Meiosis is a specialized form of cell division in eukaryotes that reduces the number of chromosome sets, usually from two to one. A single round of DNA replication is followed by two successive nuclear divisions, meiosis I and meiosis II, ordinarily producing four haploid nuclei. In sexual life cycles, this reduction counterbalances the doubling of chromosome sets at fertilization. Meiosis also reshuffles hereditary information through chromosome exchange and the distribution of parental chromosomes into different daughter nuclei. (genome.gov)
Chromosome sets and DNA content
Ploidy describes the number of complete chromosome sets in a cell. A diploid cell, designated 2n, has two sets; a haploid cell, designated n, has one. In a typical diploid organism, homologous chromosomes carry corresponding genes at corresponding positions, although they may contain different alleles. One homolog generally comes from each parent. Human diploid cells normally contain 46 chromosomes, whereas gametes contain 23. (openstax.org)
Chromosome number must be distinguished from DNA content. During the preparatory S phase of the cell cycle, each chromosome is replicated into two sister chromatids. Replication doubles DNA content without changing ploidy. Using C for the DNA content of one unreplicated haploid set, a typical meiotic sequence is 2n, 4C before division; n, 2C after meiosis I; and n, 1C after meiosis II. Thus, the first division reduces chromosome sets, while the second separates the replicated copies. (openstax.org)
Meiosis I: pairing and reduction
Meiosis I differs fundamentally from mitosis because homologous chromosomes pair and then segregate from one another. Its prophase is conventionally divided into five stages: leptotene, zygotene, pachytene, diplotene, and diakinesis. These subdivisions describe changes in chromosome condensation, pairing, and the connections between homologs. (ncbi.nlm.nih.gov)
During prophase I, replicated homologs align closely in a process called synapsis. In many organisms, a synaptonemal complex, a ladder-like protein structure, assembles between them. Each paired unit is called a bivalent, or tetrad, because it contains two chromosomes and four chromatids. Crossing over exchanges DNA between nonsister chromatids of homologous chromosomes. As the synaptonemal complex disassembles, homologs remain connected at visible sites called chiasmata, which help maintain their association until segregation. (ncbi.nlm.nih.gov)
At metaphase I, bivalents align on the division apparatus. The meiotic spindle attaches to chromosomes so that homologs face opposite poles, while sister chromatids normally move together. During anaphase I, homologous chromosomes separate; the sisters remain associated near their centromeres. Telophase I and cytoplasmic division often produce two cells, each containing one homolog from each original pair. Nuclear reformation and chromosome decondensation between divisions vary among organisms. (ncbi.nlm.nih.gov)
Meiosis II: separation of chromatids
There is no intervening round of DNA replication before meiosis II. Its chromosome movements resemble those of mitosis, but each dividing nucleus already contains only one chromosome set. During prophase II, the division apparatus forms or reorganizes. At metaphase II, chromosomes align individually rather than as homologous pairs. (openstax.org)
At anaphase II, sister chromatids separate and move to opposite poles; each separated chromatid is thereafter considered a chromosome. Telophase II and subsequent cytoplasmic division ordinarily yield four haploid products. Because crossing over has exchanged chromosome segments, the sisters entering this division need not be genetically identical. The products therefore commonly differ from one another and from the original diploid cell. (openstax.org)
Genetic variation and inheritance
Meiosis contributes to genetic diversity through two principal mechanisms. Crossing over creates new combinations of alleles along individual chromosomes. Independent assortment results from the orientation of each homologous pair at metaphase I, independently of other pairs. Ignoring crossing over, n chromosome pairs allow 2ⁿ combinations of parental chromosome origins among gametes. For humans, 2²³ equals 8,388,608 possible combinations. This figure describes chromosome-origin combinations, not necessarily that many distinct DNA sequences. (genome.gov)
Crossing over is an outcome of homologous recombination, which uses corresponding DNA sequences during exchange. Genes close together on a chromosome tend to be inherited together because a crossover between them is less likely than between more widely separated genes. Consequently, chromosome assortment does not mean that every gene is inherited independently. (genome.gov)
Position in sexual life cycles
In most animals, meiosis occurs in the germ line and produces haploid cells that develop into gametes. In plants, meiosis generally produces spores rather than gametes directly. These spores develop through mitotic divisions into a haploid generation that produces gametes, forming part of alternation of generations. Many fungi have predominantly haploid life cycles, with meiosis following a brief diploid stage after cell fusion. (openstax.org)
The four meiotic products need not become four equivalent reproductive cells. During mammalian sperm formation, they develop into sperm. During egg formation, unequal cytoplasmic divisions retain most cellular material in one large cell and produce small polar bodies. Human oocytes begin meiosis before birth, pause in prophase I, and later undergo a second arrest at metaphase II; fertilization normally triggers completion of meiosis II. (ncbi.nlm.nih.gov)
Errors in chromosome segregation
Failure of chromosomes to segregate correctly, termed nondisjunction, can produce reproductive cells with extra or missing chromosomes. After fertilization, such errors can result in aneuploidy, an abnormal number of individual chromosomes. An extra copy of chromosome 21 is the usual chromosomal basis of Down syndrome. Correct chromosome segregation is therefore distinct from, but coordinated with, the genetic reshuffling that occurs during meiosis. (genome.gov)