INTRODUCTION TO VIROLOGY.
Virology is the study of viruses.
Viruses are:
1. Viruses are a heterogenous intracellular group of microorganisms.
2. Vary with respect to size, morphology, and chemical composition that
contain either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA).
3. Acellular, and metabolically inert outside the host cell and are
obligatory parasites.
4. Obligate intracellular parasites
5. No ATP generating system
6. No Ribosomes or means of Protein Synthesis
- Host range:
Host range is determined by Viruses ability to interact with its host cell,
Binding Sites should match Receptor Sites
 Binding Sites - on viral capsid or envelope
 Receptor Sites - on host cell membrane
- Most viruses have a narrow host range:
o Polio virus - nerve cells
o Adenovirus - cells in upper Respiratory Tract
- Some viruses only infect:
o plants
o invertebrates
o protists
o fungi
o bacteria (Bacteriophages)
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 Viruses are unique life forms,,Why?
Because the Virus has a different from all other living organisms, either
eukaryotes or prokaryotes, for three fundamental reasons:
1. The nature of environment in which they grow and multiply.
2. The nature of their genome.
3. The mode of their multiplication.
First:
 Viruses can function and multiply only inside another living organism,
which may be either a prokaryotic or eukaryotic cell or any other living
cells depending on the virus.
Second:
 Whereas all other living forms can use only DNA (and not RNA) as
their genetic material (genome) for information transmission from
parent to progeny, viruses can use either DNA or RNA as their genome,
that is, some viruses can use only RNA (and not DNA) as their genetic
material.
 Therefore, these classes of RNA viruses have developed new sets of
enzymes for replicating and transcribing RNA from an RNA template.
Finally:
 All eukaryotic and prokaryotic cells divide and multiply as a whole
unit, that is, 1 ~ 2 -- 4 ~ 8 and so on. However, viruses do not multiply
as a unit.

In fact, they have developed a much more efficient way to multiply just
as complex machines are made in a modern factory. Different viral
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components are made separately from independent templates, and then
these components are assembled into the whole and infectious units,
also called virus particles (virions).
 Virions is the virus particles which consist of two or three parts: the
genetic material made from either DNA or RNA, a protein coat that
protects these genes; and in some cases an envelope of lipids that
surrounds the protein coat when they are outside a cell.
Viral Structure:
1. Nucleic Acid
- DNA or RNA (But never both)
o The nucleic acid is the genome that contains the information necessary
for viral function and multiplication.
o The composition of the protein shell called the viral "nucleocapsid"
surrounding the genome.
o ssDNA - ds DNA - ss RNA - ds RNA
2. Capsid (Coat Protein)
o protects viral genome from host endonucleases
o Consists of Capsomeres which are structural the units of capsid
o Contains binding Sites
3. Envelope
o Derived from the membrane of the host cell.
o Contains binding Sites
o The absence of viral membrane gives the naked virus.
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Size of viruses:
 The size of virions ranges from 20 nm (parvovirus) to -300 nm
(poxvirus) in diameter, and therefore, too small to be seen with the light
microscope. However, it can be studied with the electron microscope,
but this microscope is very costly and special training facilities are
required to use it.
Shape of viruses:
 The capsid consists of a number of identical units called capsomers.
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Viral Morphology:
2- Polyhedral (icosahedral)
1- Helical
23-Enveloped
4- Complex
A. Enveloped Helical
B. Enveloped Polyhedral
1-Helical Capsids
 Meaning the capsid is spiral in shape.
 It surrounds a spiral shaped core of nucleic acid.
 Helical capsids are usually flexible and rodlike.
 The length of the helical capsid is usually determined by the length of
the nucleic acids, that is, viruses having shorter nucleic acids will have
a shorter helical nucleocapsid.
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 Helical capsids can package only single-stranded RNA, but not doublestranded DNA or RNA, possibly because of the rigidity of the doublestranded nucleic acids.
2. Polyhedral or Icosahedral Capsids
 The term cubic is used to describe Icosahedral viruses.
 Viruses with icosahedral capsids possess a closed shell enclosing the
nucleic acid inside. An icosahedron has 20 triangular faces, 30 edges.
