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"DNA molecule. Computer artwork of the molecular structure of a DNA (deoxyribonucleic acid) double helix. DNA carries the genetic code, which controls and transmits an organism's hereditary traits. The helix is composed of two twisting sugar-phosphate strands (down centre), linked on the inside of the helix by complementary pairs of nucleotide bases (horizontal 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)