Cytology Chapter 3.3 PDF

III. Viruses  Multimolecular complexes  Not cells = not living things  cannot divide by their own(even with nutrients)  Lack metabolic enzymes & pathways  Need host cells to multiply  Infect animals, plants, & bacteria Cause many diseases (AIDS – smallpox – chi...

III. Viruses  Multimolecular complexes  Not cells = not living things  cannot divide by their own(even with nutrients)  Lack metabolic enzymes & pathways  Need host cells to multiply  Infect animals, plants, & bacteria Cause many diseases (AIDS – smallpox – chickenpox – rabies – poliomyelitis – mumps – measles hepatitis – mononucleosis – influenza – common cold) Some can cause cancers  III. Viruses Composition Virus = Nucleic acids (DNA/RNA, linear/circular, ss/ds) + Capsid (made of protein subunits: capsomeres) + Few proteins (polymerases, integrase) + Some: envelope (lipid bilayer) eg: HIV, influenza, smallpox + Some: tail linked to head  Viroid = only RNA molecule = very simple virus (no proteins)  III. Viruses Virus-Host Relationship  Viruses deviate / alter the cell metabolism (transcription, translation, replication) for their own benefit  Virus is specific: to a certain species, organ, tissue, cell, & even sub-cell type since: requires specific receptors on the cell BUT: Specificity is not absolute. Influenza (humans) may infect other species if the cell has the same receptors  Viruses change after leaving the host. Eg: influenza changes every year: H1N1: infects humans & pigs H5N1: infects humans & birds H: hemagglutinin, N: neuraminidase (antigens on envelope)  III. Viruses 1. Structure & Classification of Viruses Classifications according to: - Size: 20  200 nm - Nucleic acids: DNA (adenoviruses) / RNA (HIV, influenza) - Envelope: present or absent - Capsid: helical (TMV), polyhedral (adenovirus), complex (bacteriophage), or no symmetry  determines viral shape: rod (TMV), globular (HIV), polyhedral (adenovirus), helical, filamentous, or complex Bacteriophage: complex: = polyhedral head + tail (proteins arranged helical) + tail fibers (attachment to cell receptor) - Host cell: infecting prokaryotes (bacteriophages/phages) / eukaryotes  III. Viruses 2. Proliferation of Viruses 1. Viral capsid / membrane proteins recognize host cell receptor 2. Virus adheres to host surface 3. Virus injects its nucleic acid + proteins (polymerases…) into the host cytoplasm - enveloped virus: fuses its lipid bilayer with cell membrane  nucleocapsid enters the cytoplasm  capsid dissociates  nucleic acid released - non-enveloped virus: only nucleic acid is injected through a channel (capsid doesn’t enter) 4. Nucleic acid in cytoplasm performs lytic or lysogenic cycle 5. Thousands of new viruses are produced 6. Host cell dies  III. Viruses RNA Viruses Retroviruses: reverse transcriptase RNA DNA (provirus) No reverse transcriptase: RNA replication by “RNA-dependent RNA Polymerase” / “replicase” Lytic or Lysogenic cycle? Choice determined by: specific viral proteins – host cell conditions  III. Viruses Lysogenic Cycle Viral DNA is inserted into cell chromosome by “integrase”  DNA integrated is silent not active  cell seems normal As cell divides, cellular DNA & viral DNA is replicated  transmitted to daughter cells  silent reproduction of virus ! Lysogenic cycle can transform cells & cause cancer ! Lysogenic can transform to lytic cycle due to signals (eg: UV)  III. Viruses Lytic Cycle Cell metabolism is deviated  viral genes are preferably transcribed over cell genes 1. RNA is produced 2. translated to proteins by host cell equipment 3. viral nucleic acids produced by replication / transcription 4. assembly of viral particles: cage of capsid around nucleic acids 5. release of produced viruses - no lipid envelope: released by causing cell lysis using their enzymes - enveloped: released by budding: formed nucleocapsid interacts with host cell membrane (now has viral proteins)  takes a fragment of cell membrane as an envelope  detaches from cell ! Integration might occur  IV. Prions Unusual pathogens = only proteins (no nucleic acids)  Protein  Abnormal conformation  Can transform its abnormal conformation to other normal proteins  Protease resistant  accumulate in cell  cause toxicity  IV. Prions TSE (Transmissable Spongiform Encephalopathies)  Disease in animals & humans due to prions  Death of nerve cells (neurodegeneration) Disease examples: - Old: Scarpie (in sheep) - new: BSE (Bovine Spongiform Encephalopathy) Creutzfeldt-Jacob disease: from eating BSE-infected food  contains PrPres (resistant prion) PrPc (Prion-related Protein Cellular): normal protein – sensitive to proteolysis – found in nerve & blood cells PrPres (resistant): abnormal protein – resistant to proteases  forms insoluble fibrils o When normal PrPc interacts with abnormal PrPres  normal PrPc becomes also abnormal & resistant to proteases