Cell Divisions - Nuclear division and reduction division

Cell Division 
The two types of cell division are termed mitosis and meio­sis. Mitosis occurs during growth and repair, and meiosis occurs during gametogenesis, the production of gametes­that is, the sperm and eggs.
Mitosis 
During ordinary cell division, called mitosis, a mother cell divides, producing two daughter cells. In humans, the mother cell has 46 chromosomes, and the two daughter cells that result also have 46 chromosomes.
A mother cell, has only two pairs of chromosomes. These chromo­somes are at first single, containing a single DNA helix. Be­fore cell division takes place, DNA replication occurs as the
chromosomes duplicate. A duplicated chromosome contains two identical parts (called chromatids). During mitosis, these portions separate, becoming daughter chromosomes. A daughter chromosome contains one DNA helix again.
Mitosis actually requires several stages, dur­ing which the nuclear envelope disappears and a spindle apparatus with spindle fibers forms. The chromosomes are attached to the spindle fibers by structures called cen­tromeres. Once the chromosomes have moved to the cen­ter of the mother cell the centro meres split, and the daughter chromosomes move toward the poles. Daughter nuclei form, and the cytoplasm divides by furrowing.
Mitosis is the type of cell division required for growth and tissue repair. The process of mitosis assures that each cell in the body has the same number and kinds of chro­mosomes and, th ere rore, the same genes (since the genes are on the chromosomes).
Ordinarily, a cell divides only about 50 times; with maturity, a cell stops dividing. However, cancer cells never mature, and instead, continue to divide indefinitely.
Mitosis ensures that each cell in the bdy in generically identical. At the time of division, a chromosome has two portions. When these separate, each daughter celll receives the same number of the mother cell.
Meiosis 
Meiosis is a special type of cell division that occurs only during the production of eggs and sperm. During meio­sis, which takes two rounds of cell division, the chromo­some number is reduced from 46 chromosomes to 23 chromosomes.
In females, meiosis occurs during oogenesis (o"o-jen'e-sis) (egg production) in the ovaries. In males, meiosis occurs during spermatogenesis (sper"mah-to­jen'e-sis) (sperm production) in the testes. Following sexual intercourse, a sperm fertilizes an egg, and a new indi­vidual begins development. Because a sperm carries 23 chromosomes and an egg carries 23 chromosomes, the new individual has 46 chromosomes, which is the normal number for human beings. In this way, both parents con­tribute 23 chromosomes to the new individual.
Meiosis is a special type of cell division that reduces the chromosome number. Meiosis occurs during oogenesis and spermatogenesis.

Posted in Human Anatomy and Physiology