Introduction to Viruses

Classification involves placing organisms in groups according to their traits or evolutionary origins. This allows scientists to store and retrieve the vast amount of knowledge accumulated about the biological world and organisms. The classification of organisms is often referred to as taxonomy.

A virus is a microscopic infectious agent that is fundamentally different from living cells. It is not considered a living organism because it lacks the cellular machinery required for metabolism, energy production, and independent reproduction. Instead, viruses are obligate intracellular parasites, meaning they can only reproduce by infecting a host cell and hijacking its molecular machinery.

Higher Level 

Structural features of Viruses

• Very small size (much smaller than bacteria).

• No growth or metabolic processes on their own.

• Nucleic acid: either DNA or RNA (not both).

• Capsid: protein coat enclosing the genetic material.

• No cytoplasm, organelles, or viral enzymes (until inside host).

Higher Level 

Higher Level 

Diversity Among Viruses

1. Genetic Material

May contain DNA or RNA, and it can be single or double stranded.

• Further variation in how RNA is processed:

  • Positive-sense RNA: acts directly as mRNA.

  • Negative-sense RNA: must be transcribed to mRNA.

  • Retroviruses: RNA → DNA via reverse transcriptase (e.g. HIV).

2. Enveloped vs. Non-Enveloped

• Enveloped viruses: have a lipid membrane derived from the host.

• Non-enveloped viruses: lack a membrane, typically more stable.

Higher Level