At what phase of mitosis does the number of chromosomes double

Chromosomes actually duplicate in the S phase of interphase. Interphase is split into three phases, G1 phase, S phase and G2 phase. S phase is the synthesis phase, which is when DNA molecules (chromosomes) double.

Some concept fix will happen in university, because in university level, we say that chromosome never doubles, it is the amount of DNA molecule that doubles. Also, interphase is part of the cell cycle, but not mitosis (IN UNIVERSITY LEVEL, so don't worry about it)

But for high school, the answer to your question will be interphase.

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