The process of decoding mrna into a polypeptide chain is known as

Translation

Explanation:

The entire process is called gene expression and it involves transcription and translation.

In Transcription,a particular portion of DNA is copied into RNA by the enzyme RNA polymerase.

In translation, messenger RNA (mRNA) is decoded in the ribosome decoding center to produce a specific amino acid chain, or polypeptide. The polypeptide later folds into an active protein and performs its functions in the cell