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Unformatted text preview: VIRUSES VIRUSES Chapter 13 VIRUSES VIRUSES Latin for poison Can infect bacteria, fungi, plants & animals 1892 ­ Iwanoski ­ tobacco mosaic virus (TMV) Filtrate of diseased plant contained infectious material Filtrate did not contain bacteria ∴ smaller than bacteria 1898 ­ Beijernick ­ continued TMV experiments Last plant contaminated as severely as the first Infectious material is replicating inside plant GENERAL CHARACTERISTICS GENERAL CHARACTERISTICS Obligate intracellular parasites Can not replicate outside of host Very small (<0.3 µ m), filterable Requires electron microscope to see Contain one type of nucleic acid, either DNA or RNA Nucleic acid surrounded by a protein coat called the capsid Have very little enzymes of their own Use host cell enzymes for replication and packaging Molecular, nonliving entities (acellular that is not cells) May have lipid envelopes Susceptible to disinfectants Many have a range of host cells able to infect Due to specific receptors on cell surfaces Virus Sizes Figure 13.1 STRUCTURE STRUCTURE VIRION = complete infectious particle Nucleic acid surrounded by capsid +/­ lipid envelope SIZE: 20 ­ 1000 nm Pox virus = largest virus GENOME: contain a single type of nucleic acid Contain either DNA or RNA NEVER BOTH RNA and DNA Amount varies from a few genes to ~250 genes CLASSIFICATION 1. Based on nucleic acid content then 2. Capsid 3. Envelope Some categories of viruses Some categories of viruses DNA or RNA 1. NUCLEIC ACID 1. NUCLEIC ACID ssRNA genomes: have a polarity (+ or ­) Can be single stranded, double stranded, linear or circular; continuous or segmented + RNA: can go in the cell and directly function as mRNA ­ RNA: must first be transcribed into a +RNA strand which then serves as the mRNA Retrovirus or non­retrovirus Retrovirus: RNA transcribed into DNA inside the host False­color electron micrographs False­color electron micrographs of representative RNA viruses 2. CAPSID 2. CAPSID CAPSID: protein coat surrounding the nucleic acid Each capsid is composed of units called CAPSOMERS CAPSID SHAPE or SYMMETRY HELICAL POLYHEDRAL COMPLEX May be of one protein of several different proteins Cold virus Cold virus HELICAL SYMMETRY HELICAL SYMMETRY Look like a long rod Many copies of the same protein wrapped in a helix Nucleic acid surrounded by a hollow, helical, cylindrical capsid Tobacco mosaic virus (TMV) Rabies virus Morphology of a Helical Virus Figure 13.4 POLYHEDRAL SYMMETRY POLYHEDRAL SYMMETRY Many sides ICOSAHEDRAL Regular polygon with 20 sides with 12 corners Each side = equilateral triangle Polio virus Adenovirus Herpes virus Virion Structure Virion Structure Nucleic acid Capsid DNA or RNA Capsomeres Envelope Spikes Figure 13.2a Morphology of a Polyhedral Virus Figure 13.2 COMPLEX SYMMETRY • Complicated structures • Often bacteriophages • Have a capsid, a tail and a sheath • Pox virus ­ only one • No capsid but several coats surrounding the nucleic acid Figure 13.5 - Overview 3. ENVELOPE 3. ENVELOPE Surrounds the nucleocapsid of some animal viruses Made up of host cell lipids and viral proteins Enveloped viruses May contain viral glycoproteins called PEPLOMERS or SPIKES Lipid bilayer membrane surrounding the capsid May help virus get away from the cell Keeps segments of a segmented genome together Antigenic – host immune system can mount an May be used for attachment to host cell Morphology of an Enveloped Virus Figure 13.3 OTHER STRUCTURES • ENZYMES: some viruses carry them with them – Retroviruses – Pox viruses • PROTEINS: to neutralize the negative charge of the nucleic acids TAXONOMY of VIRUSES TAXONOMY of