Organisms store information about the structures of their proteins in macromolecules called nucleic acids. Nucleic acids are long polymers of repeating subunits called nucleotides. Each nucleotide is made up of three smaller building blocks (Figure 17):
Figure 17 The structure of a nucleotide and the formation of nucleic acid chains. As shown in the inset, a nucleotide is composed of a five carbon sugar, a phosphate group, and an organic nitrogen base. The sugar and phosphate groups make up the back bone of a nucleic adic chain, while the nitrogenous bases link the two sides of the chain. the five nitrogen bases that occur in the nucleic acids of DNA and RNA are shown on the right.
1. A five-carbon sugar
2. A phosphate group (-PO4-2)
3. An organic, nitrogen-containing molecule called a base
To form the nucleic acid chain, the sugars and phosphate groups making up the nucleotides are linked; a nitrogenous base protrudes from each sugar as shown in Figure 17. The order in which the nucleotides are linked together forms a code that ultimately specifies the order of amino acids in a particular protein.
Organisms have two forms of nucleic acid. One form, deoxyribonucleic acid (DNA), stores the information for making proteins. The other form, ribonucleic acid (RNA), directs the production of proteins.