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VIRUSES
What are Viruses?
• A virus is a non-cellular
particle made up of genetic
material and protein that can
invade living cells.
• The smallest biological
particle that is capable of
causing diseases in living
organisms
A Virus Has Two Essential Features
• A Nucleic Acid
– DNA or
– RNA
– But not both
• A Capsid – a protein
coat surrounding the
nucleic acid.
Viruses with RNA are called
retroviruses.
What are Viruses?
A virus is an infectious agent made up of nucleic acid
(DNA or RNA) wrapped in a protein coat called a capsid.
Viruses have no nucleus, no organelles, no cytoplasm or
cell membrane. This is why it does not belong to any
kingdom.
vs
Not Considered Living
• A virus is not a bacteria,
fungus, protist, plant or
animal.
It needs a host!
• They can not carry out
cellular functions.
• A virus can not replicate
without infecting cells
and then using the
organelles and enzymes
of the cells of the host.
Viruses are parasites —
an organism that depends entirely
upon another living organism for
its existence in such a way that it
harms that organism.
Who do viruses infect?
Viruses usually infect a specific host including:
• Viruses infect Bacteria
– These viruses are called bacteriophages
• Viruses infect Plants
– One example is the Tobacco Mosaic Virus
• Viruses infect Animals
– One example is the common cold
Bacteriophage—viruses that infect bacteria
Capsid (protein coat)
– inside contains either
RNA or DNA
Flu (influenza), HIV
DNA or RNA
Surface
Marker
Capsid (protein coat)
Acute Virus Infection
Amount of virus
Symptoms
Virus
Time
Why are some viruses harmful?
When your cells make viruses instead of operating
normally, you get sick
Virus invades cell
Virus forces cell to make copies of virus
Eventually so many copies are made, the
cell explodes,
releasing all of the new viruses
Replication Phases
I, II, III - Viruses enter cell
-
Phase I
Attachment to cell membrane
Penetration inside cell
Losing virus protein coat
IV - Replication
-
Tricks cell into making
more viral DNA
Tricks cell into making
viral protein coat
Phase II
V - Release
-
-
Assembly of virus
DNA and protein coat
into whole new
viruses
Leaving the cell
Phase III
Phase IV
Phase V
Bacteriophage Replication
Bacterial
cell wall
Bacterial
chromosome
Capsid
DNA
Capsid
1
Sheath
Attachment:
Phage
attaches to
host cell.
Tail fiber
Base plate
Pin
Cell wall
Tail
Plasma membrane
2
Penetration:
Phage pnetrates
host cell and
injects its DNA.
Sheath contracted
Tail core
3
Merozoites
released into
bloodsteam from
liver may infect
new red blood cells
11
Bacteriophage Replication
Tail
DNA
4 Maturation:
Viral components
are assembled into
virions.
Capsid
5 Release:
Host cell lyses
and new virions
are released.
Tail fibers
12
Examples of some viral diseases:
DISEASE
VIRUSES
AIDS
HIV
Wart
Herpes Simplex
Virus
Flu
Influenza
Measles
Morbillivirus .
Cancer
Hepatitis B
Viral Shapes
Viruses come in a variety of shapes
•Some may be helical shape like the Ebola virus
•Some may be polyhedral shapes like the influenza virus
•Others have more complex shapes like bacteriophages
Helical Viruses
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Polyhedral Viruses
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Complex Viruses
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Where viruses have come from?
4 Billion Years Ago
The surface of the planet is just cooling and beginning to harden into a crust. Rain forms
pools containing many organic molecules and the first simple life forms appear.
The first viruses also appear. It is not clear where they have come from.
Where viruses have come from?
•Regressive evolution - maybe these early viruses are degenerate lifeforms which have lost many functions that other organisms possess and
have only retained the genetic information essential to their parasitic
way of life.
•Cellular origins - perhaps they are sub-cellular, functional assemblies
of macromolecules which have escaped their origins inside primitive
cells.
•Independent entities - or maybe they just evolved from the selfreplicating molecules believed to have existed in the primitive prebiotic
'RNA world' along a parallel course to cellular organisms.
100 Million Years Ago. The Cretaceous Period.
Dinosaurs roam the earth.
The supercontinent Pangaea has begun to break up, but the
continents have still not drifted into their present positions.
The climate is much hotter and drier than today.
Way out in space, a big meteorite is tumbling towards Earth ...
The Year 3700 BC
The first written record of a virus infection
consists of a hieroglyph from Memphis,
the capital of ancient Egypt, drawn in
approximately 3700 BC, which depicts a
temple priest called Ruma showing typical
clinical signs of paralytic poliomyelitis.
