Cell division is a fundamental process that allows living organisms to grow, repair tissues, and reproduce. While many cells divide through a process called mitosis, producing identical copies, a specialized form of cell division known as meiosis is essential for sexual reproduction.
The Final Count
Meiosis ultimately produces four daughter cells from a single parent cell. These resulting cells are haploid, meaning they contain half the number of chromosomes found in the original parent cell. For instance, in humans, a parent cell with 46 chromosomes will undergo meiosis to yield daughter cells each containing 23 chromosomes. These four daughter cells are genetically unique, differing from each other and from the original parent cell.
The Divisional Stages
Meiosis occurs through two distinct rounds of cell division: Meiosis I and Meiosis II. Before Meiosis I begins, the cell undergoes a preparatory phase where its DNA is replicated, resulting in chromosomes that each consist of two identical sister chromatids.
Meiosis I is a reductional division because it reduces the chromosome number by half. During this stage, homologous chromosomes, which are pairs of chromosomes inherited one from each parent, separate from each other. Before they separate, crossing over occurs, where homologous chromosomes exchange segments of genetic material, creating new combinations of genes on each chromosome. By the end of Meiosis I, two haploid daughter cells are formed, each with chromosomes still composed of two sister chromatids.
Following Meiosis I, the two haploid cells proceed into Meiosis II without further DNA replication. Meiosis II is an equational division, as the chromosome number does not change during this stage. During Meiosis II, the sister chromatids within each of the two cells separate, similar to how chromosomes separate during mitosis. This separation results in four daughter cells, each containing a single set of unreplicated, haploid chromosomes.
Significance for Life
Meiosis holds biological importance for sexually reproducing organisms. One function is to maintain a constant number of chromosomes across generations. When two haploid cells, such as an egg and a sperm, fuse during fertilization, they restore the complete diploid set of chromosomes, ensuring the offspring has the correct number of chromosomes characteristic of the species.
Beyond chromosome number maintenance, meiosis is a major driver of genetic diversity. The processes of crossing over during Meiosis I and the random orientation of homologous chromosome pairs during Meiosis I (independent assortment) ensure that each gamete carries a unique combination of genetic material. This genetic variation allows species to adapt to changing environments and is a fundamental component of evolution through natural selection.