Genetics of Viruses

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Genetics of Viruses Bacteria
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Chapter 18 - Microbial Models Genetics of
Viruses and BacteriaKey Concepts

• Most viruses consist of a genome enclosed in a
  protein shell
• Viruses can reproduce only within a host cell

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Phage Reproduction

• Phages exhibit two reproductive cycles Lytic
  Lysogenic

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• Animal viruses are diverse in their modes of
  infection and replication
• Viroids and Prions are infectious agents even
  simpler than viruses

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Genetics of Bacteria

• Short generation time for bacteria facilitates
  adaptation to changing environment.
• Genetic recombination produces new bacterial
  strains.
• Control of gene expression enables individual
  bacteria to adjust their metabolism to
  environment.

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Most Viruses.

• Viruses are infectious particles consisting only
  of viral genes enclosed in a shell of proteins.

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Viral Genomes

• Viral Genomes double-stranded DNA
  single-stranded DNA double-stranded
  RNAsingle-stranded RNA.
• A virus is called either a RNA or a DNA virus

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• Viral genome is either circular or linear.
• Smallest has four genes, the largest several
  hundred.

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Viral Morphology

• Capsids are proteins that enclose a viral genome.
• Protein subunits called capsomeres make the
  capsid.
• Viral envelopes are membranes that cloak capsids.

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Terminology

• Viruses that infect bacteria are called
  bacteriophages or phages.

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(No Transcript)
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T4 Phage injecting its DNA into E. coli to take
over its genetic machinery
Head
Sheath
Tail Fiber
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Viruses Only Reproduce Within a Host Cell
Factoids

• Viruses areobligate (no choice)intracellular
  (inside cell)parasites (at expense of host
• What is an obligate lactophilus, endoparasite?

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Viruses

• Lack enzymes for metabolism and have no ribosomes
  or other equipment for making their own proteins.

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Host Range for Viruses

• Viruses are host specific.
• Lock-and-Key fit between virus and host
  cell.e.g. HIV binds to a specific receptor on
  certain white blood cells.

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• A viral infection begins when the genome of a
  virus makes its way into a cell where it
  commandeers its host, reprogramming the cell to
  copy viral genes and manufacture capsomeres (Fig.
  18.3).

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Viral Reproduction Cycle (DNA) Play CD2 Figure
18.3
DNA
Entry into cell DNA uncoating
Virus
Capsid
Cell
Viral DNA
1. Replication
2.Transcription
3.Translation
Capsid proteins
Self-Assembly Exit
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Reproduction

• DNA viruses use the DNA polymerases of host cells
  to synthesize new genomes along templates
  provided by the viral DNA
• The cycle is complete when the 100s or 1000s of
  viruses emerge from the cell...sometimes the cell
  survives, sometimes it is destroyed (next topic).

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Phages Exhibit Two Reproductive Cycles Lytic
Lysogenic

• A reproductive cycle that culminates in death of
  the host cell is a lytic cycle.
• Lytic cycle this refers to the last part of the
  infection when the bacterium lyses (breaks open)
  to release phages.
• Viruses that depend on this cycle to reproduce
  are called virulent viruses.

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• Bacteria have restriction enzymes that recognize
  cut up foreign DNA.
• Natural selection favors phage DNA resistant to
  these enzymes.

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Figure 18.4 Lytic Cycle of Phage T4 - Play CD
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• Lysogenic Cycle reproduces viral genome without
  destroying the host.
• Viruses using both lytic and lysogenic cycle in
  bacteria are temperate viruses.

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??Phage Lysogenic and Lytic Reproductive Cycles

• Phage DNA circularizes and can either use the
  lytic cycle or viral genome to enter a specific
  site in the bacterial chromosome (prophage).

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Lysogenic Lytic Cycles of Phage Lambda (similar
to T4, but no tail)
Figure 18.5
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• Some prophages can alter bacterial phenotype to
  produce toxins that cause disease
• Diphtheria
• Botulism
• Scarlet fever

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Animal Viruses (See Table 18.1)

• RNA viruses called retroviruses have unique
  enzyme called reverse transcriptase, which can
  transcribe DNA from the RNA template.
• The new DNA integrates as a provirus into a
  chromosome in the cell nucleus, where RNA
  polymerase transcribes viral DNA into RNA.

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• This RNA serves as mRNA for protein synthesis and
  codes for new viral genomes. HIV is a
  retrovirus.

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Animal Viruses Grouped by Nucleic Acid Type
(Table 18.1)

• Class I dsDNA
• Papovavirus-Papilloma (warts, cervical cancer)
  polyoma (tumors in certain animal).
• Adenovirus-Respiratory diseases some tumors in
  animals.

