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Cell division is a process by which a cell, called the parent cell, divides into two cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle. In meiosis however, a cell is permanently transformed and cannot divide again.
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. Cell division also 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.[1] A human being's body experiences about 10,000 trillion cell divisions in a lifetime.[2]
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
Cells are classified into two categories: simple, non-nucleated prokaryotic cells, and complex, nucleated eukaryotic cells. By dint of their structural differences, eukaryotic 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 generally much more simple in structure when compared to eukaryotic cells. 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. 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.
Eukaryotic cells
- 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.
- Meiosis: The division of the nucleus in sex cells, making one cell into four sex cells identical to the parent sex cell.
Degradation
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, present in large quantites in cancerous cells, rebuilds the telomeres, allowing division to continue indefinitely.
See also
References
- Morgan DO. (2007) "The Cell Cycle: Principles of Control" London: New Science Press.
- J.M.Turner Fetus into Man (1978, 1989). Harvard University Press. ISBN 0-674-30692-9
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