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VIRUSES AND VIRAL LIKE
AGENTS
WHAT IS A VIRUS?
Intracellular particles that causes diseases
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Common cold
Flu
AIDS
Bird flu
Polio
HIV
Smallopx virus
Aeromonas virus 31
Herpes simplex virus
Electron micrographs of viruses.
Influenza virus
Orf virus
VIRUSES ARE NON-LIVING
In the 1930s, it was discovered that viruses are nonliving
agents composed of only nucleic acids and proteins. Viruses
cause diseases.
FIGURE 2: Size relationships among
microorganisms and viruses
CHARACTERISTICS OF VIRUSES???
• About 5, 000 viruses have been identified, but there is believed
to be around 400, 000 different viruses.
• Believed to be the most abundant biological entity on Earth!!
• Viruses are very small- they do not “grow,” just reproduce
• They cannot reproduce on their own (obligate, intracellular
particles)
• Most can only be seen with the electron microscope, to survive
they must infect and take over a host cell in order to reproduce.
• They lack the chemical machinery to generate energy and
synthesize large molecules.
• Viruses do not have organelles, have no cytoplasm, and no cell
nucleus or nucleoid.
• They do have: nucleic acid core and a protein coat.
WHAT ARE VIRUSES???
• The viral genome contains either DNA or RNA, but does not have
both (like we do).
• They have a protein coat called a capsid that gives shape to the
virus.
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The capsid is what provides protection to the nucleic acid core.
The amino acids make it resistant to temperature, pH and other
environmental fluctuations.
Some capsids have special proteins called spikes, that help the virus
attach to host cells and penetrate cell membranes.
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Most viruses have a membrane that surrounds the nucleocapsid that
is flexible called an envelope.
ORIGIN OF VIRUSES
There Are three hypotheses for the origin of viruses.
• The regressive evolution hypothesis
• Viruses are degenerate life-forms, basically they are derived
from intracellular parasites that have lost life functions.
• The cellular origins hypothesis
• Viruses are derived from subcellular components and
macromolecules that escaped from cell walls and replicated
inside hosts.
• The independent entities hypothesis
• Viruses coevolved with cellular organisms from a selfreplicating molecule present on primitive Earth.
VIRAL STRUCTURE
1) Nucleic acid core
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the genetic material of the virus.
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instructions to make all the viral parts and viral enzymes.
• either DNA or RNA.
2) Capsid layer
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made of protein that surrounds and protects the nucleic acid
core.
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can also be involved in attaching the virus to the cell
membrane of its host.
STRUCTURE CONTINUED
3) Envelope
- not seen in all viruses.
- made of lipid or fat.
- surrounds the capsid layer.
- can also be used to attach to the CM of the host cell.
4) Spike proteins
- not seen in all viruses.
- embedded in the envelope of the virus.
- have a specific shape.
- used to attach the virus to the host’s CM.
FIGURE 5: The components of viruses
VIRAL SHAPE
• Viruses are grouped by the shape of their nucleocapsid symmetry
FIGURE 6: Various viral shapes
Structures compared
From Medical Microbiology, 5th ed., Murray, Rosenthal & Pfaller, Mosby Inc., 2005, Fig. 6-4.
PROPERTIES OF VIRUSES
Various morphologies
polyhedral
helical
spherical
filamentous
complex
Poliovirus
Herpes virus
Ebola virus
Coronavirus
Rabies virus
Lassa virus
SPREAD OF INFLUENZA VIRUS
INFECTION STRATEGIES
• Lytic Cycle:
• The virus enters the cell, makes copies of itself and causes the
cell to burst.
• Uses the materials and cell machinery of the host cell to
replicate itself.
• Lysogenic Cycle
• The virus enters the cell, integrates itself into host DNA and
replicates along with the host.
• The imbedded virus is called a prophage
• After a period of time, the prophage will enter the lytic cycle
FIGURE 14.8: Bacteriophage replication
Figure 10A: Replication of a DNA and RNA
LATENT INFECTIONS
• Many DNA viruses will establish latent infections that are
characterized by repression of most viral genes, so the virus lies
dormant.
• For example herpesviruses can generate latent infections, and
then when some stress come about the DNA of the virus can
produce a new infection.
• Retroviruses like HIV can also lie dormant
• Called a provirus
• Each time the host cells then replicates the provirus is copied.
FIGURE 11: The formation of a provirus by HIV
Viruses are host specific – a protein on the surface of the virus
has a shape that matches a molecule in the plasma membrane
of its host, allowing the virus to lock onto the host cell.
Human
adenoviruses
growing within
the nucleus
of an infected cell
VIRUSES AND TUMORS
• About 60% – 90% of cancers are due to some type of
carcinogen.
• But some viruses can act as carcinogens.
• Some viruses have been isolated from human cancers and they
can cause transformations of normal cells into tumor cells.
• Examples: herpes viruses can cause cervical cancers,
Epstein-Barr virus is linked to Burkitt lymphoma, Human
papilloma virus can cause cervical cancers as well...these
viruses are called oncogenic viruses.
EMERGING VIRUSES
• Many emerging viral infections are due to viruses that appear
for the first time in a population, or developing the ability to
expand their host range.
• Most times this due to the viruses finding new populations
in different geographical areas.
• Some new viruses can form through genetic recombination.
• Influenza virus does this that’s why vaccinations only last
for one year.
• Viruses can also mutate to cause changes in their DNA or RNA
• These mutations can cause the virus to be resistant to
drugs or allow them to affect other populations.
• Increased contact with animals can also allow viruses to
“jump” from one species to another.