You grew from a zygote, or fertilized egg, into an organism with trillions of specialized cells. Mitosis is the process that enabled you to grow and develop after the fateful meeting of egg and sperm that became “you”. Cells must divide in order for an organism to grow and develop, but cell division is also required for tissue repair and cell replacement.
* Cell Replacement *
Many tissues of the body are composed of cells that have a high turn-over rate. Think of your skin; the epidermis, or top layer, is composed of dead cells that are constantly being sloughed off and replaced from below by cells of the dermis (the living cells in the layer of skin below the epidermis). If cells of the dermis were not constantly dividing to replace dead cells, your skin would eventually wear out. This is also true of the mucous membranes of your body; the moist areas of the GI tract and urogenital tract that are the interface between your internal systems and the outside world. Skin and mucous membrane cells undergo a high rate of mitosis.
At the other end of the mitotic spectrum, there are other cell types that, once formed, do not undergo much division, such as neurons (nerve cells). Most other cell of the body fall somewhere in between, with a moderate rate of cell division.
* The Cell Cycle *
During the cell cycle, the somatic cells (non reproductive cells) of an organisms grow and divide. It is the process of a single “parent cell” splitting into two identical “daughter cells”. The daughter cells are clones of the parent, and have the same number of chromosomes as does the parent cell.
There are two major phases to the mitotic cell cycle:
* Interphase (which has 3 substages)
* Mitotic Cell Division (which has 4 substages)
~ Interphase ~
During interphase, the cell is not dividing, but is going about the everyday business of being a cell. The DNA is constantly being read and the genetic instructions translated into cellular proteins (polypeptides). During interphase the DNA exists in long strands called chromatin.
Interphase consists of 3 stages:
1. G1 phase: cell grows in size
2. S phase: DNA is copied (replicated) in preparation for cell division
3. G2 phase: cell competes preparations for division
~ Mitotic Cell Division ~
Mitotic Cell Division consists of 2 major processes:
1. Mitosis: Nuclear division (separation of the duplicated genetic material)
2. Cytokinesis: cytoplasmic division (cell divides into two daughter cells)
Mitosis has 4 basic subphases:
1. Prophase: Chromatin strands condense into chromosomes. The chromosomes consist of duplicated, condensed strands of DNA, each copy called a sister chromatid.
2. Metaphase: Duplicated chromosomes align at the cell’s equatorial plane.
3. Anaphase: Sister chromatids separate and migrate to opposite poles of the cell.
4. Telophase: Chromosomes revert to their extended state (chromatin). Nuclear envelope reforms around each of the two groups of genetic material. Cytokinesis begins.
* Cytokinesis *
The cytoplasm and its contents are then divided by a process called cytokinesis. In animal cells a cleavage furrow forms that essentially pinches the cell in two.
I teach biology, and to help my students remember the main stages of the cell cycle I give them the following acronym I-PMAT with the slightly gross sentence “I peed on the MAT.” This helps them remember interphase, prophase, metaphase, anaphase and telophase in the proper order.
* Sources *
Campbell, N. A. & Reece J. B. (2005) Biology, seventh edition. Pearson Education Inc.
Campbell, N. A., Reece J. B. & Simon, E. (2004) Essential Biology with Physiology. Pearson Education Inc.
