The Stages of Mitosis and Cell Division

Interphase

Before a dividing cell enters mitosis, it undergoes a period of growth called interphase. About 90% of a cell's time in the normal cell cycle may be spent in interphase.

• G1 phase: The period before the synthesis of DNA. In this phase, the cell increases in mass in preparation for cell division. The G1 phase is the first gap phase.
• S phase: The period during which DNA is synthesized. In most cells, there is a narrow window of time during which DNA is synthesized. The S stands for synthesis.
• G2 phase: The period after DNA synthesis has occurred but before the start of prophase. The cell synthesizes proteins and continues to increase in size. The G2 phase is the second gap phase.
• In the latter part of interphase, the cell still has nucleoli present.
• The nucleus is bounded by a nuclear envelope and the cell's chromosomes have duplicated but are in the form of chromatin.

Prophase

In prophase, the chromatin condenses into discrete chromosomes. The nuclear envelope breaks down and spindles form at opposite poles of the cell. Prophase (versus interphase) is the first true step of the mitotic process. During prophase, several important changes occur:

• Chromatin fibers become coiled into chromosomes, with each chromosome having two chromatids joined at a centromere.
• The mitotic spindle, composed of microtubules and proteins, forms in the cytoplasm.
• The two pairs of centrioles (formed from the replication of one pair in interphase) move away from one another toward opposite ends of the cell due to the lengthening of the ​microtubules that form between them.
• Polar fibers, which are microtubules that make up the spindle fibers, reach from each cell pole to the cell's equator.
• Kinetochores, which are specialized regions in the centromeres of chromosomes, attach to a type of microtubule called kinetochore fibers.
• The kinetochore fibers "interact" with the spindle polar fibers connecting the kinetochores to the polar fibers.
• The chromosomes begin to migrate toward the cell center.

Metaphase

In metaphase, the spindle reaches maturity and the chromosomes align at the metaphase plate (a plane that is equally distant from the two spindle poles). During this phase, several changes occur:

• The nuclear membrane disappears completely.
• Polar fibers (microtubules that make up the spindle fibers) continue to extend from the poles to the center of the cell.
• Chromosomes move randomly until they attach (at their kinetochores) to polar fibers from both sides of their centromeres.
• Chromosomes align at the metaphase plate at right angles to the spindle poles.
• Chromosomes are held at the metaphase plate by the equal forces of the polar fibers pushing on the centromeres of the chromosomes.

Anaphase

In anaphase, the paired chromosomes (sister chromatids) separate and begin moving to opposite ends (poles) of the cell. Spindle fibers not connected to chromatids lengthen and elongate the cell. At the end of anaphase, each pole contains a complete compilation of chromosomes. During anaphase, the following key changes occur:

• The paired centromeres in each distinct chromosome begin to move apart.​
• Once the paired sister chromatids separate from one another, each is considered a "full" chromosome. They are referred to as daughter chromosomes.​
• Through the spindle apparatus, the daughter chromosomes move to the poles at opposite ends of the cell.​
• The daughter chromosomes migrate centromeres first and the kinetochore fibers become shorter as the chromosomes near a pole.​
• In preparation for telophase, the two cell poles also move further apart during anaphase. At the end of anaphase, each pole contains a complete compilation of chromosomes.

Telophase

In telophase, the chromosomes are cordoned off into distinct new nuclei in the emerging daughter cells. The following changes occur:

• The polar fibers continue to lengthen.
• Nuclei begin to form at opposite poles.
• The nuclear envelopes of these nuclei form from remnant pieces of the parent cell's nuclear envelope and pieces of the endomembrane system.
• Nucleoli also reappear.
• Chromatin fibers of chromosomes uncoil.
• After these changes, telophase and mitosis are largely complete. The genetic contents of one cell have been divided equally into two.

Cytokinesis

Cytokinesis is the division of the cell's cytoplasm. It begins before the end of mitosis in anaphase and completes shortly after telophase. At the end of cytokinesis, two genetically identical daughter cells are produced. These are diploid cells, with each cell containing a full complement of chromosomes.