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MDCATBiology
Quick Practice Book
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,Biodiversity
• Trace the discovery of virus.
• Classify viruses on basis of their
structure or number of
strands/diseases/host etc.
• Identify symptoms, mode of
transmission and causes of viral
disease (AIDS).
,Viruses
The word virus is derived from Latin word ‘venome’ which means poisonous fluid.
Viruses are: Non-cellular, submicroscopic infectious agents which contain either
RNA or DNA, enclosed by proteinaceous coat, and reproduce only in living cells
(obligate intracellular: parasites) Viruses use biosynthetic machinery of the host to
make their materials and then transfer to their cells. Study of virus is known as
virology.
Historical Background of Virus
Some viral diseases have been known from centuries.
The first infectious disease against which presentation was developed was a viral
disease.
1. Work of Edward Jenner:
In 1796. Edward Jenner discovered an effective method for the prevention of
a viral disease small pox. He removed material from cowpox lesion on the hand
of milkmaid and injected into an 8 years old boy (James Phipps). After six
weeks the boy was injected with pus from a small pox victim. He did not
develop the disease. Jenner used material for vaccination from cowpox
lesions and successfully vaccinated 23 persons. As the material was obtained
from cow (called vacca in Latin), this method was named as vaccination by
Louis Pasteur.
2. Work of Charles Chamberland:
Charles Chamberland (1884) found that bacteria cannot pass through
porcelain filters. However, agents responsible for rabies can pass through
these filters.
Any toxic substance that caused disease was called virus. These unseen
filterable agents of rabies were called as filterable viruses.
Rabies is a disease which is transferred to human by bites of rabid dogs, foxes,
cats, bats and other animals.
3. Work of Ivanowski:
In, 1892, lvanowski discovered that the agent which caused tobacco mosaic
disease was filterable.
He obtained bacteria free filtrate from infected plants and placed it on healthy
leaves of tobacco.
The filtrate caused the disease in healthy plants.
Later these ultramicroscopic agents were also observed in victims of many
diseases including foot and mouth disease (1898) and yellow fever (1901).
In 18?8 the Dutchman Beijerink formed the name 'virus' (Latin for poison) to
describe the infectious nature of certain filtered plant fluids.
Although progress was made in isolating highly purified samples of viruses
and in identifying them chemically as nucleoproteins (nucleic acids combined
with proteins). the particles still proved mysterious because they were too
small to be seen with the light microscope. They were among the first
, biological structures to be studied when the electret, microscope was
developed in the 1930s.
Stanley (1935) crystallized the tobacco mosaic virus.
Characteristics of Viruses
Viruses are small infectious agents and can be seen under electron microscope. They
nave following characteristics:
1. Size:
They range in size from 250 nm of Pox viruses to 20 nm of Parvoviruses.
2. Filterable:
They are 10 to 1000 times smaller than bacteria. So they can pass through the
pores of filter from which bacteria cannot pass.
3. Obligate Intracellular Parasites:
Viruses cannot grow on artificial media. They can reproduce in animal cells,
plant cells or in microorganisms.
Here they reproduce by replication (a process by which many copies or
replicas of virus are formed).
Therefore, the viruses are obligate intracellular parasites.
4. No Metabolic Machinery:
Viruses have no metabolic machinery for the synthesis of their nucleic acid
and protein They depend on the host cell to complete vital functions.
5. Disease Production:
They can cause disease in the host during reproduction.
6. Resistant to Antibiotics:
They are generally resistant to many antibiotics such as penicillin,
streptomycin and others. Each type of virus will recognize and infect only
certain types of cell. In other words, viruses are highly specific to their hosts.
Structure of Virus
Virion:
The complete, mature and infectious particle is known as virion. It has following
parts:
1. Genome: The virions are composed of a central core of nucleic acid (DNA or
RNA) which is also called genome.
2. Capsid: The genome is surrounded by a protein coat, the capsid.
Nucleic acid and capsid is collectively called nucleocapsid.
Capsid gives definite shape to virion.
Capsid is made up of protein subunits known as capsomeres.
The number of capsomeres varies in a particular virus.
Examples:
• 162 capsomeres in the capsid of herpes virus.
• 252 capsomeres in the capsid of adenovirus which cause some common colds.
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