Viruses, Viroids, Lichens and Prions
Viruses, Viroids, Lichens and Prions
What is a “Virus”
- A virus (in Latin meaning toxin or poison) is the simplest form of life.
- They are small infectious agents that can only replicate inside the cells of another organism.
- Viruses do not have cells, i.e. they are acellular organisms.
- The name virus which means venom or poisonous fluid was discovered by Pasteur.
Structure of “Virus”
- Viruses are acellular organisms and characterized by having an inert crystalline structure outside the living cell.
- They are surrounded by a protective protein coating called the capsid which is made up of small subunits called capsomeres.
- Capsid protects the nucleic acid of the virus.
- These capsomeres are arranged in helical or polyhedral geometric forms.
- Along with proteins viruses also contain genetic material that could be either RNA or DNA. No virus contains both RNA and DNA.
Characteristic Features of Virus
- A virus is a nucleoprotein and the genetic material is infectious.
- Viruses are obligate parasites means they depend on a host for its nourishment, reproduction, habitat, and survival.
- The size of the viruses is too small to be seen directly with a light microscope.
- Viruses have the capability to infect all types of organisms, from animals and plants to bacteria and archaea.
- D.J. Ivanowsky (1892) recognized certain microbes as a causal organism of the mosaic disease of tobacco.
- M.W. Biejerinck (1898) demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid Contagium vivum fluidum (infectious living fluid).
- W.M. Stanley (1935) showed that viruses could be crystallized and crystals consist largely of proteins. They are inert outside their specific host cell.
- We all have suffered from the common cold or ‘flu’ which is also caused by a virus.
- Viruses that infect plants have single-stranded RNA and viruses that infect animals have either single or double-stranded RNA or double-stranded DNA.
- Once they infect a cell they take over the machinery of the host cell to replicate themselves, killing the host.
- Bacterial viruses or bacteriophages (viruses that infect the bacteria) are usually double-stranded DNA viruses.
- Viruses cause diseases like mumps, smallpox, herpes, and influenza. AIDS in humans is also caused by a virus. In plants, the symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing, and stunted growth.
What is “Viroid”
- Viroids are plant pathogens that consist of a short stretch of highly complementary, circular, single-stranded RNA without the protein coat and infectious pathogen.
- The RNA found in it is free and they lack the protein coat that is found in viruses, hence the name viroid. The RNA of the viroid was of low molecular weight.
Characteristic Features of Viroids
- Viroids were discovered and given this name by Theodor Otto Diener, a plant pathologist at the Agricultural Research Service in Maryland, in 1971.
- The genome of the smallest known viruses capable of causing an infection by themselves is around 2 kilobases in size. The human pathogen hepatitis D is similar to viroids.
- Viroid RNA does not code for any protein. The replication mechanism involves RNA polymerase II, an enzyme normally associated with the synthesis of messenger RNA from DNA, which instead catalyzes the "rolling circle" synthesis of new RNA using the viroid's RNA as a template.
- Some viroids are ribozymes, having catalytic properties that allow self-cleavage and ligation of unit-size genomes from larger replication intermediates.
- The first viroid to be identified was the potato spindle tuber viroid (PSTVd). Some 33 species have been identified.
- The smallest discovered is a 220 nucleobase scRNA (small cytoplasmic RNA) associated with the rice yellow mottle sobemovirus (RYMV).
What is “Lichen”
- Lichens are a symbiotic association that is mutually useful for both the organism of association. This symbiotic association occurs between a fungus (the mycobiont) with a photosynthetic partner (the photobiont or phycobiont), usually either a green alga (commonly Trebouxia) or cyanobacterium (commonly Nostoc).
Characteristic Features of Lichens
- Lichens occur in some of the most extreme environments on Earth—arctic tundra, hot deserts, rocky coasts, and toxic slag heaps.
- The morphology, physiology, and biochemistry of lichens are very different from those of the isolated fungus and alga in culture.
- They are also abundant as epiphytes on leaves and branches in rainforests and temperate woodland, on bare rock, including walls and gravestones, and on exposed soil surfaces (example- Collema) in otherwise mesic habitats.
- Lichens are widespread and may be long-lived, many species are also vulnerable to environmental disturbance and may be useful to scientists in assessing the effects of air pollution, ozone depletion, and metal contamination.
- Lichens have also been used in making dyes and perfumes, as well as in traditional medicines.
- Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and water for its partner.
- The association of lichens is so close that when seen in nature one would never imagine that they had two different organisms within them. Lichens are very good pollution indicators – they do not grow in polluted areas.
What is “Prions”
Prions are infectious proteins that can cause a variety of neurodegenerative diseases in humans and animals, including Creutzfeldt-Jakob disease (CJD), bovine spongiform encephalopathy (BSE, or "mad cow disease"), and Gerstmann-Sträussler-Scheinker syndrome (GSS).
Prions are thought to be caused by the misfolding of a normal protein, called PrP^C, into a pathological form, called PrP^Sc. PrP^Sc is then able to convert other PrP^C molecules into PrP^Sc, leading to a chain reaction that results in the accumulation of PrP^Sc in the brain.
The exact mechanism by which prions cause disease is not fully understood, but it is thought that they may damage neurons and disrupt brain function. Prion diseases are typically fatal, and there is no cure.