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VIRAL HISTORY
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• Discovery of
Viruses
• . In 1892 Russian
Biologist Iwanowsky
showed that disease
was due to
something smaller
than bacteria.
• Mayer in 1886
mention about
disease was tobacco
mosaic disease
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•Beijerinck (1897)
coined the Latin name
“virus” meaning poison
•He studied filtered
plant juices & found
they caused healthy
plants to become sick
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Tobacco Mosaic Virus
•Wendell Stanley
(1935) crystallized
sap from sick
tobacco plants
•He discovered
viruses were made of
nucleic acid and
protein
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Smallpox
•Edward Jenner (1796)
developed a smallpox
vaccine using milder
cowpox viruses
•Deadly viruses are
said to be virulent
•Smallpox has been
eradicated in the
world today
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Viewing Viruses
•Viruses are smaller
than the smallest cell
•Measured in
nanometers
•Viruses couldn’t be
seen until the electron
microscope was
invented in the 20th
century
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Size of Viruses
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Characteristics
• Non living structures
• Non-cellular
• Contain a protein coat called the
capsid
• Have a nucleic acid core containing
DNA or RNA
• Capable of reproducing only when
inside a HOST cell
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Characteristics
•Viral capsids
(coats) are made
of individual
protein subunits
•Individual
subunits are
called capsomeres
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CAPSOMERES
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Characteristics
•Outside of host cells,
viruses are inactive
•Lack ribosomes and
enzymes needed for
metabolism
•Use the raw materials
and enzymes of the host
cell to be able to
reproduce
EBOLA VIRUS
HIV VIRUS
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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
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Helical Viruses
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Polyhedral Viruses
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Complex Viruses
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Structure of viruses
• Viruses consist of Nucleic acid,
Capsids ,Viral Envelope and tail
fiber.
• Their nucleic acid may consists of
a single or several molecules of
DNA or RNA. The smallest viruses
have four genes while the largest
have up to two hundreds.
• Capsid : The protein coat that
encloses the nucleic acid is called
a capsid. It may be are different
shapes.
• Capsid is made up of protein
subunits called capsomeres
• The number of capsomers is
characteristics of a particular
virus.
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• Viral envelopes: These are membranous covering
around the capsid.
• It is found some viruses.
• This covering helps them to infect their hosts.
• Tail fibers: In bacteriophage virus lower part is
tail like. At the posterior end of tail some fiber
like structures are present called tail fiber.
• These fibers take part in the attachment of virus
with host cell.
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Adenovirus
COMMON COLD
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Influenza Virus
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Chickenpox Virus
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Used for Virus
Identification
•
•
•
•
RNA or DNA Virus
Do or do NOT have an envelope
Capsid shape
HOST they infect
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Bacteriophages
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Phages
•Viruses that attack
bacteria are called
bacteriophage or just
phage
•T-phages are a
specific class of
bacteriophages with
icosahedral heads,
double-stranded DNA,
and tails
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Bacteriophage Replication
•Bacteriophage
inject their
nucleic acid
•They lyse (break
open) the
bacterial cell
when replication
is finished
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Lytic Cycle Review
• Attachment
• Penetration
• Biosynthesis
• Maturation
• Release
Phage attaches by tail fibers to
host cell
Phage lysozyme opens cell wall,
tail sheath contracts to
force tail core and DNA into
cell
Production of phage DNA
and proteins
Assembly of phage particles
Phage lysozyme breaks cell wall
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Bacterial
cell wall
Bacterial
chromosome
Capsid
DNA
Capsid
Sheath
1
Tail fiber
Attachment:
Phage attaches
to host cell.
Tail
Base plate
Pin
Cell wall
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
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Tail
DNA
4
5
Maturation:
Viral components are
assembled into virions.
Capsid
Release:
Host cell lyses and
new virions are
released.
Tail fibers
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Viral Latency
•Once a prophage cell is activated, host cell enters the lytic cell
•New viruses form a & the cell lyses (bursts)
•Virus said to be virulent (deadly)
ACTIVE
STAGE
INACTIVE STAGE
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Virulent Viruses
HOST CELL
LYSES &
DIES
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The Lysogenic Cycle
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Virulence
VIRUS DESTROYING HOST CELL
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Treatment for Viral
Disease
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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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Other Viral Treatments
•Interferon are naturally
occurring proteins made
by cells to fight viruses
•Genetic altering of
viruses (attenuated
viruses)
•Antiviral drugs (AZT)
•Protease inhibitors –
prevent capsid formation
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