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Class I dsDNA

• Herpesvirus-Herpes simplex (cold sores) Herpes
  simplex II (genital sores) varicella zoster
  (chicken pox, shingles) Epstein-Barr virus
  (mononucleosis, lymphoma).
• Poxvirus-smallpox cowpox vaccinia

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Class II. ssDNA

• Parvovirus-Roseola

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Class III. dsRNA (reovirus)

• Diarrhea viruses

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Class IV. ssRNA (serve as mRNA)

• Picornavirus-poliovirus, rhinovirus (cold)
  enteric (intestinal) viruses
• Togavirus-Rubella yellow fever encephalitis

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Class V. ssRNA (template for mRNA)

• Rhabdovirus-rabies
• Paramyxovirus-measles mumps
• Orthomyxovirus-influenza

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Class VI. ssRNA (template for DNA synthesis
retrovirus)

• RNA tumor viruses (e.g. leukemia)
• HIV (AIDS virus)

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Life Cycle of HIVA Retrovirus (Play CD)
Figure 18.7
White blood cell
RNA for translation Genome of next gen.
Capsid assembly
Budding
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From Where or What do Emerging Viruses Arise
(e.g. AIDS, hantavirus, Ebola, influenza)?

• An existing virus can evolve and cause disease in
  an individual who had developed immunity to the
  ancestral virus (influenza viruses evolve are
  maintained in birds then infect us by an insect
  bite, etc.).

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• An existing virus can spread from one host
  species to another (e.g. monkeypox)

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• An existing virus can disseminate from a small
  population to become more widespread (e.g.
  hantvirus, AIDS).

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Viroids and Prions Are Infectious Agents Even
Simpler than Viruses

• Viroids are molecules of naked RNA that are
  similar to introns that can catalyze their own
  incision from larger RNA molecule (escaped
  introns?)

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Nobel Prize (1997)Stanley Prusiner

• Prions are defective proteins that cause diseases
  like scrapie in sheep and degenerative diseases
  of the nervous system of humans, and most
  recently, mad-cow disease in Britain.
• Prions catalyze conversion of normal protein to
  prion protein.
• Stay tuned...

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Genetics of Bacteria

• Short generation time of bacteria facilitates
  their adaptation to changing environments.
• See Figure 1810

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Genetic Recombination Produces New Bacterial
Strains

• Recombination is the combining of genetic
  material from two individuals into the genome of
  a single individual via three processes
• Transformation
• Transduction
• Conjugation.

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Detecting Genetic Recombination in Bacteria
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• Transformation is the alteration of bacterial
  cells genotype by uptake of naked, foreign DNA
  from the surrounding environment, which replaces
  native alleles.
• Similar to crossing over in eukaryotic meiosis.

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• Taking up naked, foreign DNA by bacteria is
  intentional as surface proteins recognize and
  transport DNA from closely related species.

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• E. coli dont take up DNA normally, but placing
  calcium in the medium stimulates them to do so.
• This technique is used in biotechnology to
  introduce foreign genes into bacteria, which then
  make molecules like insulin and growth
  hormone...more on this in Chapter 20.

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• In transduction, phages transfer bacterial genes
  from one host cell to another via two forms.
• Basically, a defective phage transfers the DNA
  from one bacterium to another where the new
  replaces the homologous region, similar to
  crossing over (see Fig. 18.12).

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• Conjugation is the direct transfer of genetic
  material between two bacterial cells that are
  temporarily joined, with DNA going only from one
  to another. A special plasmid makes this
  possible.

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• So transformation, transduction and conjugation
  produce new bacterial strains and understanding
  these processes has been important in the
  development of biotechnology.
• Transferring DNA from one organism to another,
  then exploiting the new organism.

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Plasmids

• Plasmid is a small, circular DNA that is separate
  from the bacterial chromosome.
• Plasmids can reversibly incorporate into the
  cells chromosome to be an episome.
• ??phage is also an episome.

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• Plasmid genes can confer advantages for survival
  in stressful environments.
• For example, R Plasmids confer bacterial
  resistance to antibiotics and with natural
  selection we see more and more resistant
  bacterial strains that are difficult to treat.

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• Transposons can carry resistance or other factors
  from one organism to another, and unlike all
  forms of genetic shuffling, they are not
  site-specific.
• So new genes may show up in places where they
  have never been before (read about this more ).

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Operons

• When a group of task-specific polypeptides are
  required, frequently a single promoter serves all
  genes as a transcriptional unit on a single
  chromosome.

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• Transcription gives rise to one mRNA for all the
  genes that is punctuated with start and stop
  codons along the way.

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• A single switch can control the entire cluster of
  functionally-related genes.
• The switch is called an operator and it regulates
  access to of RNA polymerase.

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Operon Operation

• The entire stretch of DNA, promoter, operator and
  structural gene is called the OPERON. see Figs.
  18.19, 18.19 18.20 for details on how the
  operon operates.

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Figure 18.18
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Figure 18.19
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Figure 18.20
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Summary

• Molecular genetics is founded on the study of
  viruses and bacteria, so these microbial models
  have been the springboard for the development of
  biotechnology.