The reproduction of cells involves the division of existing cells. This process of cell division is known as mitosis. This means the equal distribution of the genetic information stored in the cell nucleus and division of the cytoplasm. This process of cell division takes between 20 minutes and 4 hours and can be broken down into five different phases, each of which smoothly transitions into the next (Fig 2-3):
- Prophase (pro = before): The nucleus absorbs water and swells up while the chromosomes from the chromatin framework develop into fine threads. This differentiation is also known as spiralization. During prophase, the nuclear membrane breaks down, and the centriole divides and migrates to the poles of the cell.
- Metaphase (meta = after): The chromosome threads become short and thick and split into two chromatids each. These chromatids come to lie on the equatorial plate.
- Anaphase (ana = onto): The halved chromosomes (chromatids) migrate to the poles, being pulled by the spindle fibers of the centri-ole. The so-called stem fibers (protein threads) between the chromatids also push them to the poles.
- Telophase (telos = target): The cytoplasm starts to narrow while migration of the chromatids to the poles is completed. A chromatin structure re-forms. The spindle and stem fibers break up so that a new nuclear membrane forms around each chromatin structure.
- Reconstruction phase: The identically structured daughter cells thus formed develop working nuclei; ie, the cell nuclei take on their typical form and start their working functions. Constriction of the cytoplasm continues until complete separation. What is known as the interphase (resting phase) now begins. For most of their lives, the cells are in the interphase, ie, in a working state with their defined functions.
There are various forms of cell division. Mitosis means the equal distribution of the genetic stock between the daughter cells. Endomitosis denotes division of the chromosomes without the nuclear membrane breaking down or the cell body dividing. This gives rise to cells with nuclei that have multiple sets of chromosomes.
Amitosis means division of the cell nucleus without ordered, equal distribution of the chromosomes. If the cytoplasm also divides, this produces daughter cells whose nuclei have different proportions of genetic material. This type of cell division happens in:
- Striated muscle cells
- Osteoclasts (cells that break down bone tissue)
- Diseased cells (eg, tuberculosis cells)
Meiosis (in Greek: reduction) is a special form of nuclear division that occurs in living creatures that procreate sexually. Mitosis is an asexual form of reproduction, whereas meiosis is a prerequisite for sexual reproduction.
In meiosis, sperm cells and egg cells with half sets of chromosomes are produced so that when an egg cell (ovum) is fertilized by a sperm cell, another complete set of chromosomes can arise. Meiosis therefore ensures that sexual reproduction does not lead to a linear increase in the number of chromosomes from generation to generation.
Meiosis always involves two different processes of division (Fig 2-4). First the sexual germ cell divides as in mitosis with doubling of the chromosomes. This means it divides normally, as described above, to give two daughter cells. This is followed by nuclear division, in which the sets of chromosomes are halved. This second process is known as reduction division. As a result of the process of meiosis, four sperm or egg cells, each with half of a set of chromosomes, are therefore produced from one sexual germ cell (gamete).
On fertilization, the half chromosomal sets lead to the normal genetic stock in the next generation. Thus, any linear increase in the number of chromosomes from generation to generation is prevented.
As the fertilized egg cell divides again and again, the cells differentiate to form tissues during human embryo development. As this development continues, cell differentiation results in cells of different formations determined by their functional specialization.