Viruses

Discipline Courses-I
Semester-I
Paper: Phycology and Microbiology
Unit-II
Lesson: Viruses
Lesson Developer: Vibha Narang
College/Department: ARSD College, University of Delhi

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 Viruses

Table of Contents

Chapter: Title

 Introduction
 Origin
 Morphology
 Classification
 Reproductive cycle of phages
 Detailed study of special type of viruses
 Viruses and diseases
 Summary
 Exercise/ Practice
 Glossary
 References/ Bibliography/ Further Reading

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Learning Outcomes

At the end of the study of this chapter a student will be able to:

 Define a virus and understand its being on the line between living and non living.

 Explain the morphology of a virus

 Explain the milestones in the origin and discovery of viruses

 Classify virus into various groups as per certain criteria

 Understand and reproduce the structure of a virus

 Explain the genome of a virus

 Describe and understand the lytic and lysogenic cycles

 Detailed study of the Tobacco Mosaic Virus.

 Explain the diseases caused by viruses

 Describe the applications of viruses.

Introduction

Viruses are considered to be at the borderline of living and non-living world. If they are
floating around in the air or sitting here and there, they're inert. But if they come into
contact with a suitable plant, animal or bacterial cell, they come alive.

The term virus has originated from a Latin word which means poison, and was given by
Louis Pasteur, a French chemist who suggested that something smaller than the
bacterium could cause the disease rabies. The Oxford English Dictionary describes virus
as a ‘morbid poison, a poison of contagious disease, as smallpox.’

Figure: Small Pox Virus

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Source:https://upload.wikimedia.org/wikipedia/commons/thumb/9/92/Smallpox_virus_v
irions_TEM_PHIL_1849.JPG/220px-Smallpox_virus_virions_TEM_PHIL_1849.JPG

Figure: Electron micrographs: An electron micrograph of a cluster of influenza viruses,

each about 100 nanometres (billionths of a meter) long;. Source:
http://upload.wikimedia.org/wikipedia/commons/a/a4/EM_of_influenza_virus.jpg; On the
right is a micrograph of the virus that causes tobacco mosaic disease in tobacco plants.
Source:http://www.biologie.uni-hamburg.de/b-
online/library/micro229/terry/images/other/tmv.jpg

Viruses have existed for thousands of years. The first evidence comes from historical
records showing evidence of small pox like disease in Asia. In the 19th century, germ
theory was developed, which states that all infections are caused by a particular microbe
that can be grown, are microscopic and can be retained by filters.

Iwanowski, a Russian botanist in 1892 found that sap from diseased tobacco plant was
capable of inducing mosaic disease in healthy plants even after the sap was made sterile
and was passed through bacteria proof filters. He concluded that the disease causing
agent was smaller than the bacterium. Later, a German scientist Adolf Mayer also
innoculated the sap from an infected plant into healthy ones, observing the onset of
mosaic disease in the healthy plant.

Both of them believed that either bacteria (very small in size and were submicroscopic)
or toxin were the cause of the disease. .

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Figure:Dmitri Iwanowsky (1864-1920)

Source:http://upload.wikimedia.org/wikipedia/commons/8/8f/Dmitry_Iosifovich_Ivanovs
ky.jpg

In 1898, Martinus Beijernick (1851-1931) confirmed Iwanowski's results on tobacco

mosaic virus. He rejected the idea of the causative organism being a bacteria and
developed the term "contagium vivum fluidum" which means “contagious living fluid” as
first the idea of the virus.

Figure: Martinus Beijernick(1851-1931)

Source: http://commons.wikimedia.org/wiki/File:Martinus_Beijerinck.png

He placed some sap from the infected plant on an agar plate for several days. He
expected the causative organism to penetrate into the agar. After a few days, the upper
layer of agar was removed and healthy plants were infected by two successive

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underlying layers of agar. The inoculated plants gradually developed the disease and
thus convincing Beijernick of a liquid or soluble nature of the causative agent.

Bacteriophages were first described by Frederick Twort (1915) and Felix D Herelle
(1917). In 1935, W Stanley was able to crystallize the virus causing Tobacco Mosaic
Virus disease. These crystals could produce disease in healthy plants. The crystals had
the property of self replication and could be stored like any other chemical.

Viruses are minute, infectious agents, smaller than the smallest bacteria. They do not
have a cellular structure or any metabolic activity and cannot reproduce outside living
cells. The reproduction of viruses is dependent on the host cells that they infect. They
cannot perform functions on their own. When present outside a host cell they exist as
protein coat or capsid. The capsid encloses either RNA or DNA.

