A codon is a series of three nucleotides which code for a specific amino acid in protein biosynthesis. Most of the standard 20 amino acids have several different codons that encode for a single amino acid. For RNA strands, the four different available nucleotide bases are A (adenine), G (guanine), U (uracil), and C (cytosine). There are six codons that represent serine: UCU, UCC, UCA, UCG, AGU, AGC. For the corresponding DNA strands, substitute T (thymine) for U. In double stranded DNA, A always pairs with T, and G always pairs with C.