Codon most commonly refers to any group of three nucleotides that specifies the addition of one of the 20 amino acids during translation of an mRNA into a polypeptide. There are also DNA codons that code for the transcription of mRNA. Strings of codons form genes and strings of genes form chromosomes.
Codon is a fundamental component in a coded message, corresponding to a single character. It is the basic unit of the genetic code, consisting of three adjacent bases on a strand of DNA or RNA that supply genetic code information for a particular amino acid.
There are 64 varieties of codon of which 61 specify the merging of an amino acid into a polypeptide chain while the remaining three are stop codons, also known as termination codons or nonsense codons, that are the signal to stop protein synthesis.
Most codons have an equivalent alternative, where the substitution of 'silent' nucleotides will not change the encoded amino acid, apart from (in the universal genetic code) Methionine (AUG) and Tryptophan (UGG). Since the first nucleotide sequence repositories were being assembled, investigators have noticed that not all codons for the same amino acid are used with equal frequency. It is often spoken of "the" codon usage pattern of an organism, but it is rarely true that all of the genes demonstrate the same codon usage patterns. Within an organism it is usual for there to be significant differences in synonymous codon usage between genes.
Web Resources On Codon
The Genetic Code
Book Resources On Codon
Transfer RNA in Protein Synthesis by Hatfield, Lee, Pirtle
Introduction to Genetic Analysis by Griffiths, et. al.