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The Genetics of Viruses and Bacteria Ch 18

The Genetics of Viruses and Bacteria Ch 18. Discovery of the virus (Mayer). 1883: tobacco plants stunted or mottled leaves. Contagious disease through sap contact Pathogen not seen through microscope Ivanowsky filtered sap to remove bacteria, still sap is contagious.

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The Genetics of Viruses and Bacteria Ch 18

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  1. The Genetics of Viruses and BacteriaCh 18

  2. Discovery of the virus (Mayer) • 1883: tobacco plants stunted or mottled leaves. • Contagious disease through sap contact • Pathogen not seen through microscope • Ivanowsky filtered sap to remove bacteria, still sap is contagious. • 1897: Beijerinck discovered agent could reproduce • 1935: Wendell Stanley crystallized agent now known as TMV (Tobacco Mosiaic Virus)

  3. Viral Structure • Virus only 20nm diameter, smaller than ribosome. • Genetic Material • Either DNA or RNA • Single strand or double strand • Single linear or circular genetic material • 4-100 genes • Capsid • Protein shell surrounding genetic material • Various shapes • Envelope (Influenza virus) • Membranes surrounding the capsid • May be host or virus derived • Proteins and glycoproteins (some have enzymes w/i)

  4. Reproduction Overview • Viruses needs a host cell to reproduce • Obligate intracellular parasites • Limited host range (receptor recognition of host) and can be tissue specific • Zoonotic: jumps/crosses species (HIV) • Viral DNA reprograms host cell to make viral proteins and DNA/RNA (hostile takeover)

  5. Two Reproductive Cycles • Lytic Cycle • Last stage of infection • Host cell death • Virulent Virus/phage • Virus or phage that only goes through the lytic cycle • T4 phage infect bacteria using lytic cycle

  6. Lysogenic Cycle • Replicates virus DNA w/I host’s • Temperate viruses • Insert DNA into host forming a Prophage • Each time host cell undergoes mitosis, viral DNA is also replicated. • Eventually enters lytic cycle • Prophage leaves host DNA causing lytic cycle to occur • Some of prophage DNA may be expressed w/i host cell • Diptheria, botilism, scarlet fever would be harmless to humans if prophage genes did not make toxins in host bacteria.

  7. Viral Envelopes • Lipid bylayer w/ glycoproteins • Glycoproteins act as receptors for binding to host cell • Fuses with host cell • Herpes Virus (viral envelope) • Remains latent in host cell’s nuclei (provirus)(lysogenic) • Stress (emotional or physical) causes to go lytic • Blister are active form of virus

  8. Retroviruses (RNA) • Types • ssRNA serves as mRNA • ssRNA template for mRNA • ssRNA template for DNA (retroviruses) • Reverse Transcriptase • Enzyme transcribes DNA from RNA • Inserted into host’s DNA • Lysogenic Cycle (eventually lytic (AIDS))

  9. Emerging Viruses • HIV (1980, California) • Ebola (1976) Central Africa, hemorrhagic fever • Smallpox (eradicated by WHO in 1977) • Worlwide vaccination • Measles • Polio

  10. Virus and Cancer • 1911: Rous discovered that some viruses cause cancer • Tumor Viruses • Hepatitis B causes liver cancer • Oncogenes in viruses affect mitosis of host cell

  11. Viroids and Prions • Prions: • Protein molecules • replication unknown • yet is transmissible • Mad Cows Disease/Creutzfeldt Jakob • Converts normal proteins to the prion version • Normal Cow • Cow w/ Creutzfeldt Jakob • Viroids • Naked circular RNA • Infect plants • Several hundred nucleotides • No proteins • Replicates in host cells • Disrupts metabolism of plants

  12. Bacteria

  13. Genetics • ds DNA, circular • Forms dense region in cell called Nucleoid • No membrane • Binary Fission preceded by duplication of DNA • Asexual forming daughter cells identical to parent

  14. Recombinations • Recombination generates diversity (natural selection) • Transformation • Changing of genotypes by uptake or foreign naked DNA • Bacteria recognizes specific fragments of DNA that MAY contain alleles needed. • Transduction • Phages carry bacterial genes from one host to another • Generalized • Specialized

  15. Conjugation • Direct transfer of genetic material b/w two bacteria • “F-Factor”; DNA segment that codes for the formation of a sex pilus (male) • Can exist as either • w/i bacterial chromosome or • As a plasmid • Episome: genetic section that can exist as either a plasmid or w/i chromosome

  16. F Plasmid • 25 genes that control production of sex pilus • F+ has F plasmid and is inheritable • F- has no F plasmid • F+ condition is “contagious” • Hrf cell has incorporated the F plasmid into the bacterial genome

  17. R plasmids • Plasmids that contain genes that confer resistance to antibiotics • Has genes that encode for sex pilus • Can carry multiple genes for many antibiotics

  18. Transposons • “Jumping Genes” • Genes that can move locations w/i chromosome/plasmids • Allows for multiple genes in the R plasmid • Has inverted sequence on either side of gene • Needs transposase enzyme to transpose gene.

  19. Composite Transposons • Inverted repeats surround “hitchhiker genes” • Antibiotic resistance genes are often these hitchhikers

  20. Gene Therapy • Uses transposons to “cure” genetic disorders • Insert working gene to replace damaged ones • Controversial Space Doctor. Test your Gene Therapy Skills Try this at home Treatment of SCID in humans

  21. Hyperlink for more information • HIV Statistics

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