Final answer:
The element that would not be found on an mRNA molecule is the promoter. It is a DNA region essential for transcription initiation, but it is not part of the mRNA which includes regions such as the protein-coding sequence, 5' and 3' UTRs, and start and stop codons.
Step-by-step explanation:
The element that would not be found on an mRNA molecule is the promoter. While the promoter is crucial for transcription initiation, it is actually a region on the DNA, not the mRNA. The mRNA molecule will include 1) a protein-coding region, 2) a 3′ untranslated region (3' UTR), 3) a 5′ untranslated region (5' UTR), 4) start and stop codons, but not the promoter. The 5' cap and 3' poly-A tail are also features of the mature mRNA molecule. These elements are crucial for the stability and functionality of the mRNA in the process of protein synthesis.