Why do chromosome seperated into two parts during interphase of mitosis

Answer 1

I think you're asking why chromosomes double during the interphase of mitosis. If you're asking why the chromosomes separate during mitosis, that actually happens during anaphase of mitosis.

Think about our DNA, and our cells. If our cells constantly split in mitosis without replicating the DNA before each split, each daughter cell (or resulting cell) would have only half the amount of DNA as the parent cell (or cell that it split from). Each of your cell needs the entirety of your DNA within it. Half of your DNA is not sufficient for a cell to survive and function. This is why DNA is doubled/replicated/copied during interphase.