Viruses,Viroids & Lichen Characters

• In the five-kingdom classification, acellular organisms (viruses & viroids) and lichens are not mentioned.
• Viruses are not truly ‘living’. So they are not included in five-kingdom classification.
• Viruses are non-cellular organisms having an inert crystalline structure outside the living cell.
• Viruses are obligate parasites.
• When they infect a cell they take over the machinery of the host cell to replicate themselves, killing the host.
• The name virus (means venom or poisonous fluid) was given by Pasteur.
• D.J. Ivanowsky (1892) recognized certain microbes that cause mosaic disease of tobacco.
• They were smaller than bacteria because they passed through bacteria-proof filters.
• M.W. Beijerinek (1898) demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid as Contagium vivum fluidum (infectious living fluid).
• W.M. Stanley (1935) showed that viruses could be crystallized and crystals consist largely of proteins.
• In addition to proteins, viruses also contain genetic material (RNA or DNA).
• No virus contains both RNA & DNA. A virus is a nucleoprotein and the genetic material is infectious.
• 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.
• Bacteriophages (viruses that infect bacteria) are usually double stranded DNA viruses.
• The protein coat (capsid) made of small subunits (capsomeres) protects nucleic acid.
• Capsomeres are arranged in helical or polyhedral geometric forms.
• Viruses cause diseases like mumps, small pox, herpes, influenza and AIDS.
• In plants, the symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.