Mitosis and Meiosis

All cells come from other cells by one of two different kinds of cell division.

mitosis division

When a cell divides, its nucleus goes through a very careful process to assure that the vital genetic information is distributed to the daughter cells as perfectly as possible.

After the nucleus divides, the rest of the cell generally also divides (cytokinesis).

The two kinds of nuclear division are called mitosis and meiosis. It is not correct to describe either of these processes as cell division. They are specifically the division of the nucleus of the cell.

These two kinds of nuclear division have different purposes. Mitosis exactly duplicates the parent nucleus, producing two daughter nuclei which have the same number of chromosomes as the parent nucleus did, and which have exactly the same genetic information on those chromosomes as each other, and as the parent. Your book shows how this process occurs.

When the nucleus of a cell divides by mitosis, the division of the cell is called mitotic cell division. This kind of cell division is what produces the trillions of genetically identical cells in a human body. It's also the kind of division used for repair processes like healing wounds and regenerating the tails of certain kinds of lizards. There are organisms which use mitotic cell division in their asexual reproductive techniques. (NOTE: Mitotic cell division is not asexual reproduction; it is used in asexual reproduction processes.)

While mitosis is almost always accompanied by cytokinesis, there are situations in which one of these processes may occur without the other. For example, skeletal muscle cells are very large cells with several nuclei in each cell. During the differentiation of these cells, the nucleus of a cell divides, but cytokinesis never occurs. The cell just gets bigger. Then the two nuclei divide again, and again no cytokinesis occurs. The cell just gets even bigger. So mitosis can happen without any cell division.

The other kind of nuclear division, meiosis, accomplishes quite a different result. Unlike mitosis, which can occur in any cell which has chromosomes, meiosis can only occur in a cell with an even number of chromosome sets, usually two (diploid). Meiosis clearly evolved from mitosis.

In meiosis, the nucleus actually divides twice. In the first division, the process carefully separates the chromosomes into two clusters, each containing one of each kind of chromosome (one set). This means that the daughter nuclei produced by this first division are haploid, not diploid. The second division works pretty much like mitosis. At the end of this second division, there are four daughter nuclei, all haploid. So one way in which meiosis is different is that it reduces the chromosome number from diploid to haploid.

Something else different happens in meiosis. During the first meiotic division, a couple of kinds of events mix and match the genes of the two sets of chromosomes, resulting in daughter nuclei which are genetically different from each other.

Cell division in which the nucleus divides by meiosis is called meiotic cell division. This kind of cell division is pretty much always part of a sexual life cycle, though it is not correct to call meiosis "sexual reproduction," or "sexual cell division." In animal life cycles, meiotic cell division results in the production of gametes. In plant life cycles, there is an additional stage of the life cycle between the meiosis process and the production of gametes, but meiotic cell division still serves to reduce the chromosome number from diploid to haploid.

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Updated 09 Sept 2004