Gametes, or reproductive cells, are produced in a special kind of cell division called meiosis. During this process the normal number of the cell's chromosomes is reduced by half. Each gamete ends up with a copy of one chromosome from each pair.
Meiosis is a continuous process, but for convenience it is usually examined as a series of separate events, with two divisions.
As the cell prepares for meiosis, the chromosomes gradually become more visible. Their long, thin strands coil back on themselves again and again until they form a thick structure. As the chromosomes coil up, a structure outside the nucleus, called a centriole, divides. The two centrioles move apart, and slender fibers assemble around the outside of the nucleus, forming the spindle. The nuclear membrane disappears.
Each chromosome is made up of two identical chromatids. As the chromatids of hom*ologous chromosomes lie side by side, genetic material may be exchanged. This is known as crossing over and has great significance in heredity.
The hom*ologous chromosomes line up in the center of the cell before moving toward opposite poles. A chromosome from each hom*ologous pair ends up in each cell, but, because of crossing over, some paired chromatids may no longer carry identical genetic information. New nuclear membranes form, and the chromosomes uncoil.
Quite soon the two cells begin the second division. The chromosomes again coil and line up in the centers of the two cells. The chromatids separate and enter into individual gametes. The nuclei re-form, and new nuclear membranes develop.
This process results in four new cells, or gametes. Each gamete contains only one chromosome from each hom*ologous pair. This makes the cell haploid, meaning that it has half the chromosome number of the original diploid cell.