Origin

There are three theories to explain the origin of viruses:

 Regressive evolution

The first hypothesis is the theory of ‘‘regressive evolution’’, which proposes that
viruses descend from free-living and more complex parasites. According to this
theory viruses were dependent on host cell machinery through evolution but
retained the ability to auto replicate. Viruses are degenerate life-forms which
have lost many functions that other organisms possess & have only retained the
genetic information essential to their parasitic way of life.

 Cellular origins

The second hypothesis is the theory of ‘‘cell origin’’, which assumes that viruses
originate from cell DNA and/or messenger RNA, which acquired the ability to
auto-replicate, create extracellular virions, exist and function independently.

 Independent entities

The theory of ‘‘independent’’ or ‘‘parallel’’ evolution of viruses and other

organisms, which assumes that viruses appeared at the same time as the most
primitive organisms.

Morphology

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The morphological characteristics of a virus can be studied using an electron microscope.

Viruses display a wide diversity of shapes and sizes. A virus particle capable of infecting
a host is called virion. The size of the virion varies from 20 to 300 nm. Viruses are
composed of a central core of nucleic acid surrounded by a protein coat called a capsid.

According to their shape, the viruses are classified into the following types:

• Icosahedral (the viral particle appears spheroidal)

• rod like

• Mixed or combination particles with shapes of both spheroidal and rod like
(Complex virus)

Capsid

The protein shell that encloses the nucleic acid is called capsid. It consists of several
protein subunits known as capsomeres. When the nucleic acid is enclosed by protein
shell then it is called nucleocapsid.

The capsid has three functions:

i. it protects the nucleic acid from digestion by enzymes,

ii. contains special sites on its surface that allow the virion to attach to a host
cell,

iii. provides proteins that enable the virion to penetrate the host cell
membrane.

Figure: Structure and Electron Micrograph of a Bacteriophage

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Source: http://upload.wikimedia.org/wikipedia/commons/e/e7/Phage.phiPP2.jpeg,
http://pad2.whstatic.com/images/thumb/7/7c/Know-the-Difference-Between-Bacteria-and-Viruses-Step-
1Bullet1.jpg/550px-Know-the-Difference-Between-Bacteria-and-Viruses-Step-1Bullet1.jpg

Envelope
In some viruses the capsid is enclosed with a lipid membrane called the envelope. The
envelope is lipoproteinaceous in nature. It has a lipid bilayer with interspersed protein
molecules. The envelope has the lipid molecules of the membrane of the host cell. The
proteins maybe derived both from the virus and the host cell. The host proteins are
present in the envelope. Most viruses have glycoproteineceous spikes on their envelope
which help them to attach to specific cell surfaces at the time of initiation of infection in
a new host cell.
The proteins are of two types: Glycoproteins and Matrix proteins. Glycoproteins are
integral membrane proteins which have enzymatic activity. They are exposed on the
outer surface of the membrane. Matrix proteins are found on inner face of envelope.

Figure: Basic structure of a virus depicting the envelope

Source: http://cnx.org/content/m44595/latest/Figure_21_01_04ab.jpg

Functions of the Envelope

1. It assists in viral attachment to the host cell through External Glycoproteins
2. It provides the virus with the ability to infect.
3. It maintains the structural integrity of the virus.

Nucleic Acid (Genome)

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Chemically viruses are basically nucleoproteinaceous. The protein coat surrounds the
nucleic acids. The property of viruses to multiply is because of its nucleic acid content.
The nucleic acid may either be DNA or RNA. The DNA may either be circular or linear
whereas RNA mostly exists only as linear double or single stranded molecules.

Depending on the presence of DNA or RNA it may be called a DNA virus or an RNA virus
respectively. The length of nucleic acid in a virus varies from virus to virus. Larger the
size of virus particle more will be the content of nucleic acid and vice versa.

Table: Shapes and sizes of some animal viruses

VIRUS SHAPE SIZE (mm) SPECIAL FEATURE

Adenoviruses Icosahedral 70-90 Without envelop and With fibers
Herpesviruses icosahedral 100-150 With envelope
Papoviruses icosahedral 43-53 Without envelope
Paroviruses icosahedral 18-22 Without envelope
Myxoviruses icosahedral 100-350 With envelop and projecting spikes
Arena Viruses icosahedral 50-150 With envelope
Rhabdo viruses Bullet shaped 75 x 130- With envelope
230
Toga viruses icosahedral 40-60 With envelope
Pox Viruses Brick shaped 230 x 300 Enveloped

Table: Shapes and sizes of some plant viruses

VIRUS SHAPE SIZE (mm) SPECIAL FEATURE

Cauliflower Mosaic icosahedral 50 Nonenveloped
Cucumber Mosaic icosahedral 30 Nonenveloped
Tobacco necrosis icosahedral 28 Nonenveloped
Tomato spotted wilt icosahedral 70-80 Enveloped
Tobacco Mosaic Rod Shaped 18x300 Cylindrical rod,
nonenveloped
Beet Yellow Flexuous rods 10 x 1250 Nonenveloped
Potato Yellow Dwarf Rod shaped 50-100 x 200-300 With envelope
Brome mosaic icosahedral 25 Nonenveloped

Classification

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Viruses can be classified on the basis of their geometry, presence or absence of

envelopes, type of genome, mode of replication of the genome, mode of transmission, or
by the type of disease they cause.

