Why does methionine only have one codon

Translation explained in an understandable way ...

In the course of translation, proteins are synthesized by reading the mRNA that was previously produced in the transcription. Translation means "to translate", and in fact the genetic code is "translated" into protein chains.

The place where this process takes place in the cell is the ribosome.
But let's first take a closer look at the mRNA strand: This consists of a long chain of bases. Three consecutive bases (base triplet / codon) always code a special amino acid (which triplets code which amino acid can be read off in the code sun).
A distinction is made between the start codon, normal codons and the stop codons. The translation will always begin at the start codon AUG (base triplet consisting of adenine, uracil and guanine) and end at one of the three stop codons (UGA, UAA or UAG).
All other base triplets that are neither start nor stop codons and each encode a specific amino acid are referred to as 'normal' codons. The start codon AUG also encodes an amino acid (methionine) in contrast to the three stop codons, which are really only responsible for terminating the translation.

The task of the tRNA (transfer RNA) is to transport the individual amino acids to the ribosome and then to connect them to another amino acid so that peptide chains are created.
tRNA consists of several arms. An amino acid binds to one of these arms, and an anticodon that matches the corresponding base codon of the mRNA is located on the opposite arm. Example: The tRNA for methionine has the anticodon UAC; this only matches the base triplet AUG in the mRNA. The base sequence AUG in the mRNA thus codes for the amino acid methionine.
There are now many more tRNAs: Each of the amino acids requires a specific tRNA in order to be transferred to the corresponding codon on the mRNA. Because each tRNA is only responsible for one amino acid, according to its anticodon.

Back to the ribosomes: The first tRNA now attaches to the mRNA at the start codon (because the start codon is AUG, the first tRNA attached has accordingly taken up the amino acid methionine). This is followed by a second tRNA with a specific amino acid, which attaches to the first tRNA. A peptide bond links the two neighboring amino acids. The first tRNA then leaves the ribosome without an amino acid, which is now located at the end of the arm of the second tRNA together with its amino acid.
The third tRNA 'flies' with its specific amino acid and attaches itself to the mRNA. The process is repeated until a base triplet appears in the mRNA that encodes a stop codon. There are no suitable tRNAs for stop codons, so that the resulting peptide chain then becomes detached.