Differentiated cells remain at which stage?

After which phase does the cell enter mitosis?

During which phase of the cell cycle are the proteins and microtubule systems required for mitosis synthesized?

In the cell cycle,which phase changes cannot be observed under a microscope?

What would be the change in the chromosome number during $S$-phase?

Given below are the two statements.
Statement $I$: Replication of $DNA$ occurs in the '$S$' phase of interphase in the cell cycle.
Statement $II$: Replication of $DNA$ takes place only once in the cell cycle.
In the light of above statements,choose the correct answer from the options given below: