Meiosis is a specialized form of cell division fundamental for sexual reproduction. Its primary function involves creating reproductive cells, known as gametes (sperm in males, egg cells in females). These gametes contain half the number of chromosomes found in other body cells. This ensures that when two gametes fuse during fertilization, the resulting offspring has the correct total number of chromosomes, maintaining the species’ chromosome number across generations. Meiosis is also a significant source of genetic diversity.
Meiosis: A Two-Part Process
Meiosis unfolds in two distinct stages: Meiosis I and Meiosis II. These sequential divisions reduce the chromosome number and generate genetic variation. Meiosis I separates homologous chromosomes, which are pairs inherited one from each parent. Meiosis II then separates sister chromatids, the identical halves of a duplicated chromosome.
Meiosis I: Separating Homologous Chromosomes
Meiosis I begins after a cell duplicates its DNA during interphase, with each chromosome consisting of two identical sister chromatids. During Prophase I, homologous chromosomes pair up and exchange genetic material through crossing over, contributing to genetic diversity. In Metaphase I, these paired homologous chromosomes align at the cell’s center. Anaphase I then separates the homologous chromosomes, moving them to opposite ends of the cell; sister chromatids remain attached. Telophase I concludes Meiosis I, resulting in two haploid daughter cells, each with chromosomes still composed of two sister chromatids.
Meiosis II: The Separation of Sister Chromatids
Meiosis II closely resembles mitosis, the process of cell division for growth and repair in the body; the two haploid cells produced in Meiosis I proceed into Meiosis II without further DNA replication. Prophase II involves chromosome condensation and nuclear envelope breakdown. In Metaphase II, the chromosomes, each still made of two sister chromatids, align individually along the cell’s central plane. The separation of sister chromatids occurs in Anaphase II, where the sister chromatids finally pull apart from each other and move to opposite poles of the cell. Telophase II concludes the process, with nuclear membranes forming around the separated chromatids, now considered individual chromosomes, and cytokinesis then divides the cytoplasm, leading to the formation of four new haploid cells.
The Outcome of Meiosis
Meiosis culminates in four genetically unique haploid cells, such as sperm or egg cells. Each contains a single set of chromosomes, half the number of the original parent cell. This reduction ensures that when gametes combine, the fertilized cell (zygote) restores the species’ full chromosome complement. Crossing over in Prophase I and independent assortment in Metaphase I ensure these four cells are genetically distinct. This genetic diversity provides variation for populations to adapt.