How can there be 64 codon combinations but only 20 possible amino acids?
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 .
The number of amino acids coded by codons is
Here's a chart that gives the codon assignments for the amino acids.