Meiosis is a specialised form of cell division that produces reproductive cells, or cells that will give rise to reproductive cells, containing half the chromosome number of the original cell. In animals, meiosis produces sperm and eggs, while in plants and many other organisms it produces spores or cells involved in sexual reproduction.
Unlike ordinary mitotic cell division, which normally produces two genetically similar daughter cells with the same chromosome number as the parent cell, meiosis involves two successive divisions after a single round of DNA replication. These divisions are called meiosis I and meiosis II. The overall result is usually four genetically different haploid cells from one diploid starting cell. The reduction from diploid to haploid is essential because fertilisation combines genetic material from two parents and restores the diploid chromosome number. Without meiosis, chromosome numbers would generally double in every generation of sexually reproducing organisms.
A central feature of meiosis is the separation of homologous chromosomes during meiosis I. A diploid cell contains pairs of homologous chromosomes, with one chromosome of each pair originally inherited from each parent. Before meiosis begins, the chromosomes are replicated, so each chromosome consists of two sister chromatids joined at a region called the centromere. During prophase I, homologous chromosomes pair with one another in a process called synapsis and form structures called bivalents or tetrads.
Genetic material can then be exchanged between non sister chromatids through crossing over. The physical sites where crossing over has occurred are called chiasmata. This exchange creates chromosomes carrying combinations of genetic variants that were not present together in either original chromosome. Crossing over is therefore one of the major sources of genetic variation produced by meiosis and contributes to the diversity seen among sexually reproducing organisms.
Meiosis I is often called the reduction division because homologous chromosome pairs are separated and distributed into different daughter cells. During metaphase I, homologous chromosome pairs align at the cell equator, but their orientations are not predetermined. Each pair can orient in either direction, producing independent assortment of maternal and paternal chromosomes. During anaphase I, homologous chromosomes move towards opposite poles, while sister chromatids remain together.
Telophase I and cytokinesis then produce two cells, although the exact details vary among organisms. Each resulting cell is haploid because it contains one chromosome from each homologous pair, but each chromosome still consists of two sister chromatids. The random orientation of chromosome pairs and the recombination events that occurred earlier mean that the two cells are genetically different. These mechanisms explain why meiosis is not merely a numerical reduction in chromosome number but also a major generator of inherited variation.
Meiosis II resembles mitosis because sister chromatids are separated rather than homologous chromosome pairs. There is normally no second round of DNA replication between meiosis I and meiosis II. During prophase II, chromosomes condense again and a spindle forms. During metaphase II, chromosomes align individually at the equator, and during anaphase II the sister chromatids separate and move to opposite poles. Cytokinesis then produces four haploid cells in the typical meiotic pattern.
Each cell contains one copy of each chromosome, and because crossing over and independent assortment have occurred, the genetic combinations in the resulting cells can differ substantially. In animals, the products of meiosis may undergo further maturation rather than immediately functioning as gametes. For example, meiosis in males generally produces four sperm precursor cells, whereas meiosis in females involves highly unequal divisions of cytoplasm and generally produces one large egg and smaller polar bodies.
Meiosis is fundamental to sexual reproduction because it links chromosome number, fertilisation and genetic diversity. In a typical diploid organism, meiosis reduces chromosome number from two sets to one set. Fertilisation subsequently combines two haploid gametes and restores two chromosome sets in the resulting diploid zygote. This alternation prevents chromosome numbers from increasing indefinitely across generations. Meiosis also provides mechanisms that reshuffle inherited genetic information.
Independent assortment distributes whole chromosomes in different combinations, while crossing over exchanges corresponding DNA segments between homologous chromosomes. These processes mean that genetically distinct gametes can be produced by the same individual. Mutations provide another source of genetic variation, but meiosis determines how existing variants are recombined and distributed. Natural selection can then act on variation arising through these processes, making meiosis an important biological foundation for evolutionary change.
Errors in meiosis can have major biological consequences because chromosomes may fail to separate correctly. The general term for failure of chromosome segregation is nondisjunction. If homologous chromosomes fail to separate during meiosis I, or sister chromatids fail to separate during meiosis II, gametes with abnormal chromosome numbers can result. Such conditions are described as aneuploidy when individual chromosomes are present in abnormal numbers.
In humans, an example is trisomy 21, in which cells have three copies of chromosome 21 instead of two. Other chromosome-number abnormalities can cause infertility, developmental disorders, miscarriage or severe disease, although their consequences depend on the chromosome involved and the organism. Meiotic errors can also occur during recombination, chromosome pairing or DNA repair. Cells possess surveillance and repair mechanisms that reduce such errors, but these mechanisms are not perfect. Thus meiosis is both a highly controlled process and a potential source of chromosomal abnormalities.
The term meiosis derives from the Greek word meaning reduction, reflecting the reduction in chromosome number that characterises the process. The distinction between meiosis and mitosis is especially important. Mitosis generally involves one division and produces daughter cells that retain the chromosome number of the parent cell, whereas meiosis involves two divisions following one DNA replication event and ultimately produces haploid products.
Meiosis is also different from simple binary fission in prokaryotes, where cells reproduce without the elaborate chromosome-pairing process found in eukaryotic meiosis. The precise details of meiosis vary between animals, plants, fungi and other eukaryotes. In some organisms, the haploid stage is a major part of the life cycle, while in others the diploid stage dominates. Plants commonly alternate between multicellular haploid and diploid generations, so meiosis produces spores rather than directly producing gametes. These differences show that meiosis is a conserved cellular mechanism whose biological role is adapted to different life cycles.
Meiosis can therefore be understood as an integrated process involving DNA replication, homologous chromosome pairing, recombination, chromosome segregation and two rounds of cell division. Its most immediate function is to generate haploid cells, but its wider importance includes maintaining chromosome number across sexual generations and producing genetic diversity. The process depends on precise coordination of chromosome structure, spindle formation, DNA repair and cell-cycle regulation. Specialised proteins help chromosomes recognise homologous partners, exchange DNA and attach correctly to the spindle.
The resulting cells are not usually genetically identical to the cell from which meiosis began or to one another. This genetic difference has consequences for inheritance, reproduction, evolution and population diversity. Understanding meiosis is also essential in genetics because many patterns of inheritance arise from how chromosomes behave during meiotic division. Consequently, meiosis is not simply a type of cell division but a central mechanism connecting cellular reproduction with heredity and biological variation.
