Viruses are the smallest infectious agents that can only replicate inside host cells. They contain nucleic acid (DNA or RNA) as their genome and do not have cells or metabolic machinery. Viruses come in a wide range of sizes and shapes. They enter host cells and use the host's cellular machinery to produce new viral components and assemble new virus particles. The replication cycle involves adsorption, penetration, uncoating, biosynthesis of viral components, assembly, and release of new virus particles. Viruses can be cultivated using animal inoculation, embryonated eggs, or tissue culture methods.

1. Viruses
• General introduction and
classification

2. • Virus are the smallest obligate intracellular infective
agents containing only one type of nucleic acid
(DNA and RNA) as their genome.
• They have no metabolic activity outside the living
cells. They do not possess a cellular organization
and lack the enzymes necessary for protein and
nucleic acid synthesis.
• Viral genome (nucleic acid) diverts the host’s
metabolism to synthesise a number of virus specific
macromolecules required for the production of
virus progeny.
• They multiply by a complex process and not by
binary fission. They do not grow in inanimate
media. They are resistant to antibotics.

3. The major differences between prokaryotes and viruses are
shown in table below:
Properties Bacteria except
chlamydiae
Chlamydiae Viruses
Cell well + + -
Ribosomes and
cellular anzymes
+ + -
DNA and RNA Both present Both present Only one present
Binary fission + + -
Growth on
inanimate media
+ - -
Sensitivity to
antibacterial
antibiotics
+ + -
Sensitivity to
interferon
- + +

4. Morphology of virus
• Size:
• Viruses are much smaller than other organisms. The
extracellular infectious virus particle is called the virion. The
size of viruses ranges from 20 to 300nm in diameter.
• The largest virus is the smallpox virus (300nm) and the
smallest is the parvovirus (20nm).
• In earlier days the virus particles were measured by passing
them through the collodion membrane filters of different pore
size (gradocol membranes).
• With the development of ultracentifuge, the virus size could
be calculated from the rate of sedimentation of virus in the
ultracetrifuge.
• The latest and the most direct method for measuring virus size
is electron microscopy. By this method, both size and the
shape of viruses can be made out.

6. Structure and symmetry
• Structure:
• The virion consists of a nucleic acid core (genome)
surrounded by a protein coat, the capsid.
• The capsid together with the enclosed nucleic acid is
known as the nucleocapsid.
• The capsid is composed of a large number of protein
subunits (polypeptides) which are known as capsomers.
• Two major functions of capsid are, forming an
impenetrable shell around the nucleic acid core and to
introduce viral genome into the host cells by adsorbing
readily to cell surfaces. Certain viruses also contain
envelope that surrounds the nucleic acid.

7. • The envelope is acquired by the progeny virus
during release by budding through the host cell
membrane. It is lipoprotein in nature.
• The lipid is largest of host cell origin while the
protein is virus coded. Protein subunit are exposed
as projectile spikes on the surface of the envelope.
These structures are called peplomers (from peplos
meaning envelope).
• Enveloped viruses are susceptible to the action of
lipid solvents like ether and chloroform.
• Envelopes confer antigenic, biological and chemical
properties on viruses.

8. Symmetry
• Three types of symmetry are determined by the
arrangement of capsid around the nucleic acid core.
• Icosahedral (cubical) symmetry: An icosahedron is a
polygon with 12 vertices or corners and 20 facets in the
shape of equilateral triangular faces. Icosahedral
symmetry has a rigid structure. This type of symmetry
is found in papova, picorna, adenoviruses (all naked or
non-enveloped) and herpes, togaviruses(enveloped).
• Helical symmetry: The nucleic acid and the capsomers
are wound together to form a helical viruses are
enveloped and all are RNA viruses.
• Complex symmetry: Some viruses do not show either
icosahedral or helical symmetry due to the complexity
of their structures. These are referred to have complex
symmetry e.g. poxvirus.

