Viriods and prions

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VIRIODS AND PRIONS

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Viriods and prions

  1. 1. VIRIODS AND PRIONS PRESENTED BY- HASNAHANA CHETIA M.Sc Ist Sem. DEPTT. OF BIOTECHNOLOGY, G.U.
  2. 2. VIRIODS Viriods are a group of virus-like pathogens much smaller and distinctly different from viruses, consists solely of small single- stranded circular RNA molecule and have no protein coat.
  3. 3. STRUCTURE Viriods are covalently closed, circular, ssRNAs, about 250-370 nucleotides long.The circular RNA normally exist as a rod-like shape due to intra- strand base pairing, which forms double- stranded regions with single-stranded loops.The RNA of a viriod doesn’t encode any gene products;so they can’t replicate themselves. It is believed to adopt the host RNA Polymerase for this purpose which synthesizes a negative RNA strand.Then it serves as a template for the synthesis of the other viriod RNAs.
  4. 4. DISEASES CAUSED BY VIRIODS Viriods causes over 20 different plant diseases of which the most studied are- 1. POTATO SPINDLE-TUBER DISEASE 2. EXOCORTIS DISEASE OF CITRUSTREES 3. CHRYSANTHEMUM STUNT DISEASE The only human disease caused by viriods is Hepatitis-D.
  5. 5. PATHOGENICITY Though the pathogenicity of viriods is not well understood,it is presumed that viriods cause disease by triggering a eukaryotic response called RNA SILENCING, which normally functions to protect against infection caused by double stranded viruses, by degrading it.Viriods may usurp this response by hybridizing to specific host mRNA molecules to which they have a complementary sequence forming Hybrid viriod:host mRNA ds molecule which will elicit RNA silencing.This results in destruction of the host message and thus, silencing of the host gene.This leads to disease in the host plant.
  6. 6. POTATO SPINDLE TUBER VIRIOD Potato SpindleTuberViroid (PSTV) is the first recognized viroid. PSTV is infectious RNA of low molecular weight, approximately 100,000 Daltons. The RNA is single stranded molecule of 359 nucleotides with extensive regions of base pairing. Under the electron microscope, purified but denatured PSTV appears as short strands about 50 nm long and thickness of double stranded DNA PSTVd is a small, circular RNA molecule.The natural hosts are potatoes (Solanum tuberosum) and tomatoes (Lycopersicon esculentum). All potatoes and tomatoes are susceptible to PSTVd and there is no form of natural resistance.
  7. 7. Electron microscopic picture of potato spindle tuber viroid (PSTVd)
  8. 8. PSTVd INFECTED POTATOES
  9. 9. HEPATITIS D Hepatitis D is a disease caused by a viriod called Hepatitis delta virus(HDV). HDV is considered to be a subviral satellite because it can propagate only in the presence of HBV.The HDV genome exists as an enveloped negative sense, single- stranded, closed circular RNA. Because of a nucleotide sequence that is 70% self-complementary, the HDV genome forms a partially double stranded RNA structure that is described as rod-like.With a genome of approximately 1700 nucleotides, HDV is the smallest viriod known to infect animals.
  10. 10. PRIONS Prions are infectious agents composed primarily of protein.They contain no nucleic acid.They cause a variety of neurodegenerative diseases in humans and animals.
  11. 11. PATHOGENICITY The most studied prion is scrapie prion that causes the scrapie disease in sheeps. Its studies show that the causative agent,PrPSc, enters the brain of an animal converting the normal PrPc to PrPSc which continue this conversion chain by changing its folding patterns. Recent evidences suggest that PrPSc crosslinks PrPc ‘s to trigger apoptosis.
  12. 12. EC graph of Scrapie prion
  13. 13. Sheep infected with Scrapie prion
  14. 14. MODE OF TRANSMISSION It has been recognized that prion diseases can arise in three different ways: 1. Acquired 2. Familial 3. Sporadic Current research suggests that the primary method of infection in animals is through ingestion.
  15. 15. OTHER DISEASES CAUSED BY PRIONS In cattles-  BOVINE SPONGIFORM ENCEPHALOPATHY or Mad Cow disease in cows
  16. 16. In Humans-  Kuru  Creutzfeldt-Jakob disease(CJD)  Fatal Familial Insomnia  Gerstmann-Strassler-Scheinker syndrome
  17. 17. REFERENCES  MICROBIOLOGY BY PRESCOTT  WIKIPEDIA  SCIENCE DIRECT

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