Virology Introduction PDF

Asmaa Omar Professor of Clinical Pathology- Faculty of Medicine- Assiut University Prof.Asmaa Omar Viruses: General Properties 1. Small size: o The smallest infectious agents o (20-300 nm in diameter) o Bacteria (300-1000nm); RBC (7500nm) 2. Genome: o Either DNA or RNA 3. Metabolically in...

Asmaa Omar Professor of Clinical Pathology- Faculty of Medicine- Assiut University Prof.Asmaa Omar Viruses: General Properties 1. Small size: o The smallest infectious agents o (20-300 nm in diameter) o Bacteria (300-1000nm); RBC (7500nm) 2. Genome: o Either DNA or RNA 3. Metabolically inert: o Do not posses active protein synthesizing apparatus o Do not have a nucleus, cytoplasm, mitochondria or ribosomes o No metabolic activity outside host: obligate intracellular parasites o Can replicate only inside living cells; NOT on inanimate media Basic virus structure components of complete virus particle : A) Enveloped virus with icosahedral symmetry. B) Virus with helical symmetry. Viruses: Structure Genome: o A core of DNA or RNA o May be single-stranded (ss) or double stranded (ds) o May be circular or linear Capsid: protein coat surrounding the genome o Provides structural symmetry o Participates in attachment to susceptible host o Facilitates transfer of viral nucleic acid in to host cell o Protects the viral genome from nucleases in blood stream Capsomeres: the structural units making up capsid: consist of one or several proteins Viruses: Structure Envelope: o A lipoprotein surrounding the capsid in some viruses o May contain material of host cell as well as viral origin. o Virus-encoded glycoproteins are exposed on the surface of the envelope o Most human helical viruses are enveloped while icosahedral are either enveloped or non-enveloped Nucleocapsid o The protein-nucleic acid complex Virion o The complete infective virus particle  Presence of specific enzymes E.g: - RNA and DNA polymerase - Neuraminidase - Reverse transcriptase Host tissue or cell tropism E.g: Hepatitis viruses, HIV, etc Properties of naked capsid viruses Properties of enveloped viruses Significance of Envelope o Enveloped viruses are more unstable i.e. are more sensitive to heat, drying, detergents and alcohols o All viruses transmitted by feco-oral route like HAV, poliovirus rotavirus are non-enveloped (those have to survive in environment) o Enveloped viruses are often transmitted by direct contact as by blood, sexual contact like HIV, HBV, HCV, rabies virus, measels, mumps, rubella viruses etc Viruses: Symmetry (Shapes) 1.Cubic Symmetry (Icosahedral) Have exactly 60 subunits on the surface of an icosahedron Have fivefold, threefold and twofold rotational symmetry 2. Helical Symmetry The virion contains an elongated nucleocapsid The capsomeres are arranged round the spiral of nucleic acid Most helical viruses are enveloped 3. Complex symmetry Does not conform to cubic or helical symmetry Atypical Virus-like Agents Defective Viruses Are composed of viral nucleic acid and proteins but cannot replicate without a ‘helper’ virus During growth many defective viruses are produced in addition to infectious viruses Pseudovirions Contain host cell DNA instead of viral DNA within the capsid Can infect cells but do not replicate Atypical Virus-like Agents Viroids Consist solely of a single molecule of circular RNA without a protein coat or envelope RNA is small and does not code for any protein. Cause several plant diseases but are not implicated in human diseases Prions Are infectious particles that are composed solely of protein and no detectable nucleic acid Are cause of certain slow diseases like Creutzfeldt-Jacob Disease (CJD) in human and scrapie in sheep Major diseases caused by human viruses  Virus infectious cycle Viral life cycle (Virus Replications) consists of six stages within the host cell Attachment Penetration Uncoating Multiplication or SYNTHESIS OF VIRAL NUCLEIC ACID AND PROTEIN Assembly or Maturation Release 1. Adsorption or attachment Reactive sites on viruses surface interact with specific receptors on susceptible host cells Receptors on the virus capside or envelope irreversibly binds to cellular receptors on the cell member ► Limit the host spp. & cells infected 2. Penetration The coat of enveloped viruses fuse with host cell member & release the virus nucleo- capsid into host cytoplasm Other viruses enter into cell by endocytosis 3.Uncoating Viral capsid is broken by viral or cellular enzyme; Viral NA is released; transported to within the host cell ► Transcribed to form new progeny virions  4. Biosynthesis or genomic activation m-RNA transcribes from viral DNA or; formed directly from some RNA viruses & codes for viral proteins such as:  Capsid / Envelope: - Encode structural proteins - Are building blocks of virion  Enzymes encode for: - DNA / RNA polymerase - Other replication enzymes  NA replication produces new viral genomes Note: -Except Poxviruses, DNA viruses replicate in the nucleus; - RNA viruses mainly in cytoplasm 5. Assembly Assembly of viral nucelo - capsid may take place in: A) Nucleus: E.g: Herpes virus, Adeno virus & others B) Cytoplasm: E.g: Poliovirus C) At the cell surface E.g: Influenza virus N.B: Accumulation of virions at sites of assembly may form inclusion bodies ► Visible in stained cells with light microscope 6. Release Release of new intact infectious virions May occur by:  Budding E.g: Enveloped viruses  Lysis of infected host cells/ tissues