VIRUSES AT FIRST: classified by host Plant viruses Animal viruses Bacteriophage THEN: classified by disease caused Respiratory diseases Enteric viruses NOW: classified according to structure NUCLEIC ACID type REPLICATION STRATEGY MORPHOLOGY VIRAL TAXONOMY VIRAL TAXONOMY “FAMILY” ­ viridae “SPECIES” ­ group of viruses Have same nucleic acid and infect same host cells Table 13.2 (1 of 4) Table 13.2 (2 of 4) Table 13.2 (3 of 4) Table 13.2 (4 of 4) DNA FAMILIES of VIRUSES #1 DNA FAMILIES of VIRUSES #1 1. PARVOVIRIDAE ssDNA, icosahedral, naked 2. ADENOVIRIDAE Gastroenteritis, fetal death, fifth disease Human parvovirus, B19 dsDNA, icosahedral, naked Respiratory diseases DNA FAMILIES of VIRUSES #2 DNA FAMILIES of VIRUSES #2 3. PAPOVAVIRIDAE dsDNA, icosahedral, naked PA ­ papilloma = warts PO ­ polyoma = tumors VA ­ vacuolating = causes vacuoles in host cell HPV: human papilloma virus – more than 60 types, benign warts, plantar warts, laryngeal warts, and cervical warts. DNA FAMILIES of VIRUSES #3 DNA FAMILIES of VIRUSES #3 4. POXVIRIDAE dsDNA, complex, enveloped Largest of all of the viruses 5. HEPADNAVIRIDAE Smallpox virus – variola, only disease eradicated by vaccination efforts. Cowpox virus ­ vaccinia Circular dsDNA, icosahedral, enveloped Unusual replication cycle Hepatitis B virus DNA FAMILIES of VIRUSES #4 DNA FAMILIES of VIRUSES #4 6. HERPESVIRIDAE dsDNA, icosahedral, enveloped Very large viruses, become latent Ubiquitous in animals Epstein­Barr virus (EBV) ­ infectious mononucleosis Cytomegalovirus (CMV) Varicella­zoster virus (VZV) ­ chickenpox & shingles Herpes simplex viruses (HHV) Large number of these have been discovered RNA FAMILIES of VIRUSES #1 RNA FAMILIES of VIRUSES #1 1. PICORNAVIRIDAE +ve RNA, icosahedral, naked PICO = very small Polio virus – polio Coxsackieviruses Enteroviruses (“intestine”) Hepatitis A virus (HAV) ­ acute hepatitis Rhinoviruses ­ common cold RNA FAMILIES of VIRUSES #2 RNA FAMILIES of VIRUSES #2 2. TOGAVIRIDAE +ve RNA, icosahedral, enveloped TOGA = cloaked or envleoped Transmitted by arthropods (mosquitoes) Arboviruses Encephalitis viruses Rubiviruses ­ respiratory transmission Rubella = German measles +ve RNA, icosahedral Common cold, SARS 3. CORONAVIRIDAE 4. FLAVIVIRIDAE RNA FAMILIES of VIRUSES #3 RNA FAMILIES of VIRUSES #3 +ve RNA, polyhedral, enveloped Flaviviruses – includes Yellow fever Hepatitis C virus (HCV) 5. RETROVIRIDAE Diploid +ve RNA*, icosahedral, enveloped *Only diploid virus ­ 2 identical copies of ssRNA Each strand ~10kb Reverse transcriptase ­ makes a DNA intermediate to insert into host chromosome RNA tumor viruses (oncoviruses) Leukemia viruses (HTLV­1) Immunodeficiency viruses (HIV) RNA FAMILIES of VIRUSES #4 RNA FAMILIES of VIRUSES #4 6. PARAMYXOVIRIDAE (“near” + “mucus”) ­ve RNA, helical, enveloped 7. RHABDOVIRIDAE (“rod”) Paramyxovirus ­ parainfluenza Mumps virus Rubeola virus – measles ­ve RNA, helical, enveloped Shaped like bullets with spiked envelope Rod with one flat & one curved end Rabies virus RNA FAMILIES of VIRUSES #5 RNA FAMILIES of VIRUSES #5 8. ORTHOMYXOVIRIDAE (“straight”) Segmented –ve RNA, helical, enveloped Influenzaviruses Influenza virus A ­ 1° virus causing disease Can infect swine, birds and horses Influenza virus B Influenza virus C INFLUENZA VIRUS INFLUENZA VIRUS Orthomyxovirus: segmented ssRNA genome Has 8 nucleocapsids Each contains one ssRNA segment Envelope contains 2 different PEPLOMERS H = hemagglutinin peplomers Attachment to host cell N = neuraminidase peplomers Helps virus get away from the cell 9. FILOVIRIDAE RNA FAMILIES of VIRUSES #5 RNA FAMILIES of VIRUSES #5 ­ve RNA, helical, envleoped Long, thread­like Ebola virus ­ hemorrhagic fever ­ FATAL Marburg agent ­ hemorrhagic fever 10. BUNYVIRIDAE Segmented (3) –ve RNA, helical, enveloped Associated with rodents Hanta virus ­ hemorrhagic fever and hantavirus pulmonary syndrome (HPS) RNA FAMILIES of VIRUSES #6 RNA FAMILIES of VIRUSES #6 11. ARENAVIRIDAE Segmented (2) ­ve RNA, enveloped • Carried by rodents • Causes respiratory diseases • Arenaviruses – Lassa fever 12. REOVIRIDAE Segmented dsRNA*, icosahedral, naked *dsRNA = exception R = Respiratory diseases E = Enteric diseases O = Orphan ­ not associated with any disease Rota virus – severe diarrhea in young children Colorado tick fever virus CULTIVATION OF VIRUSES CULTIVATION OF VIRUSES Obligate intracellular parasites ∴ depends on the virus and it’s host cell type BACTERIOPHAGE PLANT VIRUSES Bacteria: suspension or solid media to form plaques Whole plant or plant cell culture ANIMAL VIRUSES Whole animal, embryonated eggs or animal cell culture Figure 13.6 Growing Viruses Animal viruses may be grown in living animals or in embryonated eggs Figure 13.7 CELL CULTURE CELL CULTURE Most common way of culturing animal viruses Three cell lines can be used Primary cell lines : derived from tissues by enzymes , generally die after a couple of generations Diploid cell lines: derived from human embryos, multiply for about 50 – 100 generations and then die. Continuous cell lines: Immortal lines are derived from transformed or cancerous cells. Can multiply indefinitely in culture and are immortal. These are Growing Viruses Animal and plant viruses may be grown in cell culture Continuous cell lines may be maintained indefinitely Figure 13.8 Figure 13.9a Figure 13.9b VIRAL MULTIPLICATION VIRAL MULTIPLICATION Viral genes code for capsid (structural) proteins and some enzymes needed for replication Virus uses host cell enzymes needed for protein synthesis, energy production & ribosomes Viruses ONLY replicate inside a cell Viruses invade host cell and controls metabolic and replicative “machinery” Cell now primarily makes viral proteins and VIRUS ­ HOST INTERACTIONS VIRUS ­ HOST INTERACTIONS 1. LYTIC CYCLE Host cell is lysed after viral replication T bacteriophage 4 LYSOGENIC or LATENT CYCLE Host cell stays alive after replication Viral genome is integrated into host’s genome Lambda bacteriophage TRANSFORMING INTERACTION Viral genome is integrated into host’s genome Integration alters host cell growth & morphology Cancer­like cell 2. 3. Lytic Cycle of a T­Even Lytic Cycle of a T­Even Bacteriophage 1 2 3 Figure 13.11 Lytic Cycle of a T­Even Bacteriophage 4 Figure 13.11 The Lysogenic Cycle Figure 13.12 THE VIRAL REPLICATIVE THE VIRAL REPLICATIVE CYCLE OF ANIMAL VIRUSES 1. 2. 3. 4. 5. 6. Attachment or absorption Penetration or Entry Uncoating Biosynthesis Maturation or assembly Release All the steps are similar for DNA and RNA viruses except for nucleic acid synthesis. Nucleic acid synthesis is different A Viral One­Step Growth Curve Figure 13.10 1. ATTACHMENT OR ABSORPTION 1. Viral particle attaches to host cell via specific receptors on on the surface of the host cell Some proteins and glycoproteins on host cell membranes can function as receptors for some viruses Viral particles have attachments sites made of spikes or fibers that bind these receptors Naked viruses ­ nucleocapsid Enveloped viruses – envelope on spikes Viral recognition of an animal host 2. PENETRATION 2. PENETRATION Entry of viral nucleic acid into host cell Can be achieved by pinocytosis Naked and enveloped can enter this way Can be achieved via fusion of envelope with host cell plasma membrane Then viral nucleocapsid enters the cytoplasm 3. UNCOATING 3. UNCOATING Uncoating ­ separation of nucleic acid from the