General characteristics of viruses

General characteristics of viruses

Viruses are widely distributed in nature there are viruses that infect animal or plant cells and others that infect microorganisms. Whatever type of host cell they invade all viruses are obligate parasites. That is, they can reproduce only within a metabolizing host cell, using the protein-synthesizing and energy-generating systems of the cell. However, viruses differ in their degree of dependence on the host cell for replication, which is the production of new viruses inside the host cell. For instance, some viruses that infect bacteria-called bacteriophages, or phages-have fewer than 10 genes and depend almost entirely on bacterial cell functions for replication. Other have 30 to 100 genes and are more independent of host-cell functions.

Replication is not the only viral process that must involve the host cells. Viruses lack metabolic machinery of their own to generate energy or to synthesize proteins, and so they depend on host cells to carry out their vital functions. Once inside the cell, viruses have genes for usurping the host cell’s energy-generating and protein-synthesizing systems. In additional to their intracellular form, viruses have and extracellular form that carries the viral nucleic acid from one host cell to another. In this infection form, viruses are simply small packets of genes wrapped in a protein coat. The coat protects the genes outside the host cell; it also serves as a vehicle for entry into another host cell because it binds to receptors on cell surfaces. The structurally mature, infectious viral particle is called a virion.

Outside the host cell, in the extracellular form, the virion is inert; that is, it is not metabolizing or replicating. Thus viruses have been considered nonliving entities. However, once inside a host cell, the nucleic acid becomes active and the virus “comes to life.” A host organism needs no convincing that the virus is inactive entity-just ask anyone afflicted with the miseries of the common cold virus! Thus, during replication in the cells, viruses may cause disease just as bacteria, fungal, and protozoan pathogens do.

Basic Morphology of Viruses

With the electron microscope, it is possible to determine the morphological characteristics of viruses. The virions range in size from 20 to 300 nm (1nm = 1/1000 /Am). They thus represent the smallest and simplest of infectious agents. Since most viruses measure less than 150nm, they are beyond the limit of resolution of the light microscope and are visible only by use of the electron microscope. By using materials of known size for comparison, microscopists can use electron microscopes to determine the size and structure of individual virions.

Viruses are composed of a center core of nucleic acid surrounded by a protein coat called a capsid . The viral proteins self-assemble to give the capsid its characteristic symmetry- usually either icosahedral or helical. The nucleic acid and capsid together constitute the nucleocapsid of the virion.

Most viruses that appear polyhedral or spherical have a capsid whose basic framework is that of an icosahedron, which means that its surface is made of 20 triangular facets and 12 vertices. Each triangular facet is an equilateral triangle; these facets come together to form the 12 vertices . In viral capsid , the viral proteins (protomers) form the clusters known capsomeres, which are visible by electron microscopy. In the larger and more complex polyhedral capsids, the triangular facets of the basic icosahedrons are subdivided into a progressively larger number of equilateral triangles. Thus, a capsid may be composed of hundreds of capsomeres, but it is still based on the simple icosahedrons model. The total number of capsomeres forming the capsid is characteristic of each virus group. The polyhedral heads of some phages are greater in length than in. Width, resulting in a distorted icosahedral shape, and may have trails attached.

Viruses with helical symmetry have a capsid whose capsomeres are packed with the nucleic acid in the form of a helix. Plant viruses with helical symmetry are typically rod-shaped. One of the first viruses studied by electron microscopy is the now well-known tobacco mosaic virus, which is helical virus. Animal viruses with helical capsids include the causative agents of measles, mumps, influenza, and rabies. In these viruses, the capsid is enclosed by a lipoprotein envelope with radially projecting spikes.

There are animal viruses with complex or uncertain symmetries. For example, the arenaviruses and the poxviruses have no recognizable capsid symmetry. Although these viruses have nucleic acid within the virion core , either there is no discrete structure enclosed it (as in the arenaviruses) or it is surrounded by multiple unique membranes (as in the poxviruses.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.