Mitosis

A cell undergoing mitosis goes through Prophase, Metaphase, Anaphase and Telophase.

Interphase:This stage prepares the cell for mitosis. The nucleus in this stage is well defined and has DNA loosely packed in long chromatin fibers. This stage is when the cell is preparing for mitosis, and replicates chromosomes and produces proteins and organelles. During the synthesis phase of interphase, the dividing cell replicates and must separate DNA copies correctly to two daughter cells. The DNA is then organized into chromosomes. After the DNA is copied, the chromatin condenses and makes a smaller package. A mitotic chromosome is a duplicated chromosome and it contains two sister chromatids which contain identical copies of DNA.

Prophase: In this stage, the chromatin condenses and the chromosomes are visible. The centrioles move to opposite poles of the cell (in the animal cell) and the protein fibers cross the cell to form mitotic spindles, also called microtubules which are made of actin and myosin. During this stage, the nucleolus disappears and the nuclear membrane breaks down.

Metaphase: the early stages of this phase are known as prometaphase. This is when the spindle fibers attatch to the centromeres, creating kinetochores. Microtubules attatch at the kinetochores and connect the centromers to the centrioles. Then the chromosome begins moving, and enters into Metaphase. Metaphase is when chromosomes align along the middle of the cell. The spindle fibers coordinate the movement and help ensure that the chromosomes separate properly.

Anaphase: The sister chromatids separate at the kinetochores. They are pulled at the centromeres and move to opposite poles. They are pulled by motor proteins "walking" along microtubules. These proteins are actin and myosin, and this also phase also has an increasement of ATP by mitochondria. The poles move farther apart and the microtubules lengthen. A separation off chromatids occurs in anaphase as well, where the proteins holding together sister chromatids are inactivated.

Telophase: This is when chromosomes arrive at opposite poles and daughter nuclei form. The nucleoli then form and the chromosomes disperse and are no longer visible under light microscope. The spindle fibers disperse and cytokinesis begins.

Cytokinesis is cell division. In animals it is a constriction belt of actin microfilaments around the equator of the cell. The cell is split into two. In plants a cell plate forms, and the vesicles line up at the equator. A new cell wall is laid down between the membranes, and the new cell wall fuses with the existing cell wall.