 Unlike helical nucleocapsids that package only single-stranded nucleic
acid, icosahedral capsids can be used to package either single- or
double-stranded RNA and DNA molecules.
 An icosahedral virus can be either naked or enveloped; but, unlike the
helical enveloped viruses, the icosahedron capsid structure is rather
rigid.
3- Complex:
 Means the capsid symmetry is neither icosahedral nor helical.
 Only the poxviruses among the medically important viruses show
complex symmetry.
 The overall shape of poxviruses is usually described as brick-shaped.
Many of the helical viruses and a few of Icosahedral viruses are surrounded by
an envelope.
Dependence of viruses on host cells:
 Because viruses possess neither cellular structure nor organelles, they
are unable to make their own proteins and essential enzymes. They are
therefore completely dependent on their host cells for energy and
multiplication.
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 Outside of living cells, viruses are metabolically inactive. The
information required for a virus to replicate is contained in its nucleic
acid. This information is used by the host cell to produce new viruses.
Effect of viruses on cells
In animal host the viruses can cause:
1. Cell lyses:
 The virus leave the host cell by breaking the cytoplasmic membrane and
is released out as viroins to attack other cells (enveloped viruses come
out of the cells like budding without rupturing the cell.)
 As the virus comes out of the cell the host cell will rupture i.e lyses
(killed) and die.
2. Cell transformation:
 A small group of viruses are able to change, or transform their host
cells from normal cells into abnormal ones with properties of
cancerous cells, here the cell is not dead.
3. Immune complex diseases:
 Immune system of host cell produces antibodies to fight the virus
.Antibodies will react with virus (antigen), and Antigen-Antibody
complex is produced that deposit in different places of body e.g.
Hepatitis B deposit in glomeruli of kidney
4. Asymptomatic Disease:
 Some viruses after infecting cells do not replicate, or they become
active for a time and then become inactive (latent). In response to
certain stimuli, latent viruses can be reactivated and become active
replicating particles, for example herpesviruses.
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CLASSIFICATION OF VIRUSES
 The internationally agreed system of virus classification is based on the
structure and composition of the virus particle (virion).
 In some cases, the mode of replication is also important in
classification.
 Viruses are classified into various families on this basis.
INTERNATIONAL CLASSIFICATION OF VIRUSES
Primary characteristics used in classification
Viruses are classified according to the nature of their genome and their
structure
VIRAL CLASSIFICATION
RNA or DNA
single-stranded or double-stranded
non-segmented or segmented
Nucleic acid
linear or circular
If genome is single stranded RNA,
can it function as mRNA?
whether genome is diploid (such as
in retroviruses)
symmetry (icosahedral, helical,
complex)
Virion structure
enveloped or not enveloped
number of capsomers
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PRINCIPAL EVENTS INVOLVED IN REPLICATION
1. Adsorption
 The first step in infection of a cell is attachment to the cell
surface .
 The viral attachment protein recognizes specific receptors, which
may be protein ,carbohydrate or lipid, on the outside of the cell.
 Cells without the appropriate receptors are not susceptible to the
virus.
2- Penetration
 Penetration occurs almost rapidly after attachment and is a next step for
gaining entry into the cytoplasm by crossing the plasma membrane.
 Thus, penetration allows the viruses to deliver their genome into the host
cells to initiate replication.
 The penetration process differs for different viruses:
a) Penetration by the process of endocytosis is common to both
enveloped and non-enveloped viruses.
b) Direct penetration is observed only in non-enveloped viruses,
membrane fusion is observed only in enveloped viruses. Some of the
non-enveloped viruses like picornaviruses and phages are capable of
directly injecting their genome into the host cell.
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3-Uncoating:
 Release of the viral genome from its protective capsid to enable the
viral nucleic acid to replicate.
4-Transcription:
 Synthesis of m-RNA
5-Translation:
 The viral m-RNA is translated on cell ribosomesinto structural and nonstructural proteins.
6-Replication of the viral genome
7-Assembly:
 New virus genomes and proteins are assembled to form new virus
particles.
8-Release:
 Enveloped viruses are released by budding
 Unenveloped viruses are released by rupture and lysis of the infected
cells.
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