VIRUSES.pptx

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Vision
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Love of Fellow Beings | Social
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VIRUSES

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Vision
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Essential Readings
1.Haitet.al. A Text book of Botany, New Delhi: NCBA, 2012.
2.Pandey, B. P. College Botany, Vol. I: Algae, Fungi, Lichens, Bacteria,
Viruses, Plant Pathology, Industrial Microbiology and Bryophyta. New
Delhi: S. Chand & Company Ltd, 2001.
Recommended Readings
1. Kumar, H.D. (1999). Introductory Phycology. Affiliated East-West.
Press Pvt. Ltd. Delhi. 2nd edition.

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Core Values
2. Tortora, G.J., Funke, B.R., Case, C.L. (2010). Microbiology: An Introduction, Pearson
Benjamin Cummings, U.S.A. 10th edition.
3. Sethi, I.K. and Walia, S.K. (2011). Text book of Fungi & Their Allies, MacMillan
Publishers Pvt. Ltd., Delhi.
4. Alexopoulos, C.J., Mims, C.W., Blackwell, M. (1996). Introductory Mycology, John
Wiley and Sons (Asia), Singapore. 4th edition.
5. Raven, P.H., Johnson, G.B., Losos, J.B., Singer, S.R., (2005). Biology. Tata McGraw Hill,
Delhi, India.
6. Vashishta, P.C., Sinha, A.K., Kumar, A., (2010). Pteridophyta, S. Chand. Delhi,
India.
7. Parihar, N.S. (1991). An introduction to Embryophyta. Vol. I. Bryophyta. Central
Book Depot, Allahabad.

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GENERAL CHARACTERISTICS OF VIRUSES
1. Viruses are the smallest and simplest of all known organisms.
They can only be seen with an electron microscope.
2. They are able to pass through filters that prevent the passage of
most bacteria.
3. Viruses cannot be cultured on any synthetic medium, i.e., they
are acellular or noncellular particles that completely lack
structural characteristics of cells.
4. A striking feature of many viruses is that they can be
crystallised. Virus crystals redissolved in water, can initiate an
infection cycle.

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5. They lack the cellular machinery necessary for acquiring
nutrients producing energy and synthesizing proteins.
6. Viruses absolutely require living host cells to perform nearly
every biological function necessary for their survival and thus
they are described as obligate intracellular parasites.
7. They are incapable of any independent metabolism, a property
that distinguishes them from certain bacteria such as rickettsias
and chlamydias which are also very small (filterable) and
obligate intracellular parasites.

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8. Viruses infect every kind of organisms on earth, i.e., plants, fungi, protozoa,
insects, birds, mammals and humans. Even bacteria are attacked and
killed by viruses. These bacterial viruses are called bacteriophages (or
simply phages). Viruses which attack fungi are called mycophages.
9. Each virus particle consists of a core of genetic material (nucleic acid) that
is covered by a protein coat, called a capsid. They consist of nothing more than
nucleic acid and protein.
A very characteristic feature that is peculiar to only viruses is that their
genetic material consist of either DNA or RNA, but never both; the genetic
material of all other forms of life contain both RNA and DNA.
The complete mature virus particle is referred to as a “virion”.

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10. Most viruses are highly specific for particular organisms. They
can ordinarily invade cells of one type (e.g., plant, animal,
bacterial), often a single host species, or even strains within a
species, and only one tissue or organ in that species.
11. Like other organisms, viruses are capable of self-reproduction.
But the process of virus reproduction is different from that of
other organisms in the sense that viruses do not divide to form
new individuals. Without exception, viruses only multiply
inside living host cells by using the synthetic machinery of the
cell. The method of virus multiplication is called Replication.

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12. Viruses have an “eclipse phase” in their cycles. While
reproducing in a host cell, the virus particle disintegrates
into its molecular constituents. The eclipse phase ends with
the reappearance of progeny viruses.
13. Viruses are very easily transmitted from one generation to
another by mechanical means or biological vectors.

