2. Introduction
• Viruses are the smallest, subcellular agents that
are unable to replicate outside host’s cell.
General properties:-
Viruses are not cell, they do not have a nucleus,
cytoplasm, mitochondria, or ribosomes
They contain one kind of nucleic acid (RNA or
DNA) covered by protective protein coat
(known as capsid).
Some viruses have outer lipoprotein membrane
(called envelope).
3. They are obligate intracellular parasites, because
they can’t generate their energy or proteins.
They do not undergo binary fission or mitosis.
Property Virus Cell
Type of nucleic acid DNA or RNA DNA and RNA
Proteins Few Many
Ribosomes Absent Present
Multiplication by binary fission
or mitosis
No Yes
Comparison between viruses and cells
4. Viral structure
Viruses are consist of two or three major
components:
1. Nucleic acids (DNA or RNA).
2. Capsid (protein coat).
3. Envelope (lipoprotein)(exist in some viruses).
Note:
Viruses have no envelope known as naked.
5. Viral nucleic acids (genome):
• The viral genome is located internally.
• The nucleic acid of a virus may occur in various
forms.
1. It may be composed of DNA or RNA.
2. It can either be single-stranded or double-
stranded.
3. The nucleic acid may occur as a continuous
molecule, or it may be segmented.
6. Viral capsid
Its a protein coat covered the genome.
It is made up of subunits known as capsomers.
Capsomer is consist of single or several proteins.
• Nucleic acid + Capsid Nucleocapsid
• The arrangement of capsomers gives the virus its
geometric symmetry.
7.
8. Viral symmetry:-
• Viral nucleocapsids have three forms of
symmetry
1. Icosahedral, in which the capsomers are
arranged in 20 triangles (icosahedron).
• The viruses that cause chickenpox and flaccid
paralysis have icosahedral symmetry
9. 2. Helical, in which the capsomers are arranged in
a cavity coil that appears like rod-shaped (influenza
virus).
10. • There are some viruses that have neither
icosahedral nor helical symmetry such as smallpox
and Bacteriophage. These viruses have symmetry
known as complex symmetry.
Bacteriophage
11. Function of the capsid:
1. Protect the viral genetic material.
2. Mediate the attachment of the virus to specific
receptors on the host cell surface (organ
specificity).
3. They can induce antibody and cytotoxic T cells
response.
12. Viral envelope:-
• It is a lipoprotein membrane composed of lipid
derived from the host cell membrane and
protein that is virus-specific.
• The viral envelope is acquired as the virus exits
from the cell in a process called budding.
14. Classification of medically important viruses
Criteria used for viruses classification:
I. Virion morphology, including:
Size
Type of symmetry
Presence or absence of envelope.
II. Viral genome properties:
Type of nucleic acid (DNA or RNA)
Strandedness (ss or ds)
Whether linear or circular, sense (positive,
negative)
15. III. Physicochemical properties of the virion,
including susceptibility to physical and chemical
agents.
IV. Biologic properties, including natural host
range, mode of transmission, tissue tropisms,
and pathology.
Viruses are separated into major groups—
called families:
Family–viridae
Subfamily –virinae
16. DNA viruses
The medically DNA viruses are classified into 7
families:
1. Four naked icoshedral viruses families:-
Parvoviridae
Polyomaviridae
Papillomaviridae
Adenoviridae
2. Three enveloped families:-
Hepadanviridae
Herpesviridae
Poxviridae
17. RNA viruses
There are 14 families of RNA viruses:
1. Three naked icosahedral viruses families
• Picornaviridae
• Caliciviridae
• Reoviridae.
2. Three enveloped icosahedral viruses families
• Flaviviridae
• Togaviridae
• Retroviridae.
21. Viral replication
Virus replicates only inside living cell.
Virus doesn’t reproduce by binary fission or
mitosis.
One virion infects a cell, it can reproduce into
hundreds of virions.
22. 1. Attachment and penetration
2. Uncoating of the viral genome
3. Genome replication & expression
4. Progeny virion assembly
5. Virion release from cell
Stepwise of viral replication
23. 1. Attachment and penetration:
• Viral capsid or glycoprotein bind to specific
receptor proteins on the host cell surface
through noncovalent bonding.
• After binding, the virus nucleocapsid is taken up
inside the cell by endocytosis.
2. Uncoating:
• After penetration, the viral nucleic acid
physically separated from the capsid.
24. 3. Genome replication and expression:
• DNA viruses replicate in the nucleus.
• RNA viruses replicate in the cytoplasm.
• The first step gene expression is mRNA
synthesis and then, translation it into proteins.
4. Assembly:
• Assembly is packaging of the viral nucleic acid
within the capsid proteins.
25. 5. Virion release from cell
• Virus particles are released from the cell by
either of two processes;
1. By rupture of the cell membrane and release of
the mature particles (naked viruses).
2. By budding through the outer cell membrane
or nuclear membrane (enveloped viruses).
28. Stepwise of viral pathogenesis:-
1. Transmission of the virus and its portal of
entry into the host.
2. Replication of the virus and damage of the cells.
3. Spread of the virus between host cells and
organs.
4. The immune response, both as a host defense
and as a contributing in causing of diseases.
5. Persistence of the virus infection.
29. Transmission and portal of entry:
Person-to-person (Horizontal transmission) by
respiratory aerosol, saliva, blood, or semen.
Fecal-oral route.
Transmission of virus between mother and
offspring (vertical transmission)
Animal-to-human transmission (zoonotic) (e.g.,
rabies, yellow fever virus, dengue virus etc).
30.
31.
32.
33. Localization or disseminated infection
Viral infections are either localized at the
portal of entry or spread to other organ
(systemic).
1. Localized infection:
• Rhinoviruses causes common cold, which
involves only the upper respiratory tract.
2. Systemic infection:
• Poliovirus cause poliomyelitis
34.
35. Immunopathogenesis:
• Immune system attacks viral infected causing tissue
damage and loss of function.
Example pathogenesis of hepatitis caused by
hepatitis A, B, and C viruses.
The skin rash caused measles virus.
Arthritis seen in hepatitis B, parvovirus B19 and
rubella virus.
36. Persistent Infection:-
• Certain viruses can persist for long periods either
intact or in the form of subviral component
(genome).
• Viruses established persistent infections by one of
the following mechanisms:
1. Integration of a DNA provirus into host cell
DNA (e.g., retroviruses).
2. Immune tolerance (unresponsiveness).
3. Rapid antigenic variation
37. Types of persistent infection:
• There are 3 types of persistent infection.
1. Chronic carrier infection:
• Carrier state refers to asymptomatic people who
produce virus for long periods of time and can
serve as a source of infection for others (e.g.,
HBV & HCV).
38. 2. Latent infections:
• Those infections that are not producing virus at
the present time but can be reactivated at a
subsequent time (e.g., herpesviruses).
3. Slow virus infections:
• The term "slow" refers to the prolonged period
between the initial infection and the onset of
disease (progressive multifocal
leukoencephalopathy caused JC virus).
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