Viruses are unique
particles that lack the structures characteristic of cells and are considered
non-cellular. They are unable to move independently or carry out metabolic
activities. The study of viruses is known as virology.
The term
"virus" originated from the Latin word "venome" meaning
"poison". In 1884, Charles Chamberland discovered that bacteria
cannot pass through porcelain filters, while the agent responsible for rabies,
a disease transmitted through bites of rabid animals, can. In 1892, Russian
biologist Iwanowsky conducted an experiment on tobacco plants with tobacco
mosaic disease. He extracted juice from the infected leaves and passed it
through a fine porcelain filter to remove bacteria. He then rubbed the filtered
juice on healthy plants, expecting no disease to develop. However, the healthy
plants soon showed symptoms of the disease, indicating the presence of a
disease-causing substance that could pass through the filter. Similar
disease-producing substances were later discovered in both plants and animals,
and they were named filterable viruses, as they could pass through filters with
pores too small for bacteria.
Bacteriophages, which
are viruses that infect bacteria, were independently discovered by Twort in 1915
and D'Herelle in 1917. Twort observed that bacterial colonies sometimes undergo
lysis, and this lytic effect can be transferred from one colony to another,
even in highly diluted material. He concluded that the lytic agent might be a
virus. D'Herelle rediscovered this phenomenon in 1917 and coined the term
bacteriophages, meaning "bacteria eater".
In 1935, W.M. Stanley
isolated tobacco mosaic viruses (TMV) from infected tobacco leaves. Under a
compound microscope, the TMV appeared as rod-shaped structures with
silver-shaped crystals. It was later discovered that the virus crystals were
nucleoprotein, consisting of nucleic acid and protein.
Attempts were made to
reproduce TMV outside of the tobacco plant in culture medium, but they were
unsuccessful. Stanley dissolved a small amount of purified TMV crystals in
water and rubbed them on an uninfected tobacco leaf. Soon, the typical mosaic
pattern of the disease developed, indicating that the virus had reproduced in
the living cells of the host. Viruses require enzyme systems of the host to
carry out their life processes.
Characteristics of
Viruses
Viruses are the
smallest living things.
They lack cellular
structure.
They have a simple
structure, consisting of either DNA or RNA surrounded by protein.
Viruses contain either
DNA or RNA as their hereditary material, but not both.
Viruses generally do
not have enzymes or coenzymes.
They lack cell
organelles.
Viruses are
crystallizable.
Viruses live and
reproduce only within the living cells of specific hosts, making them obligate
parasites.
There is genetic
continuity in viruses.
Viruses can cause
diseases in plants and animals.
Viruses are on the
boundary between living and non-living entities.
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| Ultrastructure Of virus |
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