The Genetics of Viruses and Bacteria Ch 18

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The Genetics of Viruses and BacteriaCh 18
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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)

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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)

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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)

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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

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• Lysogenic Cycle
• Replicates virus DNA w/I hosts
• 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.

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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 cells nuclei
  (provirus)(lysogenic)
• Stress (emotional or physical) causes to go lytic
• Blister are active form of virus

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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 hosts DNA
• Lysogenic Cycle (eventually lytic (AIDS))

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Emerging Viruses

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

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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

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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

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Bacteria
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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

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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

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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

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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

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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

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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.

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Composite Transposons

• Inverted repeats surround hitchhiker genes
• Antibiotic resistance genes are often these
  hitchhikers

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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
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