Meiosis is a specific type of cell division performed by sexually reproducing organisms to create reproductive cells, known as gametes (sperm and egg cells). This process is fundamentally distinct from the cell division that produces the body’s non-reproductive cells. Meiosis involves two sequential rounds of nuclear and cellular division, designated Meiosis I and Meiosis II, which collectively produce four daughter cells. The primary function is to reduce the number of chromosomes by half, transitioning from a diploid state (two sets of chromosomes) to a haploid state (one set of chromosomes). This two-step process also facilitates the necessary mixing of genetic material, which introduces genetic diversity within the species.
Meiosis I: The Reductional Division
Meiosis I is the reductional division because it is the stage where the number of chromosomes is cut in half. Before division, the cell duplicates its DNA, so each chromosome consists of two identical sister chromatids. The primary goal is the separation of homologous chromosomes, which are pairs inherited from each parent that carry the same genes.
In Prophase I, homologous chromosomes physically pair up (synapsis) to form structures known as tetrads. While paired, non-sister chromatids exchange segments of genetic material in an event called crossing over, or recombination. This exchange creates new combinations of alleles, ensuring the resulting gametes are genetically unique. The exchange points are visible as chiasmata, which hold the homologous pairs together.
In Metaphase I, the paired homologous chromosomes line up along the cell’s central plate. The alignment of each homologous pair is random and independent (independent assortment), which is a significant source of genetic variation. In Anaphase I, the spindle fibers pull the entire homologous chromosomes apart, moving one full chromosome (still composed of two sister chromatids) toward each opposing pole. This physical separation reduces the chromosome number from diploid to haploid.
The first division concludes with Telophase I and cytokinesis, resulting in two daughter cells. Each new cell contains a haploid number of chromosomes, but each chromosome is still duplicated, consisting of two sister chromatids. The cells then move into a brief resting period, or interkinesis, without undergoing further DNA replication, before proceeding to Meiosis II.
Meiosis II: The Equational Division
Meiosis II is the equational division because the chromosome number does not change during this stage. It begins with the two haploid cells produced by Meiosis I, each containing duplicated chromosomes. The division acts to split the remaining duplicated chromosomes.
The two daughter cells move into Prophase II, where the chromosomes recondense and new spindle fibers form. In Metaphase II, the chromosomes line up individually along the metaphase plate in each cell. These chromosomes are positioned to have their sister chromatids pulled apart.
The action of this second division occurs in Anaphase II, where the centromeres holding the sister chromatids finally separate. Once separated, the former sister chromatids are pulled toward opposite poles and are now considered individual, unreplicated chromosomes. This separation ensures the final cells contain a single chromatid for each chromosome, fulfilling the requirement for a functional gamete.
Meiosis II finishes with Telophase II and a second round of cytokinesis, resulting in four final daughter cells. These four cells are all haploid, containing one complete set of single-chromatid chromosomes. In human males, these four cells mature into sperm, while in human females, the process is modified to produce one large egg and smaller polar bodies.
The Essential Role of Two Divisions in Genetic Variation
The two-division structure is directly responsible for generating the extensive genetic variation necessary for sexual reproduction. This complexity allows for the shuffling of parental genetic material in multiple distinct ways.
The first mechanism, crossing over, occurs during Prophase I. This physical exchange of segments between homologous chromosomes creates “patchwork” chromosomes that carry a novel combination of alleles from both the paternal and maternal lines. This recombination would be impossible with a single division.
The second mechanism is independent assortment, which takes place when homologous pairs randomly align at the metaphase plate in Metaphase I. Because the orientation of one pair does not influence the orientation of any other pair, there is a vast number of possible combinations of paternal and maternal chromosomes that can be distributed to the daughter cells. For instance, humans have 23 pairs of chromosomes, which mathematically allows for over 8 million different combinations of chromosomes in the resulting gametes, even before considering the effects of crossing over.
These two events, only possible because of the reductional nature of Meiosis I, ensure that every resulting gamete is genetically unique. The subsequent separation of sister chromatids in Meiosis II then distributes these unique, recombined chromosomes into four separate cells.
Key Differences Between Meiosis and Mitosis
The two-division structure of meiosis is the fundamental difference that distinguishes it from mitosis, the process of cell division for tissue growth and repair. Mitosis involves only one round of nuclear division, producing two daughter cells that are genetically identical to the parent cell. Meiosis, by contrast, involves two successive divisions (Meiosis I and Meiosis II), which begin with one cell and produce four.
The final cellular outcome is also significantly different. Mitosis maintains the original chromosome number, resulting in diploid daughter cells. Meiosis reduces the chromosome number by half, yielding haploid cells. Furthermore, meiosis is characterized by the pairing of homologous chromosomes and crossing over in Prophase I, events that do not occur in mitosis. The purpose of meiosis is to create sex cells with genetic variation, while mitosis generates genetically uniform cells for growth and replacement.