The cell grows to its mature size during interphase, specifically during the G1 phase (Gap 1).

Here's a breakdown:

* Interphase is the period in the cell cycle where the cell grows and replicates its DNA.

* G1 phase is the first growth phase of interphase. During this phase, the cell increases in size, produces new organelles, and synthesizes proteins needed for DNA replication.

While the cell continues to grow slightly during the S phase (DNA synthesis) and G2 phase (Gap 2) of interphase, the majority of its growth occurs during the G1 phase.