VIROLOGY-PART-1 PDF - Basic Concepts in Virology Midterms

BASIC CONCEPT IN VIROLOGY MIDTERMS  VIROLOGY is the study of viruses.  Dmitri Iwanowski (1892)- Discovery of VIRAL STRUCTURE AND CHARACTERISTICS virus; 1st noted the filterable organism  Viruses particles, referred to as virions,...

BASIC CONCEPT IN VIROLOGY MIDTERMS  VIROLOGY is the study of viruses.  Dmitri Iwanowski (1892)- Discovery of VIRAL STRUCTURE AND CHARACTERISTICS virus; 1st noted the filterable organism  Viruses particles, referred to as virions, consist of two or three parts: VIRUSES  Inner nucleic acid core-  Considered the smallest filterable consisting of either DNA or RNA infectious particles/agents. but never both  They can be living or non-living, and are  Capsid- protein coat that neither eukaryotic nor prokaryotic surrounds and protects the  OBLIGATE INTRACELLILAE PARASITES that nucleic acid are dependent on host cells for survival; o The genome & its protein coat infect only cellular forms together is called the  Mode of replication: Assembly of nucleocapsid individual components  Envelope- lipid containing  Lack the capacity to make energy structure surrounding the virus; substrates and replicate their genome present in some of the larger independently. viruses  Must adapt to the biochemical rules of the cell in order to successfully use the cell’s biosynthetic machinery. Some viruses are useful:  Phage typing of bacteria  Pesticides  Source of enzymes in molecular biology  Gene vectors NAKED (CAPSID) VIRUSES  Anti-cancer agents  Antibacterial agents- page therapy to  Capsid is a rigid structure able to treat infections withstand harsh environmental conditions for the protection of the genome ORIGIN OF VIRUSES  Composed of repeating structural subunits which associate into protomers,  Derived from DNA or RNA nucleic acid capsomeres, into a recognizable components of host cells that have procapsid or capsid acquired the ability to exist  Generally resistant to drying, acid, and independently detergents ENVIRONMENTALLY STABLE  Viral genome is composed of one of the  Released from cell by lysis following:  Transmitted by the fecal-oral route and  Double-stranded DNA can endure transmission even in sewage  Single-stranded DNA  Antibody may be sufficient for  Double-stranded RNA immunoprotection  Single-stranded RNA MYCOVIRO 1  INTRODUCTION TO MYCOLOGY Types of Capsid: VIRAL REPLICATION  Symmetric Capsids o Helical- appear as rods  The major steps in viral replication are the o Icosahedral- approximation of same for all viruses a sphere assembled from  Viruses are strict intracellular parasites symmetric subunits that rely upon components of the host  Nonsymmetric Capsids – complex forms cell to replicate - capable of replication and are associated with certain only within a host cell. bacterial viruses.  The cell acts as a factory, providing the substrates, energy, and machinery necessary for the synthesis of viral proteins and replication of the genome  VIRAL TROPISM - specificity of virus types that only infect limited number of host Phases of Viral Replication: 1. Early Phase  Virus must recognize an appropriate target cell, attach to the cell, penetrate plasma ENVELOPED VIRUSES membrane and be taken up by  Envelope is a membrane composed of the cell, release (uncoat) its lipids, proteins, and glycoproteins genome into the cytoplasm, and  Glycoproteins may extend through the deliver the genome to the envelope away from the surface of the nucleus. virion that can be observed as spikes. 2. Late Phase  Modifies cell membrane during  Start of genome replication and replication; released by budding (does viral macromolecular synthesis not need to kill the cell to spread) and and proceeds through viral cell lysis assembly and release.  Membranous structure can be 3. Eclipse Period maintained only in aqueous solutions.  Ends with the appearance of new  Most cannot survive harsh condition - virions after virus assembly ENVIRONMENTALLY LABILE 4. Latent Period  Readily disrupted (inactivated) by  Includes the eclipse period and ends drying, acidic conditions, detergents, with the release of new viruses and solvents  Must remain wet and are generally INFECTIOUS CYCLE- six steps of virus transmitted in fluids, respiratory droplets, replication blood, & tissue. 