It has been obvious for as long as humans have sown crops or raised animals that each seed or fertilized egg must contain a hidden plan, or design, for the development of the organism. In modern times the science of genetics grew up around the premise of invisible informationcontaining elements, called genes, that are distributed to each daughter cell when a cell divides.
Therefore, before dividing, a cell has to make a copy of its genes in order to give a complete set to each daughter cell. The genes in the sperm and egg cells carry the hereditary information from one generation to the next.
The inheritance of biological characteristics must involve patterns of atoms that follow the laws of physics and chemistry: in other words, genes must be formed from molecules. At first the nature of these molecules was hard to imagine. What kind of molecule could be stored in a cell and direct the activities of a developing organism and also be capable of accurate and almost unlimited replication?
By the end of the nineteenth century biologists had recognized that the carriers of inherited information were the chromosomes that become visible in the nucleus as a cell begins to divide. But the evidence that the deoxyribonucleic acid (DNA) in these chromosomes is the substance of which genes are made came only much later, from studies on bacteria. In 1944 it was shown that adding purified DNA from one strain of bacteria to a second, slightly different bacterial strain conferred heritable properties characteristic of the first strain upon the second. Because it had been commonly believed that only proteins have enough conformational complexity to carry genetic information, this discovery came as a surprise, and it was not generally accepted until the early 1950s. Today the idea that DNA carries genetic information in its long chain of nucleotides is so fundamental to biological thought that it is sometimes difficult to realize the enormous intellectual gap that it filled.
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