The genetic message contained in DNA is formed by a four letter alphabet that corresponds to the four nucleotides: A, T, C and G.
With these four letters it is necessary to form "words" that have the meaning of "amino acids". Each protein corresponds to a "phrase" formed by the "words", which are the amino acids. How could only four letters of the DNA alphabet be combined to correspond to each of the twenty "words" represented by the twenty different amino acids that occur in living things?
A brilliant proposal suggested by several researchers, and later confirmed by experimental methods, was that every three letters (a crack of bases) of DNA would correspond to a "word," that is, an amino acid. In that case, there would be 64 possible three letter combinations, which would be more than enough to encode the twenty different types of amino acids (Mathematically, using the combination method would then be 4 letters combined 3 to 3, ie 43 = 64 possible combinations).
The genetic code of DNA is expressed by base cracks, which were called codons. Each codon, consisting of three letters, corresponds to a certain amino acid.
The correspondence between the DNA base trio, the RNA base trio and the amino acids they specify is a coded message that has come to be known as a "genetic code."
But, a problem arises. There are twenty different amino acids, so there are more codons than types of amino acids! It must be concluded, then, that there are amino acids that are specified by more than one codon, which has been confirmed. The table below specifies the mRNA codons that can be formed and the corresponding amino acids they specify.
We say that the genetic code is universal, because in all organisms on earth today it works the same way, whether it is bacteria, carrots or humans.
The codon AUG, which codes for the amino acid methionine, also means start of reading, that is, it is a codon that indicates to ribosomes that it is by this trio of bases that mRNA reading should be started.
Note that three codons do not specify any amino acids.. These are the codons UAA, UAG and UGA, called codons and stop during reading (or stop codons) of RNA by ribosomes in protein synthesis.
The genetic code is said to be degenerate because each "word" (meaning amino acid) can be specified by more than one set.