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14. Viruses are causal agents of highly infectious diseases in a
wide variety of organisms. They cause severe economic
damage to several crop plants.
Some viruses are known to cause tumours in a number of
animal species, including rabbits, frogs, fowl and mice.
Viruses also cause severe human diseases such as infectious
hepatitis, poliomyelitis, herpes and AIDS.
15. All viruses are generally insensitive to the broad range of
available antibiotics such as penicillin, streptomycin, and
others.

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Some Milestones in Virology
❏ 1892: Ivanowsky reported that the agent of tobacco mosaic
disease was filterable.
❏ 1898: Beijerinck, recognised that the agent of tobacco mosaic
disease was filterable.
❏ 1898: Friedrich Loeffler and P Frosch demonstrated that the
agent of foot-and-mouth disease of cattle was filterable
❏ 1915: F W Twort discovered the lysis of bacteria by filterable
agents
❏ 1917: F.d’Herelle coined the word bacteriophage (viruses
which destroy the bacteria)

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❏ 1935: W M Stanley isolated a virus in a crystalline form and
reported that viruses were made exclusively of proteins
❏ 1936: N W Pirie and F C Bawden established that viruses were
made up of proteins and nucleic acids
❏ 1942-48: S Luria and M Delbruck; A Lwaff elaborated the
process of replication in bacteriophages
❏ 1947, Alexander John Haddow made a discovery that didn't
seem particularly important. He was part of a team doing
research on yellow fever in Uganda, and he identified a new
virus that was making a monkey in his lab sick. (Zika virus).

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❏ 1952: M Zinder and J Lederberg discovered Transduction
❏ 1952: A Hershey and M Chase showed that the protein part of
virus was non-infectious and that the nucleic acid was
responsible for infection.
❏ 1953: A Lwoff and E Wellman discovered temperate phages;
initiated elucidation of the lysogenic cycle in bacteriophages.
❏ 1955: Takahashi and Conrat and Williams reconstituted TMV
from its proteins and RNA
❏ 1956: Gierer and Schramm showed that RNA is the genetic
material in TMV

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❏ 1957: Elucidation of ultrastructure of TMV by Franklin
and others
❏ 1959: R L Sinrsheimer discovered bacteriophage 0x174, a
virus having single-stranded DNA.
❏ 1963: R S Shafferman and M E Morris discovered
cyanophages.
❏ 1964: Tenin and Baltimore demonstrated RNA-
dependent DNA synthesis.

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❏ 1977: Harrison et al., coined the term ‘Gemini viruses’
for plant viruses containing single-stranded DNA
genome.
❏ 1982: Aoran Klug described the structure of TMV.
❏ 1983: Luc Montagnier’s team at the Pasteur Institute in
Paris discovered HIV-1 and In 2008, Luc Montagnier and
Françoise Barré-Sinoussi from his team were awarded
the Nobel Prize for the isolation and characterization of
HIV-1.

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DEFINITION OF VIRUS
❏ Contain only a single type of nucleic acid, either DNA or RNA
❏ Contain a protein coat (sometimes itself enclosed by an
envelope of lipids and proteins) that surrounds the nucleic
acid.
❏ Are incapable of any independent metabolism
❏ Multiply in only living host cells, by using the biosynthesis
machinery of the cell.
❏ Cause the synthesis of specialised particles (virions) that
transfer the viral nucleic acid to other cells.

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ARE VIRUSES NON-LIVING?
Following features show that viruses are non-living:
1. They are non-cellular particles that completely lack all
structures characteristic of cells.
2. They have no associated metabolism necessary for
acquiring nutrients, generating energy and synthesising
proteins. They are incapable of any independent
metabolism and are therefore, incapable of performing life
functions outside of their living hosts.

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3. They require a living host cell to perform nearly every
biological function necessary for their survival.
4. Unlike cellular organisms, viruses cannot reproduce by
fission. They depend on the cells they infect for their
multiplication. Multiplication process (replication) of viruses
is entirely different from reproducing of living organisms.
5. Some viruses can be crystallized. Virus crystals are
chemically inert and can apparently be kept indefinitely in a
dry state, without losing their infectivity.

