1. VIRUSES
Dr Saji Mariam George
Associate Professor
Assumption College Autonomous
Changanacherry.
2. VIRUSES
• The term Virus was coined by
Louis Pasteur (1884).
• The word virus is derived from
the Latin word ‘vira’ meaning
poisonous fluid. (L. virus =
poison)
• Discovered the first virus –
Tobacco Mosaic Virus (TMV)
from Tobacco leaf by a Russian
scientist Iwanowski(1892)-
described as ‘filterable viruses’
• Study of viruses - Virology
• Obligate intracellular parasites –
require living cells of the host.
Outside the living cells of host,
viruses can be crystallized and
stored like a chemical substance.
Iwanowski(1892
3. • Viruses are called acellular microbes as they
lack the typical cellular organization -They
have no cell membrane, cytoplasm,
organelles etc.
• They possess their own genetic material
containing genetic codes for the synthesis of
nucleic acids and proteins needed for their
own replication.
4. • They utilize the ribosomes of the host cell for
protein synthesis during replication – hence
can not replicate outside the host cell.
• A fully formed virus particle after release
from the host cell does not increase in size or
number , but retain its infectious nature for
very long periods ranging from a few months
to several years.
5. • Viruses are considered as intermediate between
living organisms and non- living things.
Viruses have the properties of living organisms –
i. They have their own genetic material either
DNA or RNA
ii. They are capable of undergoing replication
iii. They have the ability to undergo mutation.
Viruses have the properties of non - living things –
i ) They are acellular microbes – they have no typical
cellular organization.
ii) They have no protein synthesizing machineries,
Ribosomes-They depend host ribosomes for
replication
iii) Outside the living cells of the host, viruses are
inactive and lifeless and can be crystallized as a
chemical substance.
6. Wendell Meredith Stanley
• W.M Stanley (1935)
crystallized virus –
Tobacco Mosaic Virus
(TMV) for the first
time.
• W. M Stanley is known
as the ‘Father of
Virology’
Image : https://www.nobelprize.org/
7. SIZE OF VIRUSES
• Vary widely in size – 10 to 20
nm (e.g. Parvo
viruses( Latin parvus = small)
-single-stranded DNA
viruses) to 250 – 400 nm (Pox
viruses )
• Smaller than bacteria
• Visible only through an
Electron Microscope
Pox viruses(DsDNA
viruses)
8. SHAPE OF VIRUSES
• Vary widely in shape
Spherical
Examples :
Arboviruses
Polio virus
Japanese encephalitis
Measles
Influenza virus
Mice leukaemia
Cuboid
Adenovirus
Enteroviruses
Retroviruses
Arboviruses
17. Rod shaped -Tobacco Mosaic Virus (TMV)
Image : Plant Pathology Department of the Rothamsted Experimental Station collection
of plant virus pictures
24. NATURE OF VIRUSES
A virus can exist in two states –
• An extracellular phase as an infectious virus
particle or virion
• An intracellular phase as the genetic material
• Viruses are metabolically inert outside the
living cells.
25. STRUCTURE OF VIRUSES
• Structure of a virus is simple
• It consists of a central core of nucleic acid - the
genome - either DNA or RNA surrounded by a
protein coat , capsid and with or without an
envelope.
• The capsid with the enclosed nucleic acid is
known as the nucleocapsid.
• A virus contains only one type of nucleic acid ,
either DNA or RNA , but never both in the same
virion and all the members of the same virus
family have the same type of nucleic acid.
26. Types of viruses based on nucleic acid :
1. RNA viruses (Riboviruses)
The virus containing RNA as the genetic
material.
Most of the plant viruses are RNA viruses.
Examples :
Tobacco Mosaic virus (TMV )
Influenza virus
Measles virus
Polio virus
27. Types of RNA viruses
i) Single stranded RNA viruses (SsRNA viruses)
• RNA is usually single stranded
Examples :
Tobacco Mosaic virus (TMV)
Influenza virus
Measles virus
Mumps virus
Brome Mosaic virus virus
Papaya Mosaic virus
Polio virus
Picorna virus
Toga virus
Orthomyxovirus
Rhabdovirus
Foot & mouth disease virus
Yellow fever virus.
Ebola virus
Rabies virus
HIV virus
28. ii)Double stranded RNA viruses (DsRNA viruses)
• Viruses with double stranded RNA as the
genetic material
Examples
Reo virus
Wound tumour virus
Rota virus
Rice dwarf virus
Blue tongue virus
29. Distinct groups of RNA viruses :
i) Viruses with Anti- sense RNA (asRNA) or
negative – sense RNA
• Viruses with single stranded RNA with a base
sequence complementary to that of messenger
RNA (mRNA)
• During replication, it serves as a template for
the transcription of viral complementary RNA.
