THE TOPIC IS PRESENTED BY FATIMA KHAN:
TOPIC:CULTIVATION & STRUCTURE OF VIRUSES:
DEFINITION:“Viruses are simple acellular entities of one or more molecules of either
DNA or RNA.They are reproduced only within living cells & are
obligatory intracellular parasites”
Viruses lead a kind of “borrowed life”
Viruses exist in two phases:
Extra cellular phase:- in this phase ,they posses few enzymes
.Virions are regarded as extracellular phase of virus ,they take viral
genome from cell to cell & protect the genome in inhospitable
environment to which virus cannot replicate.
Intra cellular phase:-in this phase virus exist as replicating nucleic
acid that induce host metabolism to synthesize its components
&eventually complete virus particles are released.
GENERAL PROPERTIES:Viruses differ from other living cells by at least two ways:
1. Their simple , acellular organization.
2. Their inability to reproduce independently of host cells & carry
out cell division as prokaryotes & eukaryotes do.
CULTIVATI ON:Detection of viruses---------disease identification:Diseases like flu & cold are viral diseases & are easily
diagnosable[straight forward diagnosis].
Koch`s postulate are applicable to bacteria & other pathogens but for
viral detection these postulates are non-applicable because like
bacteria ,viruses are non-cultureable.So to resolve this problem
THOMAS M. RIVER`S expanded Koch`s postulates to include virus.
“filtrates of infectious material shown not to contain bacterial or
other cultivable organisms must produce specific antibodies in
Virus cultivation using an EMBRYONATED EGGS:for many years ,researchers have cultivated animal viruses by
inoculating suitable host animals or embryonated eggs.To prepare the
eggs for virus cultivation ,the shell is first disinfected with iodine &
penetrated with small sterile drill. viruses may be reproduce in certain
parts of embryo eg: myxoma virus grows well on chorioallantoic
membrane ,whereas mump virus prefer alantoic cavity.After
inoculation ,hole is sealed by gelatin.The infection cause a local lesion
known as pock,whose appearance is characristics of virus.
Virus cultivation of MONOLAYERS OF ANIMAL CELLS:Animal viruses have been grown in tissue culture on monolayers of
animal cells.This is made possible by development of growth media for
animal cells & by advent of antibiotics that can prevent bacterial &
fungal contaimination.A layer of animal cells & viruses inoculum is
covered on petri dish & viruses are allowed time to settle & attach to
cells.Then cells are covered by thin layer of agar to limit virion spread
so that only adjacent cells are infected by newly produced virions. As a
result localized areas of cellular destruction & lysis called plaques are
formed. Some cytological techniques may be used in clinical laboratory
for identification of viruses from sample of patients.
CYTOPATHIC EFFECT:- “when viruses replicate in host cells often a
noticeable detorition or structural changes occurs , this is called
AGAR CULTURE OF BACTERIOPHAGES:Bacterial cells are cultivated in both broth & agar culture
containing young & active growing cells. Due to lysis so many cells
are destroyed that turbidity of bacterial cells become clear.Agar
cultures are prepared by mixing bacteriophages sample with
cool,liquid agar & suitable bacterial culture.this mixture is poured
in petri dish & after hardening ,bacteria in top of agar grow &
reproduce forming a lawn.When virion come on top layer ,it
infect adjacent host cells & lysis result in clearing of lawn(plaque).
PHAGE PLAQUE (ON LAWN OF E.COLI):
PLANT VIRUSES CULTIVATION:Plant viruses are cultivated in variety of ways.Plant tissue culture,
culture of separated cells , or culture of protoplast may be used.Virus
can be grown in whole plant.leaves are mechanically induced by viruses
through rubbing. A localized NECROTIC LESION ,may developed due to
rapid death of cells in infected area. Besides lesions pigments may
Necrotic lesions on plant:-
STRUCTURE OF VIRUSES:SIZE:- In 1950`s TMW & other viruses were finally identified.Viruses
ranges in sizw from 17-300 nm in diameter.Smallest virus is 17nm in
size (the size of ribosomes) & largest is 1000nm(the size of smallest
bacteria).Viruses are barely visible to light microscope & mostly are
visible via EM.
GENERAL STRUCTURAL PROPERTIES:1. NUCLEOCAPSID CORE:- The nucleocapsid is composed of nucleic
acid either DNA or RNA,held within a protein coat called
capsid,which protect viral genetic material & aid in transfer b/w
host cells.The capsid surrounds the virus & is composed of finite
no. of protein subunits called “capsomers” which usually associate
with close to nucleic acids.
TYPES OF CAPSIDS:- There are following types of capsids:-
A- ISOCAHEDRAL CAPSID: The capsid of most isocahedral viruses is in the shape of a regular
polyhedron with 20 triangular faces and 12 corners. The
capsomers of each face form an equilateral triangle. An example
of a isocahedral virus is the adenovirus. Another is the poliovirus.
B-HELICAL CAPSID: Other capsids are helical and shaped like hollow protein
cylinders, which may be either rigid or flexible.
Eg: Tobacco mosaic virus.