Approximately 80 families and 4000 species of viruses are known. Nobel Prize winner
David Baltimore classified viruses on the basis of the type of nucleic acid a virus contains
and the way it expresses and replicates. Viruses must generate positive stranded mRNA
from their genomes to produce proteins and replicate themselves. The RNA strand of a
single stranded genome is either a sense strand (plus strand), which functions as mRNA
(messenger RNA) or an antisense strand (minus strand). This is complimentary to the
sense strand and cannot function as mRNA and undergo protein translation.

The taxonomy of viruses is similar to that of cellular organisms:

Order (-virales)

Family (-viridae)

Subfamily (-virinae)

Genus (-virus)

Species

This classification places viruses into seven groups:

I: dsDNA viruses (e.g. Adenoviruses, Herpesviruses, Poxviruses)

II: ssDNA viruses (+) sense DNA (e.g. Parvoviruses)

III: dsRNA viruses (e.g. Reoviruses)

IV: (+)ssRNA viruses with (+)sense RNA (e.g. Picornaviruses, Togaviruses)

V: (-)ssRNA viruses with (-)sense RNA (e.g. Orthomyxoviruses, Rhabdoviruses)

VI: ssRNA-reverse transcribing viruses with (+) sense RNA that replicates via DNA
intermediate in life-cycle (e.g. Retroviruses)

VII: dsDNA-reverse transcribing viruses that replicates via RNA intermediate in life-cycle
(e.g. Hepadnaviruses)

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Figure : Baltimore’s Classification

Source:http://upload.wikimedia.org/wikipedia/commons/thumb/0/07/Baltimore_Classific
ation.png/720px-Baltimore_Classification.png

Viruses are also classified on the basis of morphology, chemical composition, and mode
of replication. The viruses that infect humans are grouped into 21 families.

a. Based on Morphology

Shape

 Helical Symmetry

In this type, similar protein subunits self assemble into helical form surrounding
the nucleic acid. The nucleic acid also follows a spiral path. These nucleocapsids
are rigid, highly elongated rods or filaments. Tobacco Mosaic Virus (TMV) is a
helical virus.

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Figure: The helical Symmetry in the Tobacco Mosaic Virus (TMV)

Source: http://upload.wikimedia.org/wikipedia/commons/3/3f/TMV_Structure.png

Icosahedral Symmetry

An icosahedron has 20 equilateral triangular faces and 12 vertices. In viruses which

show this type of shape the protein sununits called capsomeres are arranged in a hollow
spherical shell that encloses the genome of the virus.. Such a symmetry shows a
polyhedron structure.

Adenovirus is a DNA virus with a polyhedral capsid. T – even bacteriophage is a DNA

virus with a polyhedral head and a helical tail (tadpole shaped).

Figure: Icosahedral Capsid Configuration

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Source: http://wikileon-
period2.wikispaces.com/file/view/Icosahedral_Virus.jpg/134227705/383x342/Icosahedra
l_Virus.jpg, http://cnx.org/content/m44597/latest/Figure_21_02_05.jpg

b. Based on Chemical Composition and Mode of Replication

RNA Virus Genomes

70% of all viruses are RNA viruses. The viral RNA maybe single stranded (ss) or double
stranded (ds)..

DNA Virus Genomes

DNA viruses generally contain a single genome of linear dsDNA. Some, however, have
a circular DNA for eg. Papova virus.

Single-stranded linear DNA, is found with the members of the Parvovirus family that
comprises the parvo-, the erythro- and the dependoviruses. The virion contains 2–4
structural protein species which are differently derived from the same gene product.

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Figure: Diversity in shape of viruses

Source:https://lifescitpjhs.wikispaces.com/file/view/VirusShapes.png/316332668/759x5
17/VirusShapes.png

REPRODUCTIVE CYCLES OF PHAGES

Bacteriophage multiplication has been studied by Max Delbruck and S. Luria in the
United States and by Prof Andre Lwoff’s school in France. Viruses cannot grow on their
own as they are acellular, they need the machinery and metabolism of a host cell for
producing multiple copies. According to them, viruses exist in 3 stages as:

 Extracellular Virions

 Vegetative Phage

 Prophage

The extracellular virions are complete particles which exist before infection. Vegetative
phage and prophage are intracellular. They only have nucleic acid. Vegetative phage
shows autonomus replication but when it is inserted in the bacterial chromosome it is
known as prophage,. Prophage replicates with the bacterial DNA. All bacteriophages
exist as vegetative phages in the host cell for sometime. However, not all become
prophages. Those bacteriophages that become prophages are called temperate whereas
those that do not do so are virulent.

