Too small to be seen with a electron microscope.
The simplest viruses are composed of
1. A small piece of nucleic acid
2. Surrounded by a protein coat
Obligate parasites that depend on the cellular machinery of their
Not active outside of their hosts.
Organisms including animals, plants, fungi, and bacteria are hosts
for viruses, but most viruses infect only one type of host.
3. History of virus discovery
• In the late 1800s, botanists had been trying to find the cause of
tobacco mosaic disease.
• In 1892, D. IWANOWSKI tried to filter the sap of infected tobacco
plants (Filter capable of removing particles the size of all known
5. • The filtrate was Fully capable of producing the
ORIGINAL DISEASE in new hosts.
• Nothing could be seen in the using the most
powerful microscopes, nor could anything be
cultivated from the filtrates.
• Iwanowski concluded that the bacteria was so
small / or they made a filterable toxin.
• A Dutch botanist named Martinus Beijerink ruled
out the filterable toxin conclusion because the
filtered sap are capable of causing undiluted
• The agent cannot be cultivated on nutrient media
(need a host).
• In 1935, Stanley discovered this agent after
6. General properties
• Obligate intracellular parasites.
• Do not have the molecular machinery to replicate without a
• Pathogenic to higher plants.
• Plant viruses infect plants.
• A virus particle, also known as a virion is an extremely small
• Essentially a nucleic acid (DNA or RNA) enclosed in a protein
coat called a capsid.
• Viral genetic material can be
1. Double-stranded DNA
2. Double-stranded RNA
3. Single-stranded DNA or
4. Single-stranded RNA
• Most plant viruses are classified as single-stranded RNA or double-
stranded RNA virus particles.
• Cause various types of plant diseases.
• These diseases do not typically result in plant death.
8. • Plant diseases produce symptoms such as
Mosaic pattern development
Leaf yellowing and distortion
• Some plant viruses are not limited to one particular plant host.
• May infect different varieties of plants.
• Plants including tomatoes, peppers, cucumbers, and tobacco may all
be infected by the tobacco mosaic virus
10. Morphology of Viruses
• About 50 % of all known plant viruses are elongate (flexuous
threads or rigid rods).
• About 50 % of all known plant viruses are spherical (isometric or
• A few viruses are cylindrical bacillus-like rods.
11. Chemical composition of plant
• Protein (Capsid)
• Nucleic acids
+ve strand RNA
-ve strand RNA
• 60-95% of the virion.
• Repeating subunits, identical for each virus type but varies from
virus to virus and even from strain to strain .
• subunits - 158 amino acids with a mass of 17,600 Daltons (17.6
kDa, kd or K)
• TYMV – 20,600 Dalton protein
Nucleic acid is 5-40% of the virion
• Spherical viruses: 20-40%
• Helical viruses: 5-6%
13. • Nucleic acid (5-40%) represents the genetic material, indispensable
for replication .
• Nucleic acid alone is sufficient for virus replication – Fraenkel-
• Protein (60-95%) protects virus genome from :
facilitates movement through the host and
transmission from one host to another
15. Terminology for virus components
• Capsid is the protein shell that encloses the nucleic acid.
• Capsomers are the morphological units seen on the surface of
particles and represent clusters of structure units.
• Capsid and enclosed nucleic acid is called the nucleocapsid.
• The virion is the complete infectious virus particle
• Tobacco mosaic virus is typical, well-studied example .
• Each particle contains only a single molecule of RNA (6395 nt) and
2130 copies of the coat protein subunit (158 aa; 17.3 kDa)
• 3 nt/subunit
• 16.33 subunits/turn
• 49 subunits/3 turns
• TMV protein subunits + nucleic acid will self-assemble in vitro in
an energy-independent fashion
• also occurs in the absence of RNA
18. Plant viruses are diverse,
but not as diverse as
animal viruses –
probably because of size
constraints imposed by
requirement to move
plasmodesmata of host
19. Icosahedral arrangement is typical in
• An icosahedron has 20 triangular
(equilateral) faces (facets), 12
vertices (corner), 30 edge.
