Animal and Human Viruses? Type of Viruses
Type of Viruses
Plant Viruses :
Plant viruses infect plant cells, disturb their metabolism and cause severe diseases in them. Like other viruses, plant viruses are obligate intracellular parasites that do not have molecular machinery to replicate without a host. Viral diseases result in huge losses in crop production and quality. Infected plants show varied symptoms and most often there is leaf yellowing, leaf distortion, etc. The important plant viruses are Tobacco Mosaic Virus (TMV), Potato Virus, Southern Bean Mosaic Virus (SBMV), Turnip Yellow Virus (TYV), etc. Plant viruses are spread mainly by insects, e.g., aphids.
Animal Viruses :
Animal viruses infect the animal cells and cause different fatal diseases in animals including man. Generally, they have a polyhedron or spherical shape. Animal viruses complete their reproductive cycle in animal cells. They enter the host cell through endocytosis or viral penetration, then take over the defense mechanism of the host and hijack the host cellular machinery to synthesize viral components such as nucleic acids and viral proteins. The viral components are then assembled into mature viruses which are released through rupturing host cell or budding the cell membrane. The released virus then infects other cells in the body or remains dormant. The protein coat or capsid of animal viruses is surrounded by an envelope. Certain common viral infections of human beings are the common cold, influenza, mumps, measles, rubella (German measles), chickenpox, small! pox, polio, viral hepatitis, herpes simplex, viral encephalitis,
fever blisters, warts and some types of cancer. Among livestock and fowl, viruses cause encephalitis, foot and mouth disease, fowl plague, Newcastle disease, pseudorabies, hog cholera and a variety of warts and other tumors. A virus usually displays some specificity
for a particular animal group.
Bacterial Viruses or Bacteriophages :
Viruses that infect the bacterial cells are called bacteriophages or simply phages. The phages have specific hosts and they are of variable shapes, sizes, and structures. The most widely studied phages are T-even bacteriophages such as T2,T4,T6, etc., which infect the colon bacillus, Escherichia coli and are also known as coliphages.
T4 bacteriophage is a large-sized tadpole-shaped complex virus. The capsid of the virus consists of an icosahedral head, a short neck with a collar and a long helical tail. The tail is made up of a thick and hollow mid-piece, a hexagonal base plate or end plate to which six spikes and six long tail fibers are attached. A dsDNA is present inside the head.
Viruses are Intermediate Between Living and Non-Living
Viruses are considered as intermediate between living and non-living entities. Viruses do not give rise directly to new viruses. They are not cellular, although they multiply their multiplication is very different from cell replication mechanism.
They resemble non-living objects in -
(1) lacking protoplast.
(2) ability to get crystallized, e.g., TMV, Poliomyelitis Virus.
(3) inability to live independent of a living cell.
(4} high specific gravity which is found only in non-living objects.
(5) absence of respiration.
(6) absence of an energy storing system.
(7) absence of growth and division. Instead of different parts of viruses are synthesized separately.
They resemble living objects in-
(1) presence of genetic material
(2) formed of organic macromolecules
(3) ability to multiply or reproduce
(4) the occurrence of mutation(the most mutable virus is HIV)
(5) occurrence of enzyme transcriptase in most viruses
(6) some viruses contain vitamins like riboflavin and biotin (@.g., Poxvirus)
(7) the occurrence of antigenic properties
(8) infectivity and host specificity
(9) they breed true to their type.