The decoding of an mRNA message into a polypeptide chain and protein is called translation.

Here's why:

* mRNA: This is the messenger RNA, which carries the genetic code from the DNA in the nucleus to the ribosomes in the cytoplasm.

* Polypeptide chain: This is a chain of amino acids linked together, which is the primary structure of a protein.

* Protein: The final, functional three-dimensional structure formed by the polypeptide chain.

During translation, ribosomes read the mRNA sequence in codons (groups of three nucleotides) and use this information to assemble the correct amino acid sequence in the polypeptide chain. This chain then folds into a specific shape, forming the functional protein.