10. Shape
• The overall shape of virus particles varies in
different groups.
• Pox virus is brick-shaped, rabies virus is
bullet-shaped and tobacco mosaic virus is rod-
shaped.
• Some are irregular and pleomorphic in shape.

11. The overall shape of virus particles varies in different groups. Pox virus is brick-shaped, rabies
virus is bullet-shaped and tobacco mosaic virus is rod-shaped. Some are irregular and
pleomorphic in shape.

12. Chemical properties
•Nucleic acid:
• Viruses contain only one kind of nucleic acid, either
single or double stranded DNA or RNA. Viral
nucleic acid may be extracted by treatment with
detergents or phenol.
• In some viruses (for example picornavirus,
papovavirus), extracted viral nucleic acid is capable
of initiating infection when introduced into the host
cells.

13. • Viral protein and lipids
• Viruses contain protein which makes up the capsid.
• Viral protein, besides protecting the nucleic acid,
also determines the antigenic specificity of the virus.
• In case of enveloped viruses, they contain lipids
(present in the envelope) derived from the host cell
membrane.

15. Susceptibility to physical and chemical
agents
• Temperature:
• Most viruses are heat labile and are inactivated within
seconds at 56ºC, minutes at 37ºC and days at 4ºC.
However, hepatitis ‘B’ virus resists heating at 60ºC for
one hour and some strains of scrapie (slow virus) resist
autoclaving at 121ºC for one hour.
• They are stable at low temperatures. For long term
storage, they are kept frozen at -70ºC.
• Another better method for prolonged storage is
lyophilisation or freeze drying (drying the frozen virus
under vaccum). Lyophilised virus can be reconstituted
by addition of water. Some viruses (e.g. poliovirus)do
not withstand freeze drying.

16. pH:
• The viruses remain viable in a pH range of 5-9, but
are killed by extreme acidity and alkalinity.
Enteroviruses are very resistant to acidic pH while
rhinoviruses are very susceptible.
• Lipid solvents:
• Chloroform, ether and detergents destroy all
enveloped viruses which contain lipoprotein
envelopes. Naked viruses are resistant to these
agents.

17. Disinfectants
• Most viruses are destroyed by oxidising agents such
as H2O2, hypochlorite, and iodine compounds.
Formaldehyde and β- propiolactone (BPL) are
actively virucidal and are commonly used for the
preparation of killed viral vaccines.
• However, most viruses are resistant to phenol.
• Chlorination of drinking water kills most viruses but
hepatitis A and polioviruses are relatively resistant to
chlorination, particularly if present with organic or
faecal material.

18. Radiations
• Viruses are inactivated by sunlight, ultraviolet
rays and ionising radiations

19. Replication of viruses(lytic cycle of
virus)
• Due to lack of biosynthetic enzymes, viruses replicate
by taking over the biochemical machinery of the host
cell to synthesise virus specific macromolecular
required for the production of virus progeny. The
genetic information necessary for viral replication is
contained in the viral nucleic acid. The replicative cycle
can be divided into six sequential phases.
• Adsorption
• Penetration
• Uncoating
• Biosynthesis
• Maturation and
• Release

21. Adsorption and attachment
• The viruses come in contact with a cells by random
collision but adsorption or attachment is mediated
by the binding of virus surface structures, known as
ligands, to the receptors on cell surface.
• In case of influenza virus, the haemagglutinin ( a
surface glycoprotein )binds specifically to sialic acid
residue of glycoprotein receptor sites on the surface
of respiratory epithelium.
• With the human immunodaeficiency virus (HIV),
attachment is between the viral surface glycoprotein
gp 120 and the CD4 receptor on host cells.

22. Penetration
• After attachment, the virus particle may be
engulfed by a mechanism resembling
phagocytosis, a process known as viropexis.
• Alternatively, in case of the enveloped viruses,
the envelope may fuse with the plasma
membrane of the host cell releasing the
nucleocapsid into the cytoplasm.

