Branches of Microbiology Bacteriology Virology Mycology Parasitology Immunology Recombinant DNA technology

viruses Are the smallest infectious agents Intracellular parasite

The distinguishing properties are ***Size ****Nucleic acid content & structure ****Metabolic capabilities & reproduction

Size –electron microscope nm

Nucleic acid content – contain only a single type of nucleic acid ether DNA or RNA (never both) They are either single or double stranded They are + or - sense

structure A complete virion is composed of nucleic acid core surrounded by protein coat == capsid Some viruses are surrounded by an envelope consisting of lipid, protein, carbohydrate

structure viruses DNA RNA envelopednonenvelopedenveloped nonenveloped

Metabolic capabilities & reproduction Viruses have no metabolic machinery of their own They are obligatory intracellular parasites Only growing within living cells

replication

11 The DNA or RNA genome may be : –ss – single stranded or DNA & RNA –ds – double stranded Genomes may be either: –(+) sense: Positive-sense viral RNA is identical to viral mRNA and thus can be immediately translated into protein by the host cell.translated OR –(-) sense: Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an RNA polymerase before translation.RNA polymerase

Steps of viral replication 1.Adsorption ( Attachment) – binding of virus to specific molecule on host cell 2.Penetration–genome enters host cell uncoating- getting rid of protein coat 3.Biosynthesis replication – viral components produced 4.assembly - viral components assembled 5.maturation – completion of viral formation 6.release – viruses leave cell to infect other cells

13 Attachment –specific binding of a virion protein (the anti-receptor) to a constituent of the cell surface (the receptor) e.g. hemagglutinin of influenza virus Penetration –energy-dependent step –occurs almost instantaneously after attachment

14 Host range Spectrum of cells a virus can infect –cell has to have a specific structure (receptor) on its surface for viral attachment –cell has to contain all of the enzymes and materials needed to produce new virions May be one species or many –HIV (only humans) vs rabies (many animals) May be one tissue or many within a host –Hepatitis (liver) vs polio (intestinal & nerve cells)

15 Entry by: Pinocytosis Enveloped viruses by fusion

16 Uncoating –at same time as penetration or shortly after –separation of viral nucleic acid (NA) from outer structural components ( by cellular enzymes) Released as: –Free nucleic acid inside the cytoplasm of the infected cell Expression of viral genome and synthesis of viral components

Viral replication

18 After the viral nucleic acid is released inside the host cell:  The transcription and translation processes of the host cell are redirected for the production of viral proteins and nucleic acids  The different types of nucleic acid genomes are expressed and replicated in several ways: DNA genomes undergo replication-using processes similar to cellular replication RNA genomes may be +ssRNA; Can be read directly as an mRNA or reverse transcribed by reverse transcriptase into DNA RNA genomes may also be -ssRNA; The RNA must first be used as a template to form +mRNAs

19 Assembly and Release Components of capsid synthesis directed by late genes Assembly of enveloped viruses needs interaction with plasma membrane which has been modified Final stage of infection Enveloped viruses released gradually by budding or exocytosis Naked viruses accumulate in cytoplasm and released during lysis

20 When a virus infects a cell, nucleic acid must be uncoated and gain access to metabolic machinery of cell. Virus life cycle is characterized by: –attachment –penetration, with entry of nucleic acid into cell –early expression of virus genes (either directly by translation, if virus contains "+" RNA, or indirectly after transcription and then translation) –replication of virus nucleic acid –synthesis of new virion components –packaging and assembly of new virions –exit from cell Viral Replication

Effect of virus on cells The production of new viral particles generally results in ***death of the host cell (cytopathic effect) ***transformation

22 Cytopathic effects- virus- induced damage to cells 1.changes in size & shape 2.cytoplasmic inclusion bodies 3.nuclear inclusion bodies 4.cells fuse to form multinucleated cells 5.cell lysis 6.alter DNA 7.transform cells into cancerous cells

How do we grow viruses? Obligate intracellular parasites require appropriate cells to replicate.

25 Growing animal viruses 1.Living animals 2.Bird embryonated eggs – chicken, duck; intact, self-supporting unit, sterile, self- nourished 3.Cell culture

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Cell culture 27

28 Diagnosis of viral diseases More difficult than other agents Consider overall clinical picture Take appropriate sample –Infect cell culture- look for characteristic cytopathic effects –Screen for parts of the virus –Screen for immune response to virus (antibodies)

Direct detection of viruses E.M. Agglutination == influenza Fluorescent antibody test ELISA=enzyme linked immunosorbent assay RIA=radio immuno assay Hybridization --- in Situ hybridization ===detection of DNA sequence in cancer cells