Why Do Our Cells Divide?
Human Life Cycle
You grew from a zygote, or fertilized egg (the fusion of two cells: an ovum and a sperm) into an organism with trillions of specialized cells. Mitosis is the process that enabled you to grow and develop after that fateful meeting of egg and sperm became ‘you’. That is the human life cycle.
Cell Replacement
So cells must divide in order for an organism to grow and develop, but cell division is also required for tissue repair, maintenance, cell turnover and replacement.
Many tissues of your 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 mucus membranes of your body; the moist areas of the GI tract and urogentialtract that are the interface between your internal systems with the outside world.
Some cell types of your body, once formed, do not undergo much division, like neurons (nerve cells), for example. Others, like the skin cells mentioned above, undergo a high rate of mitosis. Most other cell types are in between, with a moderate rate of cell division.
Eukaryotic Cell Cycle
During the cell cycle, somatic cells (non reproductive cells) of eukaryotic organisms grow and divide. It is the process of a single cell (‘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 cell cycle:
- Interphase (3 substages)
- Mitosis (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 polypeptides, so the DNA exists in long strands called chromatin.
Interphase consists of 3 stages:
- G1 phase: cell grows in size
- S phase: DNA is copied (replicated) in preparation for cell division
- G2 phase: cell competes preparations for division
Mitosis
Mitotic Cell Division consists of 2 major processes:
- Mitosis: Nuclear division (separation of the duplicated genetic material)
- Cytokinesis: cytoplasmic division (cell divides into two daughter cells)
Mitosis has 4 basic subphases:
- Prophase – Chromatin strands condense into chromosomes. The chromosome consists duplicated, condensed strands of DNA, each copy called a sister chromatid.
- Metaphase – Duplicated chromosomes align at the cell’s equatorial plane.
- Anaphase – Sister chromatids separate and migrate to opposite poles of the cell.
- Telophase – Chromosomes revert to their extended state (chromatin). Nuclear envelope reforms around each of the 2 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.
How to Remember the Stages of the Cell Cycle
Students trying to remember the main stages of the cell cycle often use 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.
More Information
See the science education website Science Prof Online to learn more about cell biology, or look to additional Suite101 biology articles, including Asexual & Sexual Reproduction, Nucleotides & Nucleic Acids, and Prokaryotic & Eukaryotic Cells
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.
Tami Port - Tami Port is a college professor of cell and microbiology and creator of ScienceProfOnline.com, a free science education website.
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