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ARE VIRUSES LIVING?
Following features show that viruses are living:
1. They are particulate and strictly obligate intracellular
parasites, that is, they depend upon specific hosts for
their reproduction and development.
2. Viruses like cells, can mutate and change
characteristics, for example, virulence.
3. They occur in definite races, or strains, each with its
specific character.

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4. The crystallised viruses do not loose their activity, for
when redissolved, they infect the healthy cells and
replicate themselves inside the host cell.
5. They respond to heat, chemicals and radiations.
6. They are transmitted from the diseased host to the
healthy ones.

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THE DIFFERENCE BETWEEN VIRUS AND CELLULAR
ORGANISMS
1. Unlike the organisms making up the taxonomic kingdoms, the
viruses are acellular, that is, they do not consist of cells. Unlike
the cellular organisms, they lack metabolic machinery to
acquire nutrients to generate energy, or to synthesize proteins.
They are incapable of fermentation, cellular respiration, or
photosynthesis.

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2. The possession of only one kind of nucleic acid, either RNA or
DNA, distinguishes viruses from other organisms, which have
both the nucleic acids.
3. The cell always arises directly from a pre-existing cell,
whereas whole viruses never arise directly from pre-existing
viruses.
4. The cell is reproduced from an integrated sum of all its
constituent parts. To multiply, viruses must take over the
metabolic machinery of the host cell.

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5. Cells multiply by the orderly growth of cell parts, followed
by cell division. Viruses reproduce by replication (a process
in which many copies or replicas are made of each viral
component and are then assembled to produce progeny virus).

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COMPARISON BETWEEN VIRUSES AND BACTERIA
Sl
No.
Bacteria Viruses
1. Most bacteria are either
saprophytes or parasites.
Certain bacteria, e.g., Rickettsias
and Chlamydias are obligate
intracellular parasites
Viruses are
obligate
intracellular
parasites
2. Plasma Membrane present Absent
3. Binary fission occurs Absent

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Sl
No.
Bacteria Viruses
4. Most bacteria cannot pass
through bacteriological filters. A
few, e.g., rickettsias and
chlamydias are filterable
They can pass
through
bacteriological
filters
5. Possess both DNA and RNA Either DNA or
RNA, but never
both
6. APT generating metabolism is
present
Absent

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Sl
No.
Bacteria Viruses
7. Ribosomes present Absent
8. They are sensitive to antibiotics Insensitive to the
available
antibiotics.

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Viroids
Virusoids
Prions

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THE STRUCTURE OF VIRUSES
Virion Size:
❏ Virions range in size from about 10 to 400 nm in
diameter.
❏ The smallest viruses are a little larger than
ribosomes, whereas the poxviruses, which include
vaccinia, are about the same size as the smallest
bacteria and can be seen in the light microscope.

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General Structural Properties
❏ All virions, even if they possess other constituents, are
constructed around a nucleocapsid core (indeed, some viruses
consist only of a nucleocapsid).
❏ The nucleocapsid is composed of a nucleic acid, usually either
DNA or RNA, held within a protein coat called the capsid,
which protects viral genetic material and aids in its transfer
between host cells.

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❏ Capsids are large macromolecular structures that self-
assemble from many copies of one or a few types of
proteins.
❏ The proteins used to build the capsid are called
protomers.
❏ The various morphological types of viruses primarily
result from the combination of a particular type of
capsid symmetry with the presence or absence of an
envelope, which is a lipid layer external to the
nucleocapsid.

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There are three types of capsid symmetry: helical,
icosahedral, and complex. Those virions having an
envelope are called enveloped viruses; whereas those
lacking an envelope are called naked viruses.

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Helical Capsids
Helical capsids are shaped like hollow tubes with protein walls.
The tobacco mosaic virus provides a well-studied example of
helical capsid structure.

