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. 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. 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. 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. 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.
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. 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. 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.
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. OTHER DISEASES CAUSED BY PRIONS
In cattles-
BOVINE SPONGIFORM ENCEPHALOPATHY
or Mad Cow disease in cows
16. In Humans-
Kuru
Creutzfeldt-Jakob disease(CJD)
Fatal Familial Insomnia
Gerstmann-Strassler-Scheinker syndrome
