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VIRUSES. - Montgomery College

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VIRUSES.
Introduction.
DISCOVERY OF VIRUSES.
• The discovery of viruses resulted from the
search for the infectious agent causing
tobacco mosaic plants and gives their leaves
a mosaic coloration.
ADOLF MEYER.
• A German scientist demonstrated that the
disease was contagious and proposed that
the infectious agent was an unusually small
bacterium that could not be seen with a
microscope.
• He successfully transmitted the disease by
spraying sap from infected plants onto the
healthy ones.
• Using a microscope, he examined the sap
and was unable to identify a microbe.
D. IVANOWSKY.
• 1890: A Russian scientist proposed that
tobacco mosaic disease was caused by a
bacterium that was either too small to be
trapped by a filter or that produced a
filterable toxin.
• To remove bacteria, he filtered sap from
infected leaves.
• Filtered sap still transmitted disease to
healthy plants.
MARTINUS BEIJERINCK.
• 1897: A Dutch microbiologist proposed that
the disease was caused by a reproducing
particle much smaller and simpler than a
bacterium.
• He ruled out the theory that a filterable
toxin caused the disease by demonstrating
that the infectious agent in filtered sap could
reproduce.
• Plants were sprayed with filtered sap from
disease plants-----> sprayed plants
developed tobacco mosaic disease----->sap
from newly infected plants was used to
infect others.
WENDELL. M. STANLEY.
• 1935:An American Biologist, from the
Rockefeller Institute, crystallized the
infectious particle now known as Tobacco
Mosaic Virus(TMV).
• The purified virus precipitated in the form
of crystals.
• He was able to show that viruses can be
better regarded as chemical matter, than as
living organisms.
• Crystals retained the ability to infect healthy
tissue.
• Subsequent determination of chemical
nature of TMV: Protein in combo with
nucleic, TMV were rods 300 nanometers
long, TMV was RNA surrounded by protein
coat.
GENERAL
CHARACTERISTICS.
• Obligatory Intracellular Parasites.
• Protein coated fragments of DNA or RNA
that have become detached from the
genomes of cells. Because they cannot
replicate on their own, they are not
organisms.
• Viruses are generally host-specific.
• They reproduce only within a certain host.
• Should be as many viruses as there are
kinds of organisms.
VIRUS-HOST RANGE.
• The host range of a virus is the spectrum of
host cells the virus can infect.
• Some viruses have broad host ranges which
may include several species(e.g. swine flu
and rabies).
• Some viruses have host ranges so narrow
that they can:
• infect only one species(e.g. phages of E.coli
• Infect only a single tissue type of one
species(e.g. human cold virus infects only
cells of the URT; AIDS virus binds only to
specific receptors on certain white blood
cells.
VIRAL SIZE.
• In the 1950’s, TMV and other viruses were
finally observed with electron microscopes.
• Smallest = 17 nanometers in diameter.
• Largest = 1000 nanometers(1 micrometer)
in greatest dimension.
• Few are barely visible at light microscope
level.
• Most are visible only via EM.
VIRAL STRUCTURE.
• The virus or virion, is just nucleic acid
enclosed by a protein coat, it’s a complete,
fully developed infectious viral particle that
is a vehicle of transmission from one host to
another.
• Viruses are classified by differences in the
structures of these coats.
NUCLEIC ACID- VIRAL
GENOMES.
• Depending on the virus, viral genomes:
• May be double-stranded DNA, singlestranded DNA, double-stranded RNA or
single-stranded RNA.
• Are organized as single nucleic acid
molecules that are either linear or circular.
• May have as few as four genes or as many
as several hundred.
CAPSID AND ENVELOPE.
• Capsid: Protein coat that encloses the viral
genome.
• It’s structure may be rod-shaped, polyhedral
or complex.
• Composed of many capsomeres: protein
subunits made from only one or a few types
of protein.
ENVELOPE.
• Membrane that cloaks some viral capsid:
• Helps viruses infect their host.
• Derived from host cell membrane which is
usually virus-modified and contains
proteins and glycoproteins of viral origin.
GENERAL MORPHOLOGY.
•
•
•
•
Helical viruses.
Polyhedral viruses.
Enveloped viruses.
Complex viruses.
HELICAL VIRUSES.
• Resemble long rods.
• Maybe rigid or
flexible.
• Viral Genome found
inside a hollow
cylindrical capsid.
• E.G.: Ebola virus and
Rabies virus.
POLYHEDRAL VIRUSES.
• Many-sided Capsid is
in the shape of
icosahedron( a
polyhedral with 20
triangular faces).
• E.G.: Adenovirus, and
poliovirus.
ENVELOPED VIRUSES.
• Roughly spherical.
• Enveloped-helical or
enveloped polyhedral
viruses.
• Enveloped helical =
Influenzae virus.
• Enveloped polyhedral
= Herpes simplex
virus.
COMPLEX VIRUSES.
• Bacteriophage.
• Capsid(head) is
polyhedral, tail sheath
is helical.
• Tail fibers, plate and
pin.
