The formation of double helix from two short single stranded DNA molecules CGCGA

Question

The formation of double helix from two short single stranded DNA molecules CGCGATAT and ATATCGCG is spontaneous in 1 M NaCl at pH 7.0 and 25 C. The delta G for the reaction is -54 kJ/mol and 251 kJ/mol of heat is released from the reaction. Please calculate the entropy change of this reaction and explain why it is negative. What kind of noncovalent interactions are involved in the formation of the double helix? You can break up the DNA double helix by heating the solution, at what temperature will this double helical DNA start to melt? (Hints: the delta H of the reactions is -251 kJ/mol since the heat is released rather than absorbed in the reaction. Use the free energy equation to do the calculations).

Explanation / Answer

Intermolecular attractions involves in formation of double helix

DG = DH-TDS

DG = -54 kj/mol

DH = -251 kj/mol

T = 25 c =298 k

(-54*1000) = ((-251*1000)-(298*DS))

DS = -661.07 j/k.mol

so that for the reaction both DH,DS is negative.

if T>DH/DS .The reaction is non-spontaneous.so that opposite reaction will stats

becoming spontaneous.then double helical DNA start to melt.

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