Viruses: Structure, Classification & Historical Discoveries

Target Exam: NEET / AIPMT Prep • Read time: 5 mins

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).

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:

(1) DNA enclosed in a protein coat
(2) Prokaryotic nucleus
(3) Single chromosome
(4) Both DNA and RNA

✅ Answer: (1) DNA enclosed in a protein coat

AIPMT Prelims 2012

Q2. Which statement is wrong for viruses?

(1) They have ability to synthesize nucleic acids and proteins.
(2) Antibiotics have no effect on them.
(3) All are parasites.
(4) All of them have helical symmetry.

✅ Answer: (4) All of them have helical symmetry.

AIPMT 2014

Q3. Which of the following shows coiled RNA strand and capsomeres?

(1) Polio virus
(2) Tobacco mosaic virus
(3) Measles virus
(4) Retrovirus

✅ 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?