KENNETH EWARD / BIOGRAFX / SCIENCE PHOTO LIBRARY KENNETH EWARD / BIOGRAFX / SCIENCE PHOTO LIBRARY
DNA, computer artwork. DNA (deoxyribonucleic acid) consists of two strands (blue) of sugar phosphates forming a double helix (spiral). The strands are held together by interactions between nucleotide bases (white, chemical structures) on each strand. Although there are four types of nucleotide base, only two base pairings are usually possible: adenine bonds to thymine, and cytosine bonds to guanine. This means each DNA strand is a complementary copy of the other, a vital part of DNA replication. The base sequence along the molecule is the inherited genetic code that controls growth and development.
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