Viruses intimate parasites
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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