DEPARTMENT OF MICROBIOLOGY
FEDERAL UNIVERSITY OF LAFIA
MCB 425: VIROLOGY AND TISSUE CULTURE
3 CREDIT UNITS
1. The Classification and Nature of Viruses
1.1. Introduction to Viruses
Viruses are unique biological entities that occupy a gray area between living
and non-living things. They are non-cellular, submicroscopic infectious agents
that can only replicate within living cells. The term "virus" is derived from
the Latin word meaning "poison" or "slimy liquid". First observed indirectly
due to their ability to cause disease, viruses are now known to infect all forms
of life: animals, plants, fungi, bacteria (bacteriophages), and archaea.
Although viruses share some similarities with living organisms (such as the
possession of genetic material and the ability to evolve), they lack many
features that are considered essential for life, such as cellular structure,
metabolism, and autonomous replication. Hence, they are often referred to
as "obligate intracellular parasites."
2. Nature of Viruses
2.1. General Characteristics of Viruses
Acellular Structure: Viruses do not possess cellular components. They are
made up of a nucleic acid core surrounded by a protein coat (capsid) and, in
some cases, a lipid envelope derived from the host cell. Viruses cannot
1
,generate ATP or carry out protein synthesis. All biosynthetic and metabolic
processes required for viral replication are hijacked from the host cell. Viruses
are extremely small, typically ranging from 20 to 300 nanometers (nm),
although some giant viruses like Mimivirus and Pandoravirus can exceed 400
nm in diameter. A viral genome may consist of: DNA or RNA, but never both.
The genome may be single-stranded (ss) or double-stranded (ds). It may be
linear or circular, segmented or non-segmented. RNA genomes may be
positive-sense (+) or negative-sense (−). Viruses are often species-specific or
even tissue-specific due to the precise interaction between viral surface
proteins and host cell receptors. Viruses can cause a wide range of diseases in
their respective hosts, from the common cold and influenza in humans to leaf
mottling and mosaic diseases in plants.
2.2. Viral Components and Structure
Viruses are structurally simple but functionally complex. The key components
include:
a) Nucleic Acid (Genome): Encodes the genetic instructions for making new
viruses. Determines replication strategy, classification, and pathogenicity.
b) Capsid: Protein coat surrounding the nucleic acid. Composed of repeating
units called capsomeres. Protects the viral genome and aids in attachment to
host cells.
c) Envelope (in some viruses): A lipid membrane derived from the host cell
membrane during viral budding. Contains viral glycoproteins essential for
attachment and fusion. Makes enveloped viruses more sensitive to detergents
and environmental factors.
2
, d) Enzymes: Some viruses carry their own enzymes, such as reverse
transcriptase in retroviruses or RNA-dependent RNA polymerase in RNA
viruses, necessary for replication.
3. Classification of Viruses
The classification of viruses is a crucial aspect of virology. It helps in
understanding their diversity, evolution, host specificity, and disease
associations. Viral classification has evolved from rudimentary methods based
on disease symptoms to highly refined molecular techniques.
3.1. Historical Perspective of Virus Classification
Historically, viruses were classified based on: Host range (animal, plant,
bacterial), Mode of transmission (e.g., arthropod-borne, airborne), Clinical
symptoms (e.g., hepatitis viruses) and Virus morphology (shape and size under
electron microscopy). However, these approaches were limited by overlaps
and exceptions. Modern classification now incorporates molecular and genetic
characteristics.
3.2. International Committee on Taxonomy of Viruses (ICTV)
The ICTV is the official body responsible for virus classification. Its system is
based on a combination of genetic, structural, and biological properties. ICTV
Criteria for Classification include: Type and polarity of nucleic acid (DNA or
RNA, single- or double-stranded), Capsid symmetry (helical, icosahedral,
complex), Presence or absence of an envelope, Genome organization and
replication strategy, Host range and pathogenicity and Phylogenetic
relationships.