1. Attachment (Adsorption) - recognition of  May need antibody and cell-mediated the target cell immune response for protection and o Viral attachment proteins (VAPs) control on enveloped viruses, or structures  Elicits hypersensitivity and inflammation on the surface of the virion capsid to cause immunopathogenesis binds to the receptors on the cell. o Viruses that bind to receptors expressed on specific cell types may be restricted to certain species (host range) or specific cell type (viral tropism) NAME 2  INTRODUCTION TO MYCOLOGY 2. Penetration (Virus Entry) - process by several complementary strands which serve as which viruses enter the host cell mRNA o Interactions between multiple 5. Viral Assembly - process by which VAPs and cellular receptors structural proteins, genomes, and in initiate the internalization of the some cases viral enzymes are assembled virus into the cell. into virus particles. o Mechanism of internalization o Site and mechanism of virion depends on the virion structure & assembly in the cell depend on cell type: where genome replication - Most NONENVELOPED occurs and whether the final viruses enter the cell by structure is a naked capsid or an receptor-mediated enveloped virus. endocytosis (phagocytosis o Capsid viruses may be of host cell) or by viropexis assembled as empty structures - ENVELOPED viruses (procapsids) to be filled with the involved fusion of the viral genome or they may be enveloped with the host assembled around the genome. cells that can lead to fusion o Envelopes are acquired during between the infected host viral "budding" from a host cell cell and additional nearby membrane, commonly in the host cell, forming nuclear endoplasmic multinucleated cells called membranes. reticulum, and syncitia cytoplasmic 3. Uncoating - process by which the capsid 6. Release - Viruses can be released from is removed cells after lysis of the cell, by exocytosis, o Release of the viral genome for or by budding from the plasma delivery of the viral DNA or RNA to membrane. its intracellular site of replication. o Enveloped viruses are released  Genome of DNA viruses (except after BUDDING from the plasma poxviruses) delivered to the NUCLEUS membrane without killing the cell.  Most RNA viruses remain in the o Naked capsid viruses are CYTOPLASM generally released after LYSIS of 4. Macromolecular Synthesis (transcription the cell (Lytic virus) and translation) EPIDEMIOLOGY OF VIRAL DISEASES o Early mRNA and non-structural proteins synthesis: genes for  Pandemic - emergence of a new viral enzymes and nucleic acid- disease across a very large geographic binding proteins region (worldwide) with prolonged o Replication of the genome human-to-human transmission o Late mRNA and structural protein  Genetic Shift - genetic re-assortment of synthesis viral genes resulting to a completely new o Post-translational modification of or "novel" virus, leading to the proteins occurrence of pandemics. Nucleic Acid: Positive (+) or Negative (-) sense  Antigenic Shift - MAJOR changes ✔ Positive-sense: Isolated RNA genomes which that result in a completely new o functions as mRNA within the -infected cells; are "novel" viral antigens that are not INFECTIOUS recognized by the host's immune ✓ Negative-sense: Isolated RNA is system; responsible for most of the NONINFECTIOUS, virions usually carry RNA PANDEMIC infections. polymerase that transcribes genomic RNA to  Antigenic Drift - MINOR changes that occur infrequently as a result of NAME 3  INTRODUCTION TO MYCOLOGY mutation in the pathogen strains and facilitate 5. Replication in the target tissue - which pathogen to avoid hast immune response; causes the characteristic disease signs responsible for most EPIDEMIC infections. 