Lecture Notes: Animal Viruses

Introduction to Viruses

Viruses are the smallest forms of life and pathogens.
Transmission electron micrograph images allow visualization of viruses, such as the influenza virus.
False colors in images enhance structure visualization.

Louis Pasteur: Developed a rabies vaccine before viruses were understood, using dried spinal cords from infected animals.
- Rabies is 100% fatal, making the vaccine significant.
Dimitri Ivanovsky: Credited with discovering viruses while studying tobacco mosaic disease.
- Used porcelain filters to differentiate bacteria and viruses.
- Found that infectious agent passed through filters, indicating a virus.

Debate among virologists about whether viruses are living.
- Some say viruses are living because they can direct cell processes to replicate.
- Others argue they are non-living as they cannot independently replicate.
Viruses are obligate intracellular parasites, requiring a host cell to replicate.
Structure: Composed of genetic material (DNA or RNA) and a protein capsid.
- Some have a lipid envelope derived from the host cell membrane.

Viruses can alter genetic makeup and contribute to evolution.
Human genome contains viral DNA remnants, illustrating past viral integration.
- 8% of human DNA consists of ancient viral remnants; bacteria have 10-20% viral DNA.
Viral gene transfer contributes to horizontal gene transfer in the same generation.
- Basis for gene therapy ideas: Replacing disease genes using engineered viruses.

Viruses are much smaller than bacteria; cannot be seen with a light microscope.
Basic structure includes:
- Genetic material: DNA or RNA, not both.
- Capsid: Protein shell around genetic material.
- Envelope: Lipid layer in some viruses, derived from the host cell.
Surface molecules determine host cell specificity (e.g., influenza affects respiratory cells).
Virus replication requires host cell machinery.
- Lack enzymes for metabolism and cannot synthesize proteins on their own.

Electron microscopes are needed to visualize viruses due to their tiny size.
Viruses vary in size; an example is 50 million polio viruses fitting in one human cell.

All cell types (bacteria, archaea, eukaryotes) are susceptible to viral infection.
Bacteriophages infect bacteria and often have complex structures.