, Diagnosis an-&& of Shrimp Diseases - Central Institute of ...

hydrolyzed by dilute alkali. The sugar moiety contains 2' hydraxj4 group. Dilute sodiumhydroxide produces a mixture of nucleosides, 2' and 3"phsphates. Cyclic 2', 3'lnonophosphates are the first products of the action of alkali on RNA. They are furtherhydrolyzed by ailtali, which attacks either one of tile two P-0-C linkages to yield a mixture of2' and 3' nucleoside phosphates. Since DXA has no 2' OH group it can not be hydrolyzed byalkali.DNA can be denatured by heat a~zd acids but not by alkaliGentle acid hydrolysis of DNA at pH 3.0 causes selective hydrolytic removal of all its purinebases without affecting the pyrimidine deoxyribose bonds or the phosphodiester bonds of thebackbone. The resulting DNA derivative, which is devoid of purine bases is called apurinicacid. Selective removal of the pyrimidine baes, accomplished by some what differentchemical conditions produces apyrimidinic acid. As mentioned above alkali cannot hydrolyzethe DNA. Nucleases also hydrolyze the nucleic acids. Nucleic acids can be denatured byheating. This process is called melting. When nucleic acid is denatured the stacking is lostand hence the UV-absorbance increases. This increase is called hyperchromic shift. For ,total6denaturation of the double stranded DNA, the hyper chromic effect is of the order of 30%.The temperature at which the solution contains 50% denatured and 50% double strandedDNA is called the melting temperature (Tm) .The value of Tm is the function of the nature ofthe DNA, ions in the solutions and the ionic strength. Renaturation is not simply the reversalof denaturation. If the denatured DNA solutions are maintained 5-10 "C below the meltingtemperature, the comple~nentary strands will slowIy reassociate and the double helicalstructures will reform. The whole process is concentration dependent. Thermal denaturation isone of the properties exploited in polymerase chain reaction (PCR).DNA replicates by semi-conservative mechanisntThe most striking feature of the Watson-Crick model of DNA,from the genetic point of viewis, that the two strands of double helical DNA are complementary. The replication of each toform new complementary strands results in formation of two daughter duplex DNAZmolecules, each of which. contains one strandfiom the parental DNA. This process is calledsemi conservative replication. This model was conclusively proved by Meselson and Stahl in1957 by ingenious experiment using bacteria. The same is true in other dividing cells.