There are two types of cell division.The one taking place in somatic cells is called Mitosis and the one taking place in sex cells is called meiosis.

Mitosis.

Most eukaryotic cells divide their nucleus by mitosis. In this process, the nucleus divides and forms two identical nuclei. Usually, the cytoplasm divides soon after mitosis, producing two daughter cells with identical nuclei. Most one-celled organisms and most of the cells in multi cellular organisms reproduce by mitosis.

Mitosis takes place in four stages: (1) prophase, (2) metaphase, (3) anaphase, and (4) telophase.

The period between the completion of one nuclear division and the beginning of the next one is called interphase. During interphase, the cell grows and carries on its normal activities, and its chromosomes are difficult to see with an optical microscope. Each chromosome and centriole makes a copy of itself at a particular time in interphase. The original chromosome and its copy are called sister chromatids. They are joined by a structure called a centromere. After duplication of the centrioles and chromosomes, the cell is ready to undergo mitosis.

The first stage of mitosis is called prophase. At this time, the chromosomes begin to coil up, condensing into visible threads that become progressively shorter and thicker. As the chromosomes condense, part of the cytoskeleton organizes into a network of fibres extending across the cell. This network is called the spindle. The centrioles move apart along the fibres of the spindle until they are at opposite sides of the cell. The centrioles mark the poles of the spindle. Toward the end of prophase, the nuclear membrane breaks apart.

In metaphase, the second stage of mitosis, the sister chromatids move to the middle of the spindle, called the equator. They are still joined, but they line up on opposite sides of the equator. Each sister chromatid is attached at its centromere to at least one spindle fibre.

In the third stage, called anaphase, the centromeres divide, and each sister chromatid becomes a new chromosome. The new chromosomes separate and move to opposite poles.

In telophase, the final stage of mitosis, individual chromosomes uncoil and again become hard to see. A new nuclear membrane forms around each new daughter nucleus. Also, the spindle breaks down, and the proteins from spindle fibres form part of the networks of cytoskeleton in the daughter cells.

Usually, division of the cytoplasm also begins during telophase. In animal cells, cytokinesis occurs when the cell membrane pinches between the two daughter nuclei to form two daughter cells. In plant cells and other cells that have a cell wall, a cell wall grows between the daughter nuclei, forming two cells. In either case, each new cell has as many chromosomes as the original cell and contains the same hereditary information.

Cytokinesis does not always create two identical cells. Sometimes, one of the daughter cells receives more of one kind of organelle than does the other cell. Cytokinesis may also result in two different sized cells. In addition, if mitosis occurs more than once in the same cell without cytokinesis, the cell can have more than one nucleus.

Mitosis in plant cells differs somewhat from that in animal cells. Cells in multicellular plants do not have centrioles, but they do form a spindle similar to that formed in animal cells.

Chromosome is a threadlike structure found in the cells of all organisms. It can be seen through a microscope as a particle only when the cell is ready to divide into two cells. Before division begins, the chromosomes are duplicated. During division, each duplicated chromosome forms into a pair of rods. The new cells that are formed receive one rod from each pair. The new cells then have a set of chromosomes exactly like those of the original cell. Each species (kind of organism) has a characteristic number of chromosomes in each body cell. Human beings typically have 46 chromosomes (23 pairs) in most of their cells. Chromosomes are made up largely of DNA (deoxyribonucleic acid) and proteins. DNA is the coded information for every living thing's heredity (the passing of characteristics from parents to offspring). Chromosomes consist of many DNA units called genes. For a more detailed discussion of chromosomes, see CELL;

Meiosis.

It consists of two divisions. (1) Reduction Division (2) Equal Division.

Human beings and many other living things reproduce sexually. A new individual can be created only if a male sex cell, called a sperm, unites with a female sex cell, called an egg. Sex cells, also called germ cells, are produced in special reproductive tissues or organs. At first, new sex cells are produced by mitosis. These cells then go through a special kind of cell division called meiosis. To understand why, we must understand something about heredity.

Every species of life has a certain number of chromosomes in each of its somatic (body) cells. These chromosomes exist in pairs. For example, human beings have 23 pairs of chromosomes; frogs, 13 pairs; and pea plants, 7 pairs. The members of each pair are similar in size, shape, and hereditary content. Suppose the egg and sperm cells had the same number of chromosomes as all the other cells in an organism. If they united, the somatic cells in the offspring would have twice the number of chromosomes that they should have.

For example, human beings have 46 chromosomes in their somatic cells. If the father's sperm cells and the mother's egg cells also contained 46 chromosomes, their child's somatic cells would have 92 chromosomes. The next generation would have 184, and so on. To prevent this from happening, the sex cells have half the chromosomes found in the somatic cells. This is accomplished by meiosis.

Meiosis consists of two separate nuclear divisions of sex cells. Each chromosome duplicates before the first division. Then each chromosome, which now consists of two joined sister chromatids, lines up side by side with the other chromosome of its pair. Each pair of doubled chromosomes moves to the equator. The paired chromosomes then separate. One chromosome, still consisting of two chromatids, goes to one pole. The other chromosome moves to the opposite pole. Cytokinesis occurs, dividing the cytoplasm into two. Each daughter cell thus receives one chromosome, made up of two sister chromatids, from each original pair. These new cells then divide.

In this second division, one of each of the sister chromatids goes to each new daughter cell. Thus, the two divisions of meiosis produce a total of four cells. Each cell contains half the number of chromosomes found in all the other cells of the organism. The second division is exactly identical to mitosis.

Human sperm and egg cells have 23 chromosomes each. When a sperm and an egg combine in a process called fertilization, they produce a single cell--the fertilized egg--with 46 chromosomes, or 23 similar pairs. A child develops from this egg.

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