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Icosahedral Capsids
❏ The icosahedron is a regular polyhedron with 20 equilateral
triangular faces and 12 vertices.
❏ The capsids are constructed from ring- or knob-shaped units
called capsomers, each usually made of five or six protomers.
❏ Pentamers (pentons) have five subunits; hexamers (hexons)
possess six.
❏ Pentamers are usually at the vertices of the icosahedron,
whereas hexamers generally form its edges and triangular
faces.

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Viruses with Capsids of Complex Symmetry
❏ The poxviruses and large bacteriophages are two important
examples.
❏ The poxviruses are the largest of the animal viruses (about 400
240 200 nm in size).
❏ They possess an exceptionally complex internal structure with
an ovoid- to brick shaped exterior.

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Viral Genomes

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VIRUS REPRODUCTION
❏ Attachment to a host
❏ Entry of either the nucleocapsid or the viral nucleic acid
into the host.
❏ Synthesis of viral particles
❏ Assembly and maturation
❏ Release of mature virions

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DNA VIRUS

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❏ Most DNA viruses use double-stranded DNA (dsDNA) as
their genetic material. However, some have single-
stranded DNA (ssDNA) genomes. In both cases, the
genomes can be either linear or circular.
❏ Some DNA genomes can switch from one form to the
other. For instance, the E. coli phage lambda has a
genome that is linear in the capsid, but is converted
into a circular form once the genome enters the host
cell.

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❏ Another important characteristic of DNA viruses is that
their genomes often contain unusual nitrogenous bases.
For example, the T-even phages of E. coli have 5-
hydroxymethylcytosine instead of cytosine, and the
hydroxymethyl group is often modified by attachment
of a glucose moiety.

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A BACTERIOPHAGE
Bacteriophage is the virus
that infect bacteria.
Bacteriophages were
discovered by Frederick
Twort(1915) and Felix
d'Herelle (1917).

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Bacteriophages have nucleic acid
surrounded by capsid.
The capsid is made up of capsomeres.
Capsomeres consist of protein subunit
known as protomeres.

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COLIPHAGES
Coliphages were discovered by Herelle.
Bacteriophages attacking E.coli are called as
Coliphages and are designated T-type.
These were numbered T1,T2,T3,.....T17 by Max
Delbruck (1938).
T2,T4,T6... are known as T-even phages.
Rest are called as T-odd phages.

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Bacteriophages may be classified into
different morphological class
• A - These are the most complex have hexagonal head
and a rigid tail with a contractile sheath and tail fibres.
Example - Coliphages T2,T4,T6
• B - These are similar to A but lack contractile sheath
on the tail and may or may not have tail fibres.
Example - Coliphages T1,T5

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• C - These are same as B type but their tail is
shorter than head.
Example - Coliphages T3,T7.
• D - These have head made up of large capsomeres
but no tail.
Example - ¢X 174 ,S13.
• E - These are similar to D type instead they have
small capsomeres.
Example - f2 , MS2.

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• F - These are filamentous bacteriophage.
Example - Coliphages fd , f1.
• G - These have lipid envelope and are
polymorphic.
Example - MV - L2

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STRUCTURE OF T4 BACTERIOPHAGE

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Phage T4 has a complex morphology, consisting of
head, neck, tail, baseplate and tail fibres.

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Head
❏ Head is icosahedron with about 2000 capsomeres and contain
a 53 micrometer long double stranded DNA in it.
❏ Head is prism like hexagonal having 950 X 650A° dimension.
❏ The elongated icosahedral head is made of multiple copies of
24 different proteins
❏ 16 involved in prohead formation and maturation
❏ 5 for DNA packing
❏ 3 complete and stabilize head
❏ Hoc (highly antigenic outer capsid protein) and Soc (small
outer capsid protein)

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Neck
The neck is composed of gene products 13, 14, wac.

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Contractile tail
❏ Tail is helical symmetry with a core tube (80 angstrom
in diameter) surrounded by a protein sheath (consist of
144 subunits of gp19 arranged in 24 rings of six
subunit each).
❏ • Sheath is connected to collar at its upper end and a
base plate at its lower end and consists of 144 subunits
of gp18 arranged in the same manner.
❏ Tail is 950A° in length joined to head by neck and collar.

