VIRUSES. Introduction. DISCOVERY OF VIRUSES. вЂў The discovery of viruses resulted from the search for the infectious agent causing tobacco mosaic plants and gives their leaves a mosaic coloration. ADOLF MEYER. вЂў A German scientist demonstrated that the disease was contagious and proposed that the infectious agent was an unusually small bacterium that could not be seen with a microscope. вЂў He successfully transmitted the disease by spraying sap from infected plants onto the healthy ones. вЂў Using a microscope, he examined the sap and was unable to identify a microbe. D. IVANOWSKY. вЂў 1890: A Russian scientist proposed that tobacco mosaic disease was caused by a bacterium that was either too small to be trapped by a filter or that produced a filterable toxin. вЂў To remove bacteria, he filtered sap from infected leaves. вЂў Filtered sap still transmitted disease to healthy plants. MARTINUS BEIJERINCK. вЂў 1897: A Dutch microbiologist proposed that the disease was caused by a reproducing particle much smaller and simpler than a bacterium. вЂў He ruled out the theory that a filterable toxin caused the disease by demonstrating that the infectious agent in filtered sap could reproduce. вЂў Plants were sprayed with filtered sap from disease plants-----> sprayed plants developed tobacco mosaic disease----->sap from newly infected plants was used to infect others. WENDELL. M. STANLEY. вЂў 1935:An American Biologist, from the Rockefeller Institute, crystallized the infectious particle now known as Tobacco Mosaic Virus(TMV). вЂў The purified virus precipitated in the form of crystals. вЂў He was able to show that viruses can be better regarded as chemical matter, than as living organisms. вЂў Crystals retained the ability to infect healthy tissue. вЂў Subsequent determination of chemical nature of TMV: Protein in combo with nucleic, TMV were rods 300 nanometers long, TMV was RNA surrounded by protein coat. GENERAL CHARACTERISTICS. вЂў Obligatory Intracellular Parasites. вЂў Protein coated fragments of DNA or RNA that have become detached from the genomes of cells. Because they cannot replicate on their own, they are not organisms. вЂў Viruses are generally host-specific. вЂў They reproduce only within a certain host. вЂў Should be as many viruses as there are kinds of organisms. VIRUS-HOST RANGE. вЂў The host range of a virus is the spectrum of host cells the virus can infect. вЂў Some viruses have broad host ranges which may include several species(e.g. swine flu and rabies). вЂў Some viruses have host ranges so narrow that they can: вЂў infect only one species(e.g. phages of E.coli вЂў Infect only a single tissue type of one species(e.g. human cold virus infects only cells of the URT; AIDS virus binds only to specific receptors on certain white blood cells. VIRAL SIZE. вЂў In the 1950вЂ™s, TMV and other viruses were finally observed with electron microscopes. вЂў Smallest = 17 nanometers in diameter. вЂў Largest = 1000 nanometers(1 micrometer) in greatest dimension. вЂў Few are barely visible at light microscope level. вЂў Most are visible only via EM. VIRAL STRUCTURE. вЂў The virus or virion, is just nucleic acid enclosed by a protein coat, itвЂ™s a complete, fully developed infectious viral particle that is a vehicle of transmission from one host to another. вЂў Viruses are classified by differences in the structures of these coats. NUCLEIC ACID- VIRAL GENOMES. вЂў Depending on the virus, viral genomes: вЂў May be double-stranded DNA, singlestranded DNA, double-stranded RNA or single-stranded RNA. вЂў Are organized as single nucleic acid molecules that are either linear or circular. вЂў May have as few as four genes or as many as several hundred. CAPSID AND ENVELOPE. вЂў Capsid: Protein coat that encloses the viral genome. вЂў ItвЂ™s structure may be rod-shaped, polyhedral or complex. вЂў Composed of many capsomeres: protein subunits made from only one or a few types of protein. ENVELOPE. вЂў Membrane that cloaks some viral capsid: вЂў Helps viruses infect their host. вЂў Derived from host cell membrane which is usually virus-modified and contains proteins and glycoproteins of viral origin. GENERAL MORPHOLOGY. вЂў вЂў вЂў вЂў Helical viruses. Polyhedral viruses. Enveloped viruses. Complex viruses. HELICAL VIRUSES. вЂў Resemble long rods. вЂў Maybe rigid or flexible. вЂў Viral Genome found inside a hollow cylindrical capsid. вЂў E.G.: Ebola virus and Rabies virus. POLYHEDRAL VIRUSES. вЂў Many-sided Capsid is in the shape of icosahedron( a polyhedral with 20 triangular faces). вЂў E.G.: Adenovirus, and poliovirus. ENVELOPED VIRUSES. вЂў Roughly spherical. вЂў Enveloped-helical or enveloped polyhedral viruses. вЂў Enveloped helical = Influenzae virus. вЂў Enveloped