Cell division
Kategorie: Biológia (celkem: 966 referátů a seminárek)
Informace o referátu:
- Přidal/a: anonymous
- Datum přidání: 23. února 2007
- Zobrazeno: 1623×
Příbuzná témata
Cell division
Cell divisionOne of the most important natural needs of every being is its reproduction - a part of the life (or cell) cycle. Mitosis - specified as a cell division (division of nucleus) - is the major stage in the reproduction of quite a lot of organisms and their parts, as well.
Firstly, I would like to explain the phases occurring in the mitosis. Those are the prophase, metaphase, anaphase and telophase. In the prophase (“before form”), chromosomes condense (become shorter and thicker); both the nucleolus and the nuclear envelope disappear. Centrioles replicate and separate, and form a bundle of microtubule fibers (each of them "catches" one chromosome). In the later metaphase (“middle form“) chromosome align together on a central (metaphase) plate and start doing dancelike movements. Anaphase (“again form”) begins, when centromeres that link the sister chromatids, together creating the whole chromosome, split. Each spindle fiber pulls one chromatid (one half of the particular chromosome) apart and toward opposite poles; it moves with floppy arms lagging behind the centromeres. Then chromatids start behaving like separate chromosomes. The final telophase (“end form”) starts as the spindle fibers disperse and chromatids arrive at the opposite poles. Then the nucleolus and nuclear envelope reappear, chromosomes decondense and the cytoplasm starts dividing. Mitosis is the shortest phase of the cell cycle (which is a part of the life cycle), though, it is not less important than the other phases (interphase and later cytokinesis-division of cytoplasm). There are three main importance of mitosis: growth of multicellular organisms, repairing or replacing of old and damaged tissues and cells and finally asexual reproduction. Every cell has a maximum size that it cannot grow bigger than. Usually it is very small and that is why every tissue or organ in multicellular organisms is built by a great amount of specialized cells. Because of the upper border of cells’ size, the only chance for the organ how to grow up is multiplying cells by their dividing. It would be very complex if every cell needed to combine specialized male and female cells (gametes) for replicating itself. In this case, no meiosis (sexual reproduction) is needed and therefore mitosis is the easiest and fastest way of dividing cells and then growth of certain organs. An example of this kind of growth is almost every being – plants, animals and fungi, as well.
Very similar is the situation by repairing (or mostly replacing) tissues and cells that are either damaged, bad, missing or old. That happens not only by some “strange” organisms (for example by the sea star when one or more of its arms are being cut), but you can also observe it on yourself, for example when you cut your finger. Well, if you cut it down as a whole, it will not grow back to you, of course, that can happen neither to most plant nor to most animal or fungi. Therefore I used the word strange for describing the skill of some beings to replace the whole organ. This ability is present by only a small part of the whole multicellular organisms system. The third, also very important function of mitosis is the asexual reproduction of organisms as whole (not only their parts), when the new offspring grows directly from the parent and is genetically identical to it. This kind of reproduction, used by many types of plants (different kinds of berries, banana palm, potato, the flower fuchsia and many other) and a few kinds of animals (for example one sea brown sun-like animal with a very interesting name which I cannot remember on), is exploited very often as the only way how to reproduce certain, mentioned upper, plants.
Although being very important, mitosis has a bad side, sort of a bad importance, as well; I mean the uncontrolled division, which ends with the disease cancer. Normally, there is a major external factor for control of the cell division – the physical contact. When, for example, filling a gap of skin being cut, cells divide until they touch each other, then they stop dividing and that is how a layer one cell thick is formed. By the uncontrolled division cells fail to stop growing and even dividing on contact with other cells. They create masses called malignant tumors that attack and invade healthy tissues. These tissues are then “killed” (destroyed) – that is why and how cancer is so dangerous for the organism.
The mitosis, consisting of its four phases, is very important not only by reproduction of whole organisms, but also by repairing some of their parts. It is considered to be at least equal to the meiosis and the sexual reproduction. However, it has a bad side, as well, and that is the maybe most problematic disease, the cancer. .