How Genetic Code Degeneracy Works
To get more specific about how translation works, let's talk about tRNA.
A tRNA molecule is a small strip of nucleotides folded into a series of loops. One end, called the anticodon end, matches up with a codon in the mRNA. Adenine matches with uracil, guanine matches with cytosine, and vice versa.
The other end is attached to an amino acid. As protein synthesis occurs, tRNA molecules take turns matching up to the mRNA codons and transferring their amino acids to the forming polypeptide chain that will eventually become a protein.
There are a lot of different kinds of tRNA molecules - not just one per codon, or one per amino acid, as you might expect. Some amino acids associate with only one tRNA, while others associate with several - each with a different anticodon. Some tRNA molecules have anticodons that can match up with multiple codons - this phenomenon is known as wobble. And some tRNA molecules have the same anticodon and same amino acid, but different nucleotides in other areas, indicating that they come from different tRNA genes.