Meiosis is the division of cells for the purpose of making gametes, or sex cells. Sex cells are different from somatic, or body cells. Sex cells have half the chromosomes of somatic cells. This halving of chromosomes is achieved through the steps of meiosis.
In normal somatic body cells, like a liver cell or a kidney cell, chromosomes are arranged in pairs. You get one copy of your chromosomes from mom, and one copy of the chromosomes from dad. These two copies work together. The genes on each pair of chromosomes match, and the chromosome pairs are known as homologous pairs, meaning that they have the same genetic information in the same location.
Having pairs of chromosomes is known as being diploid. Somatic cells are diploid. Human diploid cells have 23 pairs of chromosomes, or 46 total chromosomes. Gametes, on the other hand, are haploid, a term referring to only having one copy of each chromosome. Human haploid gametes have 23 un-paired chromosomes.
To successfully reproduce, your homologous pairs in somatic cells must be split in half to send to your offspring. Two haploid gametes will unite to make a diploid zygote, the first initial cell of a new individual. The zygote is the fertilized egg cell. Thus, meiosis and the reduction in chromosomes from diploid to haploid becomes necessary for successful reproduction. Having even a single extra copy of a chromosome can be fatal or genetically disabling. For example, an extra copy of the 21st chromosome is the cause of Down Syndrome. An extra copy of the 23 chromosome, the ones that contribute genes for gender determination, can result in disorders like Klinefelter's Syndrome. Successfully reducing chromosomes from pairs to single copies is essential to successful reproduction.
Somatic cells divide through a process called mitosis. Mitosis consists of steps that duplicate the genetic information, condense it into easily sort-able chromosome form, arrange it along the mid-line of a cell, split the duplicated chromosomes in half, and the whole process concludes when the entire parent cell splits in half, resulting in identical daughter cells. Each new daughter cell is a somatic cell with identical copies of genetic material, and the chromosomes are still in pairs.
The cell's life cycle begins with Interphase. Interphase is like the "childhood" stage for a somatic cell. During this time a cell grows, does its job, and begins to ready itself for cell division. By doubling the genetic material, the DNA, the cell ensures that its offspring daughter cells will get complete copies of instructions.
After Interphase, Mitosis truly begins. Mitosis, or the division of the nucleus of a cell where the genetic material is contained, has four steps. Prophase, the first phase, is when the DNA condenses into a sort-able form, the chromosome. In prophase, the membrane around the nucleus also dissolves, and cell structures known as centrioles begin moving to opposite sides of the cell in order to make a spindle apparatus, which has fibers for sorting chromosomes in later steps.
The second step of mitosis is Metaphase. Meta, meaning "middle", refers to the chromosomes movement to the center of the cell. As they arrange themselves along the equator of the cell, the centrioles for the spindle apparatus, and spindle fibers attach themselves to the center portion of each chromosome. The chromosomes, at this time, are X-shaped, and each side of the X is identical. Remember, earlier in Interphase the DNA replicated itself, and so the identical copies are held together in one single X-shaped chromosome.
In the third step of Mitosis, Anaphase, the copied DNA on the X-shaped chromosomes is split apart. One set of DNA is pulled to one pole of the cell by the spindle fibers contracting, and the identical copy of the DNA is pulled to the other pole of the cell. In this way, two nuclei are beginning to form.
Finally, mitosis concludes with Telophase, the ending phase. During telophase, the two sets of DNA each get their own nuclear membrane, and for a short period of time there is one cell with two identical nuclei inside it. However, this does not last long as the cell will quickly enter a stage known as Cytokinesis, or cell splitting. The parent cell's exterior cell membrane pinches in, and if there is a cell wall a new portion known as a cell plate begins to grow through the center of the parent cell. In this way, the parent cell is split in half. Each new daughter cell, at the end of cytokinesis, contains a nucleus with DNA that is identical to the original parent cell and to its clone, the other daughter cell.
Meiosis is very similar to somatic cell division, except in that the cell goes through the steps two times. This enables one parent cell to make four daughter gametes, and it also allows the genetic material to be split in half twice. One time, the doubled X-shaped chromosomes are split. The other time, the homologous pairs of chromosomes are split, ensuring that the gametes only have one copy of each. Thus, when parents' gametes units, sperm and egg, each contributes one half of the new homologous chromosome pairs.
Meiosis begins after Interphase, similar to mitosis. The parent cell has homologous pairs of chromosomes, and the pairs are X-shaped indicating two identical copies of the DNA attached at the center point of each chromosome. Similar to mitosis, the first stage is Prophase, but in Meiosis it is known as Prophase I. This indicates it is the first of the two times the parent cell will go through Prophase. In Prophase I, the same thing happens as in prophase of Mitosis. The DNA condenses into the chromosomes, and the nuclear membrane dissolves. The centrioles also move to the poles of the cell.
In Metaphase I the chromosomes line up along the equator of the cell, but in Meiosis they stay paired. Instead of a single line of chromosomes, meiosis Metaphase I has a double line of paired chromosomes. The spindle apparatus separates homologous pairs of chromosomes in Anaphase I, and in Telophase I two daughter cells temporarily form with a single copy of each chromosome, still in the X-shape with doubled genetic information.
The cell then moves into Prophase II instead of cytokinesis. In Prophase II the cell readies itself to divide once again. Metaphase II causes the X-shaped chromosomes to line up along the equator of the cell, single file this time. Anaphase II splits the X-shaped chromosomes in half, and Telophase II causes each of the 2 daughter cells to have 2 nuclei inside. When cytokinesis finally occurs, the original parent cell has divided into 4 gametes with nuclei that only have a single copy of the genetic information for each chromosome. These gametes will be sperm cells in a male individual, and egg cells in a female individual.
Thus, a parent cell will make sex cells through the process of meiotic cell division. This enables the individual to successfully reproduce.