|I | |INTRODUCTION |
DNA (deoxyribonucleic acid), molecule that acts as the mechanism of biological inheritance in almost all living creatures. DNA is found in nearly all cells and contains the coded instructions that control the workings of the cell. DNA is passed from parents to offspring, and contains the coded instructions that enable the offspring to develop from a single cell into an adult body. DNA is the most important molecule in life, and an understanding of the structure and function of DNA has been the most important development in biology during the past half-century or more. |II | |HISTORY |
Biologists had known since the late 19th century that the structures called chromosomes carry the hereditary material from parents to offspring. Chromosomes exist in a particular part of the cell—the cell nucleus—and are large enough to be visible at certain times with a light microscope. When visible, chromosomes are long, flexible entities resembling a tiny piece of string. Chromosomes contain two molecular constituents: proteins and DNA. The molecular basis of heredity was likely to reside in one or other of these molecules. For many years biologists suspected that the chromosomal proteins were more likely to be the hereditary molecules because proteins were known to be highly variable in their molecular sub-structure, whereas DNA seemed to have a rather uniform structure. It was difficult to see how a uniform molecule could give rise to the great variety of life. However, DNA was shown to be the molecule of inheritance by an experiment in 1944. (In fact the 1944 experiment is just the most famous landmark in a series of related experiments by several teams of biologists.) Then a group of American biochemists, O.T. Avery, C. M. MacLeod, and M. J. McCarty showed that DNA causes a phenomenon called ”transformation” in bacteria. In transformation, the properties of bacterial cells are altered when the bacteria are mixed with other bacteria of a different form. Something is passed between the two kinds of bacteria, causing transformation. Avery, MacLeod, and McCarty purified the various kinds of molecule in the cells—fats, proteins, and sugar as well as nucleic acids—and showed that only DNA causes transformation. The next important advance was the discovery by the American biochemist James Watson and British biophysicist Francis Crick in 1953 that DNA has the structure of a double helix. DNA is made up of three kinds of sub-molecule: phosphate, deoxyribose (a sugar), and various bases. DNA contains four kinds of base: adenine (A), cytosine (C), guanine (G), and thymine (T). The Austrian biochemist Erwin Chargaff had noticed that in DNA the amount of A equals the amount of T, and the amount of G equals the amount of C. This suggested that each A is bonded to a T, and each G to a C. (The two rules—that in DNA A bonds with T, and G with C— are called the “base-pairing rules”.) The other main kind of evidence used by Watson and Crick was that obtained by X-ray diffraction. X-ray diffraction is a method used to deduce the structure of molecules that are too small to observe directly. In the case of DNA, X-ray diffraction suggested that the molecule was some sort of helix. In Watson and Crick's model, DNA consists of two strands (or “backbones”) made up of alternating phosphate and deoxyribose molecules; the bases are attached to the deoxyribose and stick out from the two strands. G bases in one strand are bonded to C bases in the other, and A bases to T bases. The bonds between the bases are hydrogen bonds. In all, the two strands are bonded together on the inside of the molecule by the bases; each strand is twisted into a helical shape, making a double helix. Knowledge of a molecule's structure does not always help in understanding how the molecule works, but in the case of DNA, Watson and Crick's discovery was hugely important. The double helix pointed to ways in which DNA could be...
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