Final answer:
mRNA is synthesized during the G1 phase of interphase, but transcription can occur throughout all the phases of interphase as the cell prepares for division.
Step-by-step explanation:
The stage of interphase during which mRNA is synthesized is primarily the G1 phase (First Gap). During the G1 phase, the cell grows and performs routine metabolic processes. It also produces mRNA which is necessary for protein synthesis. mRNA synthesis continues, as needed, throughout interphase for the production of proteins.
It's important to note, however, that while mRNA synthesis is initiated in the G1 phase, gene expression and thus mRNA transcription can occur throughout interphase as the cell prepares for division, making G1, S, and G2 all phases during which mRNA may be synthesized.