Understanding Viruses: Structure, Types, and Life Cycle

What is a Virus?

• The study of viruses is called Virology.

• Wendell Stanley, who first crystallized the Tobacco Mosaic Virus (TMV), is considered the Father of Virology.

• The word virus comes from Latin, meaning “poison.”

• Ancient diseases like smallpox and polio are considered prehistoric viral diseases.

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Living and Non-Living Nature of Viruses

Viruses blur the line between life and non-life.

• Living property: They possess either DNA or RNA (never both).

• Non-living property: They can be crystallized outside a host.

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Viruses and Their Hosts

Viruses infect almost every type of organism:

• Plants → Phytophage

• Bacteria → Bacteriophage

• Fungi → Mycophage

• Algae → Phycophage

• E. coli → Coliphage

• Cyanobacteria → Cyanophage

• Yeast → Zymophage

• Animals → Zoophage

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Viral Shapes

Viruses come in diverse shapes:

• Brick-shaped → Smallpox virus

• Tadpole-shaped → Bacteriophage

• Bullet-shaped → Rabies virus

• Spherical → Influenza virus

• Rod-shaped → Tobacco Mosaic Virus (TMV)

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Forms of Viruses

• Viroid: Incomplete virus with only RNA.

• Prion: Incomplete virus with only protein.

• Virion: Complete (mature) virus with both nucleic acid and protein.

• Virusoid: Non-infectious RNA fragment.

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Structure of a Virus

1. Capsid

   • Protein coat surrounding nucleic acid.

   • Made up of repeating units called capsomers.

   • Responsible for antigenic properties.

2. Envelope

   • Loose covering outside the capsid (mainly in animal viruses).

   • Viruses without an envelope are called naked viruses.

   • Envelope may have spikes for host attachment.

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Types of Viruses Based on Nucleic Acid

1. Caulimovirus – Double-stranded DNA viruses

   • Examples: Cauliflower mosaic virus, Dahlia mosaic virus, Hepatitis-B, Chickenpox, Smallpox.

2. Geminivirus – Single-stranded DNA viruses

   • Examples: Bacteriophage M13, E. coli phage ɸ×174.

3. Reovirus – Double-stranded RNA viruses

   • Examples: Rotavirus, Wound tumor virus, Mycophage.

4. Retrovirus – Single-stranded RNA viruses

   • Examples: Rubella virus, HIV (AIDS), Influenza, Polio, Dengue, Mumps, Yellow fever.

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Life Cycle of Viruses

Viruses multiply inside a host through two main cycles:

• Lytic Cycle – Virus actively multiplies, causing the host cell to burst (lysis). These viruses are virulent.

• Lysogenic Cycle – Viral DNA integrates into the host genome and stays dormant for generations. Such inactive viruses are called temperate phages (provirus/prophage).

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Reverse Transcription (Teminism)

Some viruses, like retroviruses, use reverse transcriptase to synthesize DNA from RNA. This unique process helps them integrate into host DNA.

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Interesting Facts About Viruses

• Viral diseases can be diagnosed through antigen-antibody reactions.

• Virus-free plants can be developed through meristem culture.

• Interferons, glycoproteins secreted by host cells, provide natural defense against viruses.

• The smallest known plant pathogen is a viroid.

• Some viruses (like poxvirus) even contain vitamin-like molecules such as riboflavin and biotin.

• The purity of the Ganga river has been attributed, in part, to the presence of bacteriophages.

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Conclusion

Viruses are tiny yet powerful entities that shape ecosystems, health, and even evolution. While they cause some of the deadliest diseases, their unique biology has also led to groundbreaking discoveries in medicine, biotechnology, and genetics.