KENNETH EWARD / BIOGRAFX / SCIENCE PHOTO LIBRARY KENNETH EWARD / BIOGRAFX / SCIENCE PHOTO LIBRARY
DNA. Computer artwork of a DNA (deoxyribonucleic acid) double helix. This is beta or B-DNA, which spirals clockwise and is the more common form. The spiralling strands (blue) are composed of chemical units called nucleotides. These are made up of a constant sugar-phosphate group and a variable base group (coloured rods). Complementary hydrogen bonding (base pairing) between the base groups of nucleotides on opposite strands holds the helix together. The sequence of the four base groups along the DNA helix is unique for every individual and provides the instructions necessary for the organism.
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