• They are subviral pathogens / agents.
  • Their replication machinery depends upon their helper virus.
  • They can be seen in animals, protists and plants.
  • Plant satellite viruses can be classified as :
  1. Albetovirus
  2. Papanivirus
  3. Virtovirus
  • They have a nucleic acid genome that encodes a structural protein.
  • This protein then encapsidates the satellite .
  • Capsid protein is not encoded by the satellite RNAs.
  • The encoded protein in the helper virus genome is involved in the packaging of satellite RNAs.
  • Satellite Tobacco necrosis virus is an example of satellite virus.

  • Its helper virus encodes an RNA Polymerase that replicates its genome and also the satellites genome.
  • The range of length of satellite RNA genome is 200 – 1500 nucleotides.
  • There are 3 classes of satellite RNA genome.
  • Class I satellite RNA are 800 – 1500 nucleotide.
  • Class II satellite RNA are less than 700 nucleotides.
  • Class III satellite RNA are less than 350 – 400 nucleotides.
  • Satellite viruses make diseases worse caused by the helper virus.
  • Necrosis, synthetic chlorosis, reduced chlorophyll production are example of such diseases.
  • Symptoms caused by satellite RNAs are thought to be due to silencing of host genes.
  • Satellite that is associated with a human helper virus is hepatitis delta satellite virus. Its helper virus is hepatitis B virus.


  • It refers to proteinaceous infectious particle.
  • This mysterious agent is resistant to ultraviolet radiation.
  • On the other hand it is susceptible to substances that disrupt proteins.

  • This distorted protein can bind to other proteins of its type.
  • This can induce them to change their conformation.
  • Prion is responsible for several neuro degenerative diseses found in mainly mammals.
  • In humans it causes Creutzfelt Jackob disease (CJD).
  • Prion was termed by Prusiner.
  • They are misfolded proteins.
  • Centuries ago a disease was reported named scarpie.

  • This disease could survive high temperatures.
  • Later the damge caused from CJD was a  lot like those of kuru and scarpie.
  • Later it was examined that the agent causing scarpie has no DNA or RNA.
  • Prusiner showed this agent as an infectious protein and named it prion.
  • The proteins in prions are called prion protein / PrP for short.

  • PrP is a protein which is present in every persons body even they are not sick.
  • On chromosome 20 the gene of the DNA instructs each cell to produce PrP.
  • The gene is called PRNP (prion protein gene).
  • Prp is present in healthy state inside our body which is called cellular prion protein / PrPc .
  • This PrPc can be converted into PrPsc by misfolding.
  • PrPsc refers to as scarpie prion protein.