Welcome to Virology Lecture

Welcome to Virology
Lecture
Dr Shyamal Kr Paul
Assoc. Prof.
Microbiology
MMC
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Topics covered
Overview; History of virus
Definition
Classification.
Genomes.
Structure.
Replication.
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Review questions
What is a virus?
Basic structure of a virus?
Classification of virus?
What is the principle of viral replication?
What are the steps of viral replication?
Define – capsomere, capsid, virion, envelop
Atypical virus like particels- Prions, viroid,
pseudovirion, defective virus
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Origins of Virology
Viruses are probably as
old as life on earth.
Ancients were aware of
viral diseases
Perhaps the first written
record of a virus infection
drawn
in
approximately
1400BC, which depicts a
temple priest called Siptah
showing typical clinical signs
of
paralytic poliomyelitis
.
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Viral diseases were treated
Smallpox, endemic in China
by 1000BC. Recognizing
that survivors of smallpox
outbreaks were protected
from subsequent infection,
the practice of variolation
developed.
Involved
inhalation of dried crusts
from smallpox lesions, or in
later
modifications,
inoculation of the pus from
a lesion into a scratch on
the
forearm.
Practice
survived until this century
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Vaccines were developed
On 14th May 1796, Edward
Jenner used cowpoxinfected material obtained
from the hand of Sarah
Nemes, a milkmaid from
Berkley
in
Gloucestershire
to
vaccinate 8 year old
James
Phipps.
On 1st July 1796, Jenner
challenged the boy by
deliberately
inoculating
him with material from a
real case of smallpox !
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He did not become
infected!
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The virus concept.
In 1892, Dmitri Iwanowski, a Russian botanist,
showed that extracts from diseased tobacco
plants could transmit disease to other plants
after passage through ceramic filters fine
enough to retain the smallest known bacteria.
Generally recognised as the beginning of
Virology.
In 1898, Martinus Beijerinick confirmed &
extended Iwanowski's results on tobacco mosaic
virus & was the first to develop the modern idea
of the virus, which he referred to as contagium
vivum fluidum ('soluble living germ')
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Origin of Viruses
Regressive theory: viruses are degenerate
forms of intracellular parasites. The leprosy
bacillus, rickettsiae and chlamydia have all
evolved in this direction. Begs the question
of RNA virus evolution ?
Progressive theory: Normal cellular nucleic
acids that gained the ability to replicate
autonomously and therefore to evolve. DNA
viruses came from plasmids or transposable
elements.
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What is a Virus?
sub-microscopic, obligate intracellular
parasites.
size typically 20-300nM. Rod shaped
or spherical.
DNA or RNA making up the genome
Cannot
generate
energy
or
synthesize protein .
particles produced from the assembly
of pre-formed component
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Terminology for virus components
CAPSID is the protein coat that encloses the
nucleic acid
CAPSOMERS are structural units that are
building units of the capsid
NUCLIEC ACID is the DNA or RNA making up
the genome inside the capsid
ENVELOPE encloses the capsid
VIRION is the complete infective virus particle
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What is the virus structure ?
Very small - 20-300 nanometers
They have a core of nucleic acid with a
protein coat for protection
They are composed of non-structural and
structural proteins
The genome is either DNA or RNA
Some have envelopes
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Virus particle
virion
capsid
capsomeres
nucleic acid
envelope
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Atypical virus like agent
Viroids small, circular RNAs, possessing no capsid or
envelope. Associated with certain plant diseases. They
are infectious obligate intracellular parasites
Prions. “pree ons” believed to consist of a single type of
protein with no nucleic acid component. The prion
protein & the gene which encodes it are also found
in normal 'uninfected' cells.
These agents are associated with infectious and
inherited diseases, such as Creutzfeldt-Jakob disease
in humans, scrapie in sheep & bovine spongiform
encephalopathy (BSE) in cattle.
Defective virus
Pseudovirion
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Classification by genetic properties
DNA or RNA
Single stranded or double stranded
Sense (+) or antisense (-) nucleic acid
Linear or circular genome
Intact or segmented nucleic acid
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Baltimore classification
Classifies viruses into groups on the basis of their nucleic
acid and their mRNA production’
1)
2)
3)
4)
5)
6)
7)
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Double stranded DNA viruses
Single stranded DNA viruses
Double stranded RNA viruses
Sense (+) RNA viruses
Antisense (-) RNA viruses
Single stranded RNA viruses with DNA intermediate
Double stranded DNA viruses with RNA intermediate
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Types of virion structures(Symmetry)
The main criteria for viral classification are
nucleic acid, morphology, size and presence of
an envelope
1. Helices
2. Icosahedrons
3. Complex structures
All the above three can be surrounded by
a thick lipid bi-layer envelope
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Types of virion structures(Symmetry)
Helical Virus Enveloped Helical Virus An icosahedral virus Complex structure
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Icosahedral
Helical
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Envelopes
Non-enveloped viruses:
They are relatively stable and resistant to environment
insult i.e. they can resist desiccation, dry heat and soap
They generally kill the cells they infect as part of their life
cycle i.e. cell lysis
Enveloped viruses:
Can shed from infected cells in long term chronic
infection. This is called budding
Envelopes are usually derived from the host cell
membrane
The envelope is relatively sensitive to desiccation, dry
heat and detergents
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Viral replication
Reproduce only in live cells
ATTACHMENT – Recognise and bind to host
cell
PENETRATION
Direct through cell membrane
(Nonenveloped)
Fusion with cell membrane
(Enveloped)
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Viral replication cont..
Uncoating – Relesed from capsid by
enzymes
Expose nucleic acid;
Released into cytoplasm (RNA) OR
Nucleus (DNA)
Eclipse period – Virions undetected for
several hours while synthesis occures
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Replication cont.. MATURATION/RELEASE
BUDDING – No cell destruction, slow
release, may form envelope (most RNA)
CELL LYSIS – Spontanious release of
mature and incompleteviral particlcles
(more characteristics of DNA viruses)
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Replication of an enveloped virus
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‘Budding’
HIV leaving the infected T-cell
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Adsorption of an Enveloped Virus to the Host Cell Membrane
Attachment sites on the virus bind to
corresponding receptors on the host cell
membrane.
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Penetration and Uncoating of an Enveloped Virus Entering
by Fusion of Its Envelope with the Host Cell Membrane
During penetration, the viral envelope fuses with host cell membrane
and nucleocapsid enters the host cell. This is followed by uncoating
during which the viral capsid is enzymatically degraded and the viral
genome
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Enveloped Virus Replication
The viral genome directs the host cell's metabolic
machinery (ribosomes, tRNA, nutrients, energy,
enzymes, etc.) to synthesize viral enzymes and viral
parts. The capsids assemble around the viral genomes
and viral coded proteins and glycoproteins are inserted
in the host cell's membranes
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Release of an Enveloped Virus by Exocytosis
After the virus obtains its envelope by budding
from membrane bound organelles within the cell,
transport vesicles carry the virus to the cell
surface where it is released by exocytosis.
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Life Cycle of an Enveloped Virus Entering by
Endocytosis and Exiting by Budding.
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Virus Replication
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DNA virus families
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RNA virus families
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?
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