Nucleic acids (DNA and RNA) are heterologous polymers of nucleosides monophosphates (Figure 4.3). In order to help you remember the difference between nucleosides and nucleotides, keep in mind the following:

1. Nucleosides are composed of a sugar and a base.

2. Nucleotides are composed of a sugar, a base, and a phosphate.

3. Nucleoside monophosphates are the same as a nucleotides.

The last point frequently confuses students. However, it is just another way of saying that a nucleoside with a phosphate covalently attached to it is a nucleotide. Note also that it is redundant to refer to a nucleotide monophosphate - a nucleotide has at least one phosphate by definition.

The terms nucleoside and nucleotide are used to refer generically to these molecules. Nucleosides containing the sugar ribose are referred to precisely as ribonucleosides. Similarly, nucleotides containing the sugar ribose are called ribonucleotides. If the sugar present is deoxyribose, the precise names are deoxyribonucleosides and deoxyribonucleotides.

Nucleotides contain at least one phosphate, but they may have more. For example, ATP (adenosine triphosphate) contains three phosphates, making it a nucleoside triphosphate. Though the repeating units of DNA and RNA are nucleoside monophosphates, cells use nucleoside triphosphates to build nucleic acids because the energy derived from hydrolyzing two of the phosphate groups in the triphosphate helps to drive the reaction.