18.09 Viruses

Overview

• Definition: Viruses are acellular microorganisms visible only with an electron microscope.
• Structure:
  • Composed of a nucleic acid core (DNA or RNA) surrounded by a protein coat (capsid). Some viruses also have a lipid envelope.
  • Lack cellular structure seen in organisms like bacteria and fungi.
• Classification:
  • Not included in the three-domain system due to their unique nature and lack of traditional biological features.

Key Features of Viruses

1. Acellular:

• Do not possess cells or cellular components.

1. Parasitic:

• Can only reproduce inside host cells by using the host’s biochemical machinery.
• Energy required for viral replication comes from host cell respiration.

1. Lack of Metabolism:

• No metabolic activity when free in the environment.

1. Diverse Genetic Material:

• DNA or RNA as genetic material.
• Can be single-stranded or double-stranded (unlike cellular organisms where DNA is always double-stranded, and RNA is single-stranded).

1. Infectious Agents:

• Cause diseases in a wide range of hosts, including bacteria, plants, and animals.

Why Viruses Are Excluded From the Three-Domain System

• No Cellular Structure:
  • Unlike bacteria, archaea, and eukaryotes, viruses lack cell membranes, cytoplasm, or organelles.
• No Independent Metabolism:
  • Cannot generate energy or carry out biochemical processes without a host.
• Dependence on Host Cells:
  • Require a host to reproduce, which disqualifies them as independent living organisms.

Classification of Viruses

• Based on the type and structure of their nucleic acid:
  • DNA or RNA.
  • Single-stranded (ss) or double-stranded (ds).

Examples

• T4 Bacteriophage:
  • Infects Escherichia coli bacteria.
  • Used as a vector in genetic modification.
• Tobacco Mosaic Virus (TMV):
  • Infects plants like tobacco and tomatoes, causing mosaic disease.
  • Voted the most important plant virus by plant pathologists in 2011.
• HIV:
  • Causes HIV/AIDS in humans.
• Rotavirus:
  • Responsible for severe gastroenteritis in humans.

Are Viruses Living Organisms?

• Arguments for Considering Them Living:
  • Reproduce (but only inside a host cell).
  • Contain genetic material (DNA or RNA).
  • Evolve over time through mutations and natural selection.
• Arguments Against:
  • Lack cellular structure.
  • No metabolism or energy production.
  • Cannot reproduce independently.
  • Function only as parasitic entities relying on host machinery.

Answers to Questions

a. Why are viruses not included in the three-domain system?

• Viruses lack cellular structure, metabolism, and the ability to independently reproduce, which are key criteria for the classification of life.

b. Features used to classify viruses:

• Type of nucleic acid: DNA or RNA.
• Number of strands: Single-stranded (ss) or Double-stranded (ds).

c. Should viruses be considered living organisms?

• Viruses exhibit some characteristics of life (e.g., reproduction inside host cells, genetic material) but lack others (e.g., metabolism, cellular structure).
• The debate continues, with most scientists considering them non-living infectious agents.