How can there be 64 codon combinations but only 20 possible amino acids?

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

#1 " codon" × color(white)(l)2 " amino acids" = color(white)(ll)2 " codons"#
#2 " codons" × 9 " amino acids" = 18 " codons"#
#3 " codons" × 1 " amino acid" = color(white)(X)3 " codons"#
#4 " codons" × 5 " amino acids" = 20 " codons"#
#6 " codons" × 3 " amino acids" = 18 " codons"#
#color(white)(XXXXXXXXXXXXXXXX)3" stop codons"#
#stackrel(—————————————————————————)(color(white)(XXXXXXXXXl)"TOTAL" = 64 " codons")#

Here's a chart that gives the codon assignments for the amino acids.