2.22: RNA is a universal translator, reading DNA and directing protein production.

The process of building a protein from a DNA sequence is not a direct one. Rather, it incorporates a middleman, RNA, that is also a nucleic acid (FIGURE 2-47). Although both RNA and DNA are built from nucleotides, the RNA nucleotide differs from the DNA nucleotide in three important ways. First, the sugar portion of the nucleotide differs slightly, containing an extra atom of oxygen. Second, while RNA has the bases A, G, and C, instead of thymine (T) it has a similar base called uracil (U). And third, unlike DNA, RNA is single-stranded. The sugar-phosphate-sugar-phosphate backbone is still there, as are the bases that protrude from each sugar, but the bases do not bind with bases in another RNA strand to form the ladder-like structure we see in DNA.

When the cell needs to synthesize a protein, a short strip of RNA is produced using a segment of a DNA strand as a model. The RNA nucleotides are therefore complementary to the DNA nucleotides, so the RNA molecule contains all the information present in the order of nucleotides of that DNA segment. The RNA moves to another part of the cell and then directs the linking together of amino acids to form a polypeptide chain that folds into a three-dimensional protein. We explore this in greater detail in Chapter 5.

Whether we’re looking at the nucleotides that make up RNA and DNA or the lipids used to build sex hormones and cell membranes, we see a recurring theme in the construction of biological macromolecules: from relatively simple sets of building blocks linked together, infinitely complex molecules can be formed. Complex webs of one simple sugar, bonded together as glycogen, for instance, provide fuel for organisms. Similarly, sequences of amino acids of 20 different types, joined together, specify the structure of all the proteins found in every species on earth.