4. ✓ THEODOR OTTO DIENER(1971).
COVALENTLY CLOSED, CIRCULAR, SINGLE
STRANDED RNA.
✓ No protein coat.
✓ Intra-strand base pairing.
✓ Infect many plants.
✓ Viroid RNA does not code for any protein.
✓ 250 to 370 nucleotides.
✓ Molecular weights of 1.1 to 1.3 x 106
daltons.
5. STRUCTURE -
I. CONSERVED CENTRAL DOMAIN (CCR)
II. PATHOGENICITY DOMAIN (P)
III. VARIABLE DOMAIN (V)
IV. LEFT TERMINAL DOMAIN(TL), AND
V. RIGHT TERMINAL DOMAIN(TR).
6. REPLICATION
•Viroid replicates with the help of host RNA dependent RNA polymerase.
•Replication of viroid (+) RNA TAKE PLACE to produce concatemers of (-)
strand.
•The linear (-) RNA then acts as the template for synthesis of concatemer of
(+) RNA strand.
•The (+) strand then cleaved into segments of the size of the viroid genome.
8. TRANSMISSION
1. Viroid infections are transmitted by cross contamination following
mechanical damage to plants as a result of horticultural or agricultural
practices.
2. Some are transmitted by aphids, and
3. They can also be transferred from plant to plant by leaf contact.
9. PATHOGENICITY
•Viroids infection may remain latent without causing any apparent damage.
•The same viroid may cause a severe disease in another host species.
•Evidence suggests that RNA silencing is involved in the process.
•Dicer proteins are required for RNA silencing.
12. •Citrus exocortis viroid – It can infect various species of
citrus, like orange, mandarin, lemons, limes etc. Infected
plants are characterized by vertical strips of cracks on the
stem. The yield is lowered by 40%.
•Chrysthemum stunt – Here dwarfing of the plants with
much reduced flower yield.
•Cadang-cadang is disease of coconut plant occurring in
Philippines killing more than a million palms every year
14. •Stanley Prusiner(1982), awarded with the nobel
prize in 1997.
•Prions consists of only protein.
•Susceptible to some protein hydrolyzing enzymes,
but are resistant to nucleases.
15. •Prusiner designated the normal protein as PrPc
and the abnormal Scrapie
inducing proteins as PrPsc
.
•These abnormal protein molecules tend to clump together forming aggregates
and possibly block the molecular traffic (metabolic processes) in the
infected cells ultimately leading to their death.
16.
17. TRNSMISSION OF DISEASE
•When abnormal proteins from the diseased animal is introduced into a
healthy individual containing normal prion protein (PrPc
) the individual
becomes diseased, though the incubation period is long.
18. •An animal disease commonly known as ‘Mad-cow disease’ came into
prominence in the Great Britain in 1990s. This disease, technically called
Bovine Spongioform Encephalopathy (BSE) occurred to the cattle.
•An another rare disease called Creutzfeldt-Jacob Disease (CJD) resulting to
spongioform encephalopathy, occurred at higher incidents to the humans in
the Great Britain during that period and scientist linked it with the ingestion
of meat of diseased ( mad-cow disease) cattle.
•Kuru was an once prevalent disease found only in some cannibal tribals of
Papua-New Guinea.
19.
20. REFERENCES
I. A PLANET OF VIRUSES: SECOND EDITION: CARL ZIMMER,
II. PRESCOTT’S MICROBIOLOGY.
