Viruses: Living or Non Living

Physics Chemistry  Biology  Mathematics
Science > Biology > Classification of Lower Level OrganismsYou are Here
  • In the five kingdom classification of Whittaker there is no mention of some acellular organisms like viruses and viroids, and lichens.
  • Viruses did not find a place in classification since they are not truly ‘living’, if we understand living as those organisms that have a cell structure.
  • They are non-cellular organisms that are characterized by having an inert crystalline structure outside the living cell.
  • Once they infect a cell they take over the machinery of the host cell to replicate themselves, killing the host.
  • The name virus that means venom or poisonous fluid was given by Pasteur.

Evidences that Show Viruses are Living:

  • They have genetic material (RNA or DNA)
  • They can grow
  • They can be transmitted from one host to another.
  • They are capable of multiplication in the host.
  • They can mutate.
  • They show irritability because they react to heat, radiation, and chemicals.

Evidences that Show Viruses are Non-Living:

  • They can be crystallized and stored for a very long time.
  • They don’t have cell wall or cytoplasm.
  • They are inert outside the host.
  • They don’t have cell organelles or don’t have metabolism.
  • They can not function outside the host.
  • They do not show cell division.

Structure of Viruses:

  • They were found to be smaller than bacteria because they passed through bacteria-proof filters.
  • 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, 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. Bacterial viruses or bacteriophages (viruses that infect the bacteria) are usually double stranded DNA viruses.
  • The protein coat called capsid made of small subunits called capsomeres protects the nucleic acid. These capsomeres are arranged in helical or polyhedral geometric forms.

Plant Viruses:

Viruses - Plant Virus

  • The viruses which infect and attack plants are called plant viruses.
  • They have single-stranded RNA either  ss-RNA or ds-RNA.
  • They show helical symmetry
  • They are mostly rod-shaped or cylindrical.



Animal Viruses:

Viruses - Animal Virus

  • The viruses which infect and attack animals are called animal viruses.
  • They have either single or double stranded RNA or double stranded DNA.
  • They show radial symmetry.
  • They are mostly rod polyhedral in shape.

 

 



Bacterial Viruses:

Viruses - Bacterial Virus

  • They are also called Bacteriophages.
  • The viruses which infect and attack bacteria are called bacterial viruses.
  • They have DNA strand.
  • They show radial symmetry
  • They are mostly tadpole in shape.

 

Economic Importance of Viruses:

Plant Diseases:

  • Little leaf of brinjal:

Viruses Little leaf of brinjal

  • The characteristic symptom of the disease is the marked reduction in the size of the leaves.
  • The newly formed leaves become progressively smaller. The petioles are very much shortened.
  • The leaves appear appressed to the stem and become narrow, soft, glabrous, and yellow.
  • The internodes are shortened.
  • The axillary buds are stimulated to sprout, and they grow into short branches with very small leaves.
  • In severe cases, affected plants do not bear any fruit, or, if formed, it becomes hard and tough.
  • Yellow vein mosaic of lady’s finger:

Viruses Yellow vein mosaic of lady’s finge



  • Bhendi yellow vein mosaic was first reported in okra plants in 1924 in India and Sri Lanka.
  • The symptoms include alternate green and yellow patches, vein clearing, and vein chlorosis of leaves.
  • The yellow network of veins is very conspicuous, and vein and veinlets are thickened.
  • In severe cases, the chlorosis may extend to the interveinal area and may result in complete yellowing of leaves.
  • Fruits are dwarfed, malformed, and yellow-green.
  • Potato Leaf Roll:

Potato Leaf RollThe potato leaf roll virus causes a very well visible leaf rolling up. the leaflets are curled up to form a small boat.

    • Infected plants are dwarf, the leaves are very fragile, and the whole plant is light.

 

  • Papaya leaf curl:

Papaya Leaf Curl

  • It was first reported in Tamil Nadu in 1939.
  • The most prominent symptoms are the rolling of the leaves downward and inward in the form of an inverted cup and the thickening of veins.
  • Sometimes all the leaves at the top of the plant are affected by these symptoms.
  • In advanced stages of the disease, defoliation takes place and the growth of the plant is arrested.
  • Bunchy top of banana:

Bunchy Top Banana

  • The leaves remain bunched up, with yellow fringes and stand erect.
  • The bunchy top symptom is usually most visible on young plants.
  • Bunchy top symptom can be more subtle on older banana plants.
  • The production of banana fruit becomes uneconomical and unprofitable.
  • Grassy shoot of sugarcane:

Grassy Shoot of Sugarcane



  • This disease is characterized by the production of numerous small and thin tillers having narrow leaves.
  • Diseased plants exhibit varying degrees of loss of chlorophyll, ranging from total green to white.
  • Premature & excessive tillering gives a crowded appearance like ‘grass’ to the clump.
  • The root system of the affected plant reduced and plants are usually reduced in height (stunted growth). Affected clumps hardly produce one or two weak canes.
  • Tobacco Mosaic Disease:

Tobaco Mosaic

  • Symptoms induced by Tobacco mosaic virus (TMV) are somewhat dependent on the host plant and can include mosaic, mottling, necrosis, stunting, leaf curling, and yellowing of plant tissues.
  • The symptoms are very dependent on the age of the infected plant, the environmental conditions, the virus strain, and the genetic background of the host plant.



Animal Diseases:

Viruses - Animal Diseases

Viroids:

  • 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.
  • The RNA of the viroid was of low molecular weight.
  • These are mainly plant pathogens.
  • The viroids do not show dormant state.
  • The diseses caused by viroids are  Citrus exocortis Chysanthemum stunt Cucumber bale fruit Potato spindle tuber
Science > Biology > Classification of Lower Level OrganismsYou are Here
Physics Chemistry  Biology  Mathematics

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