• Harmful agents were called viruses (Latin
virus, poison or venom ) in the nineteenth
• Louis Pasteur used the term virus to
describe any living disease causing agent
• In 1892 ,Dimitri Ivanowski gave the first
clear evidence of viruses.He observed that
leaf extract of infected plant could induce
tobacco mosaic disease.
3. • Beijerinck performed experiments on the
same line and proposed that the disease is
caused by a contagium vivum fluidum.
• During the same time Loeffler and Frosch
observed that the foot and mouth disease
of cattle was caused by a filterable virus.
4. • In 1911, Peyton Rous reported that virus was
responsible for malignant muscle tumor in chicken.
• French scientist d’Herelle discovered bacterial
• Wendell Stanley crystallized tobacco mosaic virus in
1935 and found that it was largely protein. Bawden
and Pirie separated TMV particle into protein and
nucleic acid. It is now clear that viruses are
complexes of proteins and nucleic acids capable of
replication only in living cells
• Viruses are unique infectious agents which
reproduce only in living cells.
• They occur in bacteria,algae ,protozoa and
higher plants and animals
6. General Properties
• Viruses are much smaller than bacteria.Their size
ranges from 20 to 300 nm. Most viruses are invisible
under the light microscope.
• They have very simple structure. They are
nucleoprotein particles consisting of DNA or RNA
surrounded by a protein capsid.
• They lack cellular organization.Cytoplasm,
mitochondria, Golgi body ,lysososomes, cell membrane
and ribosome are absent. They use ribosomes of host
during protein synthesis.
• Viruses cannot multiply outside a living cell. They are
7. • They are metabolically inactive. They lack enzyme
systems and protein synthesis machinery.
• Generally cell have both DNA and RNA.They have
only one nucleic acid, either DNA or RNA.
• They can be crystallized.
• Viruses donot have the power of growth and
division.A fully formed virus cannot increase in
size neither can it undergo division.
• The intact virus unit or infectious particle is called
the virion. Each virion consists of a nucleic acid core
surrounded by a protein coat called capsid.
• It is composed of a number of subunits called
capsomeres. The nucleic acid and capsid is called
nucleocapsid. The nucleocapsid may be naked or
surrounded by a loose membrane called envelope.
12. • Spherical symmetry (Polyhedral): Crick and
Watson have shown that polyhedral capsids can
have three possible types of symmetry viz
tetrahedral, octahedral and icosahedral.
• Icosahedron is the most efficient shape for the
packaging and bonding of subunits.
• An icosahedron is a regular polyhedron with 20
faces formed by equilateral triangles and 12
intersecting points or corners. Each capsids is
made up of capsomeres.
• There are two types of capsomeres, pentameres
and hexameres. Polyhedral capsids may be
– Naked : Example Poliovirus, Adeno virus and
papilloma virus or
– Enveloped: Example Herpes Simplex virus
16. Helical Symmetry
Helical capsid consists of
monomers arranged in a
helix around a single
The monomers curve into a
helix because they are
thicker at one end than the
Helical capsids may be
,tobacco Mosaic Virus
17. • Helical symmetry (cylindrical): Helical capsid
consists of monomers arranged in a helix
around a single rotational axis.The monomers
curve into a helix because they are thicker at
one end than the other.
• Helical capsids may be naked ,example
bacteriophage M13 ,tobacco Mosaic Virus
• enveloped ,example influenza virus.
19. • Complex symmetry : Complex viruses are
those which are divided into two groups :
• a)Viruses without identifiable capsids .
e.g vaccinia virus
b) Viruses whose capsids are
attached with additional
e.g Bacteriophages of
T even series (T2,T4,T6).
20. • Envelope: Many viruses are surrounded by a 100
– 150 A thick membrane called envelope. Virus
envelope contains host cell proteins and proteins
specified by virus. Some members have
glycoprotein spikes. The lipids envelopes of
budding viruses are derived from host.
• Nucleic acids: Viruses contain DNA or RNA, which
may be single stranded (ss), double stranded (ds),
linear or circular. The may have plus polarity or
21. Viral Proteins
• Structural (nucleocapsid) proteins: Viral capsids are made up
entirely of proteins. The capsid is made up of identical protein
subunits called protomers. Some viral capsids have more than
one type of protein. Capsid protein protects the viral nucleic
acid from host nucleases and also helps in attachment to
• Core (Internal) proteins: They are the proteins associated with
nucleic acid of virion. E.g. Protein V and VII of Adeno virus and
nucleoproteins of vesicular stomatitis virus and influenza virus.
