-induced cell death, changes in antigenicity and the response of the host to the presence of the virus leads to the manifestations of viral disease.  The word virus was used in medical circles as early as 1790 before the germ theory has been formulated; it then simply meant “poison.” Characteristics of a virus: 1. They are small, usually 0.01 – 0.3 µ in size, retaining infectivity after passage through filters, able to hold back bacteria. 2. They are totally dependent upon living cell (obligate intracellular parasitism), either eukaryotic or prokaryotic, for replication and existence. Some viruses do possess complex enzymes of their own such as RNA or DNA polymerases but they cannot amplify and reproduce the information in their genomes without assistance. 3. They possess only one species of nucleic acid, either DNA or RNA. 4. They are acellular (not cellular). Viruses can reproduce only by using the cellular machinery of other organisms History of virus Virus size  Function of capsid: 1. Protection of Nucleic Acid 2. Transport Nucleic Acid From Cell to Cell 3. Provides Specificity for Attachment. Viral morphology: the external shape of viral particles  Unconventional Agents There are also the 'unconventional agents' sometimes known as 'unconventional viruses' or 'atypical viruses' - the main kinds which have been studied so far are: 1- VIROIDS 2- PRIONS 1- VIROIDS Viroids contain RNA only Small (less than 400 nucleotides), single stranded, circular RNAs, these are not packaged, do not appear to code for any proteins, and so far have only been shown to be associated with plant disease. However, there are some suggestions that somewhat similar agents may possibly be involved in some human disease.  2. Prions Prions contain protein only They are small, Proteinaceous particles and there is controversy as to whether they contain any nucleic acid  Penetration of animal cells by viruses involves different processes, because animal cells are enclosed not by walls but by a flexible lipoprotein bilayer membrane.  The order of the stages of viral replication that follow the uncoating of the genome varies for different virus classes. For many virus families the third step in the cycle of infection is transcription of the genome of the virus to produce viral mRNA, followed by the fourth step, translation of viral mRNA into proteins. For those viruses in which the genomic nucleic acid is an RNA that can serve as a messenger (i.e., positive-strand RNA viruses), the third step is the translation of the RNA to form viral proteins; some of these newly synthesized viral proteins are enzymes that synthesize nucleic acids (polymerases), which carry out a fourth step, the transcription of more mRNA from the viral genome. For the more complicated DNA viruses, such as adenoviruses and herpesviruses, some regions of the genome synthesize “early” mRNAs, which are translated into polymerases that initiate the transcription of “late” regions of the DNA into mRNAs, which are then translated into structural proteins. Classification of viruses: b. RNA–containing viruses 1. According to the type of nucleic acid present (1) Picornaviruses a. DNA–containing viruses (2) REOviruses (1) Parvoviruses (3) Bunyaviruses (2) HepaDNAviruses (4) Arboviruses (3) Papoviruses (5) Togaviruses (4) Adenoviruses (6) Arenaviruses (5) Herpesviruses (7) Coronaviruses Poxviruses (8) Retroviruses (9) Orthomyxoviruses (10) Paramyxovirusus (11) Rhabdoviruses (12) Lentiviruses 2. According to symptomatology a. Generalized disease – smallpox, vaccinia, measles, rubella, chickenpox, yellow fever, dengue, etc. b. Specific organ disease (1) Nervous system – poliomyelitis, aseptic meningitis, rabies, anthropod–borne encephalitis, mumps, measles, herpes simplex, etc. (2) Respiratory tract – influenza, respiratory syncytial virus pneumonia, bronchiolitis, adenovirus, common cold (3) Skin or mucous membrane – herpes simplex type 1 (oral) and type 2 (genital), molluscum contagiosum , warts, herpangina, herpes zoster (4) Eye – adenovirus conjunctivitis, keratoconjunctivitis and epidemic hemorrhagic conjunctivitis (5) Liver – hepatitis, yellow fever, in newborns – enteroviruses, herpesvirus and rubella virus (6) Salivary glands – mumps and cytomegalovirus (7) Gastrointestinal tract – rotavirus and Norwalk virus. (8) Sexually – transmitted disease – herpes simplex, hepatitis B, papilloma, molluscum contagiosum, retrovirus associated with AIDS and cytomegalovirus. 3. According to shape a. With icosahedral symmetry – all DNA–containing viruses except poxviruses. b. With helical symmetry – single–stranded RNA viruses c. Complex – poxviruses 4. According to anatomic areas in the body a. Dermotropic (skin) – smallpox, measles, chickenpox and herpres simplex b. Pneumotropic (respiratory tract) – common cold, influenza and viral pneumonia c. Neurotropic (central nervous system) – rabies, poliomyelitis and encephalitis d. Viscerotropic (visceral organ) – hepatitis (liver), mumps (salivary glands)