Lecture Notes: Classification of Living Organisms and Viruses

Classification of Living Organisms

• Three Domains of Life:
  • Bacteria
  • Archaea
  • Eukarya
  • Note: Under Eukarya, there are four kingdoms.

Viruses and Their Classification

• Viruses are not classified within the three domains of life.
  • Viruses are often considered non-living structures.
  • Scientific debate exists:
    • Some scientists argue viruses should be considered living.
    • Others argue they should not.
    • Some scientists are undecided.
  • For Cambridge A levels, viruses are classified as non-living.

Characteristics of Viruses

• Acellular Nature:
  • Lack important cell structures:
    • No cytoplasm, ribosomes, or cell surface membrane.
    • May have a lipid membrane from the host cell.
• Inability to Perform Living Processes:
  • Cannot respire or reproduce independently.
  • Must hijack host cells to replicate.
• Basic Structure:
  • All viruses have:
    • A protein coat known as a capsid
    • Nucleic acid (either DNA or RNA, but not both simultaneously)

Classification of Viruses

• Main Classification Criteria:
  1. Type of Nucleic Acid:
     • DNA or RNA
  2. Strand Nature:
     • Single or double-stranded nucleic acids
• DNA and RNA Behavior in Viruses:
  • DNA is typically double-stranded, but can be single-stranded in viruses.
  • RNA is typically single-stranded, but can be double-stranded in viruses.

Important Points for Exams

• Understand the classification methods for viruses:
  • Based on type and nature of nucleic acid.
  • Be aware that viral DNA and RNA can be atypical compared to general biological rules.