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12.22
A greater explanation of the Virus
 
  Virus (biology) (Latin, "poison"), any of a number of organic entities consisting simply of genetic material surrounded by a protective coat. A virus is a submicroscopic infectious particle composed of a protein coat and a nuclei acid (DNA or RNA) core. Viruses, like cells, carry genetic information encoded in their nuclei acid, and can undergo mutations and reproduce; however, they cannot carry out metabolism, and thus are not usually considered alive.  
  A virus is simply a number of genes, embodied in a nucleic acid (either DNA or RNA) genome, depending upon the type of virus, wrapped up in a protein or membrane coat. By themselves, however, viruses do not grow and divide. All viruses are parasites; they need to enter the environment of a host cell to be able to replicate. Some viruses infect bacterial cells and others infect the cells of higher multi cellular organisms such as plants and animals. Some viruses, such as the influenza virus and the human immune deficiency virus ( HIV) associated with AIDS can appear to be very clever in their capacity to rapidly mutate to evade the immune system.  
12.14.1 Origin of the concept and discovery of the Virus  
  The term "virus" was first used in the 1890s to describe agents that caused diseases but were smaller than bacteria. By itself a virus is a lifeless form, but within living cells it can replicate many times and harm its host in the process. The hundreds of known viruses cause a wide range of diseases in humans, other animals, insects, bacteria, and plants.  
  The existence of viruses was established in 1892, when Russian scientist Dmitry I. Ivanovsky discovered microscopic particles later known as the tobacco mosaic virus. The name virus was applied to these infectious particles in 1898 by the Dutch botanist Martinus W. Beijerinck. A few years later, viruses were found growing in bacteria; these viruses were dubbed bacteriophages. Then, in 1935, the American biochemist Wendell Meredith Stanley crystallized tobacco mosaic virus and showed that it is composed only of the genetic material called ribonucleic acid (RNA) and a protein covering. In the 1940s development of the electron microscope made visualization of viruses possible for the first time. This was followed by development of high-speed centrifuges used to concentrate and purify viruses. The study of animal viruses reached a major turning point in the 1950s with the development of methods to culture cells that could support virus replication in test tubes. Numerous viruses were subsequently discovered, and in the 1960s and 1970s most were analyzed to determine their physical and chemical characteristics.  
12.14.2 Types of Virus  
  Viruses vary considerably in size and shape. Three basic structural groups exist: isometric; rod shaped or elongated; and tadpole-like, with head and tail (as in some bacteriophages). The smallest viruses are icosahedrons (20-sided polygons) that measure about 18 to 20 nanometres wide (one-millionth of a millimetre = 1 nanometre). The largest viruses are rod shaped. Some rod-shaped viruses may measure several microns in length, but they are still usually less than 100 nanometres in width. Thus, the widths of even the largest viruses are below the limits of resolution of the light microscope, which is used to study bacteria and other large micro-organisms.  
  Many of the viruses with helical internal structure have outer coverings (also known as envelopes) composed of lipoprotein or glycoprotein, or both. These viruses appear roughly spherical or in various other shapes, and they range from about 60 to more than 300 nanometres in diameter. Complex viruses, such as some bacteriophages, have heads and a tubular tail, which attaches to host bacteria. The pox viruses are brick shaped and have a complex protein composition. Complex and pox viruses are exceptions, however; most viruses have a simple shape.  
12.14.3 Virus Replication  
  Viruses do not contain the enzymes and metabolic precursors necessary for self-replication. They have to get these from the host cells that they infect. Viral replication, therefore, is a process of separate synthesis of viral components and assembly of these into new virus particles. Replication begins when a virus enters the cell. The virus coat is removed by cellular enzymes, and the virus RNA or DNA comes into contact with ribosomes (cell organs that synthesize proteins) inside the cell. There the virus RNA or DNA directs the synthesis of proteins specified by the viral nucleic acid. The nucleic acid replicates itself, and the protein subunits constituting the viral coat are synthesized. Thereafter, the two components are assembled into a new virus. One infecting virus can give rise to thousands of progeny viruses. Some viruses are released by destruction of the infected cell. Others are released by budding through cell membranes and do not kill the cell. In some instances, infections are "silent"-that is, viruses may replicate within the cell but cause no obvious cell damage.  
  The RNA-containing viruses are unique among replicative systems in that the RNA can replicate itself independently of DNA. In some cases, the RNA can function as messenger RNA (see Genetics), indirectly replicating itself using the cell's ribosomal and metabolic precursor systems. In other cases, RNA viruses carry within the coat an RNA-dependent enzyme that directs the synthesis of virus RNA. Some RNA viruses, which have come to be known as retroviruses, may produce an enzyme that can synthesize DNA from the RNA molecule. The DNA thus formed then acts as the viral genetic material.  
  Bacterial viruses and animal viruses differ somewhat in their interaction with the cell surface during infection. The "T even" bacteriophage that infects the bacterium Escherichia coli, for instance, first attaches to the surface and injects its DNA directly into the bacterium. No absorption and un coating take place. The basic events of virus replication, however, are the same after the nucleic acid enters the cell.  
  The DNA or RNA of the virus is replicated in the cell and vital proteins are made. mature progeny virus particles are assembled and are exported from the cell, where they go on to infect other cells. Some really dangerous viruses, like influenza, kill the cell as they replicate. Others coexist peacefully within the living cell they infect and continuously bud from the cell membrane as progeny viruses. Still others, like herpes, may lie dormant (latent) within living cells for years before replicating and emerging. However, the ultimate viral parasites are the retroviruses.  
  Some like HIV, make a DNA copy of their RNA genome (by a process called reverse transcription) and insert or integrate this DNA copy into the chromosome(s) in the cells nucleus where it is replicated as part of normal cell division.  
     
 
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