Viruses and Viroids
Viruses
Viruses are acellular, highly infectious microorganisms
which are considered to be on the threshold of life.
Adolf Mayer (1886) had demonstrated that mosaic disease of
tobacco is infectious. When the juice from tobacco plants infected with the
tobacco mosaic virus (TMV) was injected into healthy plants, the plants showed
symptoms of mosaic disease. D.J. Ivanowski (1892) confirmed the observation of
Mayer. Ivanowski prepared prepared an extract of infected tobacco plant and
showed that this extract could infect healthy plants even after it was passed
through filter that retains bactria. M.W. Beijerink (1898) called it as
virus-venom or poisonous fluid and referred it as “contagium vivum fluidum”
meaning “contagious living fluid”. The term virus was first used by Louis
Pasteur.
Today our knowledge of viruses is due to efforts of
scientists like Ferderick William Twort, Felix d’ Herille, Schelsinger, W.M.
Stanley, Hershey and Chase, etc.
The detailed structure of viruses was studied only after the
invention of electron microscope.
Viruses are acellular, ultramicroscopic, disease causing
(infectious) entities. Their size ranges from 10 nm to 2000 nm. Some of the
larger viruses are, however as large as some of the smaller bacteria.
Viruses do not grow, do not eat food or respire.
Viruses are obligate parasites and can multiply only within
the living host cells. They have capacity or transmit the disease from diseased
to healthy organisms.
Viruses show very simple structure i.e. a nucleic acid core
surrounded by a protein coat (capsid) to form the nucleocapsid. They posses
their own genetic material in the form of either DNA or RNA (but never both).
They can be crystallized. Stanely (1935) obtained viruses in
crystalline form, from tobacco leaves infected by TMV. They can be stored for
long period. Outside the body of host they do not show any activity, they are
like non-living. They do not have their own metabolic machinery. They utilize
the ribosomes and enzymes of the host cell for synthesis of protein during
reproduction. Most of the viruses are highly resistant to germicides and
extremes of physical conditions. Viruses exhibit mutations. They can not be
cultured on artificial nutrient medium.
Viruses are host specific. Thus viruses show characteristics
of living as well as non-living. Therefore, they are said to be on the
threshold of life.
Types
of Viruses : (Classification of viruses)
Viruses are classified into three group on the basis of
their hosts.
1.Plant
Viruses :
The viruses which attack and infect plants are called plant
viruses. These viruses consist of ss-RNA or ds-RNA. They show helical symmetry
and are mostly rod-shaped or cylindrical.
2.
Animal Viruses :
The viruses which attack and infect animals are called
animal viruses. The genetic material is either RNA of DNA. They show radial
symmetry. These are mostly polyhedral in shape.
3.
Bacterial viruses or Bacteriophges :
(Greek Phagin = eater).
The viruses which attack and infect the bacteria are called
bacteriophages or bacterial viruses. The genetic material is DNA. They were
discovered by Twort (1915). Felix d’ Herelle named them as bacteriophages. They
are mostly tadpole shaped.
Economic
Importance of viruses
The plant viruses and animal viruses are of great economic
importance as they cause various diseases in plants and animals.
Plant diseases caused by viruses are like –
1)Little leaf of Brinjal
2) Yellow vein mosaic of lady’s finger.
3) Potato leaf roll.
4) Leaf curl of papaya.
5) Bunchy top of banana.
6) Grassy shoot of sugarcane.
7) Tobacco mosaic disease.
Common symptoms due to viral infection include-
Local lesions, clearing of veins, mosaic formation,
chlorosis, necrosis, stunting and premature defoliation, ting spotting, etc.
Animal diseases caused by viruses are like –
Disease Virus
1)Common cold
- Rhino virus
2) Influenza - Myxo virus
3) Small pox - Variola virus
4) Mumps - Paramyxo virus
5) Mealses - Morbilli virus
6) Poliomyelitis - Polio virus
7) Yellow fever - Flavi virus
8) Swine flu - H1N1 Virus
9) AIDS - HIV (Retro
virus)
Viroids
:
Viroids were discovered and named by Theodor Diener (1971).
These are very small circular, single-stranded RNAs which are not complexed
with any protein. (without protein coat) This RNA is either covalently closed
circular RNA or single stranded linear RNA. These are mainly plant pathogens.
The viroids do not show dormant phase. The first viroid discovered was the potato
spindle tuber viroid (PSTV) which causes a disease in potato. PSTV is composed
of 359 nucleotides and thus has ten times less genetic material than the
smallest known virus. The naked RNA molecule can replicate autonomously in
plant cells. It is of low molecular weight and can disperse into the
environment. At least 11 other plant diseases have been linked to viroids.
These diseases include citrus exocortis, chrysanthemum stunt, cucumber pale
fruit, etc.