6. Host responses that limit and contribute (immunopathogenesis) to the disease 7. Virus production in a tissue that releases the virus to other people for contagion 8. Resolution (convalescence) or persistent infection/chronic disease INFECTION OF THE TARGET TISSUE  The virus gains entry into the body  Viruses are transmitted from person to through breaks in the skin or across the person by the respiratory, fecal-oral, and mucoepithelial membranes that line the sexual contact routes; by trauma or orifices of the body. injection with contaminated objects or  On entry into the body, the virus needles; by tissue transplants (including replicates in cells that express viral blood transfusions); by arthropod or receptors and have the appropriate animal bites; and during gestation biosynthetic machinery. (transplacental transmission).  Virus initially replicate at the site of entry (i.e upper respiratory tract) MECHANISM OF VIRAL PATHOGENESIS  Blood stream and lymphatic system are  Process by which viruses produce predominant means of viral transfer and disease in the host dissemination to other body tissues:  Viruses cause disease when the breach  Blood stream (primary viremia) in the host's physical barrier and natural  Within mononuclear phagocytes, protective barriers lymphocytes, or endothelial cells  Outcome of viral infection- determined (secondary viremia) by the interaction between the virus and  Virus gains access to the CNS through: the susceptible host.  the blood stream  Immune response - the best treatment,  infection in the meninges or but, it also contributes to the cerebrospinal fluid pathogenesis of viral infections.  migration of infected  Tissue target of virus (tropism) - defines macrophages the nature of the disease and its  infection of the peripheral and symptoms. olfactory neurons BASIC STEPS IN VIRAL DISEASE VIRAL CYTOPATHOGENESIS  Viral disease in the body progresses  Potential outcomes of a viral infection of through defined steps: a cell are as follows: 1. Acquisition - entry into the body  Abortive Infection 2. Initiation of infection at a primary  Persistent Infection site  Lytic Infection 3. Activation of innate protections  Latent-recurrent Infection 4. Incubation period - when the virus 1. ABORTIVE INFECTION- failed infection is amplified and may spread to a  Viral Mutants, which cause secondary site; may proceed abortive infections, do not without symptoms multiply and disappear (asymptomatic), or may produce 2. PERSISTENT INFECTION (Nonlytic nonspecific early symptoms Infection) – viral replication without cell (prodrome) death NAME 4  INTRODUCTION TO MYCOLOGY a. Chronic: non-lytic, productive initiated by antiviral the following b. Latent: limited viral macromolecular but immunity. factors: no virus synthesis  Flu-like symptoms c. Recurrent: periods of latency then virus  Delayed-type 1. The production hypersensitivity mechanism d. Trasnsforming: immortalizing - and inflammation of exposure ONCOGENIC VIRUSES  Immune complex and site of disease infection Nature of the infection is determined by the  Hemorrhagic 2. The characteristics of the virus & target cell disease immune o Nonpermissive cell – lacks receptor of  Post-infection status, age, enzyme, or express antiviral prperties that cytolysis and general will NOT ALLOW replication of virus.  Cytokine storm health of o Permissive cell- provides the biosynthetic  Immunosuppresion the person machinery to SUPPORT THE COMPLETE 3. The viral REPLICATIVE CYCLE of the virus dose o Semipermissive cell - very INEFFICIENT 4. The REPLICATION or he cell may support genetics of some but not all the steps in viral the virus and replication the host 3. LYTIC INFECTION –results when virus replication kills the target cell  Viruses damage the cell & prevent repair by inhibiting the synthesis of cellular macromolecules or producing degradative enzymes & toxic proteins. HOST DEFENSES AGAINST VIRAL INFECTION  Immune Response - best & in most cases the only means of controlling viral infection.  Innate, humoral and cellular immune responses are important for antiviral immunity.  The longer the viral replication, the greater the dissemination of infection. o Antigen-nonspecific (innate) host defenses - Cell-mediated immunity o Antigen-specific immune responses -Antibody-mediated immunity IMMUNOPATHOLOGY VIRAL DISEASE Pathologic The relative manifestations & susceptibility of a symptoms of viral disease person and y the caused by severity of the hypersensitivity and disease depend on inflammatory reactions NAME 5