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PowerPoint® Lecture Slides for MICROBIOLOGY Characterizing and Classifying Viruses, Viroids, and Prions Prof. Manal Baddour Viruses Cause many infections of humans, animals, plants, and bacteria Cannot carry out any metabolic pathway Neither grow nor respond to the environment Cannot reproduce independently Obligate intracellular parasites Characteristics of Viruses Cause most diseases that plague industrialized world Virus – miniscule, acellular, infectious agent having one or several pieces of either DNA or RNA No cytoplasmic membrane, cytosol, organelles (one exception) Have extracellular and intracellular state Characteristics of Viruses Extracellular state Called virion Protein coat (capsid) surrounding nucleic acid Nucleic acid and capsid also called nucleocapsid Some have phospholipid envelope Outermost layer provides protection and recognition sites for host cells Intracellular state Capsid removed Virus exists as nucleic acid How Viruses Are Distinguished Type of genetic material they contain Kinds of cells they attack Size of virus Nature of capsid coat Shape of virus Presence or absence of envelope Viruses depend on favorable hosts in a favorable environment. Virus Host Environment Genetic Material of Viruses Show more variety in nature of their genomes than do cells May be DNA or RNA; never both Primary way scientists categorize and classify viruses Can be dsDNA, ssDNA, dsRNA, ssRNA May be linear and composed of several segments or single and circular Much smaller than genomes of cells T7 E coli Hosts of Viruses Most only infect particular kinds of host’s cells Due to affinity of viral surface proteins or glycoproteins for complementary proteins or glycoproteins on host cell surface May only infect particular kind of cell in host Generalists – infect many kinds of cells in many different hosts Viral Hosts Tobacco mosaic virus Viral Hosts Viral Hosts Influenza Viral Hosts CD4 Sizes of Viruses Capsid Morphology Capsids – protein coats that provide protection for viral nucleic acid and means of attachment to host’s cells Capsid composed of proteinaceous subunits called capsomeres Come capsids composed of single type of capsomere; other composed of multiple types ٌٌHelical Polyhedral (icosahedral), Spherical Viral Shapes Complex Viruses Complex Viruses Complex Viruses Complex Viruses The Viral Envelope Acquired from host cell during viral replication or release; envelope is portion of membrane system of host Composed of phospholipid bilayer and proteins; some proteins are virally-coded glycoproteins (spikes) Envelope’s proteins and glycoproteins often play role in host recognition The Viral Envelope Viral Replication 1- Adsorption * 2- Penetration 3- Uncoating 4- Synthesis 5- Assembly 6- Release Replication of Bacteriophage Bacteriophage lambda Lysogeny Penetration Penetration of nonenveloped virus Penetration by membrane fusion * Penetration by receptor mediated endocytosis + Penetration by receptor mediated endocytosis + Synthesis Depends on the type of nucleic acid (DNA or RNA; single stranded or double stranded) DNA viruses are synthesized in the nucleus except one exception RNA viruses are synthesized in the cytoplasm dsDNA replicate like normal cellular DNA ssDNA viruses form a complementary strand +ssRNA forms a -ssRNA which acts as a template -ssRNA carries RNA dependant RNA polymerase dsRNA each strand acts as a template for its opposite Retroviruses use reverse transcriptase to form a DNA intermediate that serves as template for synthesis of its RNA genome (+ssRNA) Budding of enveloped viruses Budding of enveloped viruses The novel Properties of Viruses Viruses Cells Are inert macromolecules outside of a cell, but become active inside a cell Do not divide or grow Acellular Obligate intracellular parasites Contain either DNA or RNA, never both Genome can be dsDNA, ssDNA, dsRNA or ssRNA Ultramicroscopic in size ranging from 10nm to 300nm Have a proteinaceous capsid around the genome, some have an envelope around the capsid Replicate in an assembly-line manner using the enzymes and organelles of the host Metabolize on their own Divide and grow Cellular Most free living Contain both DNA and RNA Genome is dsDNA 300nm to 12cm in diameter Surrounded by a phospholipid membrane and often a cell wall Self replicating by asexual and/or sexual means Characteristics of Viroids Extremely small, circular pieces of RNA that are infectious and pathogenic in plants Similar to RNA viruses, but lack capsid May appear linear due to H bonding Viroids Figure 13.19 Characteristics of Prions Proteinaceous infectious agents Composed of single protein PrP All mammals contain gene that codes for primary sequence of amino acids in PrP Two stable tertiary structures of PrP Normal functional structure with α-helices called cellular PrP Disease-causing form with β-sheets called prion PrP Prion PrP converts cellular PrP into prion PrP by inducing conformational change Characteristics of Prions Normally, nearby proteins and polysaccharides force PrP into cellular shape Excess PrP production or mutations in PrP gene result in initial formation of prion PrP When prions present, they cause newly synthesized cellular PrP to refold into prion PrP Prion Diseases All involve fatal neurological degeneration, deposition of fibrils in brain, and loss of brain matter Large vacuoles form in brain; characteristic spongy appearance Spongiform encephalopathies – Bovine spongiform encephalopathy (BSE), Creutzfeldt-Jakob disease (CJD), kuru Only destroyed by incineration; not cooking or sterilization Scrapie in Sheep