There are two major life cycles of viruses:

 The Lytic cycle

 The Lysogenic cycle

The lytic cycle of bacteriophage

The virulent phage replicates by a process called the Lytic cycle. At the end of it the
bacterial cell gets destroyed and thus the name. This process can be divided into several
distinct phases-

Adsorption

Virion is attached to the host bacterium by the process of adsorption. This is also called
as attachment and is the most important stage. It occurs in two steps:

 Collision between virions and the bacterial cells

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 Attachment of specific virion with the cell surface

Specific proteins present on the surface of the virus attach to the host surface molecule.
These molecules which allow the attachment of virus in the cell surface are called virus
receptors. And the proteins of the virion which help in attachment are called attachment
proteins.

Penetration

After attachment penetration is the next step by which viral genome enter the
cytoplasm mostly through injection.Some of the viral structures such as tail pin can
directly penetrate the plasma membrane and inject the genetic material into the
cytoplasm. Penetration is facilitated by hydrolysis of the mucopolypeptides of E. coli cell
wall by lysozyme activity. During phage multiplication lysozyme is synthesized and the
enzyme molecules remain attached to the tips of the tails of the phages. These are then
released from the bacterial host cell.

Empty protein coats that remain outside after injection of nucleic acids are called
‘ghosts’.

Synthesis

Synthesis stage begins after the process of uncoating. It leads to production of viral
components and it involves expression of the genome that is production of viral proteins.
During synthesis, viral genomes also start replicating. Synthesis takes place in two
stages – one is by the expression of genome, by the production of viral proteins and the
second is by genome replication that is production of new copies of the viral genomes.

Assembly

After the synthesis stage, the viral components start assembling to form progeny
viruses. Stable protein coat is formed with the packing of viral genome.

Lysis and Release

This is the last stage. During this phase the viral progeny leaves the host cell, to search
new cells for infection. Viruses are released from the host cell by lysis.

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Figure: Electron micrograph of bacteriophage infecting a bacterial cell (E.coli) followed

by lysis of the host.

Source: http://upload.wikimedia.org/wikipedia/commons/thumb/5/52/Phage.jpg/220px-
Phage.jpg, http://en.citizendium.org/images/8/8e/Bacteriophage5.jpg ,
http://dinsdalelab.sdsu.edu/newsite/images/dreamy/m0900047-
tem_of_bacterial_lysis_due_to_t4_phage_infection-spl.jpg

Figure: The lytic and lysogenic Cycle

Source http://cnx.org/content/m44597/latest/Figure_21_02_03.png

The lysogenic cycle

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Some of the viruses undergo a lysogenic cycle. In this the viral genome gets
incorporated in the host chromosome at a particular place. The term lysogeny refers to a
process where the prophage DNA remains associated with the host cell genome till it can
be induced to get separated to act as a virulent phage.
Lysogenic bacteria carry a phage in its temperate state. The phage leads to destruction
of the host bacterium itself. Examples of lysogenic phages are Coliphage Lambda,
Salmonella phage P22. Lysogeny is significant because it shows a close evolutionary link
between phages and bacteria. The presence of high degree of resemblance is seen due
to the association of their DNA.

Figure: The lysogenic cycle

Source:http://arthropodsbio11cabe.wikispaces.com/file/view/bacteriophage_life_cycle.jp
g/262619786/800x527/bacteriophage_life_cycle.jpg

Detailed study of special type of viruses

RNA VIRUS – Tobacco Mosaic Virus

Tobacco mosaic virus (TMV) is one of the simplest viruses known. TMV is the classical
example of a rod-shaped virus. Its rod shape results from its basic design, namely a
regular helical array of identical protein subunits, in which frame- work is embedded a
single molecule of RNA wound as a helix.

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TMV is a positive-sense single stranded RNA virus that infects plants, especially tobacco
and other members of the family Solanaceae.

Figure : Electron Microscope image of TMV

Source:http://www.biologie.uni-hamburg.de/b-
online/library/micro229/terry/images/other/tmv.jpg

It has a capsid made of 2130 molecules of coat protein. There are about 134 turns to
one TMV protein coat and 16.3 proteins per helix turn. 158 amino acids form a protein.
Its RNA is 6400 bases long.

