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