In this technique a mixture of compounds is deposited on the edge of a special type of paper. This edge is then immersed in a solvent that slowly permeates the paper (at a constant speed) by capillary action. As the solvent moves from the immersed edge toward the other edge, which is hanging freely, it carries the mixture of nucleotides with it. Each of the compounds in the mixture travels at a different speed, however; thus, as the solvent front moves along the paper, the dissolved compounds are separated from each other and appear as distinct spots on the paper. To locate the nucleotides on the paper and to determine the percentage composition, we can use a chromatogram scanner, a device that scans the paper chromatograms, measures the radiation from them, and thus locates the labeled substances (see Figure 22). [Illustration: Figure 22 _Recording of radioactivity in a sample by radioautography and paper chromatography. The peaks of the trace prepared by a chromatogram scanner coincide with the areas of separated components on the same chromatogram, as revealed by radioautography. The radioautograph is superimposed on the chromatogram recording._] Another technique used to separate the nucleotides of RNA is column chromatography. In this method mixtures of nucleotides are separated as they pass down a column of chemicals (see Figure 23). [Illustration: Figure 23 _Students visiting Argonne National Laboratory listen to a scientist explain the column chromatography process, in which mixtures of nucleotides are separated as they pass down a column of chemicals._] We have now learned how to use radioisotopes to investigate the synthesis of RNA, the molecule that translates the DNA message into the language of proteins. Let us now see what we can learn about the synthesis and function of proteins. PROTEIN SYNTHESIS: THE MOLECULES THAT MAKE THE DIFFERENCE _If a man will begin with certainties he shall end in doubts; but if he will be content to begin with doubts he shall end in certainties._ Francis Bacon Proteins occupy a central position in the structure and functioning of living matter and are intimately connected with all the metabolic reactions that maintain life. Some proteins serve as structural elements of the body, for instance, hair, wool, and the scleroproteins of bone and collagen, the latter an important constituent of connective tissue. Other proteins are enzymes, which are extremely important since they regulate all metabolic reactions. Most of the proteins in the tissues of actively functioning organs, such as the liver and the kidney, are enzymes. Other proteins participate in muscular contraction, and still others are hormones or oxygen carriers. Special proteins called histones are associated with gene function, and the antibodies that an organism produces to defend itself from bacteria are also proteins.