Prophase

Prophase, the initial phase of mitosis, follows G2 phase of interphase.  During prophase, replicated DNA condenses into visible chromosomes; the duplicated centrosomes move apart; and the mitotic spindle assembles from the centrosome.  Finally, the nuclear envelope disintegrates and the nucleolus disappears.

The term chromatid refers to one copy of a chromosome that is still joined to the other copy at the centromere.  The two copies are referred to as sister chromatids (blue arrows).

The centromere is a highly condensed region of DNA which forms the junction point for the two sister chromatids.

The breakdown of the nuclear envelope occurs in late prophase, a stage referred to as pro-metaphase.  This step allows access for the microtubules, which are extending from the centrioles, to attach to the chromatids.

The dissolution of the nucleolus occurs in late prophase.

The centrosome, the microtubule organizing center of the cell, consists of two centrioles (diplosome) oriented at right angles that are surrounded by the centrosome matrix.  During prophase, the centrosomes move to opposite poles of the cell where they produce the microtubules (spindle fibers) forming the mitotic spindle.