One of the unsolved mysteries in biology is that of viruses. Due to the absence of a cellular organization viruses cannot be replaced with either prokaryotes or eukaryotes. They are considered as an intermediate between living and non living systems. Viruses are active and show reproduction only inside the host cell. Outside the host they are just a non living thing, they may even be purified and crystallized like chemical substances. Viruses have a genetic material represented by either DNA or RNA, surrounded by a protein sheath. Viruses reproduce by using the metabolic machinery and raw materials of the host cell.
It is because of these peculiarities that viruses do not fit into any of the five kingdom of life.
Prions and Viroids
The discovery of viruses showed that there are entities containing nucleic acids, without a definite cellular organization. Such nucleic acids, are capable of exhibiting the process of replication, using the machinery of their host cells. However, in the recent years scientists have discovered certain agents of infectious diseases, which can replicate independently. They have been called as prions and viroids.
Prions
Prions represents the first exception to the rule that self replicating entities should contain nucleic acids. Prions contains only proteins. Prions were discovered during the search for the cause of a peculiar disease called scrapple in sheep. In this disease, the sheep enters into a neurological state in which they keep scraping their body against hard objects like rocks. This resulted in considerable loss of wool. In the year 1983, Stanley Prusiner and his associates, while investigating this disease, found some entities which appeared to contain no genetic material of their own. They were found to replicate even in the absence of a DNA or RNA as their nucleic acid. The only substance present in these entities was a protein molecule of about 250 amino acids. The name prions was given to these infectious agents, considering their purely proteinaceous nature.
Prions are known to cause several disease to humans, cattle, sheep etc.
Viroid
Viroid are pathogenic RNAs which are known to cause virus like diseases in some plants. They are extremely small, circular molecules, with only a few hundred nucleotides. They do not code for any polypeptides, but replicate independently of any associated plant virus. The viroids wer first discovered in 1971, by Diener, during the investigation on spindle tuber disease in potato.
Viroid are smallest known entities among the infectious agents. Viroids, like prions can not be identified by electron microscope, in the infected tissue. They lack a protein coat hence, occur as free RNA molecules with about 200-400 nucleotides. Unlike viruses viroids can not be crystallized. They are known to become associated with either chromatin or chromoplasts in the host cells. Even a change of one or two nitrogen bases in the viroid RNA, is known to transform the disease from mild to severe or vice-versa.
Some plant diseases which are caused by viroid are Potato spindle tuber, citrus exocortis, cucumber plate fruit, and chrysanthemum stunt.
Lichens
Lichens are a group of organisms formed by the symbiotic association of algae and fungi. The fungal component is called mycobiont and the algal component is called phycobiont. These component are involved in an inseparable symbiotic association. The fungal component provides a structural cover and moisture while the algal component is involved in manufacturing food.
Lichens normally found growing on tree barks, decaying wood, soil rich in humus, stones, rocks etc. some are totally aquatic.
Lichens vary in shape and size. Three main types of lichens are
- Crustose lichens which have a flattened thallus, resembling crusts, attached to the substratum partially or wholly. example Graphics.
- Foliose lichens which have a flat, expanded, leaf like thallus with root like rhizoids. example. Gyrophora.
- Fruticose lichens which have a thallus that is branched and bushy. It may be erect or pendent. Example. Asnia.