Life

Another important aspect of life is its vast diversity built on a base of underlying unity. For example, organisms as outwardly dissimilar as a bacterium, a human, and an oak tree are all composed of the same basic structural element, the cell, which in turn have many similar subcellular and molecular components. All eukaryotic cells (those cells of higher organisms that have a membrane-bound nucleus) contain mitochondria (organelles that carry out oxidation and thus provide energy), Golgi apparatus (a membrane system involved in packaging newly synthesized proteins), endoplasmic reticulum (a complex system of internal membranes), and ribosomes (small structures that form the site of protein synthesis). Simpler cells known as prokaryotes (bacteria, blue-green algae, and so forth), while lacking mitochondria and other organelles, contain ribosomes and share all the basic molecular infrastructure with eukaryotes. For example, both prokaryotes and eukaryotes have their hereditary information encoded in the molecule of deoxyribonucleic acid (DNA), transcribe that message into messenger RNA (mRNA), and translate that message into proteins in exactly the same ways. Furthermore the language in which the DNA code is written is the same in all organisms: as a triplet in which specific sequences of three out of four possible bases (adenine, thymine, guanine, and cytosine) specify each of the twenty amino acids that make up all proteins in the living world. Thus beneath apparent diversity lies a major infrastructure of unity. How to interpret this obvious contradiction has motivated a wide variety of views of the nature of life since at least the early twentieth century.