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Figure :A schematic of a TMV particle. The red represents the RNA portion, and the blue
the protein coat that encases the genetic material. (repeated-- may be deleted)

Source: http://upload.wikimedia.org/wikipedia/commons/3/3f/TMV_Structure.png

Viruses enter plant cells through wounds, grafting, seed, pollen and insects. Tobacco
mosaic virus is most commonly introduced into plants through small wounds caused by
handling and by insects chewing on plant parts. The most common sources of virus
inoculum for tobacco mosaic virus are the debris of infected plants that remains in the
soil and certain infected tobacco products that contaminate workers hands.

Symptoms

Symptoms induced by Tobacco mosaic virus (TMV) are dependent on the host plant.
These include mosaic, mottling; necrosis, stunting, leaf curling and yellowing of plant
tissues. The symptoms are dependent on the age of the infected plant, virus strain,
genetics of host plant and environmental conditions.

Figure: Typical mosaic pattern on tobacco leaves infected with Tobacco Mosaic Virus.

Source:http://www.apsnet.org/publications/apsnetfeatures/Article%20Images/Tobacco_
Fig01.jpg,http://upload.wikimedia.org/wikipedia/commons/2/26/Tobacco_mosaic_virus_
symptoms_tobacco.jpg

DNA VIRUS – Coliphage T2 (Bacteriophage)

The viruses that attack bacteria are called bacteriophages. The first clue regarding these
viruses was given by Twort (1915) in England and Felix de Herelle (1917) in France who
also coined the term bacteriophage (i.e., bacteria eater commonly called as phages)..

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The genetic material of the phages can be dsDNA, ssDNA, dsRNA or ssRNA. The
bacteriophages occur in three common forms: Tailed, cubic and filamentous. The tailed
bacteriophages named T-phages (include the T-even and T-odd phages) form the largest
group.

Figure: Structure of Bacteriophage T2

Source:http://upload.wikimedia.org/wikipedia/commons/thumb/5/56/Tevenphage.svg/2
20px-
Tevenphage.svg.png,http://upload.wikimedia.org/wikipedia/commons/thumb/f/f2/Phage
_S-PM2.png/220px-Phage_S-PM2.png

The structure of T2 bacteriophage consists of a head, a tail, a basal plate and tail fibers.
In the centre of the head there is a linear double stranded DNA. The tail is hollow and
cylindrical. In the tail the molecules of protein are spirally arranged. There is a basal
plate at the end of the tail, which bears tail fibers. The tail fibers help the virus for
absorption into a bacterium. All these parts are formed of proteins.

Alfred Hershey and Martha Chase showed that DNA of viruses is injected into the
bacterial cells while most proteins remain outside. Their discovery supported that DNA
rather than proteins is the hereditary material.

Viruses and disease

Viruses are incapable of replication without a host cell thus the process of transmission
from one host to another is necessary for viruses. Ecological, cultural, and genetic
factors are responsible for incidence and prevalence of viral diseases. Viral infections
may be localized, systemic or those that are not detectable easily.

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The symptoms of viral diseases vary to a great extent. Symptoms may appear at the site
of infection or at certain distant areas not related to point of infection. A particular virus
is capable of producing different symptoms on entering different hosts.

PLANT VIRAL DISEASES

Plant diseases caused by virus are generally systemic in nature. Even though the virus
spreads throughout the plant but only a few organs exhibit the symptoms. Leaves show
the most prominent features of viral diseases. In certain cases, shoots, floral organs or
even roots can show symptoms. The most important feature of viral plant disease is the
general retardation of growth, vigor and cropping power of plant.

Table : Endemic Viral Diseases of Plants and their vectors

Disease Virus Vectors

Tobacco Mosaic Tobacco Mosaic Virus -

Tobacco leaf curl Tobacco Leaf-curl Bemisia tobacco

virus (White fly)

Tomato leaf curl Tomato leaf-curl virus

Potato leaf curl Potato leaf-curl virus Myzus solanifolia

Papaya Mosaic Papaya Mosaic virus Aphis gossypi

(Aphid)

Citrus quick decline Citrus Trisetza virus Toxoptera

citricida.

Bunchy Banana top Banana bunchy top Pentalonia

virus nigronervosa

Rice Tungro Rice tungro bacilliform Nephotettix

virus apicalis

The most important symptoms of plant viral diseases are described below :-

1. Chlorosis

Chlorosis results in infected areas becoming pale green or yellowish in color as

chlorophyll formation inhibits due to infection.

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Figure : Chlorosis

Source: http://www.scielo.br/img/revistas/sa/v67n3/a14fig04.jpg

2. Mosaic

In mosaic irregular light green, yellow or white patches with normal green plants are
observed.