• Viral Enzymes: Several enzymes are found in animal viruses.
The ds RNA viruses contain enzymes for synthesis of viral
mRNA. The major types of enzymes affect host cell surface, add
specific terminal groups to viral mRNA, transcribe DNA to
mRNA or participate in nucleic acid replication and processing.
• Envelope proteins: Viral envelopes contain host cell proteins as
well as proteins specified by virus. Some envelopes also contain
glycoprotein and lipoprotein.
• Tobacco mosaic virus : TMV is a rod shaped,
helical virus about 300 nm long and 15 – 18 nm in
diameter.It is made of 2130 identical protein subunits
which are arranged around a central hole of 4 nm
• Genome :The TMV RNA genome is single stranded and
linear, with a length of ~6400 bases. The tightly
organized genome encodes at least three nonstructural
proteins (P126, P183, and the 30-kD MP). Both P126
and P183 are components of the TMV replicase. The
genome also codes for a putative 54-kD protein of
unknown function, and the Coat Protein.
23. Life cycle
• 1)Entry : The virus enters the plant cell through stomatal openings
or through through mechanical injuries on the plant surface.
• 2) Replication : The virus passes from one cell to another via the
plamodesmata. Several particles invade the nucleus. Replication
takes place in the nucleus.
• 3) Synthesis of viral genome and proteins: Synthesis of RNA
dependant RNA occurs in the nucleus. The capsid proteins are also
synthesized in the nucleus.
• 4)Assembly : The assembly of protein subunits and nucleic acid
starts within the nucleus.However incomplete viral particles may
also come out of the cytoplasm and assembly may be completed
there.After assembly the viral particles are released in the plant
• HIV : Human Immunodeficiency Virus (HIV. is a retro virus
which causes AIDS (Acquired Immune Deficiency
Syndrome.) The immune deficiency is caused by the loss of
the CD4+ T cells that are essential for both cell mediated and
antibody-mediated immunity.HIV is spherical enveloped
virus about 90 to 120 nm in size. The nucleocapsid has an
outer icosahedral shell and an inner cone shaped core
enclosing ribonucleoproteins. The genome is diploid
composed of two identical single stranded positive RNA
copies associated with viral reverse transcriptase enzyme.
• The virion binds to both CD4 and either coreceptor with
molecules on its surface called glycoprotein 120 (gp120).
• This binding triggers an allosteric change in a second
molecule, called glycoprotein 41 (gp41), which penetrates
the host plasma membrane allowing the virion to get inside.
25. • When HIV infects a cell
• its molecules of reverse transcriptase and integrase are carried into
the cell attached to the viral RNA molecules.
• The reverse transcriptase synthesizes DNA copies of the RNA.
• These enter the nucleus where the integrase catalyzes their
insertion into the DNA of the host's chromosomes.
• The HIV DNA is transcribed into fresh RNA molecules which reenter
the cytosol where
– some are translated by host ribosomes.
• The env gene is translated into molecules of the envelope protein
Proteases of the host cell then cut gp160 into
– gp120 which sits on the surface of the virions (and is the target of
most of the vaccines currently being tested).
– gp41, a transmembrane protein associated with gp120.
• the gag and pol genes are translated into a single protein molecule which
is cleaved by the viral protease into
– 6 different capsid proteins
– the protease
– reverse transcriptase
– the integrase
• other RNA molecules become incorporated into fresh virus particles
• T4 is a coliphage belonging to the T even series. It has a
head, head tail connector, tail, baseplate and tail fibers
• The head capsid is a 95 nm long and 65 nm wide
prolate icosahedron. It has DNA associated with several
peptides and three internal proteins.
• The DNA is linear and has a molecular weight of 120 X
106 .The head tail connector has a collar with attached
• The tail has a contractile sheath surrounding an inner
core or tube. It is connected to the collar at the upper
end and baseplate at the lower end.
• The baseplate is hexagonal and has tail fibers attached