All cells develop from previously existing cells through cell division. In a unicellular organism, cell division is equivalent to the reproduction of the organism. In multicellular organisms, cell division is responsible for the growth of the organism and the replacement of old or worn-out cells. At an early stage in the life of a multicellular organism, cell specialization, or differentiation, begins; it is by this process that new tissues and systems are formed.
In cells with nuclei, an important step in cell division is the division of the nucleus. The nucleus (except in sex cells) divides by a process called mitosis. Through mitosis, each new cell receives a copy of the parent cell's genetic material. Mitosis occurs in four successive stages: prophase, metaphase, anaphase, and telophase. There are no sharp dividing lines separating the various stages, and the events of each stage vary somewhat in different organisms. The four stages are preceded by interphase, the stage during which the growth of a cell occurs. Near the end of interphase, each chromosome duplicates itself, forming two identical strands. Each strand is called a chromatid; one is attached to the other at a point called the centromere. Chromosomes are invisible during interphase because their DNA is stretched out. Centrioles, if present, also duplicate during interphaseone pair becoming two pairs. In general, mitosis works as follows:
The DNA condenses and the chromosomes become visible. At first, the chromosomes are long and thin and appear to be individual structures. As prophase continues, the chromosomes become short and thick; each chromosome can then be seen to be made up of two chromatids. Filaments collectively called an aster form around each pair of centrioles. Other filaments, called spindle fibers, form between the two pairs of centrioles, which move to opposite poles of the cell. The nucleolus and the nuclear membrane disintegrate.
The chromatid pairs align themselves in the center of the cell and each centromere becomes attached to one spindle fiber from each centriole pair. The centromere divides and the separated chromatids become independent chromosomes.
The new chromosomes move apart from each other to opposite poles. The spindle fibers between the centromeres and centriole pairs shorten.
After the chromosomes reach the poles, the spindle fibers disappear and a nuclear membrane forms around each new group of chromosomes.
The cytoplasm then divides by a process called cytokinesis, and the original large cell becomes two smaller cells. Each new cell will take on food and grow, and may in turn divide by the same process.
Gametes (sex cells) are produced by a type of cell division called meiosis. Meiosis consists of two successive cell divisions that resemble mitosis, but the chromosomes are duplicated only once. Gametes therefore have half the number of chromosomes normally found in body cells.
At a very early stage in the life of a multi-cellular organism, the organism contains certain unspecialized cells called stem cells. These cells are capable of dividing indefinitely with little change, but they can also give rise to specialized cells. Stem cells continue to exist in a mature organism and are crucial in the development and balance of an organism's many types of tissue.