23. Uncoating
• This is the process of stripping the virus of its outer
layer and capsid to release the nucleic acid into the
cell.
• With most viruses, uncoating is affected by the
action of lysosomal enzymes of the host cells.

24. Biosynthesis
• After uncoating, the viral genome direct the
biosynthestic machinery of the cell to shut down the
normal cellular metabolism and direct the sequential
production of viral components.
• In general, the nucleic acid genome of most DNA
viruses is synthesized in the host cell nucleus. However,
the poxviruses synthesis all their components in the
cytoplasm.
• Nucleic acid genome of most RNA viruses is
synthesized in the cytoplasm.
• The exceptions are orthomyxoviruses, some
paramyxoviruses and retroviruses which are synthesized
partly in the nucleus of the host cell.
• Viral protein is synthesized only in the cytoplasm.

25. Biosynthesis consists of the following
steps
• Transcription of messenger RNA (mRNA) from
viral nucleic acid.
• Translation of the mRNA into ‘early proteins’ or
‘nonstructural proteins’. These are enzymes which
initiate and maintain synthesis of virus components.
They may also induce shutdown of host protein and
nucleic acid synthesis.
• Replication of viral nucleic acid.
• Synthesis of ‘late proteins’ or ‘structural protein’
which constitute daughter virion capsids.

26. The mechanisms of nucleic acid synthesis differ
in the different type of viruses.
• Replication of single stranded DNA viruses:
• In case of these viruses (for example
parvovirus),a complementary stand is first
synthesized, producing double stranded
‘replicative forms’.
• This double stranded viral DNA acts as a
template for its replication, and also for
transcribing into mRNA which are translated
into viral proteins.

27. Replication of double stranded DNA
viruses
• Initially only a part of the viral DNA is transcribed
into early mRNA.
• This encodes for synthesis of early proteins which
are required for DNA replication.
• Late proteins are synthesized after viral DNA
replication has commenced.

28. Replication of RNA viruses
• In many single stranded RNA viruses (e.g. poliovirus),
the viral RNA can act directly as mRNA. These are
named as positive strand (plus strand, positive sense)
RNA viruses. The single stranded parental RNA
(positive strand) acts as the template for the production
of a complementary strand (negative strand), which acts
as the template for progeny viral RNA.
• In some other single stranded RNA viruses (e.g.
influenza and parainfluenza viruses), they carry their
own RNA polymerase for mRNA transcription. These
are named as negative strand (minus sense) RNA
viruses. Parental RNA produces complementary
negative strands which acts both as mRNA and as
template for the synthesis of progeny viral RNA.

29. • In the double stranded RNA viruses (e.g. reoviruses),
the viral RNA is transcribed to mRNA by viral
polymerases.
• Retroviruses exhibits a unique replication cycle.
Virus genome (single stranded RNA) is converted
into an RNA: DNA hybrid by the viral enzyme, RNA
directed DNA polymerase (reverse transcriptase).
• Double stranded DNA is synthesized from the hybrid
(RNA: DNA). The double stranded DNA form of the
virus (provirus) integrates into the host cell genome.
The integration of the provirus into the host cell
genome may lead to transformation of the cell and
development of neoplasia.

30. Maturation
• The viral nucleic acid and capsid polypeptide
assemble together to form the daughter virions.
• The assemble takes place in either the nucleus
(herpes and adenoviruses ) or cytoplasm (picorna
and pox viruses).
• In case of enveloped viruses, the envelope is derived
from the nuclear membrane when the assembly
occurs in the cytoplasm of host cell
(orthomyxoviruses and paramyxoviruses).

31. Release
• Enveloped viruses are released by a process of budding
from the cell membrane over a period of time. The host
cell is usually not affected but there are exceptions e.g.
polioviruses not only damage host cell but may also be
released by the lysis of the host cell. In case of bacterial
viruses (e.g. bacteriophages), they are usually released
by lysis of the infected batcerium.
• Eclipse phase:
• From the stage of penetration of virus into the host cell
till the appearance of the first infectious virus progeny
particle, the virus cannot be demonstrated inside the
host cell. This period is known as eclipse phase. The
duration of eclipse phase is about 15 to 30 minutes for
bacteriophages and 15-30 hours for animal viruses.

