Viruses, Viroids and Lichens
Some acellular organisms like Viruses, Viroids and Lichens are not included in the five kingdom classification of Whittaker.
1. Viruses are not included in the classification since they are not truly ‘living’.
2. Viruses are non-cellular organisms that are characterized by having a crystalline structure outside the living cell.
3. Once they infect a cell they use the machinery of the host cell to replicate themselves, thereby killing the host.
4. The name Virus means “venom or poisonous fluid” was given by Pasteur.
5. D.J. Ivanowsky (1892) recognised certain microbes as causal organism of the mosaic disease of tobacco. These were found to be smaller than bacteria because they passed through bacteria proof filters.
6. W.M. Stanley (1935) showed that viruses could be crystallised and crystals consist largely of proteins. They are inert outside their specific host cell.
7. Viruses are obligate parasites.
8. In addition to proteins viruses also contain genetic material that could be either RNA or DNA. No virus contains both RNA and DNA.
9. In general, viruses that infect plants have single stranded RNA and viruses that infect animals have either single or double stranded RNA or double stranded DNA.
10. Bacterial viruses or bacteriophages (viruses that infect the bacteria) are usually double stranded DNA viruses.
11. The protein coat called capsid made of small subunits called capsomeres, protects the nucleic acid.
12. These capsomeres are arranged in helical or polyhedral geometric forms.
13. Viruses cause diseases like mumps, small pox, herpes, influenza, AIDS.
14. In plants, the symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.
1. Discovered by T.O. Diener in 1971.
2. It was smaller than viruses and caused potato spindle tuber disease.
3. It was found to be a free RNA; it lacked the protein coat that is found in viruses, hence the name viroid.
4. The RNA of the viroid was of low molecular weight.
1. Lichens are symbiotic associations (mutually useful) between algae and fungi.
2. The algal component is known as phycobiont and fungal component as mycobiont, which are autotrophic and heterotrophic, respectively.
3. Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and water for its partner.
4. So close is their association that if one saw a lichen in nature one would never imagine that they had two different organisms within them.
5. Lichens are very good pollution indicators – they do not grow in polluted areas with Sulphur-dioxide.
At the end this module we get a clear understanding on the basics of Biological classifications of Living organisms in detail. You can access the test module for this topic by clicking on the following link.