Download What are Viruses?

Discovery of…
(1884: invention of the Chamberland filter with pores smaller than bacteria)
• 1892: Russian biologist Dmitri Ivanovsky publishes
a paper in which shows that extracts from diseased
tobacco plants remain infectious after filtration
through Chamberland filter. MEANING?
• 1898: Dutch microbiologist Martinus Beijerinck
postulates a new form of “filterable”
infectious agent – term virus introduced.
Dmitri Ivanovsky
By the end of the 19th century, viruses were defined in terms of
their infectivity, their ability to be filtered, and their requirement
for living hosts, but…
… they still could not be directly observed!
(1930: invention of the electron microscope)
• 1931: first images of viruses obtained using
electron microscopy by German engineers Ernst
Ruska and Max Knoll.
What are Viruses?
A virus is a non-cellular biological particle made of
genetic material (DNA or RNA) and protein coat.
• From the Latin vīrus referring to poison.
• Infect living cells to reproduce.
• Capable of causing diseases.
• Co-exist with life everywhere.
• About 5,000 viruses have been
studied and described in detail,
although there are millions of
different types.
• The origin of viruses remains unclear
because they do not form fossils…
Who do viruses infect?
Specific viruses usually infect a specific host.
• Viruses infect Bacteria
– These viruses are called bacteriophages.
– Harmless to animals/humans and can be studied easily.
– Lately there is growing interest in the use of
bacteriophages to treat bacterial infections in humans.
• Viruses infect Plants
– One example is the tobacco mosaic virus.
– Often they only cause a loss of crop yield, so it is not
economically viable to try to control them.
• Viruses infect Animals
– Can cause different diseases depending on the types
of cell that they infect; some can cause lifelong or longterm (chronic) infections.
– Viral infections in animals provoke an immune response
that usually eliminates the infecting virus.
Virus Structure
A virus particle, called a virion, consists of:
Capsid
• Genes – nucleic acid
Influenza
 DNA or
 RNA (retroviruses)
 But not both!
• Capsid
 a protein coat
surrounding
the nucleic acid
Bacteriophage
Envelope
• (Optional) Envelope
 some viruses are enclosed by a bubble of lipid (fat)
Virus Shapes
Helical
Polyhedral
Spherical
Complex
Influenza
Bacteriophage
Scale Sense
Most viruses
range in size from
20 nm to 300 nm;
some are almost 1 µm!
Viruses are NOT Considered Living
Viruses do not belong to any kingdom…
vs
• Have no nucleus, no organelles, no cytoplasm or cell membrane.
• Can not carry out cellular functions – they are not living cells!
• Cannot reproduce outside the host cells: need to use the
organelles and enzymes of the invaded cells.
Virus Replication
Viruses can only reproduce inside the cells of living organisms:
invade living cells and force them produce many thousands of
identical copies of the original virus.
5. New viruses
1. Virus attaches to
a specific host cell.
are released as
the host cell
bursts open.
3. Virus genetic
material is both
replicated
(copies made)
and translated
(virus proteins
made).
2. Virus genetic
material enters
the host cell.
4. New virions
assemble
inside of the
host cell.
Some viruses do not destroy
the host cell but continue to
use its resources to replicate
for a prolonged period of time.
S
days
Symptoms
S
S
S
S
S
S
Host Defense
Innate immune system
Adaptive immune system
Hours
Days
• production
of specific
killer T cells
“first-aid” cells
that recognize and
destroy pathogens
in a generic way
• production of specific proteins
(antibodies) that bind to the
virus and make it non-infectious
- unable to penetrate host cells
Vaccination
Vaccines were used to prevent viral infections
long before the discovery of the actual viruses…
• A vaccine against a viral disease can be made
from an attenuated (weakened, less vigorous), less
virulent (active/infectious) strain of the virus.
• Edward Jenner (smallpox vaccine, 1796 ) and
Louis Pasteur (rabies, 1886) were the first to
develop vaccines to protect against viral
infections.
Edward Jenner
Attenuated virus is capable of stimulating an immune
response and creating immunity, but not causing illness.
• Worldwide use of vaccines has resulted in a dramatic decline
in morbidity (illness) and mortality (death) associated with
viral infections such as polio, measles, mumps and rubella;
smallpox have been eradicated; polio is expected to be
eradicated in the nearest future.
12
Examples of Viral Diseases
DISEASE
VIRUS causing the illness
Cold
Rhinovirus
Wart,
Cold Sores
Herpes Simplex
Virus
Flu
Influenza
Measles
Morbillivirus
Ebola
Ebolavirus
Beneficial Role of Viruses
Regulation of saltwater and freshwater ecosystems:
• Most of viruses are bacteriophages, and most are in the oceans, where
they play a major role in regulating bacterial populations by killing ~20%
of oceanic bacterial biomass each day.
• Viruses are the main agents responsible for the rapid destruction of
harmful algal blooms, which often kill other marine life.
• Viruses help maintain the ecological balance of different species of marine
blue-green algae, and thus adequate oxygen production for life on Earth.
Transferring genes between different species
(horizontal gene transfer):
• It is thought that viruses played a central role in the early evolution.
• Numerous viral DNA sequences are scattered throughout the human
genome (making up around 8% of human DNA); most of it is no longer
functional, but some genes are important in human development.
• Similarly, viruses have transferred important genes to plants (some
photosynthesis-related genes were transferred from blue-green algae).
Viruses in Ancient History
• First ever written record of a virus
infection, 1580–1350 BC: an Egyptian
stele depicts a temple priest 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 disease had been a real
problem in Europe and the US during late 19th20th centuries before vaccine was introduced.
• Year 1143 BC: preserved mummy of Pharaoh
Ramses V shows lesions on his face suggesting
that he died of smallpox.
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.
Smallpox have been eradicated in the world through
vaccination (last case diagnosed in 1977).
Viruses in History
• Year 1519: smallpox was transferred
from Europe to the Americas aboard
one of Hernando Cortez ships.
3,500,000 Aztecs died in the next 2
years - effectively the end of the Aztec
empire. The epidemic, and those that
followed during 1545–1548 and 1576–
1581, eventually killed more than half
of the native population.
• Spanish Flu, 1918-1919:
unusually deadly influenza
pandemic.
It infected ~500 million people across
the world, including remote Pacific
islands and the Arctic, and killed 50
to 100 million of them—three to five
percent of the world's population.