Figure: Mottling and Mosaic leaf symptoms

Source:http://upload.wikimedia.org/wikipedia/commons/thumb/b/b0/Alfalfa_mosaic_vir
us.jpg/220px-Alfalfa_mosaic_virus.jpg

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3. Mottling

It is an irregular pattern of indistinct light and dark areas.

Source: http://upload.wikimedia.org/wikipedia/en/b/bc/BidinBid2.jpg

4. Vein chlorosis and vein clearing

Vein chlorosis is restricted to veins while the vein clearing represents the translucence of
veins rather than being chlorotic or yellow. The veins appear lighter in colour against
dark background of normal green tissue.

5. Vein-mosaic
These represent the chlorosis along the main veins with the lighter areas irregular in
shape and distribution. Sometimes, the vein mosaics extend to the adjacent areas in the
form of irregular patterns.

6. Vein-banding

In these symptoms the chlorotic area occur along the veins in a regular manner
resulting in the presence of chlorotic and regular green coloured bands.

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Figure: Yellow vein-banding symptoms on grapevine caused
Source: www.virology-microbiology-b.blogspot.com

7. Necrosis

Necrosis represents the death of cells in localized areas. These dead tissues have a dark
brown border around them.

Figure : Necrotic leaf spot on coleus leaf caused by Impatiens necrotic spot virus
Source: http://www.forestryimages.org/images/384x256/5435414.jpg (CC)

8. Ring-spots

These represent the localized circular spots formed by concentric rings of chlorotic and
normal green tissue.

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 Source: http://www.scielo.br/img/revistas/sa/v67n3/a14fg11c.jpg (CC)

9. Enations

Enations are the outgrowths generally occurring on vein or midrib on the lower surface
of the leaves. They may vary in number and shape. They may be small, large, papillate
or spine-like in shape. In some cases like 'Pea enation mosaic virus' the outgrowths
develop between or adjacent to the veins on the lower surface of the.

10.Leaf-narrowing

In these symptoms the infected leaves generally become narrow due to reduced growth
of laminar tissues. The veins and the midrib remain normal in growth.

11.Leaf-curling

These symptoms represent irregular and extensive wrinkling and furrowing of leaves due
to reduced growth of veins in comparison to the growth of laminar tissue resulting in
shrunken veins and raised up laminar tissues leading to the curling of the leaves as leaf
curl of papaya.

12.Leaf-rolling

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 In these symptoms the downward and upward rolling of leaves takes place and this
covers their entire length as in leaf roll of potato.

Figure : Tomato with physiological leaf roll

Source: http://www.forestryimages.org/images/768x512/5411467.jpg (CC)

13. Stunting or dwarfing

In this case the infected plants show general growth retardation in all its organs
resulting in stunting of plants. The morphology of the stunted plants remains normal as
in 'Chrysanthemum-stunt virus'.

14. Rosetting

Shortening of internodes due to reduced growth brings the leaves together at the tip of
the branches giving rosette-like appearance.

15.Tumors

These are large and irregular outgrowths developed due to abnormal increase in size
and number of the cells. Tumors generally occur on roots of some leguminous plants.
Tumors develop on roots of remix sp. infected with 'Wound tumor virus'.

16.Pollen abortion and pollen sterility

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 In some infected plants either the pollen may not be produced (tomato ring-spot) or
may remain sterile (Quirking virus on Datura sp.)

INTERNAL SYMPTOMS OF PLANT VIRAL DISEASES

Internal symptoms of plant viral diseases results in:-

1. Changes in parenchymatous cells as in chlorosis.

2. Changes in xylem by formation of gum like substances blocking xylem vessels.

3. Changes in Phloem resulting in necrosis of phloem cells thus disturbance in the food
supply of host.

4. Changes in cell-organelles for eg. Ribosomes and Endoplasmic reticulum are reduced
in number.

5. Inclusion bodies like crystals start forming in cells due to viral attack.

Table: Symptoms of viral diseases in plants

Disease Symptoms

Tobacco Mosaic Chlorosis leading to mosaic, leaf curl and distontions; mottling

Tobacco leaf curl Curling and distortion of leaves.

Pumpkin Mosaic Mosaics leading to mottling

Papaya Leaf Curl Leaves become curled and develop mosaic.

Banana bunchy top Leaves become short and narrow get bunched together at the
top of the plant

Apple Mosaic Yellowish patches on the leaf which coalesce later leading to
necrosis.

Bhindi yellow vein Vein clearing followed by chlorosis, fruits dwarf, malformed
mosaic

Citrus quick decline Lack of growth of new flushes, yellowing of leaves, Limbs die
back from tip downwards.

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Rice Tungro Leaves turn yellow from tip downwards, plants show stunting
bearing empty grains.