TAXONOMY OF VIRUSES.
• Viruses are not organisms and are not
classified in the kingdoms of life.
• Regarded as self-replicating portions of the
genomes of organisms.
• In comparison to living things, vruses are
acellular(not cells and do not consist of
cells), do not metabolize energy, no
photosynthesis, cell respiration or ferment.
• Viruses are cell parasites as they infect
organisms at all taxonomic levels.
• Oldest classification based on symptoms.
• ICTV group vruses into families, based on:
• Nucleic acid type, strategy for replication,
and morphology.
• E.G.: Family= ends with viridae.
• Genus= virus.
ISOLATION, CULTIVATION,
AND IDENTIFICATION.
• Cannot replicate outside a living cell: poses
problem with detection, ID, and cultivation.
• Bacteriophages are easily grown on
bacterial cultures.
• This is the reason why so much of what is
known of viral multiplication has come
from from bacteriophage.
GROWING VRUSES IN THE
LAB.
•
•
•
•
•
Plaque method: Bacteriophages.
Living animals: Animal viruses.
Embryonated eggs: Animal viruses.
Cell cultures: Animal viruses (CPE).
Primary cell lines, Diploid cell lines, and
continuous cell lines.
VIRAL MULTIPLICATION.
• Viruses are nonliving particles that
reproduce only inside specific host cells.
• Exhibit many patterns of viral life cycle, but
they generally include:
• Coopting host ceel’s resources to:
• Manufacture capsid protein,
• Assemble newly produced viral nucleic acid
and capsomeres.
• Several mechanisms used to infect host
cells with viral DNA.
• For example, T-even phages use an
elaborate tailpiece to inject DNA into the
host cell.
• Once the viral genome is inside its host cell,
it commandeers the host’s resources and
reprograms the cell to copy the viral genes
and manufacture capsid protein.
• Three possible patterns of viral genome
replication:
• 1. DNA---->DNA: If viral DNA is doublestranded, DNA replication resembles that of
cellular DNA, and the virus uses DNA
polymerase produced by the host.
• 2. RNA----->RNA: Since host cells lack the
enzyme to copy RNA, most RNA viruses
contain a gene that codes for RNA replicase
• RNA replicase is an enzyme that uses viral
RNA as a template to produce
complementary RNA.
• 3. RNA----->DNA----->RNA: Some RNA
viruses encode reverse transcriptase, an
enzyme that transcribes DNA from an RNA
template.
MULTIPLICATION OF
BACTERIOPHAGES.
• Studies on lambda phage of E. coli showed
that double-stranded DNA viruses
reproduce by two alternative mechanisms:
The lytic cycle and the lysogenic cycle.
LYTIC AND LYSOGENIC
CYCLES.
• Lytic cycle:
• Results in the death or
lysis of the host cell.
• Bacteriophage takes
over the machinery of
the cell, so viral
replication and release
occur.
• Lysogenic cycle:
• Involves the
incorporation of the
viral genome into the
host cell genome.
• Phage becomes a
prophage, integrated
into the host genome.
• Virulent bacteriophage
reproduce by a lytic
replication cycle= that
lyse their host cells.
• Later, the phage may
reenter the lytic cycle
and replicate itself.
• Temperate viruses=
integrate and remain
latent.
THE LYTIC CYCE.
•
•
•
•
•
Attachment.
Penetration.
Biosynthesis.
Maturation.
Release.
THE LYTIC CYCLE.
THE LYSOGENIC CYCLE.
•
•
•
•
Lysogeny.
Prophage.
Phage conversion.
Specilized transduction.
THE LYSOGENIC CYCLE.
MULTIPLICATION OF
ANIMAL VIRUSES.
•
•
•
•
•
•
Attachment.
Penetration.
Uncoating.
Biosynthesis of DNA viruses.
Biosynthesis of RNA viruses.
Maturation and Release.
DNA-CONTAINING VIRAL
MULTIPLICATION.
RNA-CONTAINING VIRAL
MULTIPLICATION.
RNA VIRAL
MULTIPLICATION CONTD.
VIRUSES AND CANCER.
•
•
•
•
Oncogenes.
Oncogenic viruses.
Contact Inhibition.
Tumor-specific transplantation antigen
(TSTA ).
LATENT VIRAL INFECTIONS.
•
•
•
•
Latent Infection:
Herpes simplex.
Chicken pox (varicella).
Shingles(zoster).
PERSISTENT VIRAL
INFECTIONS.
• Persistent viral infections: Encephalitis
called subacute sclerosing
panencephalitis(SSPE).
PRIONS.
• Proteinaceous infectious particle that lacks
nucleic acid.
• Nine animal diseases fall into this category.
• All nine are neurological diseases called
spongiform encephalopathies.
• These diseases run in families, which
indicates a possible genetic cause
SPOKE TOO SOON.
• However, The diseases cannot be purely
inherited because mad cow disease arose
from feeding scrapie-infected sheep meat to
cattle, and the new (bovine) variant was
transmitted to humans who ate undercooked
beef from infected cattle.
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