Morphology and chemical composition of animal and plant viruses

MORPHOLOGY AND CHEMICAL COMPOSITION OF ANIMAL AND PLANT VIRUSES

Plant and animal viruses do not have the familiar tadpole shape of some bacteriophages. Instead, they vary greatly in size . In fact, size and morphology are characteristic properties of each kind of virus.

Morphology

Animal and plant viruses that appear polyhedral or spherical have icosahedral symmetry, with the basic framework that is an icosahedron, as described previously. Viruses with helical symmetry have a capsid whose capsomeres are packet as a spiral around the nucleic acid to form a helix.Plant viruses with helical symmetry appear as rods. The causative agents of measles, mumps, influenza, and rabies have helical symmetry.

In additional to those whose helical or icosahedral symmetry, there also are plant and animal viruses with complex or uncertain symmetries. These viruses have nucleic acid within the virion core; either there is no discrete structure enclosing the nucleic acid or the nucleic acid is surrounded by multiple unique membranes.

Chemical composition

In addition to a nucleic acid core, the majority of animal viruses and many plant viruses have a protein capsid enclosed by an envelope made of lipids, proteins, and carbohydrates . (Only a few phage groups have a lipid-containing envelope). The envelope has surface projections, or spikes, made of glycoprotein. These projections may be important in viral attachment to host cells, and are studied as possible components of viral vaccines. Virions that have envelopes can be destroyed by lipid solvents such as either and chloroform. The infectivity of viruses is thus inactivated by these chemical solvents. Nonenveloped virions are referred to as naked virion; their capacity to infects cells is not affected by the lipid solvents.

Whether or not they have an envelope, most viruses have symmetrical shapes.

CLASSIFICATION AND NOMENCLATURE OF ANIMAL AND PLANT VIRUSES

There have been many classification schemes for animal and plant viruses used through the years. One of the earliest methods, still in limited use, based classification on the kind of host the normally infect (e.g., hog cholera virus, swine influenza virus, fowl plague virus, cucumber mosaic virus, tobacco mosaic virus; for example, viruses that attached to nerve cells were called neurotropic viruses and those specific for skin were called somatotropic viruses. This method was useful health investigators. However, as knowledge of the physical, chemical, and biological characteristics of viruses has accumulated, classification schemes based on more fundamental biological properties have been formulated.

System of Classification Base on physical, chemical, and Biological Characteristics

Physical, chemical and biological characteristics of viruses have been used by International committee on Taxonomy of Viruses (ICTV) to classify viruses. Like the bacterial virologists, animal virologists have subscribed to a nomenclature for various taxa. Family names agreed upon end in –viridae, subfamily names in –virinae, and genera, like species, in –virus. Prefixes to the family endings connote descriptions of the family characteristics. For example, Picornaviridae means small (pico) RNA viruses; Hepadnaviridae means “causing liver disease” (heap) DNA viruses. Other family names refer to historical origins.

However, the plant virologists have not classified their viruses in a similar manner. They use groups of viruses (instead of families and genera) that share similar properties. Names for these groups are usually derived from the name of the prototype, or most representative member, of the group. For example, the name of the group of viruses related to tobacco mosaic virus is the tobacco group or tobamoviruses. The classification of viruses that infect animals arranged according to symmetry and in descending order to sizes, and are line drawings showing how basic morphology is used to classify animal and plant viruses. An alternative system of classification.

BACTERIOPHAGES LIFE CYCLES

There are two main types of bacteriophages: lytic (or virulent) and temperate (or avirulent). Lytic phages destroy their host bacterial cells. In the lytic infective process, after replication of the virion, the host cell bursts, or lyses, releasing new progeny phages to infect other host cells. This is called Lytic cycle.

Temperate phages do not destroy their host cells. Instead, in the temperate type of infection, the viral nucleic acid is integrated into the host-cell genome and replicated in the host bacterial cells from one generation to another without any cell lysis. This process is called Lysogeny and is carried out only phages containing double-stranded DNA. However, under the right circumstances, temperate phages may occasionally become spontaneously lytic in some subsequent generation and lyse the host cells.

Host cells infected with filamentous phages generally do not release assembled phages by out folding of the cell wall. As a viral DNA is extended through the membrane, it picks up protein molecules (newly synthesized as well as those derived initially from the infecting virions). Such an extrusion process does not cause damage to the cell, which can continue to produce bacteriophages for a long time.

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.