An mRNA molecule codifies only one type of protein?
Eukaryotic cells have monocistronic mRNA, i.e., every mRNA codifies only one polypeptide chain. Prokaryotes can present polycistronic mRNA.
At the end of the assembling of amino acids into a polypeptide chain, the mRNA, by one of its terminal codons, signals to the ribosome that the polypeptide is complete. The ribosome then liberates the produced protein. In prokaryotes after this conclusion the information for the beginning of the synthesis of another dissimilar protein may follow in the similar mRNA.