A protein is a folded polymer of amino acids in specific sequence, sometimes accompanied by metal atoms and small organic molecules. The opening illustration in this chapter showed the association of an iron atom and an organic ring in the heme group of cytochrome c, a small electron-carrying protein. Every protein, in every species of living creatures, has its own unique amino acid sequence, originally coded in a sequence of organic bases in DNA, as a part of the genetic "library" of the organism. In principle, given the amino acid sequence of a protein, one not only can identify the protein, but can determine from what species it came. It is in this sense that we describe proteins and nucleic acids as "information carriers."

Nucleic acids are the information carriers par excellence. From one generation to the next, DNA is the source of the information on how to synthesize proteins, and hence on how to build a living creature. Lipids, carbohydrates, and all the other molecules that we previously have examined are not information carriers in this sense. Some cases are known in which one kind of molecule is used in vertebrates for a given purpose, and a different molecule in invertebrates; or one molecule may be peculiar to a given class of plants.

But this is a far cry from being able to say from inspection of the molecule: This protein came from the digestive machinery of a dog, this one from the respiratory system of a horse, and that one from the same respiratory system in bread mold.

The "central dogma" of molecular biochemistry, so labeled tongue-in-cheek by the men who proposed it, is "DNA makes RNA makes protein." This is a concise way of saying that the information contained in a protein molecule came from messenger RNA, and that the ultimate source of the information in RNA was analogous sequences of bases in DNA. Information flows from left to right in the illustration on the next page. Some exceptions are known to this simple one-way flow of information, but at least we can say "Nucleic acid makes protein," and add "and protein makes everything else." All of the chemical processes of living things are under the control of enzymes, which are protein molecules. Lipids, carbohydrates, and all the small molecules of the cell are products of enzymatic`syntheses. They are "second-hand" molecules, in a different category with regard to information. Carbohydrates and lipids are the props and scenery in the living drama; proteins and nucleic acids are the actors.