Meiosis

Meiosis is a specialized form of cell division that occurs during the formation of sex cells (gametes). Although meiosis may seem much more complicated than mitosis, it is really just two cell divisions in sequence. It results in the production of two sex cells (gametes) that fuse together during sexual reproduction. The other main purpose of meiosis, other than cell division, is to generate genetic diversity. This allows the cell to adapt and survive, which is very important in sexually reproducing organisms.

Meiosis starts with diploid cells, which have two sets of chromosomes. The nuclear envelope divides twice during meiosis, during meiosis I and then during meiosis II. The diploid cell is first split into two diploid cells. The two diploid cells are then split into four haploid cells, which have half the chromosomes of diploid cells.

As sex seems to be present in the vast majority of eukaryotes, the origin of meiosis is presently unknown. Protists having optional or alternative sexual and asexual cycles seem to be the best targets for research on the evolution of meiosis. Primitive forms of meiosis: the possible evolution of meiosis. Solari AJ.Biocell 2002 Apr;26(1):1-13

Meiosis I
Meiosis is the first of the two separate divisions during which the diploid cell separates into two diploid cells. This is the step of meiosis where genetic variation is created by recombination. It is often called the reduction division. This is because it is here that the chromosome complement is reduced from diploid (two copies) to haploid (one copy). Interphase in meiosis is identical to interphase in mitosis. At this stage, there is no way to determine what type of division the cell will undergo when it divides. Meiotic division will only occur in cells associated with male or female sex organs. Prophase I is virtually identical to prophase in mitosis, involving the appearance of the chromosomes, the development of the spindle apparatus, and the breakdown of the nuclear membrane. Metaphase I is where the critical difference occurs between meiosis and mitosis. In mitosis, all of the chromosomes line up on the metaphase plate in no particular order. In Metaphase I, the chromosome pairs are aligned on either side of the metaphase plate. It is during this alignment that the chromatid arms may overlap and temporarily fuse, resulting in what is called crossovers. During Anaphase I, the spindle fibers contract, pulling the homologous pairs away from each other and toward each pole of the cell. In Telophase I, a cleavage furrow typically forms, followed by cytokinesis, the changes that occur in the cytoplasm of a cell during nuclear division; but the nuclear membrane is usually not reformed, and the chromosomes do not disappear. At the end of Telophase I, each daughter cell has a single set of chromosomes, half the total number in the original cell, that is, while the original cell was diploid; the daughter cells are now haploid.

Prophase I
The chromosones first become visible in prophase I. These chromosomes combine to form tetrads, which means a group of four chromosones. After this, the chromosomes cross over at certain points called chiasmata, where they exchange genetic material and create different combinations. The nuclear envelope then disperses and the spindle moves into the center. The tetrads also connect to the spindle fibers through kinetochores.

Metaphase I
The tetrads arrange across the center by movements of the kinetochores. The two centromeres are opposite each other, but the sister chromatids are not pulled apart like in mitosis.

Anaphase I
The chromatids holding the chromosomes together loosen, and the two homologous chromatids are seperated into poles. This helps to increase genetic diversity from the parent cells.

Telophase I
The chromosomes move into opposite poles and the nuclear envelope reforms. The spindle is broken down, and the cells move into meiosis II.

Meiosis II
During Meiosis II, two diploid cells are then split into four haploid cells during the second set of stages of meiosis. It is quite simply a mitotic division of each of the haploid cells produced in Meiosis I. There is no Interphase between Meiosis I and Meiosis II, and the latter begins with Prophase II. At this stage, a new set of spindle fibers forms and the chromosomes begin to move toward the equator of the cell. During Metaphase II, all of the chromosomes in the two cells align with the metaphase plate. In Anaphase II, the centromeres split, and the spindle fibers shorten, drawing the chromosomes toward each pole of the cell. In Telophase II, a cleavage furrow develops, followed by cytokinesis and the formation of the nuclear membrane. The chromosomes begin to fade and are replaced by the granular chromatin, a characteristic of interphase. When Meiosis II is complete, there will be a total of four daughter cells, each with half the total number of chromosomes as the original cell. In the case of male structures, all four cells will eventually develop into sperm cells. In the case of the female life cycles in higher organisms, three of the cells will typically abort, leaving a single cell to develop into an egg cell, which is much larger than a sperm cell.

Metaphase II
The chromosomes move into the center again and line up. Instead of two tetrads, there are now two chromosomes. This means that the chromatids will split up instead.

Anaphase II
The kinetochores split up the sister chromatids.

Telophase II
The chromatids concentrate in the poles, and the nuclear envelope reforms again. The cells divide for the last time, and the result is four cells. Unlike mitosis, the cells produced cannot go immediately back into interphase.

Meiosis versus Mitosis


Meiosis and mitosis both divide cells, but they are not exactly alike. Mitosis performs a single cell division, while meiosis performs two cell divisions. More than that, however, is that meiosis creates genetic diversity by crossing over the chromosomes. Mitosis does not do this, so the daughter cells are almost exact replicas of the parent cells. Meiosis allows for adaptation, so it is commonly used to create the egg and sperm cells, along with the other cells in our bodies, while mitosis is used to generate cells which are genetic clones.

Other differences include:

Related References

 * Meiosis ThinkQuest
 * Meiosis vs. Mitosis NOVA Online
 * Meiosis Tutorial The Biology Project