Download Virus PowerPoint

• Virology – Study of viruses
The word virus comes from a Latin
word meaning poison.
• “Humans have long been
familiar with the ravages of
viruses, if not the creatures
themselves. Dried pustules
on the mummified face of
Ramses V testify to the fact
smallpox killed even in mighty
Egypt 3,000 years ago.”
–Time Magazine Nov 3, 1986
Edward Jenner
• (1798) He noticed that farmhands who contracted
cowpox, a mild disease related to smallpox, did not
develop the more deadly disease. Jenner inoculated
a boy with material from a milkmaid’s cowpox sore,
then demonstrated, that the boy and developed
immunity to smallpox. This may be considered the
first vaccine.
Louis Pasteur
• (Late 1800s) He developed a vaccine of a
weakened virus to protect a young boy
against rabies.
Dmitri Ivanovski
• (1892) –pinpointed the cause of tobacco
mosaic disease to the juice extracted from
infected tobacco plants.
Martinus Beijerinck
• (1897) – He determined that tiny particles
in the juice from the Tobacco leaves
caused the disease. He names
these particles viruses.
Dr. Wendall Stanley (1935)
• He was the first to isolate & crystallize a virus
(TMV). Since a virus can be crystallized, it became
clear that they were not really living things.
• He removed fluid from approximately one ton of
infected and diseased tobacco leaves. He obtained
one teaspoon of crystal substance. (He shared the
Nobel prize with for his work)
The invention of the electron microscope
allowed biologists to see viruses, which
are too small to be seen by light
microscopes.
Viruses don’t share characteristics of living
organisms. Are they Alive?
They have no nucleus, cytoplasm or organelle.
They need no nourishment.
They have no cell membrane.
They have no metabolic processes.
They have no means of locomotion.
They do not respond to their environment
(except when they come in contact with the
correct receptor site on a host cell.)
Virus Characteristics:
• A typical virus is composed of a core of either DNA
or RNA surrounded by a protein coat called a
capsid. (usually about 95% of the total mass)
• Some capsids are surrounded
by a membrane envelope. These
are referred to as enveloped viruses.
The envelope allows the virus to
attach to their host cell.
• Viruses may have a few genes
or as many as a thousand.
Viral Shape:
• Virus shape may be determined by its capsid, or its
nucleic acid. Common viral shapes are:
• Helix-Shape (rod)– Tobacco Mosaic
• Icosohedron – 20 triangular faces
Cubic Symmetry
• Example: Adenovirus – a respiratory tract virus.
It has spikes to attach to host cell. It has 252
“capsomeres” and reproduces in nucleus of the
host cell.
Bacteriophage: Virus that
infects bacterial cells.
“Phage”
HIV -Structure
(Human Immunodeficiency Virus)
• HIV is a retrovirus, which means that it carries RNA
instead of DNA, and has an enzyme called Reverse
Transcriptase. This enzyme uses the RNA as a
template to make DNA. The viral DNA becomes
incorporated in the
host cell.
• The virus, like others (such as
chicken pox or influenza)
is surrounded by an envelope of
mostly lipids. On the surface are
projections made of glycoproteins,
which are protein containing sugar
chains that the virus used to attach to
its host cell.
• This eventually destroys the cell, weakening the
immune system.
• A person gets sick from opportunistic
infections that a normal immune system could
stop. (These may include cancer, fungal
infection, bacterial infections etc.).
VACCINE (from the word “vaccinus” Latin “pertaining to cows.)
Definition:
• A preparation of pathogens or other materials
that stimulates the body’s immune system to
provide protection against that pathogen
• Attenuated – viruses that have been genetically
altered so that they are incapable of causing
disease under normal circumstances.
VIROIDS AND PRIONS:
• Viroids are the smallest known diseasecausing agents that are able to replicate. It
consists of a short, single strand of RNA
and has no capsid. They cause disease in
plants.
• Prions – normal proteins called prions can
become abnormal forms of proteins that
clump together inside a cell, perhaps by
blocking the cells “molecular traffic.”
