VIRUSES
The viruses were discovered by Louis Pasteur as a poisonous fluid.
Viruses are non-cellular inert highly infectious organisms that multiply only intra-cellularly inside the living host without involving growth and division.
Viruses have inert crystalline structure outside the living body but when they enter the living cell, they take over the machinery of living cell to replicate themselves. The viruses consist of a core of nucleic acid encapsulated by proteins. These are also called as nucleoprotein particles. These are obligate parasites and cause infection due to nucleic acid. The smallest animal virus is 10nm (foot and mouth disease of cattle) were as the smallest plant virus is 17nm (Alfa Alfa mosaic virus).
STRUCTURE (TOBACCO MOSAIC VIRUS)
TMV is the first virus that was isolated and studied under microscope by W.M Stanley in 1935-46. A typical virus consists of the central core of helical nucleoid surrounded by a protein coat called capsid. Each unit of capsid is called capsomer. The nucleoid can be of 4 types –double stranded DNA –single stranded DNA –double stranded RNA –single stranded RNA. In some viruses like HIV the capsid is surrounded by a protein envelop. The unit of envelop is called peplomer.
Viruses do not undergo division or binary fission. In general, virus that infect plants have single stranded RNA and virus that infect
Animals have double stranded DNA.
BACTERIOPHAGES; the bacteriophages were discovered by Twort in 1915 and the term bacteriophage was given by Herelle.
The viruses that infect bacteria are called bacteriophages.
A well studied bacterial virus which attacks E. coli commonly called as coli phage has the following structure;
It is a tadpole like virus consisting of a hexagonal head and a tail. The head consists of an outer coat of proteins enclosing a molecule of DNA. The tail consists of tail sheath proteins bearing a disc like hexagonal plate at its distal end. The disk bears tail fibers.
Reproduction
It can be lytic (virulent) or lysogenic (non virulent).
In lytic phase the virus enters a bacterial cell and disintegrates the bacterial DNA. It is followed by the formation of new phage particles. The bacterial cell then bursts or undergoes lysis to release new phage particles.
In lysogenic phase the bacterial cell does not undergo lysis or breakdown but the DNA of virus divides along with the bacterial DNA
V virus
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G genetic material
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Pox virus
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ds -DNA
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Hepatitis-B
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ds -DNA
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Cauliflower mosaic virus
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ds -DNA
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Adeno viruses
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ds -DNA
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Coli phage
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ss -DNA
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Reo virus, wound tumor virus
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ds-RNA
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TMV, Influenza, polio virus
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ss-RNA
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Rabies virus
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RNA
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AIDS
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RNA (retrovirus)
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VIROID;- Viroids are the smallest known agents of infection that comprise of small single stranded molecule of RNA. Viroids lack capsid and have no proteins associated with them. These are responsible for very important plant diseases such as potato spindle tuber disease, and citrus exocortis disease.
Prions;- these are the proteins that cause infectious diseases. Prions lack genetic material. Some diseases caused by them are creutzfelds-jakob disease, kuru disease in humans and scarpic in sheep and mad cow disease.
LICHENS
Lichens are symbiotic association between algae and fungi. The algal part is called
Phycobiont and fungal parts are called Mycobiont. Phycobiont prepares food for
fungi and the Mycobiont provides shelter and absorbs mineral nutrients and
Water for its partner. Lichens are good pollution indicators as they do not grow in polluted area.
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