General Properties of Viruses

General properties of viruses: Virus structure, classification and replication Properties of viruses with comparison to prokaryotes Binary fission Both RNA and DNA + + + + Rickettsia + Chlamydia Virus Cellular organization Growth in inanimate media Ribosome Sensitivity to antibiotics Sensitivity to interferon Bacteria + + + + - Mycoplasma + + + + - + + + + + - + + + + + + - - - - - - - + Differences of viruses from other cells: • 1) simple organization • 2) The absence of both DNA and RNA in the same virion • 3) Inability to reproduce independently (Completely dependent on host cell) Infectious agents simpler than viruses Viroids • Circular single-stranded RNA • are found in the nucleus of hosts • mainly causes disease in plants • not yet known how they cause disease symptoms Prions • • cause degenerative disorders of central nervous system: Kuru, Creutsfeld- Jacobs disease, scrapie Properties of Viruses  Heat sensitivity ◦ Relative to time and a function of protein denaturation (enveloped viruses more sensitive)  Lipid solvents ◦ Enveloped viruses generally more sensitive  pH sensitivity ◦ Extremes are destructive  Chemicals ◦ React with amino acids of proteins, but some inactivate DNA or RNA  Radiation and Ultraviolet light  Humidity ◦ Different viruses respond differently 5 Virion Structure Lipid Envelope Nucleic Acid Protein Capsid Virion Associated Polymerase Spike Projections Basic virus structure Terms & Definitions in Virology • Capsid: All viruses have capsids • protein coats that enclose and protect their nucleic acid. • Each capsid is constructed from identical subunits called capsomers made of protein. • Nucleocapsid : The capsid together with the nucleic acid are nucleoscapsid.  • Envelope : lipid bilayer containing viral glycoproteins. The phospholipids in the bilayer are derived from the cell.  Virion: infectious form of virus Viral Structure -Symmetry • Helical Nucleocapsids (Helical symmetry) • Icosahedral Nucleocapsids (Cubic symmetry) • Complex Nucleocapsids Viral Structure Symmetry • Icosahedral Constructed of 20 equilateral triangular faces e.g. Herpes Simplex virus • Helical Tubular construction with the subunits arranged around the nucleic acid in a coil e.g. rabies virus 10 Viral Structure (No symmetry recognized) • Complex Not fully understood e.g. smallpox virus 11 Structure of Virus Genome DNA Nucleic Acid RNA Single Stranded Double Stranded Double Stranded Single Stranded Classification of Viruses • • • • • Order ( - virales) Family ( - viridae) Sub-family ( -virinae) Genera ( - virus) Common names Classification of Viruses • • • • Virus family names – viridae (Poxviridae) Subfamily – virinae (Chloradopoxvirinae) Genus – (+ species) – virus (Poxviruses) Several genera within a subfamily divided based on immunologic features and host specificity – Example: Genus – Orthopoxvirus • Species: – Variola major (small pox) – Vaccinia (cow pox) Classification of Human Viruses "Group" dsDNA ssDNA Retro dsRNA ssRNA (-) ssRNA (+) Family Genome Genome size (kb) Capsid Envelope Poxviridae Herpesviridae Adenoviridae Polyomaviridae Papillomaviridae dsDNA, linear dsDNA, linear dsDNA, linear dsDNA, circular dsDNA, circular 130 to 375 125 to 240 26 to 45 5 7 to 8 Ovoid Icosahedral Icosahedral Icosahedral Icosahedral Yes Yes No No No Anellovirus Parvoviradae ssDNA circular ssDNA, linear, (- or +/-) 3 to 4 5 Isometric Icosahedral No No Hepadnaviridae Retroviridae dsDNA (partial), circular ssRNA (+), diploid 3 to 4 7 to 13 Icosahedral Spherical, rod or cone shaped Yes Yes Reoviridae dsRNA, segmented 19 to 32 Icosahedral No Rhabdoviridae Filoviridae Paramyxoviridae Orthomyxoviridae Bunyaviridae Arenaviridae Deltavirus ssRNA (-) ssRNA (-) ssRNA (-) ssRNA (-), segmented ssRNA (-, ambi), segmented ssRNA (-, ambi), segmented ssRNA (-) circular 11 to 15 19 10 to 15 10 to 13.6 11 to 19 11 2 Helical Helical Helical Helical Helical Circular, nucleosomal Spherical Yes Yes Yes Yes Yes Yes Yes Picornaviridae Calciviridae Hepevirus Astroviridae Coronaviridae Flaviviridae