C-VIRUSES WITH ENVELOP: Many viruses have an envelope” an outer membranous layer
surrounding the nucleocapsid.”Enveloped viruses have a roughly
spherical but somewhat variable shape even though their
nucleocapsid can be either icosahedral or helical.
Eg: Influenza virion, herpes & HIV virus.
D-VIRUSES WITH COMPLEX CAPSIDS: Have capsid symmetry that is neither purely icosahedral nor
helical.They may possess tails and other structures (e.g., many
bacteriophages) or have complex, multilayered wall surrounding
the nucleic acid (e.g., poxviruses such as vaccinia).
PROTOMERS: “Subunits of proteins which aggregate to form capsomers &
which in turn aggregate to form capsid.”
OUTER COVERING (COAT):-
HELICAL CAPSIDS:- These capsids have shape like hollow tube
with protein walls.eg:- TMV. A single type of protomer associate
together in helical or spiral arrangement to produce a long, rigid
tube, 15-18 nm in diameter & 300 nm long. The RNA genetic
material is wound spirally & toward inside of capsid. Not all helical
capsids are rigid & some are flexible too. Eg : capsids of influenza,
is enclosed in thin flexible envelop.
It is nature`s most favourite shape because it is most efficient way to
enclose a space.Hexagons & proteins Pentagons are used for
construction purposes.As we know capsomers are made up of
(pentone with 5 subunit)
(hexone with 6 subunit)
The icosahedron of adeno virus is constructed of 42 capsomers;
larger icosahedra are made ,if more hexamers are used to form
the edges and faces .
NUCLEIC ACIDS: Viruses are exceptionally flexible with respect to the nature of
their genetic material. They employ all four possible nucleic acid
a) single-stranded DNA,
b) double-stranded DNA,
c) single-stranded RNA,
d) double-stranded RNA.
All four types are found in animal viruses.Plant viruses most often
have single-stranded RNA genomes.The size of viral genetic
material also varies greatly. The smallest genomes (those of the
MS2 and QB viruses) are around 110 daltons, just large enough to
code for three to four proteins. At the other extreme, T-even
bacteriophages, herpesvirus, and vaccinia virus have genomes of
1.0 to 1.6 × 10 ^8 daltons and may be able to direct the synthesis
of over 100 proteins.
DNA VIRUSES: ss linear & circular DNA:Tiny DNA viruses like φX174 and M13 bacteriophages or the
parvoviruses possess single-stranded DNA (ssDNA) genomes. Some
of these viruses have linear pieces of DNA, whereas others use a
single, closed circle of DNA for their genome.
ds linear & closed DNA: Most DNA viruses use double-stranded DNA (dsDNA) as their
genetic material. Linear dsDNA, variously modified, is found in
many viruses; others have circular dsDNA.
The lambda phage has linear dsDNA with cohesive ends—singlestranded complementary segments 12 nucleotides long—that
enable it to cyclize when they base pair with each other.
ss RNA virus: Most RNA viruses employ single-stranded RNA (ssRNA) as their
The RNA base sequence may be identical with that of viral mRNA,
in which case the RNA strand is called the plus strand or positive
strand (viral mRNA is defined as plus or positive).
Positive-sense (5' to 3') viral RNA signifies that a particular viral RNA
sequence may be directly translated into the desired viral proteins.
Therefore, in positive-sense RNA viruses, the viral RNA genome can
be considered viral mRNA, and can be immediately translated by
the host cell.
However, the viral RNA genome may instead be complementary to
viral mRNA. and then it is called a minus or negative Negativesense (3' to 5') viral RNA is complementary to the viral mRNA and
thus must be converted to positive-sense RNA by an RNA
polymerase prior to translation. Negative-sense RNA (like DNA) has
a nucleotide sequence complementary to the mRNA that it
Polio, tobacco mosaic, brome mosaic, and Rous sarcoma viruses
are all positive strand RNA viruse.
rabies, mumps, measles, and influenza viruses are examples of
negative strand RNA viruses.
Many of these RNA genomes are segmented genomes,that is,
they are divided into separate parts.
It is believed that each fragment or segment codes for one
Usually all segments are probably enclosed in the same capsid .
Many animal viruses, some plant viruses, and at least one
bacterial virus are bounded by an outer membranous layer called
animal virus envelop.
projection of proteins in the form of spikes.
variable shapes of virus due to flexible &
bullet-shape rabbies virus have constant characteristics shape due
to rigid envelop.
VIRAL ENZYMES: Are located within the capsid, or even on capsid & envelop.
Mainly involved in nucleic acid replication.
e.g: the influenza virus uses RNA as its genetic material and carries an
RNAdependent RNA polymerase that acts both as a replicase and as
an RNA transcriptase that synthesizes mRNA under the direction of its
VIRUSE WITH COMPLEX SYMMETRY:-
pox viruses & bacteriophages
are examples of complex
symmetry viruses.pox viruses have intracellular envelop
covering the nucleocapsids & bacteriophages have tail
structure present.They posses two type of symmetry i.e;
icosahedral (on head) & helical (on tail).