Meiosis is a type of cell division
occurring in eukaryote
s in preparation for sexual reproduction
. The net result of meiosis is usually the production of four cells which have half the number of chromosome
s as the somatic
cells of the organism have. If the somatic cells are diploid
, the products of meiosis will be haploid
. In some types of organisms (such as in plant
s) it's more complicated, but you can think of meioisis as resulting in the production of gamete
Meiosis happens in such a way as to ensure that the chromosomes get divided among the resulting cells in an orderly fashion: Where the parent cell has two of each type of chromosome (they are diploid), the offspring cells end up with one of each type (they are haploid).
Here's an overview of how it happens:
(details have been dredged from foggy memory and checked against http://www.bowdoin.edu/dept/bio/bio104/mitosis/tool2.html. Some details have been omitted for the sake of simplicity.)
- During interphase (a part of the cell cycle in which the cell is metabolically active, and no division is occurring), the chromosomes replicate. Each one becomes a strand of two DNA double helices -- two identical copies. They are held together by a centromere.
- The chromosomes condense to the point that they are visible under a microscope as the familiar X shape (the centromere is at the place where the arms of the X cross); this step is called prophase I. The wall of the cell's nucleus is dissolving.
- Each pair of homologous chromosomes pairs up, and the pairs line up on a plane that bisects the cell. This step is called metaphase I.
- The homologous pairs split apart, with one member going to opposite poles of the cell. This step is anaphase I. The process of homologous pairs splitting up and going into different offspring cells is called segregation.
- At the next step, telophase I, one member of each homologous pair has arrived at opposite ends of the cell.
- New cell membranes form, dividing what was once one cell in two. New nuclear membranes might form. This step is called cytokinesis I.
- The two cells may enter a resting phase (interphase II) before continuing with meiosis.
- In each of these two cells, the nuclear membrane dissolves and the chromosomes again condense. (prophase II)
- The chromosomes line up on a plane that bisects each cell (metaphase II).
- Each doubled chromosome splits in two as the centromeres detach; the two strands move to opposite poles of the cell (anaphase II).
- At telophase II, all the single chromosomes have arrived at opposite ends of the two cells.
- New cell membranes form, dividing each of the cells in two for a total of four cells; four new nuclear membranes form around the chromosomes in each cell. This last step is called cytokinesis II.
- After this last step, there are four cells, each with one copy of each chromosome, where the parent cell had two of each.
Errors can happen in meiosis, in which the chromosomes don't segregate properly and some resulting cell has an extra copy of one chromosome (is trisomic for that chromosome) and another is missing one (is monosomic for that chromosome). This nondisjunction can occur either during the first or second divisions of meiosis. Usually, this will result in gametes that will not survive, or, if they do survive to form a zygote, that zygote won't survive. In humans, there are some well-known conditions resulting from meiotic errors that aren't always severe enough to kill gametes or zygotes, and in some cases result in people that are almost completely normal.