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Viruses
Virus Introduction
 A virus is a sub-
microscopic infectious
agent only capable of
reproducing within a
host cell.
 Virus is Latin for toxin or
poison.
Size of Viruses
 less than 0.1 µm in
diameter
 Hundreds of
thousands can fit
inside a typical
human cell.
Virus Introduction
 Biologists debate whether or not viruses are living.
 Some say they are not because they are not composed
of cells or fit any of the other criteria of life.
 Others would argue that since they have been shaped
by evolution through natural selection, they must be
living.
 A viral infection in a higher organism usually invokes
an immune response. Viruses cause disease or illness.
 Antibiotics do not work against viral infection, only
antivirals or vaccines are able to keep a virus at bay.
Viruses in History
 Viruses have been written
about since the 10th century.
 Many Muslim doctors of the
Middle Ages described viral
diseases and how to best
keep them from spreading.
Viruses in History
 The first virus identified was
in tobacco plants, the
tobacco mosaic virus
(TMV).
 The first human virus found
was that which caused
Yellow Fever.
 Scientists then began to
grow and study viruses to
better understand them.
The Origin of the Virus
 The origin of the modern virus is unclear.
 Two hypotheses exist:
1) They could be runaway stretches of nucleic acid from
a larger organism that detached and became active,
therefore new viruses are forming frequently and
many do not have ancestors
2) Viruses once lived outside of host cells, but over time
due to their parasitic lifestyle, they lost the genes
necessary to live outside the host
Classification of Viruses
 The Linnean classification system is modified for
viruses since they do not fit into any of the current
three Domains.
 There has been talk of a new Domain, Domain
Acytota, meaning ‘without cells’ for the viruses.
 Currently they have Phylum down to Species.
 Nobel Prize winner, David Baltimore has developed
his own classification system for viruses, the Baltimore
Classification System.
Baltimore Classification System
 Classifies viruses based
on:
1) Type of nucleic acid
(DNA or RNA)
2)Single stranded or
double stranded
3) Whether or not they use
reverse transcriptase.
(work backwards going
from RNA to DNA)
Structure of a Virus
 Viruses are composed of
genetic material, either
RNA or DNA, and a
protective protein layer
known as a capsid.
 Viruses can have a lipid
envelope made from the
host cell’s cell
membrane.
Diagrams of Viruses
Virus Lytic Cycle
http://www.youtube.com/watch?v=wVkCyU5aeeU&feature=related
http://www.youtube.com/watch?v=rzdwfwuVWUU&feature=related
http://www.mcgrawhill.ca/school/schoolGraphics/biology2_1.mpg
Virus Lytic Cycle (also called the infectious cycle)
 There are 5 parts to the viral lytic cycle:
1) Attachment: the virus must attach to the cell wall or cell
membrane of the host.
2) Penetration: the virus must enter the cell through cell
membrane fusion or endocytosis.
3) Uncoating: the protein coat is worn away exposing the viral
genome.
4) Replication and Assembly: the viral genome takes over the
host cell’s machinery and creates more viruses.
5) Lysis: The new virus particles rupture from the host cell.
Lysogenic Cycle
 Lysogenic Cycle: enter a host cell but do not take over
the activity by destroying the host DNA, but combines
with and becomes part of the host DNA in a way that
does not interfere with the host cell activity.
 When the host copies its own DNA, it copies the viral DNA
as well for many generations in lysogeny

Changing conditions cause the phage to enter the lytic cycle.
 Bacteria are not defenseless against phage infections.
Some bacteria have enzymes called restriction
enzymes that can recognize foreign DNA, and cut up
the DNA, making it useless.
http://www.youtube.com/watch?v=_J9-xKitsd0&feature=related
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Differences Between
Lytic and Lysogenic Cycles
In the Lytic Cycle:
 Viral DNA destroys Cell DNA, takes over cell functions and
destroys the cell.
 The virus replicates and produces progeny phages.
 There are symptoms of viral infection.
In the Lysogenic Cycle:
 Viral DNA merges with cell DNA and does not destroy the cell.
 The virus does not produce progeny.
 There are no symptoms of viral infection.
http://www.brighthub.com/science/genetics/articles/30611.aspx
DNA Viruses
 These usually infect a
host through a chance
encounter with the virus.
 These include some of
the more well known
viruses such as the
herpes virus and any of
the pox viruses such as
smallpox or chicken pox.
RNA Viruses
 Use RNA as their genetic
material or use an RNA
intermediate to
replicate.
 These include the
Rubella virus which
causes the German
Measles.
Reverse transcripting viruses
 These convert RNA into
DNA and incorporate it
into a host cell.
 Often called retroviruses
because they work
backwards.
 HIV is the most well
known of these types of
viruses.
RNA or DNA?
Viruses with RNA (higher mutation rate)
 Human immunodeficiency virus (HIV)
 Influenza viruses
 Rabies
 Measles, mumps, pneunomia, polio, common cold
 SARS
Viruses with DNA (usually stable/constant, vaccines effective )
 Chickenpox, cold sores, genital herpes
 Mononucleosis
 Hepatitis
 Respiratory infections, tumours
Viruses and Disease
 Viruses cause some of the
most common illnesses
such as the common cold,
the flu, cold sores, etc…
 Viruses also cause some of
the deadliest diseases
known to man, Ebola,
Avian Flu, AIDS, etc…
 The virulence of a virus is
its ability to cause disease.
Applications of Viruses




Viruses have been used to
study the basic
mechanisms of molecular
biology such as DNA
replication, protein
synthesis, etc…
Viruses have also been used
by geneticists to genetically
modify organisms.
Virotherapy uses viruses to
treat bacterial diseases and
some forms of cancer.
Viruses are also being used
as biological insecticides.
Applications of Viruses
 Viruses are currently
being used in
nanotechnology acting
as tools to stop or start
sequences needed in
host cells.
 Viruses have also been
explored as a viable
biological weapon. Scary
thought!
Viroids
 Small, infectious pieces of RNA that are smaller than
viruses and don’t have a capsid.
 Plant pathogens that interfere with normal
functioning of the host cells RNA.
Prions
 Abnormally shaped infectious protein responsible for
some brain diseases of mammals.
 Found in the brain and nervous tissue
 EG: bovine spongiform encephalopathy (BSE) or “mad
cow disease”
PRIONS – man-made
problem?
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End on a cuddlier, happier note…