(commonly about 250 amino acids long).
They cause other prions in the brain to
become abnormal.
Prions are linked to brain
diseases in humans and animals.
• Kuru – Anxiety yields to trembling, loss of
coordination and death in 16 months. (It was
once contracted by cannibals in New Guinea
from eating infected human brains)
• Mad Cow Disease is a Prion disease that affects
cow brains.
Creutzfeldt-Jakob (CJD)
• Human form of the Bovine spongiform
encephalopathy (BSE) AKA “Mad Cow
Disease” Begins with depression & memory
problems. In 4-6 months it progresses to
dementia, uncontrollable jerking of muscles and
finally death.
• The first known prion disease was scrapie, which has
infected sheep for many years. In the 1980s, Britain had
an outbreak of bovine spongiform encephalopathy, better
known as mad cow disease, which spread to Europe and
other areas. Two cows have been found with the illness
in the United States.
• The human form of the illness is called variant
Creutzfeld-Jakob disease and is believed to have
originated from eating infected beef. It has killed about
180 people worldwide. Symptoms can take years to
develop.
• ALSO – “Fatal Insomina” Prion disease
in humans: Progressive insomnia leads to
panic attacks, phobias, hallucinations and
finally dementia. Death occurs within 18
months.
ANTIVIRAL APPROACHS:
• Acyclovir – used against herpes simplex and
Chicken pox (Varicella)
• AZT – inhibits reverse
transcriptase in HIV
• Protease Inhibitors – interfere with the
synthesis of viral capsids during viral replication.
• ANTIBIOTICS ARE NOT USEFUL
AGAINST VIRUSES!
• Virus particles have been used to insert
genes into cells during recombinant
DNA activities.
A bacteriophage that infects E. coli is used.
The virus’ own genes are removed from the capsid
and replaced by new gene --- the gene that codes
for the human protein.
Reproduction by the Lytic Cycle
• During the Lytic Cycle a virus invades the host cell,
produces new viruses, destroys the host cells , and
releases newly formed viruses. Viruses that
undergo the lytic cycle are considered virulent
because they can cause disease.
• 1) Attachment – the
bacteriophage attaches its
tail fibers to the bacterial
cell receptor site.
• 2) Entry – The virus releases
an enzyme that weakens the
cell wall of the host cell. It then
injects its DNA into the host
cell. (It leaves the capsid
outside the host cell.)
• 3) Replication – The virus
takes control of the bacterial
cell’s protein synthesizing
machinery, transcribing mRNA
from viral DNA. Viral proteins
are formed as well a new viral
capsids. Viral genes are replicated.
• 4) Assembly – The replicated
viral genes are enclosed within
newly created virus capsids. (The assembly of
new virus particles usually occurs in the
cytoplasm of the host cell, but it may also take in
the eukaryotic cell nucleus.)
• 5) Release (Lysis) – Enzymes are produced
that cause the host cell to disintegrate, releasing
the new bacteriophages.
• In enveloped viruses, the virus leaves with a piece of the
membrane surrounding the capsid. This becomes the
viral envelope.
The Lysogenic Cycle
• A virus that replicates through the
lysogenic cycle does not kill the host cell
immediately. It may stay in the host for
days, months or even years.
• The virus in this cycle is called a temperate
virus. The virus DNA is incorporated in the
cell DNA, at a site in the host cell genome, is
called a prophage. (provirus) The prophage
is replicated with the cell DNA and is passed
to daughter cells.
• When the prophage
becomes virulent,
(reproduces & can
cause disease)
due to certain
chemicals or
radiation, it enters
the lytic cycle,
proceeding with
replication and
destroys the host
cell.
Chickenpox and Shingles
• Caused by varicella-zoster herpes virus
• Virus multiplies in lungs and travels to blood vessels
in the skin. (symptoms: fever & skin rash)
• It spreads through air or direct contact
• It can stay in nerve cells as a provirus.
• It can later cause a disease known as shingles
(causes a painful rash)