Cell division
Cell division
Cell division is the process by which a single living cell splits to become two cells. All cells divide at some point in their lives. Cell division occurs in single-celled organisms like bacteria, and in multicellular organisms like plants, animals, and fungi. Many cells continually divide; examples include the cells that line the human digestive tract and skin cells.
Cell division is can occur by means of mitosis or meiosis. Mitosis is simple cell division that creates two daughter cells that are genetically identical to the original parent cell. The process varies slightly between prokaryotic organisms (organisms whose genetic material is not enclosed within a membrane) and eukaryotic organisms (whose genetic material is enclosed within a specialized membrane). In eukaryotes, mitosis begins with replication of the deoxyribonucleic acid (DNA) within the cell to form two copies of each chromosome. Once two copies are present, the cell splits to become
two new cells by cytokinesis, or formation of a fissure. Mitosis occurs in most cells and is the major form of cell division.
The process of meiosis is the production of daughter cells having half the amount of genetic material as the original parent cell. Such daughter cells are designated as haploid. Meiosis occurs in human sperm and egg production in which four haploid sex cells are produced from a single parent precursor cell. In both mitosis and meiosis of nucleated cells, shuffling of chromosomes creates genetic variation in the new daughter cells. These very important shuffling processes are known as independent assortment and random segregation of chromosomes.
Cell division is stimulated by certain kinds of chemical compounds. Molecules called cytokines are secreted by some cells to stimulate others to begin cell division. Also, contact with adjacent cells can control cell division. The phenomenon of contact inhibition is a process where the physical contact between neighboring cells prevents cell division from occurring. When contact is interrupted, however, cell division is stimulated to close the gap between cells. Cell division is a major mechanism by which organisms grow, tissues and organs maintain themselves, and wound healing occurs. Cancer is a potentially deadly form of uncontrolled cell division.
while prokaryotes divide via mitosis that esentially involves the duplication of the genetic material and the separation of each copy into the newly divided cells, eukaryotes undergo a more complex process of cell division because DNA is packed in several chromosomes located inside a cell nucleus. In eukaryotes, cell division may take two different paths. Mitosis is a cellular division resulting in two identical nuclei is performed by somatic cells. The process of meiosis results in four nuclei, each containing half of the original number of chromosomes (the haploid cells). Sex cells or gametes (ovum and spermatozoids) divide by meiosis. Both prokaryotes and eukaryotes undergo a final process, known as cytoplasmatic division, which divides the parental cell in new daughter cells.
The series of stages that a cell undergoes while progressing to division is known as the cell cycle. Cells undergoing division are also termed competent cells. When a cell is not progressing to mitosis, it remains in phase G0. Therefore, the cell cycle is divided into two major phases: interphase and mitosis. Interphase includes the phases (or stages) G1, S, and G2, whereas mitosis is subdivided into prophase, metaphase, anaphase and telophase.
The cell cycle starts with the active synthesis of RNA and proteins, which are necessary for young cells to grow and mature. This phase varies greatly among eukaryotic cells of different species and from one tissue to another in the same organism. Tissues that require fast cellular renovation such as mucosa and endometrial epithelia have a shorter G1 period than those tissues that do not require frequent renovation or repair, such as muscles or connective tissues.
The cell cycle is highly regulated by several enzymes, proteins, and cytokines in each of its phases, in order to ensure that the resulting daughter cells receive the appropriate amount of genetic information originally present in the parental cell. In the case of somatic cells, each of the two daughter cells must contain an exact copy of the original genome present in the parental cell. Cell cycle controls also regulate when and to what extent the cells of a given tissue must proliferate, in order to avoid abnormal cell proliferation that could lead to dysplasia or tumor development. Therefore, when one or more of such controls are lost or inhibited, abnormal overgrowth will occur and may lead to impairment of function and disease.
When occurring properly, the various phases of the cell cycle result in the duplication of the genetic material and the segregation of the material in the correct amounts into the newly divided cell.
Terry Watkins