3
FEDERAL UNIVERSITY OF LAFIA
MCB 425: VIROLOGY AND TISSUE CULTURE
3 CREDIT UNITS
1. The Classification and Nature of Viruses
1.1. Introduction to Viruses
Viruses are unique biological entities that occupy a gray area between living
and non-living things. They are non-cellular, submicroscopic infectious agents
that can only replicate within living cells. The term "virus" is derived from
the Latin word meaning "poison" or "slimy liquid". First observed indirectly
due to their ability to cause disease, viruses are now known to infect all forms
of life: animals, plants, fungi, bacteria (bacteriophages), and archaea.
Although viruses share some similarities with living organisms (such as the
possession of genetic material and the ability to evolve), they lack many
features that are considered essential for life, such as cellular structure,
metabolism, and autonomous replication. Hence, they are often referred to
as "obligate intracellular parasites."
2. Nature of Viruses
2.1. General Characteristics of Viruses
Acellular Structure: Viruses do not possess cellular components. They are
made up of a nucleic acid core surrounded by a protein coat (capsid) and, in
some cases, a lipid envelope derived from the host cell. Viruses cannot
1
,generate ATP or carry out protein synthesis. All biosynthetic and metabolic
processes required for viral replication are hijacked from the host cell. Viruses
are extremely small, typically ranging from 20 to 300 nanometers (nm),
although some giant viruses like Mimivirus and Pandoravirus can exceed 400
nm in diameter. A viral genome may consist of: DNA or RNA, but never both.
The genome may be single-stranded (ss) or double-stranded (ds). It may be
linear or circular, segmented or non-segmented. RNA genomes may be
positive-sense (+) or negative-sense (−). Viruses are often species-specific or
even tissue-specific due to the precise interaction between viral surface
proteins and host cell receptors. Viruses can cause a wide range of diseases in
their respective hosts, from the common cold and influenza in humans to leaf
mottling and mosaic diseases in plants.
2.2. Viral Components and Structure
Viruses are structurally simple but functionally complex. The key components
include:
a) Nucleic Acid (Genome): Encodes the genetic instructions for making new
viruses. Determines replication strategy, classification, and pathogenicity.
b) Capsid: Protein coat surrounding the nucleic acid. Composed of repeating
units called capsomeres. Protects the viral genome and aids in attachment to
host cells.
c) Envelope (in some viruses): A lipid membrane derived from the host cell
membrane during viral budding. Contains viral glycoproteins essential for
attachment and fusion. Makes enveloped viruses more sensitive to detergents
and environmental factors.
2
, d) Enzymes: Some viruses carry their own enzymes, such as reverse
transcriptase in retroviruses or RNA-dependent RNA polymerase in RNA
viruses, necessary for replication.
3. Classification of Viruses
The classification of viruses is a crucial aspect of virology. It helps in
understanding their diversity, evolution, host specificity, and disease
associations. Viral classification has evolved from rudimentary methods based
on disease symptoms to highly refined molecular techniques.
3.1. Historical Perspective of Virus Classification
Historically, viruses were classified based on: Host range (animal, plant,
bacterial), Mode of transmission (e.g., arthropod-borne, airborne), Clinical
symptoms (e.g., hepatitis viruses) and Virus morphology (shape and size under
electron microscopy). However, these approaches were limited by overlaps
and exceptions. Modern classification now incorporates molecular and genetic
characteristics.
3.2. International Committee on Taxonomy of Viruses (ICTV)
The ICTV is the official body responsible for virus classification. Its system is
based on a combination of genetic, structural, and biological properties. ICTV
Criteria for Classification include: Type and polarity of nucleic acid (DNA or
RNA, single- or double-stranded), Capsid symmetry (helical, icosahedral,
complex), Presence or absence of an envelope, Genome organization and
replication strategy, Host range and pathogenicity and Phylogenetic
relationships.
3
