Groups of three nucleotides found in the coding region of an mRNA molecule. Each codon is translated into a specific amino acid, although due to the redundancy of the genetic code more than one codon can encode the same amino acid.
Every gene in the genome is transcribed into RNA by enzymes called RNA polymerases. RNA is a molecule consisting of a long strand of the nucleotides adenine, guanine, cytosine and uracil (denoted A, G, C and U, respectively). There are three basic types of RNA: transfer RNA, ribosomal RNA and messenger RNA. The first two types perform their own distinct roles in cells (carrying amino acids to the ribosome and catalysing the formation of proteins, respectively), while messenger RNA, or mRNA, serves as the template for the generation of proteins.
Since mRNA is made from nucleic acids and protein is composed of completely different building blocks known as amino acids, there must be a set of rules for translating one into the other. This set of rules is known as the genetic code. Each mRNA molecule contains a 5' untranslated region, a coding region, and a 3' untranslated region. The untranslated regions, or UTRs, serve regulatory functions, while the coding region contains the code that will be translated into protein.
During the process of translation, the codons in the coding region of an mRNA molecule pass in rapid succession through the active site of a ribosome. As each codon enters the active site, a tRNA molecule bearing the appropriate amino acid enters the ribosome and binds to the codon. The ribosome catalyses the formation of a peptide bond between the amino acid and its neighbours in the emerging protein. In this way, the information in the coding region of mRNA is converted into functional proteins, the "doing" molecules of cells.