bioc441 - Lecture2_2008

bioc441 - Lecture2_2008 - 1 2 Nothing in biology makes...

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3 “Nothing in biology makes sense except in the light of evolution” T. Dobzhansky
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Biochemistry 441 Lecture 2 Ted Young January 9, 2008 The utility of complementarity Biochemistry 441 Lecture 2 Ted Young January 9, 2008 The utility of complementarity Denaturation and renaturation of nucleic acids
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5 Denaturation of nucleic acids: ds DNA Denaturation of nucleic acids: ds DNA The forces that hold the two strands of DNA together are all weak forces and therefore the two strands can be easily separated. Common denaturing agents in the lab: Heat, high pH, and strong H-bonding agents such as urea and formamide. ((Why not water?)) Why not low pH? To function in vivo as a template for DNA and RNA synthesis the two strands must be separated using enzymes known as DNA helicases.
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6 Denaturation of nucleic acids: ds DNA Denaturation of nucleic acids: ds DNA The denaturation or “melting” of double- stranded DNA occurs over a very narrow temperature range, indicating that it is a highly cooperative process. The temperature at the midpoint of the melting curve is known as its melting temperature or T m .
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7 Energetics of denaturation Energetics of denaturation helix coil One molecule two molecules, Few conformations many conformations Δ G = Δ H - T Δ S For the helix to be stable Δ G must be >0. Δ S = S coil - S helix > 0 therefore T Δ S is negative. Therefore, for the helix to be stable Δ H must be larger in a positive sense than T Δ S is negative. Δ H > T Δ S Δ H is positive because there are strong interactions between bases-both H-bonds and base stacking due to van der Waals interactions between the bases. These must be stronger than the repulsive forces of the
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bioc441 - Lecture2_2008 - 1 2 Nothing in biology makes...

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