Meiosis is a specialized form of cell division in sexually reproducing organisms. Its main function is to create sex cells, or gametes (sperm and egg cells), which possess half the genetic material of ordinary body cells. This process is often called a reduction division because it halves the total number of chromosomes. Successful completion ensures that when two gametes combine during fertilization, the resulting organism will have the correct, full set of chromosomes.
The Starting Point: Diploid Cells
Meiosis begins with germ cells in the reproductive organs, which are diploid cells. Diploid cells contain two complete sets of chromosomes, one set inherited from each parent. In humans, the diploid number is 46, consisting of 23 pairs of homologous chromosomes. Before meiosis begins, during interphase, the cell duplicates its DNA. Each of the 46 chromosomes is replicated, resulting in two identical sister chromatids joined together. The cell starts the division with 46 chromosomes, totaling 92 chromatids.
Chromosome Reduction in Meiosis I
Meiosis I is the first major division where the chromosome number is significantly reduced. During this stage, the paired homologous chromosomes (one from each parent) line up together. These pairs then separate and move to opposite ends of the cell during Anaphase I. This separation halves the chromosome count, ensuring each resulting cell contains one member of each homologous pair.
After the cell divides in Telophase I and Cytokinesis, two new cells are formed. Each cell is now considered haploid because it contains only 23 chromosomes. Importantly, each of these 23 chromosomes is still duplicated, consisting of two sister chromatids. The cells thus have 23 chromosomes and 46 chromatids, but the ploidy level has dropped from diploid (46) to haploid (23).
Final Count: Meiosis II and the Formation of Gametes
The two cells from Meiosis I immediately enter Meiosis II, a division similar to mitosis. There is no further DNA replication before this stage begins. The 23 duplicated chromosomes within each cell line up single-file along the central plate. During Anaphase II, the sister chromatids finally separate from one another. Once separated, each chromatid is counted as an individual, unduplicated chromosome.
This final separation results in four distinct daughter cells. Each cell contains a single set of 23 chromosomes. These resulting cells are the mature gametes (sperm or egg cells). They are haploid, containing the single set of genetic instructions necessary for reproduction. Thus, 23 is the final, stable chromosome number found in the functional gamete, reduced from the starting 46 chromosomes.