🦠Invisible Invaders: Understanding the Classification of Viruses & Their Odd Cousins!

Classification of Viruses (with bracketed explanations)

• Viruses are not considered organisms because they are acellular (not made of cells).

• They lack characteristics of the three domains of life (Bacteria, Archaea, Eukarya) and thus, are not classified in any domain or kingdom.

• A virus consists of:

  • Nucleic acid (either DNA or RNA)

  • Protein coat (capsid) (protective outer shell of the virus)

• Viruses cannot perform metabolism on their own. They depend on the host cell (plants, animals, or bacteria) to replicate and synthesize proteins.

Basis of Virus Classification

Viruses are classified based on:

1. Genetic material (DNA or RNA)

2. Replication strategy (how they reproduce inside host)

3. Morphology (shape and structure)

4. Type of host (bacteria, plants, or animals)

ICTV (International Committee on Taxonomy of Viruses) provides official classification guidelines.

Acellular Infectious Agents

• Prions: Composed of only protein, no nucleic acid. Cause neurological diseases in animals (e.g., mad cow disease).

• Viroids: Composed of circular RNA only, no protein coat. Infect and cause diseases in plants.

🧬 "How Viruses Choose Their Targets: Host Range & Morphology Explained!"

🦠 Classification of Viruses on the Basis of Host Range

1. Animal Viruses: Infect animals and humans.
   🔹 Examples: Influenza virus, Rabies virus

2. Plant Viruses: Infect plants.
   🔹 Examples: Tobacco mosaic virus, Potato virus X

3. Bacteriophages (Phages): Infect bacteria.
   🔹 Examples: T4 phage, Lambda phage

4. Archaea Viruses: Infect archaea (primitive prokaryotes living in extreme environments)
   🔹 Example: Sulfolobus spindle-shaped virus

🔬 Classification of Viruses on the Basis of Morphology

1. Helical Viruses: Have a spiral-shaped capsid around the nucleic acid
   🔹 Examples: Tobacco mosaic virus, Rabies virus

2. Icosahedral Viruses: Have a symmetrical 20-sided (icosahedral) capsid
   🔹 Examples: Adenoviruses, Herpesviruses

3. Complex Viruses: Have a mix of icosahedral & helical structures, often with tails
   🔹 Examples: Bacteriophages

4. Enveloped Viruses: Surrounded by a lipid envelope (taken from host membrane)
   🔹 Examples: Influenza virus, HIV

5. Non-enveloped (Naked) Viruses: Lack a lipid envelope; have only a capsid
   🔹 Examples: Poliovirus, Adenovirus

Classification by Genetic Material & Replication Strategy"

🧬 Classification Based on Genetic Material

1. DNA Viruses: Viruses with DNA as genetic material

   • DNA can be:

     • ssDNA (single-stranded DNA)

     • dsDNA (double-stranded DNA)
       🔹 Examples:

   • dsDNA viruses:

     • Adenoviruses (cause respiratory infections)

     • Herpesviruses (cause herpes, chickenpox)

   • ssDNA viruses:

     • Parvoviruses (cause gastroenteritis)

2. RNA Viruses: Viruses with RNA as genetic material

   • RNA can be:

     • ssRNA (single-stranded RNA)

     • dsRNA (double-stranded RNA)
       🔹 Examples:

   • ssRNA viruses:

     • Coronaviruses (cause COVID-19)

     • Influenza viruses (cause flu)

   • dsRNA viruses:

     • Rotaviruses (cause gastroenteritis)

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