32. Abnormal replicative cycles
• Incomplete viruses:
• A proportion of daughter virions that are produced
may not be infective. This is the result of defective
assembly. One example of such defective assembly
is influenza virus. They will have a high
haemagglutinin titre but low infectivity. This is
known as Von Magnus phenomenon.
• Pseudovirions:
• The capsid occasionally encloses host cell nucleic
acid instead of viral nucleic acid. They are non-
infective and do not replicate. These are called
pseudovirions.

33. Abortive infection
• This occurs due to wrong selection of host cells by the
virus. The viral components may be synthesized but the
maturation is defective. The virus progeny either is not
released or is non-infectious. Here, the defect is in the
host cell and not in the parental viruses.
• Defective viruses:
• Some viruses are genetically defective and they are
unable to give rise to fully formed progeny. Yield of
progency virions occurs only in the presence of helper
virus, which can supplement the genetic deficiency.
Examples of defective viruses are hepatitis D virus and
adeno-asssociated satellite viruses which replicate only
in the presence of hepatitis B and adenoviruses (both
acts as helper viruses) respectively,

34. Cultivation of viruses
• As viruses multiple only in living cells, they cannot
be grown on any of the inanimate culture medium.
Three methods are employed for the cultivation of
viruses:
– Animal inoculation
– Embryonated egg inoculation
– Tissue culture
• Animal inoculation:
• Animals inoculation is used for:
• Primary isolation of certain viruses
• To study pathogenesis of viral diseases
• To study viral oncogenesis

35. • Infant (suckling) mice are used in the isolation of
arboviruses and coxsackie viruses, many of which
do not grow in any other system.
• Animals may be inoculated by several routes-
intracerebral, subcutaneous, intraperitoneal or
intranasal. After inoculation, animals are observed
for signs of disease or death.
• Later on, they are sacrificed and tissues are tested
for the presence of virus. The viruses are identified
by neutralization test using antiviral sera. In some
viruses, inclusion bodies may be observed in stained
smear.
• Besides mice, Other animals such as guinea pigs,
rabbits and ferrets are also used in some situations.

36. Embryonated egg inoculation
• Goodpasture (1931) first used embryonated
hen’s egg for cultivation of viruses.
Embryonated hen’s eggs (7 to 12days old) are
inoculated by one of the several routes such as
chorioallantoic membrane (CAM), allantoic
cavity, amniotic sac and yolk sac.
• After inoculation, eggs are incubation for 2-9
days.

38. Chorioallantoic membrane (CAM):
• CAM is inoculated mainly for growing poxviruses.
It produces visible lesions (pocks).
• Each pock is derived from a single virion. Pick
counting, therefore, indicates the number of viruses
present in the inoculum.
• Pocks produced by different viruses have different
morphology.

40. Allantoic cavity
• Allantoic inoculation is employed for growing
influenza virus for vaccine production.
• Other chick embryo vaccines include yellow fever
(17D strain) and rabies (flury strain) vaccines.
Duck’s eggs being bigger, provide a better yield of
rabies virus and were used for the preparation of the
inactivated non- neutral rabies vaccine.

41. Amniotic sac
• Inoculation into the amniotic sac is mainly used
for the primary isolation of the influenza virus.
• Yolk sac inoculation:
• It is inoculated for the cultivation of some viruses
and certain bacteria (chlamydia and rickettsiae).
• Tissue culture:
• Three types of tissue cultures are available:
• Organ culture
• Explant culture
• Cell culture

43. Question
• 1. Explain the lytic cycle of virus.
• 2. Define Virus and their replication
mechanism.
• 3. List 5 (five) viruses with disease
• 4. Describe the structure of virus.
• 5. Give the characteristic of virus.
• 6. Differentiate between bacteria and virus