BIO 150 - Overview of Viruses & Virology (1).ppt - Overview of Viruses Virology Dr Raji Subramanian General Properties of Viruses Virus genetic

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Unformatted text preview: Overview of Viruses & Virology Dr. Raji Subramanian General Properties of Viruses Virus: genetic element that cannot replicate independently of a living (host) cell (obligate intracellular parasites) Virology: the study of viruses Virus particle: extracellular form of a virus; allows virus to exist outside host and facilitates transmission from one host cell to another Virion: the infectious virus particle; the nucleic acid genome surrounded by a protein coat and, in some cases, other layers of material Viruses Intracellular parasites that utilize the host cell machinery to multiply and synthesize their proteins Contain a single type of nucleic acid (DNA or RNA), either single-stranded or double-stranded Contain a protein coat Some are enclosed by an envelope made up of lipids, proteins & carbohydrates Some viruses have spikes Most viruses infect only specific types of cells in one host Host range is determined by specific host attachment sites and cellular factors General Properties of Viruses Viral Hosts and Taxonomy Viruses can be classified on the basis of the hosts they infect • Bacterial viruses (bacteriophages) • Animal viruses • Plant viruses Viral Structure Contain either DNA or RNA, never both Nucleic acid Capsid may be single-stranded or double-stranded may be linear or circular may exist as separate molecules Protein coat surrounding the nucleic acid Capsomeres - subunits that make up the capsid Nucleocapsid Complex of nucleic acid & capsid Viral Structure Enveloped viruses Capsid covered by an envelope made of lipids, proteins & carbohydrates Some envelopes are covered with carbohydrate -protein complexes called spikes (eg. Influenza virus) Nonenveloped viruses Capsid not covered by an envelope Comparison of Naked and Enveloped Virus Particles General Morphology of Viruses Helical Viruses Shaped like long rods The capsids are hollow cylinders surrounding the nucleic acid Eg. Rabies virus, Ebola virus General Morphology of Viruses Polyhedral Viruses Viruses with capsids that are many-sided Eg. Poliovirus, Adenovirus General Morphology of Viruses Enveloped Viruses Influenzavirus General Morphology of Viruses Complex Viruses Complicated structures Eg. Bacteriophage (viruses that infect bacteria) Viral Taxonomy International Committee on Taxonomy of Viruses (ICTV) 2000 virus species placed in 3 orders, 73 families, 9 subfamilies, & 287 genera Order names end in –virales Family names end in –viridae; subfamily names in –virinae Genus names end in -virus. Species & Subspecies are designated by name & number. Herpesviridae Herpesvirus Human herpes virus HHV-1, HHV-2, HHV-3 Retroviridae Lentivirus Human immunodeficiency virus HIV-1, HIV-2 Baltimore Classification of Viruses Group I – Double-stranded (ds) DNA viruses Genome replication: dsDNA dsDNA mRNA symthesis: dsDNA mRNA Simplexvirus (HSV1 and HSV 2) Varicella-zoster virus (VZV) Chicken pox & Shingles Papillomavirus Human wart virus Human papillomavirus (cervical cancer) Orthopoxvirus (vaccinia and smallpox viruses) Baltimore Classification of Viruses Group II – Single-stranded (ss) DNA viruses Genome replication: ssDNA dsDNA ssDNA mRNA synthesis: ssDNA dsDNA mRNA Eg. Human parvovirus B19 (family Parvoviridae) Group III – Double-stranded RNA viruses Replication: dsRNa ssRNA dsRNA mRNA synthesis: dsRNA mRNA Eg. Rotavirus (family Reoviridae) – most common cause of viral gastroenteritis in infants & children Baltimore Classification of Viruses Group IV - Positive sense (+) single-stranded RNA viruses Positive sense or + strand RNA genomes can act as mRNA and be directly translated into proteins upon entry into the host cell Replication: +RNA -RNA +RNA mRNA synthesis: +RNA = mRNA Eg. Hepatitis A virus & Poliovirus (family Picornaviridae) Hepatitis C virus and West Nile virus (family Flaviviridae) Rubella virus (family Togaviridae) Coronavirus Upper respiratory infections SARS Single-stranded RNA, enveloped virus Baltimore Classification of Viruses Group V - Negative sense (-) single-stranded RNA viruses Negative sense or - strand RNA genomes cannot function as mRNA directly Replication: -RNA +RNA -RNA mRNA synthesis: -RNA mRNA Eg. Rabies virus (family Rhabdoviridae) Ebola virus (family Filoviridae) Measles virus (family Paramyxoviridae) Influenza virus Influenza viruses A and B Influenza C virus Single-stranded RNA, enveloped virus Baltimore Classification of Viruses Group VI - Single-stranded RNA - RT viruses Single-stranded RNA – Reverse Transcription viruses synthesize DNA from RNA during infection using an RNA-dependent DNA polymerase