Acellular Biological Entities: Viruses, Viroids, & Prions (Outline) • Acellular entities as infectious agents of animal and plant diseases • Structure and functional properties. • Viral structural components: genome (DNA or RNA), capsid, nucleocapsid, and envelope. Naked and enveloped viruses • Parasitic nature of viruses – Life cycle of bacteriophages and relationship to human disease – Host-cell specificity: common human diseases – Life cycle of animal viruses and association with susceptibility to infection. – Human genetic variability in susceptibility to viral infections • Life cycle of the Ebola virus • Life cycle of a retrovirus such as HIV • Role of ancient and current retroviruses: – In shaping the human genome; addition of genetic variability – as selective pressure for human genetic variability
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• Acellular entities as infectious agents of animal and plant diseases • Structure and functional properties. • Viral structural components: genome (DNA or RNA), capsid, nucleocapsid, and
envelope. Naked and enveloped viruses • Parasitic nature of viruses
– Life cycle of bacteriophages and relationship to human disease – Host-cell specificity: common human diseases – Life cycle of animal viruses and association with susceptibility to infection. – Human genetic variability in susceptibility to viral infections
• Life cycle of the Ebola virus • Life cycle of a retrovirus such as HIV • Role of ancient and current retroviruses:
– In shaping the human genome; addition of genetic variability – as selective pressure for human genetic variability
Simple infectious agents Virus- genetic material in transit from one host cell to the next
Viroid- Circular naked RNA
Prion- misfolded infectious proteins
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 • Prions are mis-folded infectious proteins, lack nucleic
acids • Prions cause disease by converting normal proteins
into the prion version
• 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) The Truth Will Out: Is vCJD Caused by BSE?
Prion • Scarpie • Mad Cow disease • Kuru- Fore tribe of Papua New Guinea
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 Capsid made of individual protein subunits
(capsomeres) gives the virion its shape • 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