Viruses, Viroids and Lichens
- The viruses are non-cellular obligate parasites that are characterized by having an inert crystalline structure outside the living cell.
- Once the viruses infect a cell, they kill the host cell and take over the machinery of the host cell to replicate themselves.
- The name virus that means venom or poisonous fluid was given by Pasteur. D.J. Ivanowsky.
- J. Ivanowsky recognized certain microbes as causal organism of the mosaic disease of tobacco, which were found to be smaller than bacteria because they passed through bacteria-proof filters.
- W. Beijerinek demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid as Contagium vivum fluidum.
- M. Stanley showed that viruses could be crystallized and crystals consist largely of proteins, which are inert outside their specific host cell.
- In addition to proteins viruses also contain genetic material that could be either RNA or DNA.
- Viruses that infect plants have single stranded RNA and viruses that infect animals have either single or double stranded RNA or double stranded DNA.
- Bacteriophages are usually double stranded DNA viruses.
- The protein coat called capsid made of small subunits called capsomeres, protects the nucleic acid.
- Viruses cause diseases like mumps, small pox, herpes and influenza, AIDS.
Fig. Different types of virus
- Viroids are smaller than viruses, which was found to be a free RNA.
- Virioids lacked the protein coat that is found in viruses, hence the name viroid.
- Lichens are symbiotic associations between algae and fungi.
- The algal component is known as phycobiont and fungal component as
- Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and water for its partner.
- Lichens are very good pollution indicators.