VIRUS :
The
VIRUS was first discovered by Iwanowski in 1892 as extremely small organism. The
viruses are submicroscopic, acellular, forms of life and are much smaller than bacteria. The size of virus is from 20 to 80 micron. The viruses possess a regular geometrical and macro-cellular organization. All viruses consist of a core of nucleic acid (DNA/RNA) which remains wrapped in a coat of protein called capsid; the capsid is composed of numerous protein molecules called capsomeres.
The DNA viruses have single DNA molecule which may be either linear or circular in shape. The linear DNA molecule is double stranded and circular DNA can be either single or double stranded. The RNA viruses are only biological system known in which RNA is genetic material.
The viruses can not lead a free living mode of life for the performance of fundamental life activities such as reproduction, genetically determined structure and functions. They have to lead a parasitic mode of existence.
Types of Viruses:
1. Bacterial Viruses or Bateriophages: These viruses parasitize bacterial cells.
2.
Plant Viruses: The plant viruses parasitize the plant cells and disturbed their metabolism and cause several
diseases in them. Examples are tobacco mosaic virus, potato virus.
3. Animal Viruses: They infect the animal cells and cause diseases in animals and human beings. They have spherical shape and genetic material in the form of DNA and RNA. Examples are Small
pox virus, Influenza virus, mumps, pea virus, etc.
Differences between Viruses and Bacteria:
1. Viruses have only one type of nucleic acid either DNA or RNA where as
bacteria have both.
2. Viruses are devoid of ribosomes and enzyme systems needed to generate ATP molecules where as bacteria have got ribosomes as well as enzymes needed in ATP synthesis.
3. Viruses do not have cell wall where as bacteria have well defined cell wall.
4. Viruses cannot multiply or synthesize their protein and enzymes independent of
host cell but bacteria can.
Tidbits:
A virus may not harm one type of host may but kill or injure another one.
Viruses may cause the wild growth and reproduction of cells rather than their direct destruction.
Mild viral diseases may grant its host immunity. Example a cow pox makes a person lightly ill but provides resistance against small pox.
Viruses pass from one host to another directly or indirectly.
A given vaccine is effective against one type of virus but forever. Viral vaccines may have either mild live virus which causes mild infection but body produces antibodies or killed virus which induce the human body produce antibodies. For vaccination purposes viruses are killed by formaldehyde.
Viral vaccines are not useful for plants because they can not produce antibodies.
Antibiotics and Sulfa drugs are used against virus.
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