Cell division

Cell division (or local doubling) is the process by which a cell, called the parent cell divides into two cells, called daughter cells. Cell divisions is usually a small segment of a larger cell cycle. In meiosis however, a cell is permanently transformed and cannot divide again.

Cell division is the biological basis of life. For simple unicellular organisms such as the Amoeba, one cell division reproduces an entire organism. On a larger scale, cell division can create progeny from multicellular organisms, such as plants that grow from cuttings. But most importantly, cell division enables sexually reproducing organisms to develop from the one-celled zygote, which itself was produced by cell division from gametes. And after growth, cell division allows for continual renewal and repair of the organism.

The primary concern of cell division is the maintenance of the original cell's genome. Before division can occur, the genomic information which is stored in chromosomes must be replicated, and the duplicated genome separated cleanly between cells. A great deal of cellular infrastructure is involved in keeping genomic information consistent between "generations".

Variants of cell division
Cells are classified into two categories: simple, non-nucleated prokaryotic cells, and complex, nucleated eukaryotic cells. By virtue of their structural differences, eurkayotic and prokaryotic cells do not divide in the same way.

Furthermore, the pattern of cell division that transforms eukaryotic stem cells into gametes (sperm in males or ova in females) is different from that of eukaryotic somatic (non-germ) cells.

Prokaryotic cells
Prokaryotic cells are simple in structure. They contain non-membranous organelles, lack a cell nucleus, and have a simplistic genome: only one circular chromosome of limited size. Therefore, prokaryotic cell division, a process known as binary fission, is fast and trauma-free.

The chromosome is duplicated prior to division. The two copies of the chromosome attach to opposing sides of the cellular membrane. Cytokinesis, the physical separation of the cell, occurs immediately.

Somatic eukaryotic cells
Eukaryotic cells, conversely, are complex. They have many membrane-bound organelles devoted to specialized tasks, a well-defined nucleus with a selectively permeable membrane, and a large number of chromosomes (humans, for example, have forty-six). Therefore, cell division in eukaryotic cells must be an order of magnitude more complex than cell division in prokaryotic cells. This is accomplished by a multi-step process:


 * Mitosis: The division of the nucleus, separating the duplicated genome into two sets identical to the parent's.
 * Cytokinesis: The division of the cytoplasm, separating the organelles and other cellular components.

Germ eukaryotic cells
Stem cells that differentiate into germ cells go through a process of cell division similar to mitosis called meiosis. In meiosis, the cell duplicates its genome once but then splits twice. The four resultant cells, called gametes, have half the number of chromosomes. Two gametes from opposing genders fuse together, creating a zygote that has the full number of chromosomes again.

It should be noted that germ-line stem cells can undergo mitosis. For example, sperm precursor cells called spermatogonia constantly reproduce by mitosis, with only a small amount undergoing meiosis to become sperm.

Cellular division degrades in time
Multicellular organisms replace worn-out cells through cell division. In some animals, however, cell division eventually halts. In humans this occurs on average, after 52 divisions, known as the Hayflick limit. The cell is then referred to as senescent. Senescent cells deteriorate and die, causing the body to age. Cells stop dividing because the telomeres, protective bits of DNA on the end of a chromosome, become shorter with each division and eventually can no longer protect the chromosome. Cancer cells, on the other hand, are "immortal." An enzyme called telomerase, absent in normal cells but present in large quantites in cancerous cells, rebuilds the telomeres, allowing division to continue indefinitely.

Phases of Cell Division
Interphase Prophase Metaphase Anaphase Telephase