Compared to eukaryotic and even prokaryotic cells, viruses are much smaller and simpler in structure. Lacking the structures and metabolic machinery found in a cell, a virus is an infectious particle consisting of little more than genes packaged in a protein coat.
Are viruses living or nonliving?
Viruses contains both living and non living characters
Living Characteristics:
They can reproduce.
They possess nucleic acid ( DNA or RNA)
They can cause infection.
They undergo mutation and Genetic recombination.
Non-living characteristics:
They are sub-cellular ( non Cellular) or Acellular.
They can be crystallised.
They do not respire or excrete.
Discovery of Virus:
Tobacco mosaic disease stunts the growth of tobacco plants
and gives their leaves a mottled, or mosaic, coloration. In
1883, Adolf Mayer, a German scientist, discovered that he
could transmit the disease from plant to plant by rubbing
sap extracted from diseased leaves onto healthy plants. After
an unsuccessful search for an infectious microbe in the sap,
Mayer suggested that the disease was caused by unusually
small bacteria that were invisible under a microscope. This
hypothesis was tested a decade later by Dimitri Ivanowsky,
a Russian biologist who passed sap from infected tobacco
leaves through a filter designed to remove bacteria. After
filtration, the sap still produced mosaic disease.
But Ivanowsky clung to the hypothesis that bacteria caused
tobacco mosaic disease. Perhaps, he reasoned, the bacteria
were small enough to pass through the filter or made a toxin
that could do so. The second possibility was ruled out when
the Dutch botanist Martinus Beijerinck carried out a classic
series of experiments that showed that the infectious agent in
the filtered sap could replicate.
In fact, the pathogen replicated only within the host it infected.
In further experiments, Beijerinck showed that unlike bacteria used in the lab at that time, the mysterious agent of
mosaic disease could not be cultivated on nutrient media in
test tubes or petri dishes. Beijerinck imagined a replicating
particle much smaller and simpler than a bacterium, and he
is generally credited with being the first scientist to voice the
concept of a virus. His suspicions were confirmed in 1935
when the American scientist Wendell Stanley crystallized
the infectious particle, now known as tobacco mosaic virus
(TMV). Subsequently, TMV and many other viruses were actually
seen with the help of the electron microscope.
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