What will happen to DNA if salt is added to it: (PGI Dec 2008)
Correct Answer: Increase Melting point (Tm)
Description: Ans : A (Increase Melting point (Tm)) & D (Melti ng lead to denaturation of DNA) "A ten fold Increase of monovalent cation increases the Tm by 16.6!'C"- Harper 27th313Denaturation & TM of DNADefinition#"The strands of a given molecule of DNA separate over a temperature range. The midpoint is called melting point (T J" It is also defined as "the temperature at which DNA Is half denatured# At this temperature, the absorbance of DNA at 260 nm is increased by 18.5% i.e. half of 37% increased in absorbance when DNA is completely denatured. This phenomenon is called hyperchromicity or hyperchromic effect.Factors Affecting TmTm depends on both the length of molecule & specific nucleotide sequence composition of that molecule.DNA rich in G-C pairs which have 3-H bond melts at higher temperature than that rich in A-T pairs ,which have 2-H bond.A tenfold increase of monovalent cation concentration increase the Tm by 16.6 degCFormamideQ which commonly used in recombinant DNA technology & urea , both of them destablise H-bonds & thus lower Tm.How' ionic concentration Affects Tm : The ion concentration also influence the Tm because the negatively charged PO4. in the two strands are shielded by positively charged ion. When ionic concentration is low this shielding is decreased, thus increasing the repulsive force b/w strands & reducing TmWhat is Annealing:Once the strands are separated they can be renatured. If the melted sample of DNA is slowly cooled the absorbance of the solution decreases. This is indicator of complementary strands being paired again. This process is called as annealing. Annealing can occur only at a temperature below Tm of DNA which is about 7GdegC. It is fastest at 20degC below (Tm) or 50degC.Application of Tm pf DNA# Amount of G & C (Guanine & Cytosine) in a genome (called "GC content") can be estimated by measuring the temperature at which the genomic DNA melts. Higher (Tm ) value mean higher GC content.# Also used in molecular biology techniques notably in PGR.# Used as a proxy for equalising the hybridisation strengths of a set of molecule e.g. the oligonucleotide probe of DNA microarray.Used to detect sequence differen ce b/w stran ds & d iffe rent DNA sequence.Also KnowDenaturation & renaturation of DNA are basis of nucleic acid hybridisation - Molecular Biology by Lodlsh 6th/117Denaturation of DNA is also called as "a helix to coil transition " - Chatterjea & Shinde 7th/228Tm value is calculated by - Nearest Neighbor methodfhttp:// en. wikipedia.org)
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