Question

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).

melting; formation

formation; melting

Answer 1

The thermodynamic parameters indicate that the formation of duplex DNA is enthalpically opposed and entropically favored, while the melting of duplex DNA is enthalpically favored and entropically opposed.

The entropy of a system decreases when 2 single stranded DNA molecules come together and form a double stranded molecule.When two strands of DNA come together, they form weak chemical bonds called hydrogen bonds between the nucleotides in the two chains. Bond formation releases energy and thus contributes to a net increase in entropy. An additional entropy boost comes from interactions between the nucleotides in the center of the helix; these are called base-stacking interactions. The negatively-charged phosphate groups in the backbone of the DNA strands repel each other. However, this destabilizing interaction is overcome by the favorable hydrogen bonding and base-stacking interactions. This is why the double-helix structure is more stable than single strands: its formation causes a net gain in entropy. In contrast, DNA breakup leads to a decrease in entropy as indicated by absorption of heat.