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A Handbook of Biology

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Viruses cause diseases like mumps, small pox, herpes, influenza &

AIDS. In plants, the symptoms can be mosaic formation, leaf rolling and

curling, yellowing and vein clearing, dwarfing and stunted growth.

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Viroid: It is an infectious agent with a free, low-molecular weight RNA

(NEET 2016) and no protein coat. These are smaller than viruses. It is

discovered by T.O. Diener (1971). He found that it caused potato spindle

tuber disease.

Capsid head

Tail sheath

DNA

Bacteriophage

Tail fibers

PRÏØÑS

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These are infections proteinaceous agents.

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The most notable diseases caused by prions are Bovine Spongiform

Encephalopathy (BSE) commonly called mad cow disease in cattle.

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Its analogues variant Cr-Jacob disease (CJD) is found in humans.

LÏÇHËÑS

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Lichens are symbiotic associations (mutually useful associations)

between algae & fungi. (AIPMT 1996)

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The algal component is called phycobiont (autotrophic) and fungal

component is mycobiont (heterotrophic).

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Algae prepare food for fungi and fungi provide shelter and absorb

mineral nutrients and water for its partner.

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Lichens are very good pollution indicators. (AIPMT 1992)

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They do not grow in polluted areas. (AIPMT 1989)

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Lichens are pioneer in the xerarch succession.