Togaviridae ssRNA (+) ssRNA (+) ssRNA (+) ssRNA (+) ssRNA (+) ssRNA (+) ssRNA (+) 7 to 9 7 to 8 7 6 to 7 28 to 31 10 to 12 11 to 12 Icosahedral Icosahedral Icosahedral Isometric Helical Spherical Icosahedral No No No No Yes Yes Yes Major diseases caused by human viruses "Group" dsDNA ssDNA Retro dsRNA ssRNA (-) ssRNA (+) Family Human pathogens (disease) Poxviridae Herpesviridae Adenoviridae Polyomaviridae Papillomaviridae Variola (smallpox); Orf (pustular dermatitis); Molluscum contagiosum (pustular dermatitis) Herpes simplex 1,2 (oral, genital herpes); Varicella-zoster (chickenpox); Epstein-Barr (mononucleosis); Cytomegalovirus (neonatal abnormalities); HHV6 (roseola); HHV8 (Kaposi's sarcoma) Adenovirus (respiratory infection, conjunctivitis) Polyomavirus (benign kidney infection, respiratory disease, leukoencephalopathy) Papillomavirus (warts, genital carcinoma) Anellovirus Parvoviradae Unknown B-19 (fifth disease, fetal death) Hepadnaviridae Retroviridae Hepatitis B ("serum" hepatitis) HIV (aids); HTLV (leukemia) Reoviridae Rotavirus (infantile gastroenteritis) Rhabdoviridae Filoviridae Paramyxoviridae Orthomyxoviridae Bunyaviridae Arenaviridae Deltavirus Rabies virus (rabies) Ebola virus (ebola) Parainfluenza virus (respiratory infection); Mumps virus (mumps); Respiratory syncytial virus (respiratory infection); Measles virus (measles) Influenza virus (influenza) Hantaan virus (hemorrhagic fever with renal syndrome) Lassa fever virus (hemorrhagic fever) Hepatitis D (fulminant acute hepatitis) Picornaviridae Calciviridae Hepevirus Astroviridae Coronaviridae Flaviviridae Togaviridae Poliovirus (polio), rhinovirus (URI), Hepatitis A ("infectious" hepatitis) Norwalk (gastroenteritis) Hepatitis E (acute hepatitis) Astrovirus (gastroenteritis) Coronavirus (respiratory infection) Yellow fever virus (yellow fever); Hepatitis C (hepatitis) Eastern Equine encephalitis virus (encephalitis); Rubella virus (rubella) Viral Multiplication (Viral Life Cycle) BASIC STEPS IN VIRAL LIFE CYCLE • Attachment: Viruses attach to cell membrane • Penetration by endocytosis or fusion • Uncoating by viral or host enzymes • Biosynthesis: Production of nucleic acid and proteins • Maturation: Nucleic acid and capsid proteins assemble • Release by budding (enveloped viruses) Virus recognition, attachment, and entry •Attachment: •Specific viral receptor •Co-receptor •Entry: •Receptor-mediated endocytosis •Fusion of the viral membrane at the cell surface Attachment This is the first step in viral replication. Surface proteins of the virus interact with specific receptors on the target cell surface. 23 RECEPTOR VIRUS ICAM-1 Rhino CD4 HIV acetylcholine rabies EGF vaccinia CR2/CD21 Epstein-Barr HVEM herpes Sialic acid Influenza, reo, corona PENETRATION • Enveloped viruses (e.g., HIV, influenza virus) • penetrate cells through fusion of the viral envelope with the host cell membrane. • • Non-enveloped viruses penetrate cells by receptor mediated endocytosis of the virion in clathrin coated pits with accumulation of viruses in cytoplasmic vesicles. PENETRATION herpesviruses, paramyxoviruses, HIV 26 UNCOATING • The viral capsid is removed and degraded by viral enzymes or host enzymes • This process makes the nucleic acid available for transcription to permit multiplication of the virus. 27 Transcription and Translation – After the viral genome has been uncoated, transcription or translation of the viral genome is initiated. – It is this stage of viral replication that differs greatly between DNA and RNA viruses – – de novo synthesis: New virion synthesis ASSEMBLY AND MATURATION • Synthesis and Assembly of Viral Capsids • After de novo synthesis of viral genome and proteins, viral proteins are packaged with newly replicated viral genome into new virions that are ready for release from the host cell. • This process can also be referred to as maturation. RELEASE • Exit/Release from host cells – Non Enveloped virions • Usually released following lysis of host cell – Enveloped virions • Most enveloped viruses bud of Plasma membrane