polyhedral = Herpes simplex virus. COMPLEX VIRUSES. вЂў Bacteriophage. вЂў Capsid(head) is polyhedral, tail sheath is helical. вЂў Tail fibers, plate and pin. TAXONOMY OF VIRUSES. вЂў Viruses are not organisms and are not classified in the kingdoms of life. вЂў Regarded as self-replicating portions of the genomes of organisms. вЂў In comparison to living things, vruses are acellular(not cells and do not consist of cells), do not metabolize energy, no photosynthesis, cell respiration or ferment. вЂў Viruses are cell parasites as they infect organisms at all taxonomic levels. вЂў Oldest classification based on symptoms. вЂў ICTV group vruses into families, based on: вЂў Nucleic acid type, strategy for replication, and morphology. вЂў E.G.: Family= ends with viridae. вЂў Genus= virus. ISOLATION, CULTIVATION, AND IDENTIFICATION. вЂў Cannot replicate outside a living cell: poses problem with detection, ID, and cultivation. вЂў Bacteriophages are easily grown on bacterial cultures. вЂў This is the reason why so much of what is known of viral multiplication has come from from bacteriophage. GROWING VRUSES IN THE LAB. вЂў вЂў вЂў вЂў вЂў Plaque method: Bacteriophages. Living animals: Animal viruses. Embryonated eggs: Animal viruses. Cell cultures: Animal viruses (CPE). Primary cell lines, Diploid cell lines, and continuous cell lines. VIRAL MULTIPLICATION. вЂў Viruses are nonliving particles that reproduce only inside specific host cells. вЂў Exhibit many patterns of viral life cycle, but they generally include: вЂў Coopting host ceelвЂ™s resources to: вЂў Manufacture capsid protein, вЂў Assemble newly produced viral nucleic acid and capsomeres. вЂў Several mechanisms used to infect host cells with viral DNA. вЂў For example, T-even phages use an elaborate tailpiece to inject DNA into the host cell. вЂў Once the viral genome is inside its host cell, it commandeers the hostвЂ™s resources and reprograms the cell to copy the viral genes and manufacture capsid protein. вЂў Three possible patterns of viral genome replication: вЂў 1. DNA---->DNA: If viral DNA is doublestranded, DNA replication resembles that of cellular DNA, and the virus uses DNA polymerase produced by the host. вЂў 2. RNA----->RNA: Since host cells lack the enzyme to copy RNA, most RNA viruses contain a gene that codes for RNA replicase вЂў RNA replicase is an enzyme that uses viral RNA as a template to produce complementary RNA. вЂў 3. RNA----->DNA----->RNA: Some RNA viruses encode reverse transcriptase, an enzyme that transcribes DNA from an RNA template. MULTIPLICATION OF BACTERIOPHAGES. вЂў Studies on lambda phage of E. coli showed that double-stranded DNA viruses reproduce by two alternative mechanisms: The lytic cycle and the lysogenic cycle. LYTIC AND LYSOGENIC CYCLES. вЂў Lytic cycle: вЂў Results in the death or lysis of the host cell. вЂў Bacteriophage takes over the machinery of the cell, so viral replication and release occur. вЂў Lysogenic cycle: вЂў Involves the incorporation of the viral genome into the host cell genome. вЂў Phage becomes a prophage, integrated into the host genome. вЂў Virulent bacteriophage reproduce by a lytic replication cycle= that lyse their host cells. вЂў Later, the phage may reenter the lytic cycle and replicate itself. вЂў Temperate viruses= integrate and remain latent. THE LYTIC CYCE. вЂў вЂў вЂў вЂў вЂў Attachment. Penetration. Biosynthesis. Maturation. Release. THE LYTIC CYCLE. THE LYSOGENIC CYCLE. вЂў вЂў вЂў вЂў Lysogeny. Prophage. Phage conversion. Specilized transduction. THE LYSOGENIC CYCLE. MULTIPLICATION OF ANIMAL VIRUSES. вЂў вЂў вЂў вЂў вЂў вЂў Attachment. Penetration. Uncoating. Biosynthesis of DNA viruses. Biosynthesis of RNA viruses. Maturation and Release. DNA-CONTAINING VIRAL MULTIPLICATION. RNA-CONTAINING VIRAL MULTIPLICATION. RNA VIRAL MULTIPLICATION CONTD. VIRUSES AND CANCER. вЂў вЂў вЂў вЂў Oncogenes. Oncogenic viruses. Contact Inhibition. Tumor-specific transplantation antigen (TSTA ). LATENT VIRAL INFECTIONS. вЂў вЂў вЂў вЂў Latent Infection: Herpes simplex. Chicken pox (varicella). Shingles(zoster). PERSISTENT VIRAL INFECTIONS. вЂў Persistent viral infections: Encephalitis called subacute sclerosing panencephalitis(SSPE). PRIONS. вЂў Proteinaceous infectious particle that lacks nucleic acid. вЂў Nine animal diseases fall into this category. вЂў All nine are neurological diseases called spongiform encephalopathies. вЂў These diseases run in families, which indicates a possible genetic cause SPOKE TOO SOON. вЂў However, The diseases cannot be purely inherited because mad cow disease arose from feeding scrapie-infected sheep meat to cattle, and the new (bovine) variant was transmitted to humans who ate undercooked beef from infected cattle.