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Cell division is a process by which a cell, called the parent cell, divides into two or more cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle. This type of cell division is known as mitosis, and leaves the daughter cell capable of dividing again. In another type of cell division present only in eukaryotes, called meiosis, a cell is permanently transformed into a gamete and cannot divide again until fertilization.
For simple unicellular organisms such as the Amoeba, one cell division is equivalent to reproduces-- an entire new organism is created. On a larger scale, mitotic 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".
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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
Eukaryotic cells
Eukaryotic cells consist of cell membrane, organelles which represent the functional components for storage, excretion, digestion and nucleus (centre of the cell), it contains all the vital information needed by the cell or the whole organism to function, grow and reproduce.
Eukaryotic cells are found in humans, plants and animals, also algae, and protozoa. Eukaryotic cells have both a cellular membrane and a nuclear membrane. Eukaryotic genome is more complex than that of prokaryotes and distributed among multiple chromosomes.
- Mitosis: The division of the parent nucleus into two daughter nuclei, separating the duplicated genome into two sets, each identical to the parent cell's genome.
- Cytokinesis: The pinching and division of the cell membrane and cytoplasm, separating the recently divided nuclei, the organelles, and other cellular components.
- Meiosis: The division of the nucleus in sex cells that reduces the diploid number of chromosomes to a haploid number in order to facilitate sexual reproduction.
Examples of cell division in multicellular eukariotic organisms include repair, growth, and development. An injury or wound is healed when the cells exposed by the injury divide at an excellerated rate until they come into contact with other cells. After this the cells return to a more typical division rate. Cell division causes an organism to grow as long as the rate of cell division exceeds normal cell death. As cells divide and become more numerous they are located in more diverse physical and chemical environments. These variations in local conditions influence the cells to alter gene expression causing the cells to differentiate and become more specialized allowing an organism to develop.
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.citation needed 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.citation needed 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
Other references
- ^ Maton, Anthea; Hopkins, Jean Johnson, Susan LaHart, David, Quon Warner, David, Wright, Jill D (1997). Cells: Building Blocks of Life. New Jersey: Prentice Hall, 70-74. ISBN 0-13423476-6.
- ^ Quammen, David (April 2008), "Contagious cancer: The evolution of a killer", Harper's 316(1895): 42.
External links
- How Cells Divide: Mitosis vs. Meiosis
- The Mitosis and Cell Cycle Control Section from the Landmark Papers in Cell Biology (Gall JG, McIntosh JR, eds.) contains commentaries on and links to seminal research papers on mitosis and cell division. Published online in the Image & Video Library of The American Society for Cell Biology
- The Image & Video Library of The American Society for Cell Biology contains many videos showing mitosis and cell division.
- Videos of the first cell divisions in Xenopus laevis embryos (side view and top view), acquired by MRI (DOI of paper)
- Images : Calanthe discolor Lindl. - Flavon's Secret Flower Garden
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- This page was last modified on 28 August 2008, at 18:56.
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