Q What are the three periods into which interphase are divided?
Interphase is the preceding phase to the mitotic division. It is divided into three periods, G1, S and G2 the letter G comes from "gap", meaning breach or interval, and the letter S comes from "synthesis", indicating the period in which the DNA replicates.
In fact, "gap" is not totally appropriate for the periods immediately after and before the DNA synthesis. The idea of "growth" would be more sufficient since in those periods (G1 and G2) the cell is growing to divide later in mitosis.