Meiosis (a Greek word meaning "decrease") is a cellular process that forms the basis for sexual reproduction, together with syngamy. It is a form of nuclear division by which a diploid parent produces four haploid daughter cells. The process includes the two stages of nuclear division (meiosis I and II), each usually accompanied by cell division. Single-stage meiosis has been suggested but not convincingly demonstrated. Duplication of chromosomes precedes the process of meiosis (during S phase). Most animals and plants are normally diploid, and use meiosis to produce sexual gametes, which fuse to form zygotes that develop into new organisms. In most animals this is often the primary or only means of proliferation. In other eukaryotes, sexual reproduction may play a more restricted role, while asexual reproduction gains emphasis (examples include plants and hydras).
The mechanistic differences between mitosis, which produces somatic cells, and meiosis, is best understood by considering mitosis first. (All jargon used in this article is defined in the article on mitosis.)
As described earlier, meiotic nuclear division consists of two stages, called meiosis I and meiosis II. It starts with a cell in the same state as does a mitotic division. However, the alignment of chromosomes for prophase is different. Homologous chromosomes join into tetrads (so called because each tetrad contains four chromatids), and the tetrads line up on the metaphase plane .
Prophase I (the prophase of meiosis I) is the longest phase. In this phase, chromosomes shorten and become visible as single threadlike structures. Beaded appearances, if any, are due to the alternation of densely stained chromomeres in comparison with non-staining areas. Chromomeres are regions where the chromosomal material is tightly coiled.
Homologous chromosomes derived from maternal and paternal gamete nuclei come together and pair up. They have same lengths, same centromere positions and in most cases same number of genes arranged in linear order. The pairing process, known as synapsis , may begin at several points along the structures and then "zip up". The paired homologous chromosomes, now known as bivalents, shorten and thicken by means of molecular packaging as well as coiling. The bivalents are now very much visible.
The homologous chromosomes then fall apart and parts appear to repel their counterparts and the structure appears as a pair of chromatids. Each chromosome becomes two chromatids. The chromosomes join at several points or crosses known as chiasmata. Genetic crossing-over results. Nevertheless, the pair of chromosomes hold until anaphase.
Prophase I is subdivided into different stages:
In the human ovary, oocytes are stored in the diplotene stage since fetal life. Only just prior to ovulation meiosis I is resumed and is then completed at the time of ovulation (meiosis II ends only just after fertilization). Oocytes that are not ovulated do not complete meiosis I.
The end of Prophase I (also known as Prometaphase I) is signified by contraction and staining of all chromosomes, as well as the polar migration of centrioles, the dispersion of the nucleoli and nuclear envelope and the formation of the spindle fibres (including their attachment to chromosomes).
In Metaphase I, the nuclear envelope has already dispersed into vesicles. Spindle fibres (microtubules) attach only to one kinetochore of both whole centromeres, lining up the bivalents along the equator so that each centromere is equidistant above and below the equator. The lining along the equator is random, maternal or paternal homologues may point to either pole, this is known as independent assortment.
Anaphase I sees the division of bivalents, thanks to microtubule shrinkage. Since microtubules are only attached to one kinetochore, whole centromeres, each attached to two (sister) chromatids, are pulled towards opposing poles. Two haploid sets of chromosomes form at opposing poles. Each chromosome consists of a sister chromatid pair, as aforementioned.
The first meiotic division effectively ends when the centromeres arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. Telophase I includes the disappearance of spindles and spindle fibres as well as the uncoiling of the chromatids and the reformation of the nuclear membrane. Pinching of the cytoplasm or the formation of cell walls occurs, as in mitosis. Cytokinesis may occur. Note that many plants simply skip telophase I and interphase II, going immediately into prophase II. Interkinesis (or Interphase II) sometimes occurs in animal cells and presents no DNA replication.
Two daughter cells are formed by the end of Telophase I.
Prophase II takes an inversely proportional time compared to telophase I. In this prophase we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the polar regions and are arranged by spindle fibres. This arrangement is rotated by 90 degrees when compared to meiosis I.
In Metaphase II, the centromeres behave as two entities, organising fibres on each side to both poles aligning on the equator. This is followed by Anaphase II, where the centromeres divide and the fibres pull the now separated chromatids (i.e. chromosomes) are pulled behind the centromere.
The process ends with Telophase II, which is similar to that found at mitosis. Uncoiling, lengthening and disappearance of the chromosomes occur as well as the disappearance of the fibres and the replication of the centrioles. Nuclear envelopes reform; cleavage or cell wall formation eventually produces a total of four daughter cells, each with an haploid set of chromosomes.
If meiosis did not occur, fusion of the gametes would not result in a diploid condition (2n) but 4n. Meiosis also provides opportunities for new combination (through crossing over) of genes, ensuring heritable variation. The reduction of chromosomes from the diploid to the haploid condition separates alleles so that each gamete carries a sole allele for a gene locus. In addition, the orientation of the metaphase I/II equatorial lining-up is random, resulting in new allelic recombinationis. Independent assortment forms the basis of Mendel's second law. Lastly, chiasmata causes genetic breaks and the establishment of new ones.