As you study Figure 4, note that a nucelotide is assymetric—it has a hydroxyl (OH) group sticking off the sugar on one side and a phosphate sticking off the other.

Organic chemists have arcane numbering systems for molecules that we don’t need to worry about. Just take my word for it: the hydroxyl group is attached to the 3’ carbon and the phosphate to the 5’ carbon. These designations are going to be helpful when we start hooking nucleotides up into polymers, which is right now.