DNA
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DNA (deoxyribonucleic acid), the basic chemical structure of the genes. It is formed of four bases, adenine (A), guanine (G), cytosine (C), and thymine (T), a sugar deoxyribose, and phosphate. Its structure was found in 1953 by James Watson and Francis Crick. The smallest unit of DNA is a nucleotide, which consists of one base, one sugar, and phosphate. The base distinguishes the four different nucleotides from each other. Three successive nucleotides form a codon, which is the smallest code word unit of DNA. One codon determines one amino acid (see this) when the message of DNA is translated to a protein. The execution of protein synthesis according to the "blueprint" in DNA is called expression of the gene. First a blueprint is copied to RNA (see this) by transcription and then according to the directions in codons, amino acids are linked to form a protein (see this) chain by translation.