There are only four types of nitrogen- containing bases that can compose RNA nucleotides: adenine (A), uracil (U), guanine (G) and cytosine (C). Amino acids however are 20 different ones.
Considering only one nucleotide (a 1:1 coding) it would be impossible to codify all amino acids.

Considering two nucleotides there would be an arrangement of 4 elements, 2 x 2, resulting in a total of only 16 possible codifier units (4 x 4). Nature may know combinatory analysis since it makes a genetic code by arrangement of the 4 RNA bases, 3 x 3, providing 64 different
triplets (4 x 4 x 4).

So each triplet of nitrogen-containing bases of RNA codifies one amino acid of a protein. As these triplets appear in sequence in the RNA molecule, sequential amino acids codified by them are bound together to make polypeptide chains. For example, a UUU sequence codifies the amino acid phenylalanine, as well the UUC sequence; the ACU, ACC, ACA and ACG sequences codify the amino acid threonine; and so on for all possible triplet sequences and all other amino acids.   _{Your Comment}