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Essentials of Anatomic Pathology

Essentials_of_Anatomic_Pathology

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tulika goyal

biology

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shannu jain

biology

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shannu jain

DNA-THE CENTRAL DOGMA

he start of a distinct modern molecular biology dates from the discovery of the structure of DNA by Watson and Crick. In his book, The Double Helix, Jim Watson writes: "Back" in my rooms I lit the coal fire, knowing there was no chance that the sight of my breath would disappear before I was ready for bed. With my fingers too cold to write legibly I huddled next to the fireplace, daydreaming about how several DNA chains could fold together in a pretty and hopefully scientific way. Soon, however, I abandoned thinking at the molecular level and turned to the much easier job of reading biochemical papers on the interrelations of DNA, RNA and protein synthesis. Virtually all the evidence then available made me believe that DNA was the template upon which RNA chains were made. In turn, RNA chains were the likely candidates for the templates for protein synthesis. There was some fuzzy data using sea urchins, interpreted as a transformation of DNA into RNA, but I preferred to trust other experiments showing that DNA molecules, once synthesized, are very stable. The idea of the genes' being immortal smelled right, and so on the wall above my desk I taped up a paper sheet saying DNA -> RNA -> protein. The arrows did not signify chemical transformations, but instead expressed the transfer of genetic information from the sequences of nucleotides in DNA molecules to the sequences of amino acids in proteins"  The term Central Dogma is ascribed to Frances Crick. In his book, What Mad Pursuit, he describes why he used this term: "The other theoretical idea I proposed was of a rather different character. I suggested that "once 'information' has passed into protein it cannot get out again," adding that "Information means here the precise determination of sequence, either of bases in the nucleic acid or of amino acid residues in the protein. I called this idea the central dogma, for two reasons, I suspect. I had already used the obvious word hypothesis in the sequence hypothesis, and in addition I wanted to suggest that this new assumption was more central and more powerful. I did remark that their speculative nature was emphasized by their names." Nearly 20 years ago, biochemists found that a separable constituent of the cell Deoxyribonucleic acid or DNA appeared to guide the cell's protein synthesizing machinery. The internal structure of DNA seemed to represent a set of coded instructions which dictated the pattern of protein synthesis. Experiments indicated that in the presence of appropriate enzymes each DNA molecule could form a replica, a new DNA molecule containing the specific guiding message present in the original. This idea when added to what was already known about the cellular mechanisms of heredity appeared to establish a molecular basis for inheritance. Proponents of the theory that DNA was a self duplicating molecule, containing a code that by itself determined biological inheritance, introduced the term 'central dogma' into scientific literature in order to describe the principles that were supposed to explain that the reverse effect is impossible, proteins cannot guide the synthesis of nucleic acids. But actual experimental observations deny the second and crucial part of this assumption. Other test-tube experiments show that agents besides DNA have a guiding influence. The kind of protein made may depend on the specific organism from which the necessary enzyme is obtained. It also depends on the test tube's temperature, the degree of acids and the amount of metallic salts present. The central dogma banishes from consideration the interactions among the numerous molecular processes that have been discovered in cells or in their extracted fluids. In the living cell, molecular processes - the synthesis of nucleic acids and proteins or the oxidation of food substances - are not separate but interact in exceedingly complex ways.  No matter how many ingredients the biochemists test tubes may contain the mixtures are nonliving; but these same ingredients organized by the subtle structure of the cell constitute a system which is alive.

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apeksha rajkumari