The mitotic phase (also known as M phase) is a multistep process during which the duplicated chromosomes are aligned, separated, and move into two new, identical daughter cells. The first portion of the mitotic phase is called karyokinesis, or nuclear division. The second portion of the mitotic phase, called cytokinesis, is the physical separation of the cytoplasmic components into the two daughter cells.

Karyokinesis (Mitosis)

Karyokinesis, also known as mitosis, is divided into a series of phases—prophase, metaphase, anaphase, and telophase—that result in the division of the cell (Figure 1).

Figure 1. Stages of mitosis

During prophase, the “first phase,” the nuclear envelope starts to dissociate into small vesicles, and the membranous organelles fragment and disperse toward the periphery of the cell. The nucleolus disappears . The centrosomes begin to move to opposite poles of the cell. Microtubules that will form the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen. The sister chromatids begin to coil more tightly and become visible under a light microscope.  Each sister chromatid develops a protein structure called a kinetochore in the centromeric region (Figure 2). The proteins of the kinetochore attract and bind mitotic spindle microtubules.

Once a mitotic fiber attaches to a chromosome, the chromosome will be oriented until the kinetochores of sister chromatids face the opposite poles. Eventually, all the sister chromatids will be attached via their kinetochores to microtubules from opposing poles.

During metaphase, all the chromosomes are aligned in a plane called the metaphase plate, or the equatorial plane, midway between the two poles of the cell.  At this time, the chromosomes are maximally condensed.

During anaphase, the sister chromatids separate at the centromere. Each chromatid, now called a chromosome, is pulled rapidly toward the centrosome to which its microtubule is attached. The cell becomes visibly elongated (oval shaped) as the polar microtubules slide against each other at the metaphase plate where they overlap.

During telophase,  the chromosomes reach the opposite poles and begin to decondense (unravel), relaxing into a chromatin configuration.  Nuclear envelopes form around the chromosomes, and nucleosomes appear within the nuclear area.

The activity below will walk you through mitosis—providing you with the chance to review the different steps of the process and how they work together.

Click here for a text-only version of the activity.