enzyme called reverse transcriptase. Replication: ssRNA dsDNA ssRNA mRNA synthesis: ssRNA dsDNA mRNA Eg. Human Immunodeficiency virus (HIV) Family Retroviridae; Genus Lentivirus Lentivirus (HIV) Oncogenic viruses (leukemia) Retrovirus Structure and Function Retroviruses Process of Replication of a Retrovirus Entrance into the cell Removal of virion envelope at the membrane Reverse transcription of one of the two RNA genomes Integration of retroviral DNA into host genome Transcription of retroviral DNA Assembly and packaging of genomic RNA Budding of enveloped virions; release from cell Baltimore Classification of Viruses Group VII - Double-stranded gapped DNA - RT viruses These viruses consist of one complete but nicked strand and one incomplete or gapped complementary strand. Upon infection, the host cell repairs the gap and seals the nick to generate a covalently, closed, circular viral genome. Replication: gapped dsDNA dsDNA +RNA -DNA gapped dsDNA mRNA synthesis: gapped dsDNA dsDNA mRNA Hepadnavirus (Hepatitis B virus) Growing Viruses Viruses must be grown in living cells. Bacteriophages form plaques on a lawn of bacteria. Quantification of Viruses Plaque assay: analogous to the bacterial colony; one of the most accurate ways to measure virus infectivity Plaques are clear zones that develop on lawns of host cells • Each plaque results from infection by a single virus particle Quantification of Bacterial Virus by Plaque Assay Growing Viruses Animal viruses may be grown in living animals or in embryonated eggs. Growing Viruses Animal and plants viruses may be grown in cell culture. Continuous cell lines may be maintained indefinitely. Virus Identification Cytopathic effects Serological tests Detect antibodies against viruses in a patient. Use antibodies to identify viruses (Western blotting). Modern molecular methods to identify viral nucleic acids Polymerase chain reaction (PCR) Virus Identification Viral Multiplication Viral nucleic acids contain only few genes necessary for viral multiplication. These genes are functional only in the host cell. Enzymes needed for protein synthesis, ribosomes, tRNA & ATP needed for viral multiplication are supplied by the host cell. For multiplication, viruses must invade the host tissue and take over the host’s cellular machinery. Multiplication of Bacteriophages 1 2 3 Viral Attachment and Penetration Bacteriophage T4: virus that infects E. coli Virions attach to cells via tail fibers that interact specifically with polysaccharides on E. coli cell envelope Tail fibers retract and tail core makes contact with E. coli cell wall Lysozyme-like enzyme forms small pore in peptidoglycan Tail sheath contracts and viral DNA passes into cytoplasm Attachment of Bacteriophage T4 to the Cell Wall of E. coli Multiplication of Bacteriophages (Virulent bacteriophage & Temperate bacteriophage) Figure 13.12 Multiplication of Animal Viruses Possible Effects that Animal Viruses May Have on Cells Viruses & Cancer Oncogenic viruses or oncoviruses Viruses capable of causing cancers Viruses are capable of activating a portion of the cellular DNA called oncogenes. Activated oncogenes transform normal cells into cancerous cells. Transformed cells have increased growth, abnormal shape, & exhibit chromosomal abnormalities. The genetic material of oncogenic viruses becomes integrated into the host cell's DNA. Oncogenic Viruses Oncogenic DNA viruses Human Papilloma Virus (HPV) – cervical cancer Hepatitis B virus (HBV) – liver cancer Oncogenic RNA viruses Human T-cell leukemia virus (HTLV-1 & HTLV-2) – leukemia & lymphoma Viroids Viroids: infectious RNA molecules that lack a protein coat Small, circular, ssRNA molecules Smallest known pathogens (246–399 bp) Cause a number of important plant diseases Do not encode proteins; completely dependent on host-encoded enzymes Prions Infectious protein particles Inherited and transmissible by ingestion, transplant, and surgical instruments Creutzfeldt-Jakob disease, fatal familial insomnia, mad cow disease Abnormal prion protein accumulates in brain cells causing brain damage Mechanisms of Prion Misfolding Host cell contains gene (PrnP) that encodes native form of prion protein that is found in healthy animals. Prion misfolding results in neurological symptoms of disease (e.g., resistance to proteases, insolubilty, and aggregation) Prions Prion disease occurs by three distinct mechanisms Infectious prion disease: pathogenic form of prion protein is transmitted between animals or humans Sporadic prion disease: random misfolding of a normal, healthy prion protein in an uninfected individual Inherited prion disease: mutation in prion gene yields a protein that changes more often into disease-causing form ...
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