Detailed Notes on Viruses

Chapter 6: An Introduction to Viruses

1. Finding the First Viruses

• Leeuwenhoek’s Microscope: Early observations had limitations; could not visualize viruses.
• Louis Pasteur: Proposed the idea that rabies was caused by a smaller agent than bacteria; coined the term "virus" meaning poison.
• 1890s Discovery: D. Ivanovski and M. Beijerinck identified the Tobacco Mosaic Virus (TMV) as the first virus.

2. Properties of Viruses

• Components: Made up of proteins and nucleic acid (either DNA or RNA).
• Size Range: Extremely small; viruses are less than 0.2 µm, typical range being 20nm to 450nm, often seen under electron microscopes.
• Shapes: Varying structures such as helical, icosahedral, and complex forms.

3. Classification of Viruses

• DNA Viruses: Usually double-stranded but can be single-stranded (linear or circular).
• RNA Viruses: Primarily single-stranded, can be double-stranded, and sometimes segmented.
• Positive-sense RNA: Ready for translation; Negative-sense RNA: Requires conversion before translation.