The cell cycle has checkpoints to ensure the appropriate sequence of events. Microtubules synthesized during S phase assemble to build the mitotic spindle. It consists of microtubules that aggregate to form a centriole, enclosed in a centrosome.
Astral rays extend from the centriole.
Chromosomes attach to the mitotic spindle with microtubule assemblies called kinetochores. Each replicated chromosome consists of two complete sets of genetic information, called chromatids, attached at a section of DNA called a centromere that also replicates. Mitosis consists of four stages. In prophase, the chromosomes condense and become visible when stained, the nuclear membrane disassembles, and the mitotic spindle forms. In metaphase, spindle fibers align replicated chromosomes down the cell's equator.