TRANSMISSION of VIRAL DISEASES

Transmission of viral diseases maybe mediated by a vector or without a vector. The

following methods of transmission are not mediated by vectors:-

Transmission not mediated by Vectors

Air Borne

Common method of transmission of animal viruses eg. Influenza and Small Pox . Plant
viruses do not transmit by this method.

Contact

It is the most important method for transmission of plant viruses, eg Potato virus X spreads
in fields by contact between roots and crowns of the infected and healthy plants.

Hereditary

It is most common in insect viruses and are spread through eggs of infected parents.

Transmission mediated by Vectors

Vectors are agents responsible for transport, in this case it is transmission of infection from
one host to another potential host. Insects are the agents which act as vectors. The vectors
of plant viruses belong to insect order Hemiptera. These insects have sucking mouth parts
and they feed by sucking the plant parts directly. The sap flows through the mouth parts
carrying the virus particles.

Viral diseases of human beings

Human viral diseases include common cold, the flu, chickenpox, smallpox, rabies, yellow
fever, measles, German measles, Herpes, etc.

Table : Human Viral Diseases

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 Diseases Virus

Measles Paramyxovirus

German measles Rubella virus

Yellow fever Flavivirus

Rabies Rhabdovirus

Polio Polio virus

Small pox Small pox virus

Mumps Paramyxovirus

Herpes Herpes Simplex 1 and 2

PREVENTION OF VIRAL DISEASES

Prevention of viral diseases can be done by taking care that the disease does not get
stabilized in the host or by making the host resistant to infection. These can also be
prevented by killing the vector by treating with insecticides .The population of vector has to
be eradicated completely. Recombinant DNA technology is helpful these days in producing
virus or disease resistant plants.

Applications

Viruses are important in the study of cell biology, molecular biology, material science,
nanotechnology to name a few. The three main applications are in the field of

1. Life sciences and medicine

Viruses have helped to study the basic mechanisms of molecular genetic like DNA
replication, transcription, translation and immunology. Viruses have been used to
manipulate and investigate functions of cells. They are being used as vectors to treat
diseases like cancer, as they can target cells and DNA specifically. They are also being used
in gene therapy.

2. Materials science and nanotechnology

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Viruses are considered to be organic nano particles. Their size, shape and chemical
structure helps them to be used as templates for organizing material on nano scale.

3. Weapons

Viruses can cause epidemic in human civilization. This is a matter of concern as they can be
used as weapons for biological warfare.

SUMMARY

Viruses are strange things considered to be between living and non-living. They cause
infectious diseases. The term virus was originated from a Latin word which means poison.
The branch of study of viruses is called virology. The credit for the discovery of viruses
goes to Beijerinck {1898}. The contributions of other pioneers like Pasteur, Mayer and
Iwanowsky cannot be overlooked. Stanley successfully crystallized Tobacco mosaic virus.
The concept of contagium vivum fluidum showed that viruses were fluid in their physical
nature.

Viruses maybe spheroidal, rod shaped or a combination of both these shapes. TMV is a rod
shaped plant virus whereas colliphage T2 is a virus having spheroidal head and rod like tail.

Chemically speaking viruses are nucleo proteinacious particles. Chemical analysis has shown
that the nucleic acid of a virus maybe DNA or RNA. Bacterial and animal viruses are mostly
DNA viruses whereas plant viruses are in most cases RNA viruses. The nucleic acid content
varies from virus to virus. The protein shell that encloses the nucleic acid is called capsid. It
consists of many structural sub-units known as capsomere. The envelope of viruses is a bi-
layer lipid coat. It assists in viral attachment by glycoproteins.

Bacteriophage is a phage which eats bacteria. Their multiplication has been studied by
Delbruck and Luria and they suggested that bacteriophages exist in three stages. Those that
become prophages are called temperate and those unable to do so are known as virulent.
The life cycle of viruses differs from species to species but has seven basic stages –
attachment, penetration, synthesis, assembly, maturation and release. The two major life
cycles are the lytic and the lysogenic cycle. The replicative process of virulent phages is
called the lytic cycle. The lysogenic cycle is the replication cycle under a temperate state.

Viruses cause innumerable diseases which maybe localized, systemic or those that are not
detected easily. The symptoms vary to a great extent. They maybe external or may appear
at the site of infection. Symptoms are widely distributed. A single virus can show a variety

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of symptoms on infection. Plant viral diseases are mainly identified by their symptoms
present on the host plant. Viruses also cause a number of human diseases. The
management and prevention of viral diseases can be done by taking care that the disease
does not get stabilized in the host. The transmission of viral diseases is mediated by a
vector. Transmission may also take place in the absence of a vector.

Viruses are important in the study of cell biology, molecular biology, material science and
nanotechnology, to name a few.

