Acellular Biological Entities: Viruses, Viroids, & Prions

Transcription

1 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

2 Acellular Disease-causing biological entities (Health Connection) Simple infectious agents Virus- genetic material in transit from one host cell to the next Viroid- Circular naked RNA Prion- misfolded infectious proteins

3 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

4 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? (5 mins)

5 Acellular Disease-causing biological entities (Health Connection) Virus Childhood diseases Age-independent disease Emerging diseases Discovery Channel Video Clip (Textbook site) Viroid Plant diseases Prion Scarpie Mad Cow disease Kuru- Fore tribe of Papua New Guinea

6 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

7 Viruses To replicate they need to infect a living cell Every living cells has one or more viruses that can infect it, specifically.

8 Naked Viral Shapes Enveloped Complex RNA Capsomere DNA Membranous envelope Capsomere of capsid Glycoprotein Glycoproteins nm nm (diameter) RNA Capsid Head nm (diameter) nm DNA Tail sheath Tail fiber 20 nm (a) Tobacco mosaic virus 50 nm 50 nm 50 nm (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

9 Viral Structures 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

10 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

11 Viral genomes are made of either DNA or RNA Flu viruses are RNA Genital warts virus (HPV) and Herpes virus are DNA viruses Membranous envelope RN A Protein coat Glycoprotein spike Influenza HPV

12 Plant viruses are serious agricultural pests Most plant viruses have RNA genomes Enter their hosts via wounds in the plant s outer layers Protein RNA

13 Emerging viruses threaten human health Colorized TEM 50,000 Colorized TEM 370,000 Figure 10.20A, B Ebola Virus (RNA) SARS Virus (RNA)

14 Bacteriophage Life Cycles Phage Phage DNA The phage injects its DNA. Bacterial chromosome Phage DNA circularizes. Daughter cell with prophage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Cell divisions produce population of bacteria infected with the prophage. The cell lyses, releasing phages. Lytic cycle Lytic cycle is induced or Lysogenic cycle is entered Lysogenic cycle Prophage The bacterium reproduces, copying the prophage and transmitting it to daughter cells. New phage DNA and proteins are synthesized and assembled into phages. Phage DNA integrates into the bacterial chromosome, becoming a prophage.

15 Bacteriophage Life Cycles Lytic Cycle Attachment Entry of DNA Degradation of host DNA Replication of nucleic acid and synthesis of viral proteins Packaging/Assembly of phage particles (DNA and proteins) Release by cell lysis Lysogenic Cycle Attachment Entry of DNA No degradation of host DNA Integration of nucleic into bacterial genome Prophage: an integrated phage Exit to lytic cycle induced by chemicals and high energy radiation

16 Prophage-mediated Diseases One of many reasons for why bacteria cause disease is the production of a toxic protein. Some proviral genes code for toxic proteins. Examples: o Diphtheria o Botulism o Scarlet fever o E. coli food poisoning

17 Animal Virus Life Cycle Attachment Entry? Uncoating of virion separate protein from NA Replication of nucleic acid and synthesis of viral proteins Maturation of virions (assembly of NA and proteins) Release: cell lysis or budding 0qdqoBFa8

18 The Ebola virus An enveloped RNA (- strand) Entry and exit Pathogenesis Transmission Links cs_284

19 The AIDS virus HIV is an RNA retrovirus It makes DNA using RNA template Inside a cell, HIV uses its RNA as a template to make a DNA copy of itself, which integrates into the host genome. Glycoprotein Protein coat Envelope RNA (two identical strands) Reverse transcriptase

20 Viral DNA in the Human Genome About 8% of our genome is derived from RNA viruses called retroviruses - This is evidence of past infection - Sequences tend to increase over time Figure 11.11

21 Roles of human endogenous retroviruses Endogenous retroviruses Placenta formation Large Brain development Powerful emotions Explorer: The virus hunters (DVD) Natgeotv.vom orer/3828/overview#tab-videos/06253_00

22 The Virus Hunters (DVD) Retroviruses in human genomes (first 12:30 mins)

23 Role for Viruses in shaping the Human Genome Past retroviral infection in a primate ancestor Insertion of sequences New proteins or protein domains for host that maybe beneficial and influence behavior Present retroviral infections add to the genetic variability of the human populations

24 The Virus Hunters (DVD) Role of ancient retroviruses in major evolutionary steps (12:30-15:30 mins)

25 The Virus Hunters (DVD) Role of integrated viruses in complex emotions and behavior (~40:30 to 46:20 mins)