Based on the picture above: Why is ribose less stable than deoxyribose?

a)    The extra hydroxyl on the ribose allows it to hydrogen bond with many more molecules, which would  decrease the stability of the macromolecule.

b)   The extra hydroxyl would cause the ribose to be in the linear sugar state instead of the cyclized sugar state, which would be unfavorable.

c)    The extra hydroxyl would increase branching and thereby cause more errors in DNA.

d)   The extra hydroxyl would be more likely to covalently bind to water, and thus, prevent DNA replication.

e)    None of the above. 

The correct answer is C. If you recall, the way DNA is polymerized is that you have the 3′ hydroxyl attacking the 5′ phosphate. Now, if we have another hydroxyl, which is present in ribose, then sometimes, you can get the 2′ hydroxyl (hypothetically) attacking the 5′ phosphate and this could actually lead to branching in DNA. Currently, the accepted model is that the DNA is a double helix, but if we had two hydroxyls attacking two different phosphates, you’d get an extremely different shape, which would essentially make DNA replication almost impossible.

A is incorrect because hydrogen bonding is usually associated with increased stability – not decreased stability. 

B is incorrect because sugars tend to prefer to remain in their cyclized state rather than their linear state (think back to those fantastic organic chemistry memories). 

D is incorrect because, although the hydroxyl might interact with water, that interaction is INTERMOLECULAR interaction and has absolutely nothing to do with covalent bonding – which is a permanent bond.