Examples: Measles virus, Ebola virus, Mumps virus,
Rabies virus , Influenza virus
30. ii ) Viruses with sense RNA or positive sense
RNA
• Viruses with single stranded RNA with the
same base sequence as mRNA.
• During replication, it functions as mRNA
serving as a template for protein synthesis.
Examples:
Polio virus, Rubella virus, Yellow fever virus,
Zika virus, Dengue fever virus, Tobacco
Mosaic Virus.
31. iii ) Viruses with a genome
made up of several pieces of
double stranded RNA
Example :
Reo virus
iv ) Retro viruses
Viruses with a single stranded
RNA that is copied by
reverse transcriptase into a
DNA genome within the host
cell.
Examples:
HIV -1 & HIV-2 that cause AIDS
32. 2. DNA virus (Deoxy – virus)
• Virus containing DNA as the genetic material.
• Majority of animal and bacterial viruses are DNA viruses.
Types of DNA viruses
1. Double stranded DNA virus (DsDNA Viruses)
Examples
Pox viruses – Small pox virus (Variola major & V. minor) ; Chicken pox virus
( Varicella zoster virus) ; Cow pox virus (CPXV)
Caulimovirus e.g Cauliflower Mosaic Virus (CaMV)
Adenovirus
Hepatitis virus
Papilloma virus
Herpes simplex virus
Bacteriophages of T – Series: T even bacteriophages – T2, T4, T6 &
T- odd phages T1, T3, T5 , T 7.
33. 2. Single stranded DNA
viruses
Examples :
Phi – Chi 174 (ΦX174)
Adeno - associated virus - 2
Parvovirus
34. RNA – DNA VIRUSES
• Rarely, a virus contains both RNA and DNA
• Such viruses are called RNA – DNA viruses
or ‘RNA-DNA hybrid virus’ or RDHV.
• A DNA virus with the capsid of an RNA virus
(inhabit Boiling Spring Lake, a city in North
Carolina, United States, Goeff Diemer and
Kenneth Stedman 2012).
35. • In most viruses , the nucleic acid is a single
molecule that may be linear or circular in
nature.
• However, some viruses have genomes that
occur in several segments as in
Orthomyxoviruses (e.g. Influenza virus ) ,
which have 7 or 8 SsRNA segments.
• The protein coat is known as capsid –
composed of many smaller units called
capsomeres (Protomers).
36. • The capsomers are made up of polypeptide
chains.
• The composition, number and forms of
capsomers vary with the kinds of viruses.
• The capsid gives shape to the virus.
• It also protects the nucleic acid from inactivation
by nucleases and other deleterious agents in
environment
• Another function of capsid is to introduce the
viral genome into the host cells by adsorbing
readily to cell surfaces.
37. • Viruses may be non - enveloped (naked) or
enveloped.
• Majority of viruses are non-enveloped – They
consist only one nucleic acid and protein coat.
• In enveloped viruses (e.g. Influenza virus), there
is an outer envelope or limiting membrane
outside the capsid , which usually contain lipids
and carbohydrates, which are generally derived
from the host cell during virus replication.
38. Structure of Tobacco Mosaic Virus
(TMV)
• The structure of a virus first
elucidated was that of TMV
(Franklin et.al., 1957).
• It is a rigid , rod - shaped,
helical virus.
• Each virus particle is 300 nm
long and 15 – 18 nm in
diameter
• TMV has a central core of
single stranded RNA
surrounded by a protein coat.
• The protein coat consists of
2130 identical protein subunits
called capsomeres
(Protomers).
39. • The capsomeres are arranged in a helical
manner around the RNA.
• The protein subunits are arranged around a
central hole of 4nm.
• There are 130 turns to the helix and 49 subunits
for every 3 turns.
• Each protein sub unit is made up of a single
polypeptide chain , made up of 158 amino acids.
• Inside the protein capsid there is a single
stranded RNA molecule which is also spirally
coiled to form helix.
40. • Virus RNA consists of 6500 nucleotides.
• Each turn of RNA helix contains 49
nucleotides
• Thus each protein sub unit is associated with
three nucleotide residues of RNA.
• The protein coat serves as a protective tube .
• The RNA is infective itself , although much
less than the intact virus because the
unprotected RNA is subjected to the action of
nucleases and thus destroyed.
41. HUMAN IMMUNODEFICIENCY VIRUS
(HIV)
• First isolated by Robert Galls (1993 , USA) and French
Scientist Luc Montagnier .
• HIV is a RNA retro virus (Oncogenic RNA viruses).
