Viruses Viroids Prions and Lichens: In the world of biology, we have to classify things much like a library classifies books. That’s because for every category of book, like math books, there are subcategories, like statistics books. And for every subcategory, there are thousands of books on the topic. Without classification, the library would be a mess. In biology, it’s no different. There are so many topics that we have to use terms to classify them into broad categories. This lesson goes over some of these categories: viruses viroids prions and lichens.
The term ‘Virus’ means poisonous fluid. The word was coined by Louis Pasteur. Viruses are very small (0.05-0.2 µm), infective, nucleoprotein particles, which can be called as living because of the presence of nucleic acid as genetic material and ability to produce their own copy-viruses. They show only some properties of living beings, otherwise they behave like non-livings. Hence, these are referred to as the connecting link between living and non-living.
On the basis of nature of genetic material, the viruses are of two types
(i)Adenovirus DNA containing, e.g., HIV, etc.
(ii) Retrovirus RNA containing, e.g., Rous sarcoma virus, etc.
On the basis of their host, the viruses can be categorised as
(i)Animal virus (Zoophagineae), e.g., HIV, sarcoma, etc.
(ii)Plant virus (Phytophagineae), e.g., TMV, etc.
(iii)Bacterial viruses (Phagineae), e.g., T4 phage, etc.
Characteristics of viruses are as follows
Living
Non-living
After the discovery of viruses, scientists found another pathogen that was smaller than a virus. It didn’t have a protein layer like a virus and caused potato spindle tuber disease. It was discovered by T.O. Diener in 1971.
These are abnormally folded proteins, which can transmit several diseases. The word prion is derived from “proteinaceous infectious particle”. They were discovered by American biologist Stanley B. Prusiner.
The most common disease caused by Prions is bovine spongiform encephalopathy (BSE), Cr–Jacob disease (CJD), etc.
Lichens are symbiotic (Mutual) relationship between algae and fungi in which algae prepare food and fungi provides shelter. In this relationship, algae are known as photobionts, and fungi are known as mycobionts. Lichens have properties different from those of their component organisms.
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