Lecture 04 2B03 2017.pptx - Lecture 4 Nucleotides and...

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Lecture 4 Nucleotides and Nucleic Acids Part III 1
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Thermostability of DNA duplex At high temperature, the UV absorbance of DNA increases by 30-40% This hyperchromic shift reflects the unwinding of DNA duplex – melting or denaturation The stability of DNA can be measured by Tm (melting temperature) at which 50% duplex is melt Higher GC content, higher Tm, more stable the duplex 2
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Hydrogen bonds in an A-T pair 2 Hydrogen bonds between A and T
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Hydrogen bonds in a G-C pair 3 Hydrogen bonds between G and C Base-stacking also contributes to increased Tm
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Denaturation and renaturation of DNA Denatured DNA will re-nature to re-form the duplex structure if the denaturing conditions are removed (increase temp) Renaturation requires re-association of the DNA strands into a double helix, a process also termed re-annealing (cooling temp) For this to occur, the strands must realign so that their complementary bases are once again in register and the helix can be “zippered up” 5
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DNA supercoils In duplex DNA, there are ~10 bp per turn of helix Confined DNA (such as circular DNA) will supercoil
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