protein capsid coat Can be due to host lysosomal enzymes or configurational changes in the capsid By pinocytosis Attachment, Penetration, Uncoating Figure 13.14a 4. SYNTHESIS 4. SYNTHESIS Virus takes over host’s biosynthetic “machinery” Period of time when new viral nucleic acid molecules, capsid proteins and other viral components are produced Synthesis of host DNA & proteins stops All replicate in nucleus except Poxviruses, uses DNA dependent DNA polymerase All replicate in cytoplasms except retroviruses and orthomyxoviruses, uses RNA dependent RNA DNA Viruses RNA Viruses Multiplication of DNA Virus Figure 13.15 Sense Strand (+ Strand) RNA Virus Figure 13.17a Antisense Strand (– Strand) RNA Virus Figure 13.17b 5. MATURATION or 5. ASSEMBLY Assembly of capsomers into capsid Newly synthesized viral DNA is inserted into the newly made capsids to form new virions 6. RELEASE 6. RELEASE Naked viruses usually released from host cell by lysing the cell Host cell dies 50 ­ 200 virions are released Enveloped viruses usually bud out of the cell As they leave the cell, they acquire their envelope from the host cell plasma membrane Budding of an Enveloped Virus Figure 13.20 Budding of an Enveloped Virus Figure 13.20 RETROVIRUSES RETROVIRUSES GENOME = RNA REPLICATE through DNA intermediate Before integrates into host genome must make a dsDNA copy of the viral RNA genome Utilize an enzyme = REVERSE TRANSCRIPTASE RNA dependent DNA polymerase RNA ­­­­­­> DNA ­­­­> integrates RETROVIRAL REPLICATION RETROVIRAL REPLICATION Attachment to host by viral peplomers Penetration by fusion with host’s membrane Biosynthesis RNA ­­­> dsDNA DNA enters nucleus ­­­> integrates DNA remains in the host’s genome, PROVIRUS Integrated DNA ­­­> RNA ­­­> proteins Assembly Release ­­­> budding RNA ­­­> DNA ­­­> RNA Figure 13.19 VIRAL LATENCY VIRAL LATENCY Many viruses can remain latent in the host cell Period of time when there is no replication of the viral genome No apparent disease occurs during this time Virus can be activated by different stimuli Now causes disease HSV­1 Remains latent in nerve cells Stimulation cold sores or blisters Chicken pox virus Remains latent but after activation shingles VIRUSES & CANCER VIRUSES & CANCER HTLV ­ I and HTLV ­ 2 Cause cancer in humans Human T­cell Lymphotrophic Virus Both retroviruses cause leukemia Other viruses cause cancer in animals EBV : Epstein­Barr virus associated with African Burkitt’s Lymphoma and nasopharyngeal carcinoma HBV: Hepatitis B virus associated with hepatocellular carcinoma ( liver cancer) ONCOGENES ONCOGENES ONCO = tumor ONCOGENE = tumor gene Originally derived from normal cellular genes ONCOGENIC VIRUS = a virus that promotes tumor formation Oncogenic viruses integrate in the host chromosome causing transformation of cells PROteinaceous INfectious particles First identified by Stanley Prusiner in 1982 DISEASES: kuru, Creuztfeld­Jacob disease, bovine spongiform encephalopathy (Mad Cow disease), Scrapie (found in sheep) All are neurological diseases, spongy appearance of the brain All are fatal (slow viral infection) INFECTIVE PARTICLE = PROTEIN ONLY IS INFECTIOUS Goes to the brain Converts the normal protein (gene on chr 20) to an infectious abnormal protein PRIONS PRIONS How a Protein Can Be Infectious Figure 13.22 Figure 13.22, step 1 Figure 13.22, step 2 Figure 13.22, step 3 Figure 13.22, step 4 Figure 13.22, step 5 Figure 13.22, step 6 Figure 13.22, step 7 Figure 13.22, step 8 VIROIDS VIROIDS Cause some plant diseases Short pieces of naked RNA Is a closed, folded molecule that protects it from cellular enzymes RNA does not code for any proteins ...
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