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DNA strand, illustration. Deoxyribonucleic acid (DNA) consists of long strands forming a double helix of phosphates and sugars. These two helices are held together by pairs of nucleotide bases (rods). There are four different nucleotide bases. It is the varying sequence of these four bases along a DNA helix that forms the genetic code for that individual. A typical DNA molecule can contain a sequence that is many millions of bases long. The genetic code in DNA is the basis of all life on Earth. (KEYSTONE/SCIENCE PHOTO LIBRARY/PATRICK LANDMANN/SCIENCE PHOTO LIBRARY)