Codons are three letter genetic words: and the language of genes use 4 letters (=nitrogenous bases). Hence 64 words are there in genetic dictionary, to represent 20 amino acids that the biological organisms use.
And you must note that more than one codon may code for the same amino acid. This is referred to as degeneracy of the code. For example, three amino acids are coded by any of six different codons, and that alone uses up 18 of the 64 combinations. Three of the codons are stop codons. They do not code for any amino acid. Instead, they act as signals to end the genetic message carried by messenger RNA.
A stop codon is a trinucleotide sequence of three nucleotides found in messenger RNA (mRNA) or DNA that instructs the cell to stop production of proteins. There are 64 distinct trinucleotide codons, of which 3 are stop codons and 61 define amino acids (i.e., UAA, UAG and UGA). A stop codon (or termination codon) is a codon (nucleotide triplet within messenger RNA) that marks the end of the current protein's translation process in molecular biology, more specifically in protein biosynthesis.
The number of amino acids coded by codons is
1 codons ×2 amino acids = 2 codons
2 codons ×9 amino acids = 18 codons
3 codons ×1 amino acid = 3 codons
4 codons ×5 amino acids = 20 codons
6 codons ×3 amino acids = 18 codons
3 stop codons
Total = 64 codons
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