What do the letters of the DNA alphabet stand for?

DNA stands for Deoxyribo Nucleic Acid, and its letters (A) (C) (G) (T) all stand for Adenine, Cytosine, Guanine, and Thymine

A and G (adenine and guanine), and two pyrimidines, C and T (cytosine and thymine); in mRNA, U (uracil) replaces T. The words of the alphabet comprise 3 letters, thus there are 43=64 permutations or words which are called codons. 61 of the 64 codons code for 1 of 20 amino acids. Since more than one codon codes for a particular amino acid the code is said to be redundant. Four of the 64 codons punctuate the message; one, AUG, is the start signal and three, UAG, UAA and UGA, are stop signals.