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Baseplate
❏ Base plate is hexagonal with single pin or peg or spike
at each corner.
❏ A thin tall fiber (1300A long) is also given out from each
corner of base plate.
❏ The baseplate is composed of six gene products (5, 27,
29, 26, 28, and 51).
❏ The gp5 and gp27 complex attached at the tip of the
tube has lysozyme like activity.

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Tail fibres and pins
The short tail fibre is a trimer of gp12 and gp9 forms the
socket of long tail fibre consisting of 4 gene products (34,
35, 36 and 37).

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Genome of T4 bacteriophage
❏ The genome comprises of about 300 probable genes and
three different types of promoters: Early (Pe), Middle
(Pm), and Late (Pl)
❏ The early and middle genes encode functions for DNA
replication and for regulating expression of the late genes.
❏ Late genes encode head and tail components for the
phage particles, and functions for cell lysis.
❏ More than 40% of the genetic information is needed for
the synthesis of T4 structure
❏ The genes responsible for each substructure are largely
clustered
❏ The genome is AT-rich and contains modified base 5-
hydroxy-methyl-cytosine (HMC), rather than cytosine.

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LIFE CYCLE OF BACTERIOPHAGES
❏ Being non-cellular the viral particle has no power of
independent metabolism and reproduction.
❏ The replication of virus takes place by two types of cycles:
❏ 1. Lytic cycle, and 2. Lysogenic cycle
❏ Based on interaction of phages with the bacterial cell, they have
been distinguished into two types - Virulent or Lytic and
Avirulent or temperate phages.
❏ The virulent phages perform the lytic cycle and the avirulent or
temperate phages perform the lysogenic cycle.

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LYTIC CYCLE
This type of cycle is seen in T-even phages which attack E. coli.
The lytic cycle consists of five steps
(a) Adsorption
(b) Infection/Penetration
(c) Synthesis of phage components in host cell
(d) Formation of new phage particles and
(e) Liberation of phages from the host cell

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The Life Cycle of
Bacteriophage
T4.
(a) A schematic
diagram depicting
the life cycle with
the minutes after
DNA injection
given beneath
each stage. mRNA
is drawn in only at
the stage during
which its
synthesis begins.

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A Map of the T4 Genome. Some of its genes and their functions are
shown. Genes with related functions tend to be clustered together.

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LYSOGENIC CYCLE
A. Lwoff (1953) discovered this type of cycle in Lambda (ƛ)
phages that attack E. coli.
The phage involved in this cycle is called Temperate
phage, the bacterium is the lysogenic strain and the entire
process is called Lysogeny.

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Lambda (ƛ) phages

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The cascade of events leading to either lysogeny or the lytic cycle
involves a number of regulatory proteins that function as
repressors or activators or both.
Two regulatory proteins are of particular importance: the
lambda repressor (product of the cI gene) and the Cro protein
(product of the cro gene).

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RNA VIRUS

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Structure and
Replication of TMV

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Electron micrograph of Tobacco mosaic virus rod-shaped particles
(300 nm x 15 nm).

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MECHANISM OF TMV MULTIPLICATION
1. Way of entry
❏ Through mechanical wounds
❏ By the vectors (Microsiphon solanifolii, Myzus pseudosolani and M. persica)
❏ Through infected pollen grains
2. Entry of Viruses
3. Synthesis of new viral RNA
4. Synthesis of viral protein
5. Formation of new viral particle

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Cell-to-cell movement of Tobacco mosaic virus (TMV). In this model, the movement protein
(MP) binds to the viral RNA [1]. Host proteins and/or other virus-encoded proteins may be
included in the MP-complex [2]. The MP-complex then moves from cell-to-cell through the
plasmodesmata [3]. When the complex is localized to a new cell, the MP (and any host proteins)
are presumably released from the TMV RNA [4], allowing for translation of the genomic RNA to
express the replicase proteins and to initiate a new round of replication [5].

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