A form of nucleic acid organized into pairs of double-helix molecules packaged into chromosomes carrying the genetic code. The molecules are made of linked nucleotides units with a sugar, a phosphate, and one of four base chemicals adenine, thymine, guanine, and cytosine. These bases join like ladder rungs--always an A to a T and a C to a G--with the sugar-phosphate forming the outside "backbone" of the strand. The sequence of these nucleotides, with each group of three spelling one anino acid "codon," determines the kind of protein manufactured when translated by strands of RNA. (James Watson and Francis Crick discovered this structure in 1953.) RNA also aids in DNA's replication. Everything living carries the same gene code, one reason scientists are so confident we are all related biologically. Some DNA sequences are identical in humans and bacteria, a fact that underlines our common biological origins.

       
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