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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) – PowerPoint PPT presentation

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Title: Viruses: intimate parasites


1
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

2
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

3
Viruses may have other parts
  • Envelope piece of organelle 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

4
Viral size and shape
  • Viruses range from 30 nm to 300 nm
  • Ribosomes are about 30 nm
  • The smallest known bacteria are about 200 nm
  • Viral shapes
  • helical, polyhedral, and complex

http//www.glencoe.com/qe/images/b136/q4323/ch18_0
_a.jpg www.blc.arizona.edu/.../
Figures/Icos_Virus.GIF http//www.foresight.org/U
pdates/Update48/Images/T4Schematic.jpg
5
Examples of virus shapes
Ebola
Adenovirus
http//www-cgi.cnn.com/HEALTH/9604/16/nfm/ebola.le
vine/ebola.reston.large.jpg http//www.virology.n
et/Big_Virology/EM/Adeno-FD.jpg
6
Specificity
  • 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?

7
Classification
  • 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.

8
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.

http//www.tthhivclinic.com/Fig_20.gif
9
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.

10
Measuring numbers of virionsthe 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.

http//dept.kent.edu/biology/Courses/30171/imageQB
T.JPG news.bbc.co.uk/.../ _230333_cell_culture_3
00.jpg
11
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.

12
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

http//www.fao.org/ag/againfo/foto/egg-facts.gif
13
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

14
Viruses and disease
  • Most of discomfort, some of damage, is due to
    host defense response to viral infection.
  • Cytopathic effects damage caused by viruses
  • Cells change shape, change size (round up, swell)
  • Cells become sticky (clump, fuse to form
    syncytia)
  • Syncytia are giant, multinucleated cells
  • Cells change internally (vacuoles, inclusions)
  • Location, appearance of inclusions characteristic
    of certain viruses comprised of viral parts or
    cell debris
  • Cells die.

15
Other effects of viruses
  • Transformation cells become cancer-like
  • Lose contact inhibition
  • Continue to multiply
  • In vivo, produce tumors
  • Certain Herpes-type viruses, genital wart virus,
    etc.
  • Some viruses cause birth defects teratogenesis
  • Damage to critical cells in fetus, failure to
    form parts
  • Typically, virus is mild and able to cross
    placenta
  • Rubella (German measles virus), CMV

16
NOT viruses
  • Viroids naked RNA
  • Cause of infectious disease in plants
  • Prions infectious protein particles
  • Got Stanley Prusiner the Nobel Prize, but still
    controversial.
  • Cause of Mad Cow Disease, Scrapie, Elk Wasting
    disease, Creutzfeldt-Jakob disease, Kuru, etc.
  • Transmissible spongioform encephalopathies
  • Misshapen protein causes normal protein to also
    fold incorrectly, increasing their numbers
  • Replication without the presence of nucleic acid
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