Metabolism takes place through the catalytic action of enzymes: without enzymes, the various biochemical reactions that take place in cells simply wouldn’t happen. In turn, enzymes are made of proteins. Proteins are therefore a vital constituent of life—at least here on Earth. As we shall see later, the instructions for creating the various proteins necessary for a cell’s existence are contained in its deoxyribonucleic acid (DNA), while the biochemical machinery of protein synthesis is based on its ribonucleic acid (RNA). In shorthand form: DNA makes RNA makes proteins.