How is it possible for both DNA synthesis and hydrolysis to be exergonic?

In exergonic reactions, free energy is released; in endergonic ones, it is absorbed.

DNA hydrolysis involves the breakdown of DNA via water and applicable enzymes. There is energy holding the bonds together, so when they're broken, that energy is released.



The first step in DNA hydrolysis involves bringing the non-bonded electron pairs on the oxygen atom of water into very close proximity of one of the phosphorus atoms in the DNA backbone. As we have previously seen, electrons repel each other. When the non-bonding electron pairs of an oxygen atom approach the phosphorus atom in a phosphate group, they are strongly repelled by the negative charges surrounding the phosphate oxygens. In other words, the abundance of electron density on the phosphate oxygens effectively shields the phosphorus atom from being attacked by water. Indeed, when DNA is chemically modified so as to remove the phosphate negative charges, it becomes much more prone to hydrolysis.