Ch. 18

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Ch. 18 Microbial Models of DNA

• Microbes microscopic organisms like bacteria
  and viruses, can include fungi and protista
• Bacteria kingdoms Eubacteria and Archeabacteria
  (formerly Monera), Prokaryotic cell, unicellular,
  one circular piece of DNA and multiple plasmids
• Viruses nonliving DNA or RNA and a protein
  coat

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Usefulness

• While we are familiar with disease causing
  bacteria and viruses (pathogens), most bacteria
  are harmless and actually helpful
• Because the have simple systems the study of
  their genetics has helped us understand more
  complex systems, like our own.
• They have some specialized systems that have
  helped us study disease
• They transfer genetic material between cells an
  important area of genetic research.

3
Discovery of Viruses

• Late 1880s Mayer, Ivanowsky and Beijerinck
  plants can spread disease to each other, disease
  is smaller than a bacteria and can be within the
  sap
• Early 1900s - Stanley crystallized TMV
  (tobacco mosaic virus)
• Crop was still big part of economy
• Actually saw virus with electron microscope

4
Viruses

• 20 nm (nanometers) smaller than ribosome
• Cells can not be crystallized
• Nucleic acid plus protein
• Protein called a capsid
• Variety of shapes. Including the icosahedron
• dsDNA, ssDNA, ssRNA and dsRNA
• Linear or circular
• 4 100s genes
• Bacteriophages are viruses that infect bacteria
• Phages named T1 T7

5
Viral Reproduction

• Viruses are host specific (range)
• Phages only bacteria, some only E.coli
• Rhinovirus (cold) only humans
• Rabies all mammals
• Proteins fit with receptor cells
• Some viruses of eukaryotes are tissue specific
  (URT)
• Lytic vs lysogenic life cycles
• See handouts (385 396)

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Viral Life Cycles

• LYTIC cycle
• Virus attaches to host
• Injects nucleic acid
• Host reads nucleic acid and then
• manufactures viral DNA and proteins
• Virus is assembled
• Escapes host cell
• Infects other cells

• LYSOGENIC cycle
• Same as lytic UNTIL
• Cell replicates BEFORE copying and making viral
  instructions
• This takes along viral nucleic acids to new
  generations of host, as part of the host genome
• At some point, lots of host offspring are
  triggered to start lytic cycle
• latent phase

7
Control of Viral Genes

• Bacteria posses restriction endonucleases, which
  are enzymes that degrade viral DNA as it enters
• Go lysogenic instead if phage is added to a
  specific site within bacterial DNA then its a
  prophage and it codes for a protein that
  represses the other phage genes.
• Some bacteria are actually more harmful because
  they posses phages that make toxins
• Viruses can also be enveloped by host to help
  them hide

8
Retroviruses

• These viruses double stranded RNA that is a
  template from which an enzyme called reverse
  transcriptionase can make DNA
• backwards
• An example is HIV

9
Vaccines

• Colds epithelial cells in throat can repair
  themselves efficiently
• Polio attaches to nerve cells that cannot be
  repaired
• 1796 Edward Jenner noticed that cow pox
  (mild) makes one immune to small pox (more
  harmful).
• Used fluid from cow (vacca) pox blister to give
  people cow pox, making them immune to small pox.
• Small pox has since been eradicated (WHO)
• Fewer cases of polio, rubella, measles and mumps
  since vaccines
• Rabies vaccines for pets
• Newer vaccines for chicken pox (varicella) and
  HPV (gardasil)
• Flu shots are vaccines against the flu varies
  from year to year and location to location
• Globalization
• Evolution of viral genome
• CDC and WHO

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Emergent Diseases

• More cases, cases in new location, more deadly
  cases and spreading to a new host make a disease
  an emergent disease
• HIV
• Hantavirus
• Ebola
• The plague (emergent diseases arent all viral)
  not currently emerging.
• Lyme disease
• Asian Bird Flu
• ???? Certain cancers

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Link between cancer and virus

• Some viruses do cause cancers in animals
• Example is feline leukemia
• Versions of oncogenes are found in normal cells
  something triggers them
• Viruses may activate proto-oncogenes
• Viroids circle of RNA that only infects plants,
  replicate in plant cell to interfere with enzymes
  
• Prions infectious proteins that convert normal
  proteins to infectious proteins
• Ex. Mad Cow aka Crueutzfeldt Jacob disease

12
History

• Viral genomes often have much in common with the
  hosts DNA
• Could have evolved as either plasmids or
  transposons
• Plasmids are small circular pieces of DNA that
  replicate rapidly with in bacteria
• Transposons are pieces of DNA that move from
  location to location with in a genome

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Bacterial Genetics

• Bacteria are cells prokaryotic cells with a
  cell membrane, cytoplasm with ribosomes, and a
  circular piece of DNA in the nucleoid region. May
  also have plasmids
• Bacteria are mm,
• genome is about 4300 genes which is 100 x virus
  and 1/1000 of a euk. Cell
• DNA is 500 longer than the cell
• Divide by binary fission
• Adaptable
• Lots of habitats and host and modes of nutrition
• Natural selection happens quickly because of
  generation time (hours to days)

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Genetic Recombination

• Transformation
• Uptake of DNA from the environment
• S. pneumonia coats
• Transduction
• Phages carry genes from one bacterial host to
  another
• May have research possibilities
• Conjugation
• Bridge called pili between two bacteria and
  plasmids are transferred. No new cells, not
  reproduction
• Transfer resistance to antibiotics called R
  factor for resistance
• Conjugation requires bacteria to have F
  (fertility) factor
• Transposons
• Regions of DNA that move within a bacteria,
  either to a new place on the chromosome or
  between the chromosome and the plasmids
• Originally called jumping genes by McClintock,
  they moved within the genome of indian corn,
  turning the color genes on and off

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Control of Bacteria genes

• Restriction endonucleases
• Transposons
• Feed back inhibition from OPERONS
• Transcription unit
• Promoter RNA polymerase attaches
• Operator acts as switch
• Regulatory gene
• Repressor keeps operator off
• Inducer lets operator be on

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Operons

• trp operon
• See pg. 338
• If no tryptophan then operon on and making
  tryp.
• If lots of trp then repressor in operator, no
  need to make if you already have

• lac operon
• See pg. 339
• No lactose, no need for lactase enzyme.
• Lactose actually inactivates the repressor to
  lactase can be made

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