Here's why:

* Codons are three-nucleotide sequences within mRNA that specify which amino acid should be added to a growing polypeptide chain during protein synthesis.

* mRNA carries the genetic information copied from DNA in the nucleus to the ribosomes in the cytoplasm, where protein synthesis occurs.

* tRNA (transfer RNA) carries specific amino acids to the ribosome, recognizing the codons on mRNA through its anticodon.

So, while tRNA plays a crucial role in translation by decoding the mRNA codons, it's the mRNA itself that actually contains the codons.