Cell The Unit of Life Test - 3

• In the five kingdom classification of Whittaker there is no mention of lichens and some acellular organisms like viruses, viroids and prions.

• Viruses did not find a place in classification since they are not considered truly ‘living’, if we understand living as those organisms that have a cell structure.

• The viruses are non-cellular organisms that are characterised by having an inert crystalline structure outside the living cell.

• The viruses are non-cellular organisms that are characterised by having an inert crystalline structure outside the living cell.

• Viruses are parasites that infect plants, animals, fungi, and bacteria.

• They consist of genetic material and a protective protein coat. Viruses are dormant without a host.

• The name virus that means venom or poisonous fluid was given by Dmitri Ivanowsky in 1892.

• He recognised certain microbes as causal organism of the mosaic disease of tobacco.

• 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 crystallised and crystals consist largely of proteins. They are inert outside their specific host cell.

• In 1971, T.O. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber disease. It was found to be a free RNA; it lacked the protein coat that is found in viruses, hence the name viroid.

• Most viruses have either RNA or DNA as their genetic material.

• The nucleic acid may be single or double-stranded.

• The entire infectious virus particle, called a virion, consists of the nucleic acid and an outer shell of protein.

• The simplest viruses contain only enough RNA or DNA to encode four proteins.

• In addition to proteins, viruses also contain genetic material that could be either RNA or DNA. No virus contains both RNA and 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.

The viruses are non-cellular organisms that are characterised by having an inert crystalline structure outside the living cell.

Lichens are symbiotic associations, i.e. mutually useful associations, between algae and fungi. The algal component is known as phycobiont and fungal component as mycobiont, which is autotrophic and heterotrophic, respectively. Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and water for its partner.

In 1971, T.O. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber disease.