What is the major event in M phase? S phase? G0 phase?

Question

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Answer 1

During the S phase the DNA replicated.  This is in preparation for the M phase, during which mitosis will take place.

The G0 phase often called the "resting stage."  The cell is not actively dividing or preparing to divide.  In some cases, the cell exits the replication cycle and differentiates into it's terminal state.