Final answer:
The primary step in gene expression regulation in both prokaryotic and eukaryotic cells is transcription, with prokaryotic regulation being largely at the transcription level, while eukaryotic regulation includes multiple stages due to cell complexity.
Step-by-step explanation:
The major step in regulating gene expression in both prokaryotic and eukaryotic cells involves transcription, which is the first stage in the process of converting genetic information into proteins. Gene expression in prokaryotes is largely regulated at the point of transcription, where the amount of transcription determines how much of each protein is produced. Eukaryotic cells, however, have more complexity in regulating gene expression due to their compartmentalization, with the DNA contained within the nucleus and various control points including epigenetic regulation, transcriptional control, post-transcriptional modifications, translational regulation, and post-translational modifications.