Cell division

From Free net encyclopedia

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 monkeys

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".

1 Variants of cell division
- 1.1 Somatic eukaryotic cells
- 1.2 Germ eukaryotic cells
2 Cellular division degrades in time
3 Phases of Cell Division
4 See also
5 References
6 External links

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Variants of cell division

Image:Three cell growth types.png 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.

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Somatic eukaryotic cells

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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.

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Germ eukaryotic cells

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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.

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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.

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