What happens to a cell during M phase of cell cycle?

During $G_1$ phase, the cell is in resting state and the nucleocytoplasmic index is stable. During the $\mathrm{S}$ phase of the cell cycle, DNA replicates, consequently nucleocytoplasmic index increases due to increase in nuclear volume. During $G_2$ phase, the cell is again stable. During M phase, nucleocytoplasmic index decreases. After $M$ phase or mitosis when the cell divides, nucleoplasmic index returns to normal.