Sexual reproduction requires that organisms produce cells that can fuse during fertilization to produce offspring. In most organisms, fertilization occurs between two haploid cells, the larger being called “female” or “egg” and the smaller being called “male” or “sperm.” In most animals, meiosis is used to produce haploid eggs and sperm from diploid parent cells so that fusion of an egg and sperm produces a diploid zygote. As with mitosis, DNA replication occurs prior to meiosis during the S-phase of the cell cycle. In meiosis, two rounds of nuclear division result in four nuclei and usually four daughter cells, each with half the number of chromosomes as the parent cell. The first division separates homologous chromosomes, and the second—like mitosis—separates chromatids into individual chromosomes. Meiosis generates variation during crossover in prophase I and random alignment during metaphase I. Cells produced by meiosis are genetically unique.

Most cells in a sexually reproducing organism have two sets of chromosomes, one set inherited from the father and one set inherited from the mother. Cells with two sets of chromosomes are called diploid cells (2n). Humans have 22 pairs of autosomes or “body chromosomes,” which are all of the non–sex chromosomes. These chromosomes are the same for both sexes. Each autosomal chromosome is paired with an equally sized chromosome. The other pair are sex chromosomes, which are not autosomes. Chromosomes X and Y are not the same size and shape.

Meiosis and mitosis share similar processes but distinct outcomes. In humans, mitosis involves division of a single nucleus to produce two genetically identical daughter cells, while meiosis involves two nuclear divisions to produce four genetically different daughter cells with only one chromosome set. The main differences between these two processes take place during the first division of meiosis, when homologous chromosomes pair, crossover, and exchange segments. The homologous chromosomes separate into different nuclei during meiosis I. The second division of meiosis is similar to a mitosis, except daughter cells do not contain identical genomes due to crossover and chromosome recombination in prophase I. During meiosis, variation in the daughter nuclei is introduced because of crossover in prophase I and random alignment in metaphase I. The cells that are produced by meiosis are genetically unique.

Crossover occurs between nonsister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes.

The crossover events are the first source of genetic variation in the nuclei produced by meiosis. Illustrated above, a single crossover event between homologous non-sister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal chromosome. Now, when that sister chromatid is moved into a gamete cell it will carry some DNA from one parent of the individual and some DNA from the other parent. The sister recombinant chromatid has a combination of maternal and paternal genes that did not exist before the crossover. Multiple crossovers in an arm of the chromosome have the same effect, exchanging segments of DNA to create recombinant chromosomes.

Random, independent assortment during metaphase I is demonstrated by considering a cell with a set of two chromosomes (n = 2). There are two possible homologous chromosome arrangements at the equatorial plane in metaphase I, that are then separated during anaphase I. The total possible number of different gametes is 2ⁿ, where n equals the number of chromosomes in a set. In this example, there are four possible genetic combinations for the gametes. With n = 23 in human cells, there are over eight million possible combinations of paternal and maternal chromosomes. Credit: Rao, A.and Fletcher, S. Department of Biology, Texas A&M University.

Sexual reproduction is used by almost all eukaryotes. The variation introduced into the reproductive cells by meiosis appears to be one of the advantages of sexual reproduction that has made it so successful. Meiosis and fertilization alternate in sexual life cycles. The process of meiosis produces unique reproductive cells called gametes with half the number of chromosomes as the parent cell. Fertilization restores the diploid condition. Sexually reproducing organisms alternate between haploid and diploid stages.

Meiosis and mitosis are both preceded by one cycle of DNA replication. However, meiosis includes two rounds of nuclear division. The four daughter cells generated from meiosis are haploid and genetically distinct. Two daughter cells generated from mitosis are diploid and identical to the parent cell.

Nearly all eukaryotes undergo sexual reproduction. The variation introduced into reproductive cells by meiosis provides an important advantage for sexual reproduction. The process of meiosis produces unique reproductive cells called gametes, which have half the number of chromosomes as the parent cell. In a fertilization event, two haploid gametes fuse to form a diploid zygote.