Icosahedron (sphere) e.g., BMV
20. The Bacteriophages
• Bacteriophages are obligate intracellular parasite on bacteria that
uses bacterial machinery system for its own multiplication and
• These are commonly referred as “phage”.
• Bacteriophages were jointly discovered by Frederick Twort (1915)
in England and by Felix d'Herelle (1917) at the Pasteur Institute in
• “Bacteriophage” term was coined by Felix d'Herelle.
Examples of phages:
• T-even phages such as T2, T4 and T6 that infect E. coli
• Temperate phages such as lambda and mu
• Spherical phages with single stranded DNA such as PhiX174
• Filamentous phages with single stranded DNA such as M13
• RNA phages such as Q beta
• Depending upon the phage, the nucleic acid can be either DNA or
RNA but not both.
• The nucleic acids of phages often contain unusual or modified bases,
which protect phage nucleic acid from nucleases that break down
host nucleic acids during phage infection.
• Simple phages may have only 3-5 genes while complex phages may
have over 100 genes.
• Certain phages are known have single stranded DNA as their nucleic
• Most phages range in size from 24-
200 nm in length.
• T4 is among the largest phages; it is
approximately 200 nm long and 80-
100 nm wide.
• All phages contain a head structure,
which can vary in size and shape.
Some are icosahedral (20 sides)
others are filamentous.
• The head encloses nucleic acid and
acts as the protective covering. Some
phages have tails attached to the
• The tail is a hollow tube through
which the nucleic acid passes during
24. • T4 tail is surrounded by a contractile sheath, which contracts during
infection of the bacterium. At the end of the tail, phages like T4 have
a base plate and one or more tail fibers attached to it.
• The base plate and tail fibers are involved in the binding of the
phage to the bacterial cell. Not all phages have base plates and tail
26. Bacteriophage replication cycle
• The first step in the infection process is the adsorption of the phage
to the bacterial cell.
• This step is mediated by the tail fibers or by some analogous
structure on those phages that lack tail fibers.
• Phages attach to specific receptors on the bacterial cell such as
proteins on the outer surface of the bacterium, LPS, pili, and
lipoprotein. This process is reversible.
• One or more of the components of the base plate mediates
irreversible binding of phage to a bacterium.
• The irreversible binding of the phage to the bacterium results in the
contraction of the sheath (for those phages which have a sheath) and
the hollow tail fiber is pushed through the bacterial envelope.
• Some phages have enzymes that digest various components of the
bacterial envelope. Nucleic acid from the head passes through the
hollow tail and enters the bacterial cell.
• The remainder of the phage remains on the outside of the bacterium
as “ghost”. Even a non-susceptible bacterium can be artificially
infected by injecting phage DNA by a process known as
29. TYPES OF LIFE CYCLE
• Depending on the life cycle, phages can either by lytic (virulent) or
• While lytic phages kill the cells they infect, temperate phages
establish a persistent infection of the cell without killing it.
• In lytic cycle the subsequent steps are synthesis of phage
components, assembly, maturation and release.
30. Lytic cycle
• There are five steps in a typical bacteriophage lytic
I. Attachment- A virus will attach to a suitable host cell.
II. Penetration- The whole virus or only the genetic material
(nucleic acid) will penetrate the cell’s cytoplasm.
A bacteriophage capsid remains on the outside of the bacterial cell
whereas many viruses that infect animal cell enter a host cell
31. III. Replication and synthesis - The viral DNA/RNA directs the host
cell to produce many copies of viral nucleic acids and proteins
necessary for its replication.
IV. Assembly - The viral nucleic acids and proteins are assembled
together to form new infectious particles.
V. Release - Newly generated viral particles are released from the host
33. lysogenic cycle
• The infection will enter a latent period.
• The host cell is not killed in this process, but the viral nucleic acid
will undergo genetic recombination with the host cell’s
• This integrated structure is called a prophage.
• When the bacterial DNA replicates, the prophage also replicates.
• Certain external condition such as UV light and x-rays cause viruses
to revert to a lytic cycle and then destroy their hosts.