Question

As GC content increases, the value for the Gibbs free energy of melting _______, which indicates that the reaction becomes more __________.

A.      decreases; exergonic

B.      increases; exergonic

C.      decreases; endergonic

D.     increases; endergonic

8.      Therefore, the thermodynamic parameters indicate that the _______ of duplex DNA is enthalpically opposed and entropically favored, while the _________ of duplex DNA is enthalpically favored and entropically opposed (hint: see the discussion of entropy in Chapter 1).

A.      melting; formation

B.      formation; melting

Answer 1

1. The Guanine-Cytosine pairing involves 3 Hydrogen bonding whereas Adenosine-Thymine pairing involves 2 Hydrogen bonds. Therefore GC pairing contributes greater stability to a Double-Stranded DNA. Therefore the Gibbs' Free Energy change during melting increases. As the solid structure is more stabilized the melting protocol becomes Endergonic i.e. involving absorption of energy. Hence, the correct option is B.

2. When a Duplex DNA is formed new Hydrogen bonds are formed. So, Enthalpy change of the process is negative. But as two strands are coiled the entropy of the system decreases. Hence the formation of duplex DNA is enthalpically favored and entropically opposed.

In case of melting the Hydrogen bonds are broken down. So, Enthalpy change of the process is positive. But as two strands are separated the entropy of the system increases. Hence the melting of duplex DNA is enthalpically opposed and entropically favored.

Hence, the correct option is A.