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The adjective viral dates to History of virology and Social history of viruses Louis Pasteur was unable to find a causative agent for rabies and speculated about a pathogen too small to be detected using a microscope.
Thus, he could pass a solution containing bacteria through the filter and completely remove them. His experiments showed that crushed leaf extracts from infected tobacco plants remain infectious after filtration.
Ivanovsky suggested the infection might be caused by a toxin produced by bacteria, but did not pursue the idea. Beijerinck maintained that viruses were liquid in nature, a theory later discredited by Wendell Stanleywho proved they were particulate.
He accurately diluted a suspension of these viruses and discovered that the highest dilutions lowest virus concentrationsrather than killing all the bacteria, formed discrete areas of dead organisms. Counting these areas and multiplying by the dilution factor allowed him to calculate the number of viruses in the original suspension.
The study of phages provided insights into the switching on and off of genesand a useful mechanism for introducing foreign genes into bacteria. By the end of the 19th century, viruses were defined in terms of their infectivitytheir ability to be filtered, and their requirement for living hosts.
Viruses had been grown only in plants and animals. InRoss Granville Harrison invented a method for growing tissue in lymphand, inE. Lambert used this method to grow vaccinia virus in fragments of guinea pig corneal tissue.
Maitland grew vaccinia virus in suspensions of minced hens' kidneys. Their method was not widely adopted until the s, when poliovirus was grown on a large scale for vaccine production.
This work enabled Jonas Salk to make an effective polio vaccine. The first X-ray diffraction pictures of the crystallised virus were obtained by Bernal and Fankuchen in On the basis of her pictures, Rosalind Franklin discovered the full structure of the virus in Inthe hepatitis B virus was discovered by Baruch Blumberg and inHoward Temin described the first retrovirus.
Viral evolution Viruses are found wherever there is life and have probably existed since living cells first evolved. This provides an invaluable source of information for paleovirologists to trace back ancient viruses that have existed up to millions of years ago.
There are three main hypotheses that aim to explain the origins of viruses: Over time, genes not required by their parasitism were lost. The bacteria rickettsia and chlamydia are living cells that, like viruses, can reproduce only inside host cells.
They lend support to this hypothesis, as their dependence on parasitism is likely to have caused the loss of genes that enabled them to survive outside a cell.
This is also called the degeneracy hypothesis,   or reduction hypothesis. The escaped DNA could have come from plasmids pieces of naked DNA that can move between cells or transposons molecules of DNA that replicate and move around to different positions within the genes of the cell.
They were discovered in maize by Barbara McClintock in Viroids are molecules of RNA that are not classified as viruses because they lack a protein coat.A sentence that follows the customary word order of English sentences, i.e., subject-verb-objects. The main idea of the sentence is presented first and is then followed by one or more subordinate clauses.
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