5.1 Viruses: Discovery and Structure

Viruses are infectious agents that are not cells. They are obligate intracellular parasites, meaning they can only replicate inside the living cells of other organisms. They are considered an exception to the Cell Theory.

Cell Theory→

• They are pathogens that cause a wide range of diseases in plants and animals (e.g., influenza, polio, dengue fever, Tobacco Mosaic Disease).
• They lack the machinery for self-replication and cannot synthesize their own energy or proteins.
• Viruses are too small to be seen with a light microscope and require an electron microscope for visualization.

History and Discovery of Viruses

The understanding of viruses evolved through the work of several key scientists. The study of viruses is called virology.

Characteristics of Viruses (Living vs. Non-Living)

Viruses exhibit characteristics of both living organisms and non-living matter, placing them on the borderline of life.

General Structure of a Virus

A complete, infectious viral particle is called a virion. It consists of a central core and an outer coat.

Core

The inner part of the virion containing the genetic material and essential enzymes.

• Genome: The genetic material, which can be either DNA or RNA, and can be single-stranded (ss) or double-stranded (ds).
• Core Proteins (Enzymes): Viruses may carry enzymes necessary for their replication cycle.
  • Example 1: Single-stranded RNA viruses have enzymes to convert their genome into a double-stranded form.
  • Example 2: Retroviruses (like HIV) and Hepatitis B virus contain reverse transcriptase, an enzyme that converts their ssRNA genome into dsDNA.

Coat

The outer protective covering of the virion.

• Capsid: A protein shell surrounding the core.
  • It is composed of identical repeating protein subunits called capsomeres.
  • The number of capsomeres is specific to the virus type (e.g., Herpes virus has 162; Adenovirus has 252).
  • Symmetry of Capsids:
    • Icosahedral (Polyhedral/Spherical): Capsomeres are arranged in 20 triangular faces.
    • Helical: Capsomeres are arranged in a hollow coil, creating a rod shape (e.g., TMV).
• Envelope: An additional lipoprotein layer that surrounds the capsid in some viruses (e.g., influenza virus).
  • It is derived from the host cell's surface membrane.
  • It may be covered with glycoprotein spikes, which help the virus recognize and attach to host cells.

Classification of Viruses

Viruses are classified based on several criteria, including the type of host they infect, their morphology (structure), and their genome.

Classification Based on Host

• Bacteriophage: Viruses that infect and attack bacteria. Typically have a polyhedral head and a tail structure.
• Plant Viruses: Viruses that infect plants. Most have an RNA genome (e.g., Tobacco Mosaic Virus (TMV)).
• Animal Viruses: Viruses that infect animals and humans, causing various diseases.
  • Papovavirus: Causes warts.
  • Poxivirus: Causes smallpox.
  • Picornavirus: Causes polio and hepatitis A.
  • Paramyxovirus: Causes measles and mumps.
  • Rous sarcoma virus: Causes cancer in animals.

Classification Based on Structure (Morphology and Genome)

This is the internationally agreed-upon system, which includes the Baltimore classification system based on genome type and replication strategy.

1. Based on Capsid:

• Helical: e.g., Tobacco Mosaic Virus (TMV)
• Polyhedral (Icosahedral): e.g., Adenoviruses
• Enveloped: e.g., Influenza viruses, HIV
• Complex: e.g., Bacteriophages

2. Based on Genome:

• dsDNA (Double-stranded DNA): e.g., Smallpox virus
• ssDNA (Single-stranded DNA): e.g., Mild rash virus
• dsRNA (Double-stranded RNA): e.g., Diarrhoea virus
• ssRNA (serves as mRNA): e.g., Rubella virus
• ssRNA (template for mRNA synthesis): e.g., Influenza virus
• ssRNA (template for DNA synthesis - Retrovirus): e.g., HIV