Exercises

At the end of the study of this chapter a student will be able to answer:

1. Define a virus and why it is considered to be on the line between living and non
living.

2. Explain the morphology of a virus.

3. Explain the milestones in the origin and discovery of viruses.

4. Mention the criteria for classification of viruses .

5. Write different classes of viruses based on Baltimores classificationExplain the

features of the Genome of a Virus.

6. Describe the lytic and lysogenic cycles.

7. Write notes on Tobacco Mosaic and the coliphage T2 virus.

8. Write the symptoms of the diseases caused by viruses in plants.

9. Describe the applications of viruses in various spheres of human life.

Glossary

Agar: A gelatinous substance obtained from certain species of red algae, for eg. Gelidium

Assembly: In viruses it is a phase during their replication in which the process of

integration of viral components into complete virus takes place.

Bacteriophage: Eater of bacteria, a virus that infects specific bacteria, multiplies and
ultimately destroys the host bacterial cell.

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Base plate: The hexagonal plate at the end of the helical tail to which tail fibers are
attached.

Capsid: The protein coat of complete virus particles.

Capsomeres: The individual protein units, which comprise the capsid of viruses.

Chlorosis: Lack of chlorophyll development due to infection by virus in this context.

Collar: It is the thin disc that serves as an attachment area between the head and the tail
of the virus.

Contagium Vivum Fluidum: It is a Latin phrase given Beijernick, which describes

diffusible, filterable and infectious suspension of tobacco, leaves infected with Tobacco
Mosaic Virus.

Core: The central nucleic acid portion of a virus particle.

Cuboidal: Having cube like appearance.

Eclipse: It is the phase during multiplication of bacteriophage which starts with injection of
nucleic acid into the host cell till the formation of first complete phage particle.

Envelope: The outer membranous lipoprotein structure covering the nucleocapsid particles.

Helical: It indicates the spiral arrangement of capsids, subunits surrounding the nucleic
acid of cuboidal viruses.

Icosahedron: Here it refers to 20-faced virus structure.

Interferon: It is a proteinacious substance, which interferes with virus multiplication in the

host cell. These are host specific and their action involves prevention of protein synthesis.

Latent Period: It is the penultimate stage of the replication cycle of bacteriophages. This
period starts with the formation of first new phage particle in the host cell and ends with
lysis.

Localized: This type of infection is restricted to a particular tissue or organ. It does not
spread throughout the entire body of the organism.

Lysis: It is the destruction of host cells (bacteria) at the time of multiplication of

bacteriophage.

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Lysogenic bacteria: Bacteria carrying a phage in this temperate state. It leads to
destruction of host bacteria.

Lysogeny: It is the process where the prophage DNA remains associated with the host cell
chromosome until it gets separated to become virulent.

Maturation: It is a process during virus multiplication in which the individual components

assemble and mature.

Necrosis: It is a pathogenic infection in which the part infected turns black leading to
disintegration and death.

Nucleic acid: these are compounds of pentose sugar, phosphoric acid, and nitrogen
containing base (purine and pyrimidine). These three units combine to form nucleotides
which join to form poly nucleotide strands by phosphodiester bonds and are called nucleic
acids.

Nucleoid: It is the central nucleic acid portion of a virus particle also called core.

Penetration: It is the process of injection of bacteriophage, nucleic acid into the host
bacterium.

Plaque: It is a clear area in the plate cultures of bacteria caused by infection from
bacteriophage.

Prophage: It is a non-infectious virion. It multiplies with the bacteria but does not bring
lysis of the host cell.

Release: It is the process in which mature viruses are released from bacterial cells.

Replication: It is the multiplication or formation of more copies of viruses.

Sense: This represents the polarity of single-stranded nucleic acid. Positive (+) sense is
that found in mRNA and negative (-) sense is complimentary to mRNA.

Symptom: It is the visible change when a virus or any other pathogen invades a host.

Temperate phage: These are bacteriophages that on entering the host bacterial cell do not
always result in lysis of the bacteria. This association is not permanent.

Transmission: It is moving of one substance from one place to another, from infected
organism to the healthy one.

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Vector: It is an organism, which carries or transmits disease-causing organism.

Virion: It is a highly infective particle.

Virulence: It refers to the infectivity of the pathogen.

References

1. Pelczar, M,J., Chan, E.C.S., Kreig, N.R. 2003. Microbiology – An application Based
Approach 5th Edition. Tata Mc Graw Hill Publ. Company Limited.

2. Prescott, L.M., Harley J.P., Klein D. A. (2008). Microbiology 7th edition, McGraw Hill,
India..

Web links

1. http://scienceblogs.com.br

2. http://www.microbiologybytes.com/

3. http://textbookofbacteriology.net/

4. http://bacteriophageucd.wordpress.com/life-cycle/

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