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BIOLOGICAL DIVERSITY CHAPTER 27 VIRUSES text pg 528 542 27 1 The Nature of Viruses all viruses have the same basic structure o core of nucleic acid RNA or DNA surrounded by a protein o no cytoplasm NOT a cell viruses are classified by the nature of their genomes o RNA viruses o DNA viruses o retroviruses Viruses are strands of nucleic acids encased in a protein coat capsid molecules repeated many times o capsomeres repeating protein molecules a protein sheath around the nucleic acid core composed of one to a few different protein in some viruses special enzymes are stored with the nucleic acid inside of the capsid ex reverse transcriptase many viruses have an envelope surrounding the capsid that is rich in proteins lipids and glycoproteins o lipids found in envelope are derived from host cell o proteins found in viral envelope are virally encoded Viral hosts include virtually every kind of organism viruses infect everything fungal cells bacterial cells protists and cells of plants and animals o however viruses can replicate in only a very limited number of cell types suitable cells for a particular virus host range tissue tropism viruses remain dormant until specific environmental trigger causes their expression once inside a multicellular host viruses will only target a specific set of ells Viruses replicate by taking over host machinery viruses can only reproduce inside of a host cell o when they are outside of the cell they are referred as virions viruses lack the necessary parts for replication on their own o no ribosomes or enzymes for protein synthesis o mostly no enzymes for nucleic acid reproduction viruses hijack a host cell s transcription and translation systems to produce viral proteins Most viruses come in two simple shapes icosahedron structure with 20 equilateral triangular facets o most animal viruses are icosahedral rod like or thread like appearance helical some envelope viruses are polymorphic without definite shape Viral genomes exhibit great variation genomes vary in nucleic acid carried and the number of strands RNA viruses o single stranded o replicated and assembled in the cytosol of infected eukaryotic cells o replication is error prone high rates of mutation o difficult targets for immune systems vaccines and antiviral drugs o retroviruses have reverse transcriptase reverse transcriptase enzyme that reverse transcribes RNA genome into DNA this DNA is integrated into the host s chromosomal DNA ex HIV human immunodeficiency virus DNA viruses o mostly double stranded o DNA is replicated in the nucleus of eukaryotic host cells 27 2 Bacteriophage Bacterial Viruses bacteriophage virus that infect bacteria o structurally and functionally diverse o bacterial hosts o DNA viruses Archaeal viruses have diverse morphologies DNA genomes unrelated to bacteriophages found in extreme conditions with extreme archaea Bacterial viruses exhibit two reproductive cycles o during infection one whisker or tail fiber contacts the protein of bacterial cell wall o other whiskers set bacteriophage perpendicular to the host cell and then bring the base plate into contact with the bacterium s surface Contact with the host o different bacteriophages target different parts of the surface of bacteria o first step is attachment bacteriophage attaches to the host bacterium o second DNA is injected into the bacterium cytoplasm o once inside of the cell third step synthesis occurs a bacteriophage may take over the replication and protein synthesis enzymes in order to synthesize viral parts o fourth during assembly the components that were made are put together o finally fifth stage the virus particles are released through enzymes that lyse the cell or budding through host cell wall The lytic cycle o similar to the above steps but the cell is ruptured or lysed at the end of the process o ends with cell death o these types of bacteriophages are called virulent or lytic The lysogenic cycle o some bacteriophages do not immediately kill the cells they infect called lysogenic temperate o integration of nucleic acid into genome of host bacterium advantage because virus will replicate as long as host cell is replicating DNA segment that is integrated into the genome is called a prophage resulting cell is called a lysogen o after penetration and the DNA is injected into the bacterial host there is integration of the DNA into the host genome o propagation reproduction of lysogenic bacteria occurs when prophage is replicated along with the host genome o cell stress such as starvation can push a prophage into induction when lysogenic prophage leaves the lysogenic cycle and goes into the lytic cycle requires turning on the gene expression required for the lytic cycle induction takes advantage of host proteins that respond to stress into producing protease enzyme that destroys the repressor protein that is keeping the viral genome silent the repressor protein that controls DNA repair genes Bacteriophage can contribute genes to the host genome during integration some viral genes may be expressed while host genes are being expressed phage conversion prophage integration when phenotype or characteristics of lysogenic bacterium is changed by the 27 3 Human Immunodeficiency Virus HIV HIV infection compromises the host immune system HIV specifically targets macrophages and CD4 cells type of helper T lymphocyte cell o without these cells body cannot fight off infections ultimately leads to death HIV infects key immune system cells viral glycoprotein gp120 fits on the cell surface marker protein CD4 on macrophages and T cells o HIV attaches to the two receptors CD4 and CCR5 o receptor mediated endocytosis is activated o HIV particle enters cell inside of cell protective coat is shed and viral RNA and reverse transcriptase are released into the cytoplasm o reverse transcriptase makes double stranded DNA that compliments the viral RNA this may be incorporated into the host genome as a provirus replicated viruses are budded off the cell by exocytosis HIV has a high mutation rate because reverse transcriptase is much less accurate than DNA polymerases o mutations lead to altered glycoprotein gp120 that now binds to CXCR4 receptor only found on CD4 cells rapid decline in T cells and immune response AIDS treatment targets different phases of the HIV life cycle drugs work to target reverse transcriptase viral entry and integration of the genome o most approved drugs are reverse transcriptase and protease inhibitors


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KSU BSCI 10110 - CHAPTER 27: VIRUSES

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