Poliomyelitis (Polio) is a viral disease which
may affect the spinal cord causing muscle
weakness and paralysis.
The Year 1193 BC
The Pharaoh Siptah rules Egypt from 1200-1193 BC when he dies
suddenly at the age of about 20.
His mummified body lays undisturbed in his tomb in the Valley of the
Kings until 1905 when the tomb was excavated.
The mummy shows that his left leg was
withered and his foot was rigidly extended
like a horse's hoof - classic paralytic
poliomyelitis
The Year 1143 BC
Ramesses V's preserved mummy shows that he died of
smallpox at about the age of 35 in 1143 BC.
The lesions on the face of the mummy are very similar to those of more recent patients.
Smallpox is a disease caused by the Variola major virus.
Some experts say that over the centuries it has killed more people
than all other infectious diseases combined.
The Year 1520
Smallpox, which had reached Europe from the East in 710 A.D., was transferred to
the Americas by Hernando Cortez.
3,500,000 Aztecs died in the next 2 years - effectively the end of the Aztec empire.
The Year 1796
On 14th May 1796, Edward Jenner vaccinated an 8 year
old boy, James Phipps, with material from a cowpox
lesion on the hand of a milkmaid, Sarah Nelmes.
James, who had never had smallpox, developed a small
lesion at the site of vaccination which healed in 2 weeks.
On 1st July 1796, Jenner challenged the boy by
deliberately inoculating him with material from a real
case of smallpox!
Deadly viruses are said to be virulent
Smallpox has been eradicated in the world today
The Year 1886
John Buist stained lymph from skin lesions of a smallpox patient and saw
"elementary bodies" which he thought were the spores of micrococci.
These were in fact smallpox virus particles - just large enough to see with
the light microscope.
•Viruses are smaller than the smallest cell, measured in nanometers
•Most of the viruses couldn’t be seen until the electron microscope was invented in
the 20th century
Size of Viruses
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The Year 1892
Dmitri Iwanowski, a Russian botanist, presents a paper to the
St. Petersburg Academy of Science which shows that extracts
from diseased tobacco plants can transmit disease to other
plants after passage through ceramic filters fine enough to
retain the smallest known bacteria.
This is generally recognised as the beginning of Virology.
Unfortunately, neither Iwanowski nor the scientific
community fully realize the significance of these results.
Walter Reed (1851-1902)
During the Spanish-American War and
subsequent building of the Panama Canal,
American deaths due to yellow fever were
colossal. The disease also appeared to be
spreading slowly northward into the
continental United States.
Through experimental transmission to mice,
in 1900 Walter Reed demonstrated that
yellow fever was caused by a virus, spread
by mosquitoes.
Karl Landsteiner (1868-1943)
Karl Landsteiner (1868-1943) and
Erwin Popper proved that
poliomyelitis was caused by a virus.
Landsteiner and Popper were the first
to prove that viruses could infect
humans as well as animals.
Francis Peyton Rous (1879-1970)
Francis Peyton Rous (1879-1970)
demonstrated that a virus (Rous
sarcoma virus) can cause cancer in
chickens. (For this work, he was
eventually awarded the Nobel Prize, in
1966.
Rous is the first person to show that a
virus could cause cancer in animals.
Treatment and Prevention of
Virus Infections
• Antivirals
• Vaccines and immunisation
Antiviral Targets
• Attachment/Entry
• Nucleic acid replication
• Virus protein processing
• Virus maturation
Vaccines
• An attenuated virus is a weakened, less vigorous
virus
• “Attenuate" refers to procedures that weaken an
agent of disease (heating)
• A vaccine against a viral disease can be made from
an attenuated, less virulent strain of the virus
• Attenuated virus is capable of stimulating an
immune response and creating immunity, but not
causing illness
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Incidence of Poliomyelitis
A
B
Number of cases (in thousands)
40
Poliovirus vaccines
A: Salk – killed inactivated
vaccine.
B: Sabin – live attenuated
vaccine
30
20
10
0
1950
1960
1970
1980
Problems with Antivirals
• Identification of virus-specific
target.
• Generation of resistant variants.
Weapons
• The ability of viruses to cause devastating epidemics has led to
the concern that viruses could be weaponised for biological
warfare.
• Further concern was raised by the successful recreation of the
infamous 1918 influenza virus in a laboratory.
• The smallpox virus devastated numerous societies throughout
history before its eradication.
• There are only two centers in the world that are authorized by the
WHO to keep stocks of smallpox virus: the Vector Institute in
Russia and the Centers for Disease Control and Prevention in the
United States.
• However as the vaccine for smallpox is no longer used routinely
in any country, much of the modern human population has no
established resistance to smallpox.