Acellular Biological Entities: Viruses, Viroids, & Prions (Outline) • Acellular entities as disease agents • Compare and contrast viruses and living cells • Parasitic nature of viruses: – Host-cell specificity: relate to common human diseases – Insertion of some viral genomes into host DNA • Life cycle of a retrovirus such as HIV • Role of ancient and current retroviruses in shaping animal genomes. • Role of ancient and current viral infections as selective pressure affecting human genetic variability
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Acellular entities as disease agents • Compare and ...
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Prion • Scarpie • Mad Cow disease • Kuru- Fore tribe of Papua New Guinea
Viroids and Prions: The Simplest Infectious Agents
• Viroids are circular RNA molecules that infect plants and disrupt their growth- Cadang-Cadang of coconut trees
• Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals
Prion diseases • Caused by mis-folded infectious proteins, lack
nucleic acids, cause molecules of the normal protein to mis-fold
• Diseases caused by prions affect the nervous system
Scrapie in sheep Mad cow disease Kuru in humans Creutzfeldt-Jakob Disease (CJD) in humans Fatal Familial Insomnia (FFI)
Viruses are genes packaged in protein – Biological non-living entities – Have no cytoplasm – Cannot self-replicate – Cannot metabolize – Genetic material either DNA or RNA never both
Viruses
• To replicate they need to infect a living cell • Every living cells has one or more viruses that
• Virion: an individual viral particle • Capsid: protein coat surrounding the nucleic acid • Nucleocapsid- nucleic acid and protein capsid
Basis of Host-Range of Viruses
• Host range is determined by “lock-and-key” fit between virus surface and cellular receptors on host cell
• Most viruses infect only specific types of cells in one host Narrow host range with tissue specificity – cold viruses: upper respiratory tract cells. – HIV, AIDS virus: a certain white blood cell. • Some have a broad host-range infecting multiple
species – rabies
Membranous envelope
RNA
Protein coat
Glycoprotein spike
Viral genomes are made of either DNA or RNA – Flu viruses are RNA – Genital warts virus (HPV) and Herpes virus are DNA