Although this reaction is energentically favorable (meaning it releases rather than stores energy) it’s important to realize that it doesn’t just happen. The formation of DNA would proceed much too slowly to be useful in living systems if it weren’t for an enzyme called DNA polymerase, which lives to hook up one nucleotide to another. In the case of RNA, the responsible enzyme is called RNA polymerase. We’ll be talking more about these enzymes when we examine the processes of DNA replication and transcription.

Figure 7. Get a spine. Nucleic acids have a sugar-phosphate backbone and two different ends.

Our brand new nucleic acid molecule, whether it’s two nucleotides long or two million, has the same overall geometry. It’s a linear molecule, in which the bases are linked to each other through the formation of bonds between the 5-carbon sugar and phosphate. Thuse, we have a sugar-phosphate backbone, from which the bases stick out like ribs.