• It causes Acquired Immuno Deficiency Syndrome (AIDS )
• AIDS is an immunoregulatory disorder that is often fatal
because it predispose the person to severe opportunistic
infections because of depletion of helper T cells, owing to
infection by HIV.
• HIV infects and destroys helper T cells leading to a number
of immunological deficiencies.
• HIV occurs in two main strains viz. HIV -1 and HIV -2.
42. HIV
• HIV is a spherical, enveloped virus, 90 – 120
nm in diameter.
44. • HIV has an outer protein coat and an inner
cone shaped core, enclosing the genome.
• The genome is diploid , composed of two
identical , single stranded, RNA copies which
is unusual among viruses.
• In association with the viral RNA is the
reverse transcriptase enzyme (RNA
dependent DNA polymerase) , which is a
characteristic feature of retroviruses.
45. • When HIV infects a host cell, the viral RNA is
transcribed by the enzyme reverse transcriptase
first into a single stranded DNA and then to a
double stranded DNA (Provirus) which is
integrated into the host cell chromosome by
another enzyme integrase.
• The virus can remain dormant for long periods.
• Sometimes, in response to viral promoters, the
provirus initiates viral replication by directing
synthesis of viral RNA and other components.
46. • The virus acquires a lipoprotein envelope
from the host cell membrane while budding
out of the cell.
• The lipoprotein envelope consists of lipid
derived from the host cell membrane and
glycoproteins which are virus coded.
• The major virus coded envelope proteins are
the projecting knob like spikes (Peplomers)
on the surface and the anchoring
transmembrane pedicles.
47. BACTERIOPHAGES
(BACTERIAL VIRUSES)
• Viruses that infect bacteria
and destroy them – hence
bacteriophages are also
called as bacteria eaters.
• First described by Twort
(1915) and later by d’Herelle
(1917).
d’Herelle
Image:https://microbiologysociety.org/publication/past-issues/world-
war-i/article/frederick-william-twort-not-just-bacteriophage.html
48. • Bacteriophages occur widely in nature in
close association with bacteria.
• Like all viruses, bacteriophages have a central
core of nucleic acid , either DNA or RNA
surrounded by a protein coat (capsid) made
up of capsomeres (protomers).
49. SHAPE OF BACTERIOPHAGES
• Six morphological types (Bradley 1967)
TYPE A :
• Most complex type
• Has a hexagonal head, rigid tail with a
contractile sheath and tail fibres.
Examples :
T2, T4, T6 Bacteriophages
(T – for Type)
50. TYPE B :
• Has a hexagonal head , a flexible tail with no
contractile sheath and with or without tail
fibres
Examples :
T1, T5 phages
51. TYPE C :
• Has a hexagonal head , a tail shorter than
head with no contractile sheath and with or
without tail fibres.
Examples:
T3, T 7 phages
52. TYPE D :
• Has a head made up of large capsomeres, but
has no tail.
Examples:
Phi X 174 ( ΦX174 - Phi Chi 174 )
53. TYPE E :
• Has a head made up of small capsomeres,
but has no tail.
Examples:
F2, MS2 phages
54. TYPE F :
• Filamentous phage
Example :
fd phage
56. NUCLEIC ACIDS
• Phages have different nucleic acids
• All tailed phages contain DsDNA
• Phages with large capsomeres (Group D) and the
filamentous phages (Group F) have SsDNA.
• Group E phages have SsRNA
• The DNAs of phages may be circular
(e.g. ΦX174) .
• In phage Lambda, the DNA is linear in the
virion, but on entering the host cell, the
cohesive ends join to form a circle.
57. MORPHOLOGY AND STRUCTURE OF
T4 BACTERIOPHAGE
• The T – even phages (T2, T4, T6) have been
studied in great detail and traditionally serve
as the prototypes in describing the properties
of bacteriophages.
• Among the T – even phages , T4
bacteriophage is the most extensively studied
coliphage.
• It has a hexagonal head and a cylindrical tail
• The virus is non – enveloped.
59. • The elongated , hexagonal head contains a
tightly packed core of circular, DsDNA, which
is 1000 times longer than the phage itself,
surrounded by a protein coat (capsid).
• The DNA contains more than 75 genes.
• The head is 100 nm long and 65 nm in
diameter and consists of about 2000 identical
protein subunits (Capsomeres).
• The neck is very short and it contains a collar.
• The neck connects the head with the tail.
60. • The tail is long, helical (cylindrical) and is
composed of a central hollow core or tube
having a hole of 25 A°, through which the
phage DNA passes into the host bacterial cell.
• This is surrounded by a contractile sheath.
• The contractile tube is spring like and it has
24 rings. Each ring is composed of 6 sub units.
Thus the contractile tube is formed of 144
protein sub units.
