Animals, and indeed plants, are com

Animals, and indeed plants, are composed of microscopic bricks called cells. The cells found in different organs and tissues of the same creature are of quite different sizes and shapes - bones are made from angular cells, kidneys from spherical cells, nerves from long, narrow cells - but all are made from similar components. Round the outside of each cell is a skin, the cell membrane, enclosing the gelatinous cytoplasm which carries a number of small structures called organelles. The most important of these is the cell nucleus, which lies at the centre of the cell and carries the information from which the entire organism is built.
This information is stored as a code, made up from a sequence of components contained in a long molecule of a complex substance known as deoxyribonucleic acid (DNA). The DNA molecule is a little bit like a ladder that has been twisted throughout its length. The shafts of the ladder are made up of sugar-phosphate molecules and each rung consists of a pair of molecules known as nucleic-acid bases. There are only four of these bases and the sequence in which they are found along the twisted ladder gives the coded instructions from which the whole organism is formed. Although repeated in its entirety in the nucleus of each eel! of the organism, only certain parts of the code are needed to build up particular organs.
The peculiar thing about the DNA molecule is its ability to reproduce itself. The molecule splits along its length and unwinds so that each half of the ladder consists of a shaft and a series of half-rungs. The missing ladder halves are built from the pool of sugar-phosphate bases, which is supplied by the creature's food and is present in each cell nucleus. As each of the four types of nucleic acid base in the strand attracts only a specific kind of nucleic acid base to itself, when two new complete strands of DNA are formed they are absolutely identical to each other in the sequence of their components. This is the most important process involved in cell multiplication and underlies the growth of all organisms.
However, to grow, organisms also require proteins in the form of either structural elements such as collagen, in the case of the packing tissue between organs, or as enzymes which aid specific biological processes. Although the production of proteins is carried on outside the cell nucleus it is controlled by