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"DNA molecules. Computer artwork of the molecular structure of DNA (deoxyribonucleic acid) double helices (down left and right). DNA carries the genetic code, which controls and transmits an organism's hereditary traits. Each helix is composed of two twisting sugar-phosphate strands, linked on the inside of the helix by complementary pairs of nucleotide bases (horizonal lines). There are four types of nucleotide base; adenine, guanine, cytosine and thymine. The sequence of base pairs, arranged in discrete segments known as genes, determines the genetic code." (KEYSTONE/SCIENCE PHOTO LIBRARY/PASIEKA)