61. • The contractile sheath and the core of the
tail remain attached to a hexagonal end plate
(base plate)
• The end plate contains six spikes or tail
fibres.
• Each of the tail fibres and tail spikes is a
bundle of polypeptide chains.
• The tail fibres help the phage to attach itself
on to the bacterial cell wall.
62. • The tail of T 4 phage is 100 nm in length and
25 nm in diameter.
• The tail has the property of contraction
which helps the phage in introducing its
nucleic acid into the host bacterial cell .
63. MULTIPLICATION OF VIRUSES
• Viruses are obligate intracellular parasites which
require the living cells of the host.
• Outside the living cells of the host , viruses are
inactive and lifeless and have no independent
metabolic activity and are incapable of
reproduction by processes characteristics of
other microorganisms.
• The multiplication of viruses take place by
replication, in which the viral protein and
nucleic acid components are synthesized within
host cells.
64. • The genetic information necessary for viral
replication is contained in the viral nucleic acid.
• However, virus has no protein synthesizing
machinery (Ribosomes) and biosynthetic
enzymes.
• Hence virus utilize the biochemical machinery of
the host cell to synthesize virus specific
macromolecules required for the production of
the virus progeny.
• The general outline of the replication is similar
in most viruses, but there are also differences
among various groups.
65. MULTIPLICATION OF
BACTERIOPHAGES
• Bacteriophages exhibit two types of life
cycles
i) Virulent or lytic life cycle
(e.g. T4 Bacteriophage )
ii) Temperate or lysogenic life cycle
(e.g. Lambda phage)
66. i) Virulent or Lytic life cycle
(e.g. T4 Bacteriophage )
• Exhibited by virulent phages.
• Infect a bacterial cell and undergo
multiplication which ends in the lysis of host
bacterial cell releasing a large number of
progeny phages.
• A lytic life cycle has five sequential phases
viz. adsorption, penetration, synthesis of
phage components, assembly and maturation
and release of progeny phages.
67. i) Adsorption
• A virulent bacteriophage comes into contact
with a susceptible bacterium by random
collision
• Attaches to a specific receptor site on the
bacterial surface with the help of the tail
fibres.
68. ii) Penetration
• The phage drill a hole in the cell wall of the
host bacterial cell by an enzyme lysozyme or
activates the degradative enzymes of the
host and injects its DNA into the bacterial
cell.
• After penetration, the empty head and tail of
the phage remain outside the bacterium.
69. iii) Synthesis of Phage components
• After the entry into the host bacterial cell, the
phage DNA directs the biosynthetic machinery
of the host cell to shut down the normal cellular
metabolism and to produce components of new
virus particles.
• The synthesis of bacterial mRNA and protein will
be stopped within a few minutes and the
bacterial DNA is quickly degraded into small
fragments and the nucleoid region of the
bacterium becomes dispersed.
• The phage DNA, head protein and tail protein
are synthesized separately in the bacterial cell.
70. iv) Assembly and maturation
• Assembly of phage DNA, head protein and
tail protein .
• Each component of phage nucleic acid
acquires a protein coat and finally the tail
structures are added forming a mature phage
progeny.
71. v) Release of phage progeny
• Phage enzyme lysozyme causes the lysis of
host bacterial cell wall resulting in the release
of mature phage progeny
• Usually , the lysis of E.coli takes place after
about 22 minutes and approximately 300 T4
phage progeny are released.
72. Lytic Life Cycle of a T4 Bacteriophage
Image http://textbookofbacteriology.net/phage.html
73. ii) Temperate or Lysogenic Life Cycle
e.g. Lambda phage
• A temperate phage does not cause the lysis of the host
bacterial cell.
• After entry into the host bacterial cell, the nucleic acid of
the phage becomes integrated with the bacterial
chromosome in a non-infectious stage known as
prophage.
• The prophage behaves like a segment of the host
chromosome and replicates synchronously with it.
• The bacterium that carries a prophage within its genome
is called a lysogenic bacterium.
• As the bacterium reproduces, the viral nucleic acid is
transmitted to the daughter cells at each binary fission.
75. • The prophage gives some new properties to the lysogenic
bacterium – Lysogenic conversion.
• This is due to the synthesis of new proteins that are coded by the
prophage DNA.
• Occasionally, during the multiplication of the lysogenic bacterium,
the prophage may become ‘excised’ from certain cells.
• This prophage may be converted into virulent phages
spontaneously and initiates the lytic replication and phage
progenies are released which can infect other bacterial cells and
render them lysogenic. This is known as spontaneous induction of
prophage – rare event.
• A lysogenic bacterium is resistant to reinfection by the same or
related phages – ‘superinfection immunity’.
