What is a virus ?

Definition of a virus

It’s in 1953 that Lwoff has defined the concept of a virion. It’s a viral particle which is mature and infectious, and free in the environment. A virion has 4 essential characters :

• It has only one type of nucleic acid which can either be DNA or RNA (but not both). Viral nucleic acid contains the whole genetic information about the virus and constitute what is called the viral genome.
• The virion reproduces itself as from its genetic material only by replicating its genome. There is no fission as in bacteria and there is no mitosis as in eucaryotype cells.
• They are obligatory intracellular parasites. They can only reproduce themselves within a living host cell. Having no enzymatic or energetic system to ensure their own replication. They use all the macromolecules of the cell that they parasite (ribosome, RNA, enzymatic activity, regulatory system). During the interaction particle/host cell, there are two possibilities:
  • the viral multiplication causes the death of the cell : it’s the cellular lyses.
  • The cellular lesions do not cause the death of the cell : that’s what is called viral persistence.
• The virion presents a particle-like structure.

A plant virus, or phytovirus, is defined as an obligatory parasite of living cells in a host plant. Those pathogenic organisms are multiplied by the contaminated plant cells. They generally induce metabolic disorders leading to the expression of symptoms (viral diseases). The infection of a crop by a virus implies that it is introduced in a living host cell virus through a wound, from protoplasm to protoplasm or through the intermediary of a living organism called vector.

Anatomy of a virus

Any viral particle is made of at least two recurrent and obligatory elements :

• the genome, varied genetic information composed of DNA and RNA, single strand or double strand , circular or linear.
• the capsid, shell of protein that surrounds the genome. It ensures the protection and the survival of the viral particle in the external environment.

The interactions between the nucleic acid and the protein of the capside are very close. The nucleoprotein structure defined by the nucleic acid and the internal proteins is called nucleoid and the group nucleoid and capsid constitute a functional unity which is called nucleocapsid. The viruses limited to their nucleocapsid are called nude viruses. In some viral families, the nucleocapsid is surrounded by a peripheral optional structure called the envelope.

The dimension and shape of plant viruses vary considerably. However, each particular virus presents particles with constant characteristics. So, the classification of viruses is partially based on the dimensions and the morphology of virions observed through an electronic microscope. The viral particles present isometric or spherical shapes, shapes of small rigid sticks or filamentous shapes.

Return to the page " Viruses and viroids : the world of the invisible"