Meiosis is the specialized form of cell division required for sexual reproduction. This process takes a single parent cell with a full set of chromosomes and prepares it to contribute to the next generation. Its primary function is the creation of gametes (sex cells), such as sperm and egg cells. Meiosis ensures that the resulting gametes contain exactly half the number of chromosomes (haploid). This halving is necessary so that when two gametes combine during fertilization, the offspring restores the species’ full, diploid chromosome number.
Meiosis I: The Reductional Division
Meiosis I is the first of two sequential rounds of division. This initial phase is called the reductional division because it cuts the chromosome number in half. The diploid parent cell contains two sets of chromosomes, one inherited from each parent. Before division, the cell duplicates its DNA, so each chromosome consists of two identical sister chromatids.
During Meiosis I, homologous chromosome pairs line up together and then separate. This separation reduces the cell’s ploidy level from diploid to haploid. Crossing over occurs early in this division, where these paired chromosomes exchange segments of DNA. The cell divides into two daughter cells, each containing a haploid set of chromosomes. However, each chromosome still consists of two sister chromatids, which is why a second division is necessary.
Meiosis II: The Equational Division
Following Meiosis I, the two resulting cells move into the second round of division, Meiosis II. This phase is termed the equational division because the chromosome number does not change during this stage. Its purpose is to separate the remaining duplicated material.
During Meiosis II, the chromosomes align along the center of the cell, and the sister chromatids are finally pulled apart. Each separated chromatid is then considered an individual chromosome. The two cells that started Meiosis II divide to produce a total of four genetically distinct cells. Each of these four final gametes is haploid, ready for fertilization.
How Meiosis Differs From Mitosis
Meiosis is distinguished from mitosis, the process most body cells use for growth and repair, by its requirement for two divisions. Mitosis involves only a single division, resulting in two daughter cells that are genetically identical and maintain the full, diploid chromosome number.
In contrast, meiosis involves two rounds of nuclear division (Meiosis I and Meiosis II), which collectively produce four daughter cells. These four gametes are not genetically identical to the parent cell or to each other, due to the genetic shuffling that occurred during Meiosis I. The ultimate goal of mitosis is to create identical body cells, while meiosis creates genetically unique sex cells with half the chromosome number.