Formatting Life’s Hard Drive:
The Structure of DNA and RNA

Sweet and Sour: the structure of nucleotides

In the previous issue, we sort of glossed over the structure of nucleic acids. This time we’re going to dig right in. Nucleic acids are polymers made up of nucleotides. Nucleotides are themselves fairly complex molecules, but as usual your Old Quantum Butcher is going to make it easy for you. A nucleotide is nothing more than a weak organic base connected to a five carbon sugar and a phosphate group. That’s it. The sugar in a nucleotide is either ribose, in which case you’ve got a ribonucleotide, or deoxyribose, in which case you’ve got a deoxyribonucleotide. The base can be one of five organic alkaloids—adenine, cytidine, uracil, thymidine or guanidine—A, C, T, G or U. The deoxyribonucleotides thus formed are called deoxyadenosine, deoxycytosine, deoxythymosine, and deoxyguanosine. If you're forming ribonucleotides you just say adenosine, etc. Uridine is used only as a ribonucleotide (which means it only shows up in RNA). The abbreviations for these nucleotides are A, C, T, G or U. The phosphate in the nucleotide is just that, a group composed of phosphorous bound to multiple oxygens. This phosphate group is acidic, which, as we discussed in Fundamentals I, just means that it can donate hydrogen to a water solution. Now, the bases in nucleotides, like all bases, can remove hyrdogen from solution, but these are weak bases, and the phosphate in nucleotides can donate hydrogens faster than the bases can suck it up—which means that solutions of nucleotides, or polymers of nucleotides, are on balance acidic.

Figure 4. Basic structure of nucleotides. All nucleotides are composed of a five carbon sugar, ribose or deoxyribose, bound to a phosphate group and an organic base. Different varieties of organic base yield different varieties of nucleotides.