Viruses

A virus is a submicroscopic infectious agent that can replicate only inside the living cells of other organisms. Viruses infect animals, plants, fungi, bacteria (bacteriophages), and even archaea.
They are non-cellular, meaning they are not considered true living organisms, but they possess some characteristics of life, such as reproduction and mutation.

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The word virus comes from Latin, meaning “poison” or “slimy liquid.”

2. General Characteristics of Viruses

1. Acellular nature – Viruses do not have cellular structure; they lack cytoplasm, nucleus, and cell membrane.
2. Obligate intracellular parasites – They can multiply only inside host cells by using the host’s biochemical machinery.
3. Size – Extremely small, ranging from 20 to 300 nanometers (nm) in diameter.
4. Structure – Composed mainly of nucleic acid (DNA or RNA) and a protein coat (capsid); some viruses have an envelope.
5. Genetic material – Contain either DNA or RNA, never both.
6. No metabolic activity outside host cells.
7. Specificity – Each virus infects specific hosts or tissues (host specificity, tissue tropism).
8. Crystallization – Viruses can be crystallized, showing they are not living in the usual sense.

3. Structure of a Virus

A typical virus consists of the following components:

A. Nucleic Acid (Core)

• Carries the genetic information required for replication.
• May be DNA or RNA, single-stranded (ss) or double-stranded (ds), linear or circular.
• Example:
  • DNA viruses → Adenoviruses, Herpesviruses
  • RNA viruses → Influenza virus, HIV

B. Capsid

• Protein coat surrounding the nucleic acid.
• Composed of subunits called capsomeres.
• Protects the genetic material and helps attach to host cells.

C. Envelope (in some viruses)

• Lipoprotein layer derived from the host cell membrane.
• Contains viral glycoproteins that help in host recognition.
• Example: HIV, Influenza virus, Herpesvirus.

D. Enzymes

• Some viruses carry their own enzymes, e.g., reverse transcriptase in retroviruses.

4. Morphological Types of Viruses

1. Helical Viruses – Rod-shaped; nucleic acid enclosed in a spiral capsid.
   Example: Tobacco mosaic virus (TMV)
2. Icosahedral Viruses – Spherical with 20-sided symmetry.
   Example: Adenovirus
3. Complex Viruses – Have complicated structures, e.g., bacteriophages (head + tail).
4. Enveloped Viruses – Enclosed in a membrane envelope.
   Example: Influenza virus, HIV
5. Filamentous Viruses – Long and flexible filament shapes.
   Example: Ebola virus

5. Classification of Viruses

Viruses are classified based on their:

• Type of nucleic acid (DNA or RNA)
• Replication method
• Morphology
• Host organism

A. DNA Viruses

• Have DNA as genetic material.
• Examples:
  • Adenoviridae → Respiratory infections
  • Herpesviridae → Herpes simplex, chickenpox
  • Poxviridae → Smallpox virus

B. RNA Viruses

• Have RNA as genetic material.
• Examples:
  • Orthomyxoviridae → Influenza
  • Retroviridae → HIV
  • Picornaviridae → Poliovirus, Rhinovirus

C. Bacteriophages

• Viruses that infect bacteria.
• Example: T4 phage of E. coli.

6. Replication of Viruses

Viruses cannot divide by binary fission or mitosis. They multiply inside host cells through specific stages:

A. Stages of Viral Replication:

1. Attachment (Adsorption)
   1. Virus attaches to specific receptors on the host cell surface.
2. Penetration (Entry)
   1. Virus or its genetic material enters the host cell (by injection or endocytosis).
3. Uncoating
   1. Viral capsid dissolves, releasing nucleic acid into the host cytoplasm.
4. Synthesis (Replication and Protein Formation)
   1. Viral genome replicates using host machinery.
   1. Viral proteins (capsid and enzymes) are synthesized.
5. Assembly (Maturation)
   1. New viral particles are assembled from nucleic acids and proteins.
6. Release
   1. New virions are released by cell lysis (non-enveloped viruses) or budding (enveloped viruses).

7. Cultivation of Viruses

Since viruses need living cells, they are cultivated in:

1. Living animals (mice, rabbits)
2. Embryonated eggs (for vaccine production, e.g., influenza)
3. Tissue culture (cell lines in laboratories)
4. Bacterial cultures (for bacteriophages)

8. Economic and Medical Importance of Viruses

A. Harmful Effects

1. Human diseases: Influenza, Measles, Mumps, HIV/AIDS, COVID-19, Herpes, Hepatitis, Rabies.
2. Plant diseases: Tobacco mosaic, Leaf curl, Potato virus.
3. Animal diseases: Foot-and-mouth disease, Rinderpest, Rabies.
4. Bacterial destruction: Some viruses kill useful bacteria in soil and water.

B. Beneficial Uses

1. Vaccine production (e.g., measles, polio, rabies).
2. Gene therapy (using viral vectors to deliver genes).
3. Phage therapy (using bacteriophages to treat bacterial infections).
4. Molecular biology tools – Retroviruses used in genetic engineering and research.

9. Control and Prevention of Viral Infections

1. Vaccination – Most effective method (e.g., polio, hepatitis B).
2. Antiviral drugs – e.g., Acyclovir (Herpes), Oseltamivir (Influenza), AZT (HIV).
3. Hygiene and sanitation – Clean water, food, and environment.
4. Vector control – Control of mosquitoes, flies, etc.
5. Isolation and quarantine – To prevent viral spread.

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