भाइरस उत्पतिकाे इतिहास || how the virus came into existence || Bishwo Ghatana

Our Next youtube channel : All History:    • The richest Man on Earth - Mansa Musa || स...   Follow on Facebook:   / bishwoghatna   Follow on twitter:   / bishwoghatana   The Basics of Viruses We know that viruses are quite diverse. Unlike all other biological entities, some viruses, like poliovirus, have RNA genomes and some, like herpesvirus, have DNA genomes. Further, some viruses have single-stranded genomes, while others have double-stranded genomes. Their structures and replication strategies are equally diverse. Viruses, do, however, share a few features: First, they generally are quite small, with a diameter of fewer than 200 nanometers. Second, they can replicate only within a host cell. Third, no known virus contains ribosomes, a necessary component of a cell's protein-making translational machinery. Are Viruses Alive? A schematic diagram shows two groups of organisms: capsid-encoding organisms and ribosome-encoding organisms. The two groups are shown on a sphere that is divided in half, and lines radiating from where the two halves meet represent different taxa within the two groups. The top half of the sphere represents the capsid-encoding organisms, which are viruses, and include viruses of Archaea, viruses of Bacteria, and viruses of Eukarya. The bottom half of the sphere represents ribosome-encoding organisms and includes Bacteria, Archaea, and Eukarya. View Full-Size ImageFigure 1 To consider this question, we need to have a good understanding of what we mean by "life." Although specific definitions may vary, biologists generally agree that all living organisms exhibit several key properties: They can grow, reproduce, maintain internal homeostasis, respond to stimuli, and carry out various metabolic processes. In addition, populations of living organisms evolve over time. Do viruses conform to these criteria? Yes and no. We probably all realize that viruses reproduce in some way. We can become infected with a small number of virus particles — by inhaling particles expelled when another person coughs, for instance — and then become sick several days later as the viruses replicate within our bodies. Likewise we probably all realize that viruses evolve over time. We need to get a flu vaccine every year primarily because of the influenza virus changes, or evolves, from one year to the next. Viruses do not, however, carry out metabolic processes. Most notably, viruses differ from living organisms in that they cannot generate ATP. Viruses also do not possess the necessary machinery for translation, as mentioned above. They do not possess ribosomes and cannot independently form proteins from molecules of messenger RNA. Because of these limitations, viruses can replicate only within a living host cell. Therefore, viruses are obligate intracellular parasites. According to a stringent definition of life, they are nonliving. Not everyone, though, necessarily agrees with this conclusion. Perhaps viruses represent a different type of organism on the tree of life — the capsid-encoding organisms, or CEOs. Where Did Viruses Come From? There is much debate among virologists about this question. Three main hypotheses have been articulated: 1. The progressive, or escape, the hypothesis states that viruses arose from genetic elements that gained the ability to move between cells; 2. the regressive, or reduction, hypothesis asserts that viruses are remnants of cellular organisms; and 3. the virus-first hypothesis states that viruses predate or coevolved with their current cellular hosts. The Progressive Hypothesis A schematic diagram shows the steps in the replication of a retrotransposon. DNA and RNA molecules are depicted as different colored rectangles, and transcription, translation, and reinsertion steps are shown with arrows. A schematic diagram shows the life cycle of a retrovirus as it infects a eukaryotic host cell, replicates, and releases its progeny. The virus's primary structural features are shown along with a simplified outline of a host cell with a nucleus. The steps in the process are depicted with key molecules and labeled arrows. Bar-headed lines indicate points where treatments can inhibit steps in the pathway. Music Credit : Martian Cowboy Kevin MacLeod (incompetech.com) Licensed under Creative Commons: By Attribution 3.0 License http://creativecommons.org/licenses/b... http://incompetech.com/ http://audionautix.com/ http://audionautix.com/