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Viruses: intimate parasites. Are viruses alive? Not made of cells, in violation of Cell Theory Do not grow (but self assemble) Do not metabolize (but steal energy) Cannot reproduce w/o a host cell (but other organisms may require another species in order to reproduce) Can evolve over time

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viruses intimate parasites
Viruses: intimate parasites
  • Are viruses alive?
    • Not made of cells, in violation of Cell Theory
    • Do not grow(but self assemble)
    • Do not metabolize(but steal energy)
    • Cannot reproduce w/o a host cell(but other organisms may require another species in order to reproduce)
    • Can evolve over time
    • Some can respond to environmental stimuli
    • Have a complex, organized structure
if not cells then what
If not cells, then what?
  • Viruses are particles
  • Some components are essential
    • A genetic material with the blueprint for making more
      • Could be: ds DNA, ss DNA, ss RNA, ds RNA
      • Space is limited, so genes are few
    • A covering to protect the genetic material
      • Capsid, made of one or more proteins
      • Capsid + nucleic acid: nucleocapsid
  • Viruses are obligate intracellular parasites
viruses may have other parts
Viruses may have other parts
  • Envelope: piece of organelle membrane or cell membrane covering capsid
    • Virus is formed by budding, pushes through membrane taking a piece.
    • Viral envelope usually contains viral proteins.
    • Envelope makes virus susceptible to some disinfectants
  • Spikes (peplomers) extend from envelope
    • Used for attachment, escape
  • Accessory enzymes
    • Reverse transcriptase, RNA RNA enzymes
viral size and shape
Viral size and shape
  • Viruses range from 20 nm to 300 nm
    • Ribosomes are about 30 nm
    • The smallest known bacteria are about 200 nm
  • Viral shapes:
    • helical, polyhedral, and complex; Figures/Icos_Virus.GIF;

examples of virus shapes
Examples of virus shapes



  • A recurring theme in biology:
    • Enzymes, membrane receptors, antibodies, etc.
  • Viruses are limited to certain types of host cells
    • Species barriers: rabies not specific, but most are
    • Tissue type: rabies specific to nerve, salivary tissue
    • Cell type: HIV infection mostly restricted to Helper T cells, a kind of lymphocyte.
  • Different characteristics of host cells involved
    • Attachment to cell surface often a major point
  • Every type of organism has a virus that infects it?
viruses across kingdoms
Viruses across kingdoms
  • A densovirus newly isolated from the smoky-brown cockroachPeriplaneta fuliginosa
  • Acanthamoeba castellanii Promotion of In Vitro Survival and Transmission of Coxsackie B3 Viruses
  • The causal organism is the Tulip Breaking Virus (TBV). The pathogen is a potyvirus and is divided into two strains,
  • Genome characterization of Botrytis virus F, a flexuous rod-shaped mycovirus
  • Microbes problematic, viruses especially so.
    • No sexual reproduction, no asexual reproduction, just “assembly”.
    • No clear evolutionary relationships
  • Classification scheme (from David Baltimore)
    • First, by nucleic acid type, e.g. ds DNA, + sense RNA
    • Next, structural characteristics (presence of envelope, capsid shape), type of organism infected, etc.
life cycle of a virus
Life cycle of a virus
  • Manner of infection and reproduction depends on whether host is prokaryotic or eukaryotic.
  • Life cycle here outlined is general:
    • ADSORPTION: following contact,

molecules on surface of virus bind

to particular molecules on host cell.

    • PENETRATION: the nucleic acid

must get access to the machinery

of the cell to replicate.

life cycle continued
Life cycle continued
  • SYNTHESIS/REPLICATION: once inside the nucleic acid issues orders leading to
    • Replication of the nucleic acid
    • Transcription (usually) and translation, producing the necessary capsid proteins.
  • ASSEMBLY: a spontaneous process
    • Capsid proteins and nucleic acid combine to make virion.
    • Cheap but highly inefficient process.
  • RELEASE: successful parasite must spread to others
    • Virus causes lysis of cell or pushes through cell membrane. Virions may acquire an envelope.
bacteriophage lytic vs lysogenic
Bacteriophage: lytic vs. lysogenic
  • Most bacteriophages multiply then lyse the host cell
    • This life cycle is called a lytic cycle
  • Others are “temperate”, enter a lysogenic cycle.
    • Lysogeny is an effective way to multiply the viral DNA
    • Viral DNA inserts into the bacterial chromosome
      • Now called a “prophage”
      • Bacterial replication also replicates viral DNA
      • Prophage may bring new genes for use by bacterium
    • Damage to bacterial DNA (e.g. UV) prompts virus to begin lytic cycle; DNA excises, virus multiplies.
measuring numbers of virions the plaque assay
Measuring numbers of virions:the Plaque Assay
  • Virus and host cells are mixed
  • Bacteria cover in a Petri dish as a “lawn”, eukaryotic cells cover bottom of a dish.
  • Multiplication of virus leads to release, spread to and destruction of nearby cells.
  • Visible as holes, plaques, on bacterial lawn; eukaryotic cells in culture are first stained for easier view to see plaques.; _230333_cell_culture_300.jpg;

growing viruses
Growing viruses
  • Obligate intracellular parasites: require a host cell!!
  • Whole organism
    • Animal models, human volunteers
      • Ethically, humans require consent, safety, pay
  • Eggs: aseptic incubator
    • Various cells and membranes support growth of viruses
    • Shell provides protection from bacterial contamination
    • Used for large batches of viruses for vaccines
      • Egg allergies a problem sometimes

growing viruses continued
Growing viruses continued
  • Organ/tissue/cell culture
    • Minced tissue, separated into cells by enzyme treatment
    • Grown in sterile plastic dish with nutrient solution
    • Cells prepared this way grow until dish is covered, stop.
      • Scrape up, use some to inoculate new culture
      • Limited number of rounds of replication
    • Transformed cells, with cancer properties, grow forever.
      • Must be subcultured when dish bottom is covered
  • Cell culture major reason for advances in virology