A codon is a series of three nucleotides used to specify a specific an amino acid. These nucleotides are labeled with G, C, and A and are found in both RNA and DNA. T-containing codons are found in only DNA, and U-containing codons only in RNA.
The nucleotides found in codons determine what purpose that codon has in the DNA or RNA sequence. Each codon indicates which amino acid is created for a given protein, following a sequence of codons that work with each other in this fashion. The codons are essentially the instructions for cells to create the proteins needed to create life, forming the basis of cell communication.
The five nucleotides found between RNA and DNA are adenine, cytosine, guanine, thymine and uracil with each respective nucleotide represented in the code by the first letter in the word. One nonstandard nucleotide exists, called inosine, which bonds with adenine, cytosine or thymine in a given sequence. These nucleotides bond with other organic molecules to create the biological compounds that every cell needs.
The start and stop codons in RNA — the codons that tell the strand the beginning and end of its sequence — are AUG for start and UAA, UGA and UAG for stop. Without these codons, it would be impossible for cells to tell where or how the sequence should be used.