Viruses are tiny infectious agents that multiply within living cells and consist of nucleic acid surrounded by a protein coat. They do not have cells and can have different structures including icosahedral, helical, and complex symmetry. Viruses are classified based on their genetic material, host range, site of replication, capsid structure, and mode of transmission. Viruses reproduce through lytic and lysogenic cycles using the host cell's machinery to replicate their components and release new viruses. They can be cultivated in animal models, cell cultures, and fertilized eggs to study and mass produce viruses. Studying viruses is important for disease prevention, emerging infection monitoring, and biotechnological applications.

1. Virus: Definition, Study
of Morphology,
Classification,
Reproduction and
Cultivation
Viruses are tiny infectious agents that multiply within living cells.In this
presentation, we will explore the definition of viruses, study their
morphology, understand their classification, uncover the mechanisms of virus
reproduction, explore methods of virus cultivation, and discuss the
importance of studying viruses.
by Pawanjot Singh
P

2. Definition of a Virus
Avirus is asmall nucleoprotein complex and submicroscopic infectious agent, replicates only inside the living
cell(host cell) of other organisms such as animals and plants.They consist of asingle type of nucliec acid that
serves as genetic material,iether DNAor RNA,surrounded by aprotein coat called acapsid.
Properties of viruses:
They do not have cellular organization.
They contain only one type of nucleic acid either DNAor RNA.
They lack enzymes for protein synthesis.
They multiply by complex process but not by binary fission.
They are unaffected by antibacterial antibiotics.

3. Structure
Viruses are acellular that means they do not have cells as it
has no nucleus, cytoplasm or organelles.They consists of
two main parts viz.nucleic acid and capsid.In nucleic acid,
DNAor RNAis found at the core of the virus .The nucleic
acid is linear or circular and either segmented.Capsid is a
protein which is surrounded by the genetic material.
protein coat is made up of repeating sub-units known as
capsomeres.

4. Study of Virus Morphology
Icosahedral Symmetry
Many viruses have a
symmetrical/icosahedral
shape with 20 triangular faces.
This geometric arrangement
provides stability and
structural integrity.
These viruses have 12
vertices(corners) and 30
edges and is made up of
equilateral triangles fused
symmetrically.
Icosahedral morphology is
sometimes also reffered to as
polyhedral/cubic
morphology. Example:
Poliovirus and Adenovirus.
Helical Symmetry
These viruses have ahelical
structure, with aspiral
arrangement of protein
subunits surrounding their
genetic material.This allows
for flexibility and versatility in
shape.
These viruses are also called
spiral symmetry.
Most of these viruses are
enveloped and all of them
contain RNAproteins.
Example:Tobacco mosaic
virus.
Complex Symmetry
They have complexicity in
their structure and do not
show icosahedral or helical
symmetry.
They do not have any 1
particular type of virus
structure and most of them
are combination of the helical
and cubical structures.
Most of these viruses consists
of head and tail.
Example:Poxvirus.

5. Classification of Viruses
1 Genetic Material
Viruses can be classified based on their genetic material, either DNAor
RNA, and whether it is single-stranded or double-stranded.
2 Host Range
Viruses can also be classified based on the types of organisms they
infect, such as animals, plants, or bacteria.
3 Site of Replication
Viruses can also be classified based on the site they attack and
replicate on, most of the viruses replicate on cytoplasm or nucleus of
the host others replicate through RNAand DNA.
4 Capsid Structure
The shape and structure of the capsid can also be used to classify
viruses into different families such as cubical, spiral and complex.
5 Mode of Transmission
Some viruses are transmitted through direct contact, while others are
transmitted through avector like mosquitoes.

6. Reproduction of Viruses
LYTIC CYCLE:
Attachment
The virus attaches to the
surface of the host cell using
specific molecules on its capsid.
Entry
The genetic material is injected
into the host cell either by
direct penetration or by the
fusion of its envelope with the
host cell membrane.
Replication
The virus uses the host cell's
machinery to replicate its
genetic material and produce
new viral components.
Assembly
The newly replicated viral
components are assembled into
complete viruses.
Release
Then finally virus enzymes causes the cell to burst and the viruses are
released from the host cell.These new viruses further infect to other
cells and the process continues.

7. Reproduction of Viruses
LYSOGENIC CYCLE :
Attachment
The virus attaches
to the surface of
the host cell using
specific molecules
on its capsid.
Entry
The virus enters
the host cell either
by direct
penetration or by
the fusion of its
envelope with the
host cell
membrane.
Integration
The viral genome
integrates into the
host cell and starts
the next phase i.e.,
the replication.
Replication
When the host
cells replicates,
the viral DNAis
copied along with
the host cell DNA.
Each new
daughter cell is
infected with the
virus
Induction
When the infected
cells are exposed
to certain
environmental
conditions, viral
DNAis activated
and enters the
lytic cycle.
Replication
The virus takes
over the cells
metabolism,
causing the
creation of new
proteins and
nucleic acid by the
host cell's
organelles.
Assembly
The newly replicated viral
components are assembled into
complete viruses.
Release
Then finally virus enzymes
causes the cell to burst and the
viruses are released from the
host cell.These new viruses
further infect to other cells and
the process continues.

8. Cultivation of Viruses
1 Animal Models
Viruses can be grown and studied in living
animals, such as mice or monkeys, to
observe their effects and develop
vaccines.
2
Cell Culture
Viruses can be cultivated using cell
cultures, where they infect and replicate
within specific types of cells grown in a
lab. 3 Egg Cultivation
Some viruses, like influenza, can be
grown in fertilized chicken eggs,
providing asystem for large-scale virus
production.

9. Importance of Studying
Viruses
1 Disease Prevention
Studying viruses helps us
understand how they cause
diseases, leading to the
development of vaccines and
antiviral drugs.
2 Emerging Infections
By studying viruses, we can
monitor and respond to
emerging infections, such as
the recent COVID-19
pandemic.
3 Biotechnological Applications
Viruses are used in biotechnology to deliver genetic material into
cells, develop gene therapies, and produce vaccines.

10. Conclusion
The study of viruses is essential for understanding infectious diseases,
developing treatments, and advancing biotechnology.By unraveling the
mysteries of these microscopic agents, we can protect human and animal
health, and improve our quality of life.