Introduction and Discovery of Viruses

Overview

This lecture explains the beginning of viruses (acellular life), their nature, characteristics, and the historical sequence of the discovery of viruses.

Structure and Nature of Viruses

• Viruses are called 'living entities' or 'living particles' because they possess qualities of both living and non-living things.
• Viruses are made of nucleic acid (DNA or RNA) and proteins; some also contain glycoproteins or lipoproteins.
• Viruses are acellular (non-cellular); they are not cells.
• They can replicate only inside the host's body; not outside.

Characteristics of Viruses

• Viruses are 'Obligate Parasites'—meaning they completely depend on their host.
• Viruses can infect all types of life forms (including bacteria).
• They have living characteristics such as DNA or RNA, reproduction (replication), mutation, and genetic recombination.
• Non-living traits: they can crystallize and do not grow on nutrient media.

History of Virus Discovery

• In 1838, Adolf Mayer transmitted Tobacco Mosaic Disease from one plant to another by sap rubbing.
• Porcelain filter experiments showed that the infectious agent is smaller than bacteria.
• Beijerinck proved that the agent is not a toxin but something else that replicates only inside the host.
• In 1935, Stanley crystallized the virus (Tobacco Mosaic Virus) for the first time.
• After the invention of the electron microscope, it became possible to see viruses.

Key Terms & Definitions

• Obligate Parasite — an organism that depends solely on its host and cannot survive outside.
• Facultative Parasite — a parasite that can survive both inside and outside the host.
• Nucleic Acid — biological molecules like DNA or RNA that carry genetic information.
• Crystallization — the process of turning a substance into solid crystals, making it easier to study.
• Sub-cellular (Acellular) — smaller than cells, non-cellular organisms.

Action Items / Next Steps

• Next lecture: Study the classification of viruses.
• Remember the events of Tobacco Mosaic Disease and the discovery of viruses.