Viruses: Structure, Classification & Historical Discoveries
1. Introduction to Viruses
- Infectious agents with simple acellular organization.
- Can reproduce only within living cells and are obligate intracellular parasites.
- They act as a connecting link between living and non-living entities.
- Have properties of both living and non-living things.
- Contain genetic material, but are unable to multiply outside the living host.
🔥 Key Exam Hotspot
Viruses are an exception to the Cell Theory.
2. Discoveries in Virology
- Pasteur (1880): Coined the term virus (which means venom or poisonous fluid).
- D.J. Ivanowsky (1892): Recognized certain microbes as the causal organism of the mosaic disease of tobacco. Found that they were smaller than bacteria because they passed through bacteria-proof filters.
- M.W. Beijerinck (1898): Demonstrated that the extract of infected tobacco plants could cause infection in healthy plants and called this fluid Contagium vivum fluidum (infectious living fluid).
- W.M. Stanley (1935): Crystallized TMV (Tobacco Mosaic Virus) for the first time. Showed that viruses could be crystallized and that crystals consist largely of proteins.
🔥 Key Exam Hotspot
Viruses are obligate parasites and are inert outside their specific host cell. An inert virus is called a virion.
3. Structural Components of Viruses
The structural components of viruses are Envelope, Capsid, and Nucleoid:
- Envelope: The outer thin, loose covering composed of proteins (from virus), lipids, and carbohydrates (both host derived). May or may not be present. (Present in HIV, Herpes virus).
- Capsid: The outer protein coat made up of small subunits called capsomeres for the protection of nucleic acid.
- Nucleoid: Viruses contain either DNA or RNA:
- Deoxyviruses (DNA viruses):
- Double-stranded DNA (dsDNA): e.g., Pox virus, Cauliflower mosaic virus, Herpes virus.
- Single-stranded DNA (ssDNA): e.g., Coliphage φ × 174, M13 phage.
- Riboviruses (RNA viruses):
- Double-stranded RNA (dsRNA): e.g., Reovirus, Wound tumour virus.
- Single-stranded RNA (ssRNA): e.g., TMV, Influenza virus, Foot and Mouth Disease virus, Retroviruses (HIV).
- Deoxyviruses (DNA viruses):
Classification by Host Specificity:
| Group | Common Type of Genetic Material |
|---|---|
| 1. Phytophagineae (Plant viruses) | ssRNA |
| 2. Zoophagineae (Animal viruses) | ss or dsRNA or dsDNA |
| 3. Bacteriophages (Bacterial viruses) | dsDNA |
📌 NOTE: No virus contains BOTH RNA and DNA.
4. Structure of Specific Viruses
A. Tobacco Mosaic Virus (TMV)
- Elongated rod-like structure, $3000\,\text{\AA}$ long and $180\,\text{\AA}$ in diameter with a molecular weight of $39.4 \times 10^6\text{ Dalton}$.
- Genetic material is ssRNA, consisting of $6400\text{ nucleotides}$.
- Capsid contains 2130 capsomeres.
B. Bacteriophage (Infects Bacteria)
- Tadpole-like structure with a polyhedral head connected to a helical tail.
- Genetic material is dsDNA.
- Head consists of nucleic acid surrounded by a protein coat or capsid.
5. Practice Questions & Solved Examples
AIPMT 2014
Q1. Viruses have:
✅ Answer: (1) DNA enclosed in a protein coat
AIPMT Prelims 2012
Q2. Which statement is wrong for viruses?
✅ Answer: (4) All of them have helical symmetry.
AIPMT 2014
Q3. Which of the following shows coiled RNA strand and capsomeres?
✅ Answer: (2) Tobacco mosaic virus
Q4. Viruses are non-cellular organisms but replicate themselves once they infect the host cell. To which of the following kingdom do viruses belong to?
✅ Answer: (4) None of the above