Meiosisis a type of cell division that reduces the number of chromosomes in the parentcell by half and produces four gamete cells. This process is required toproduce egg and sperm cells for sexual reproduction. During reproduction, whenthe sperm and egg unite to form a single cell, the number of chromosomes isrestored in the offspring.
Meiosisbegins with a parent cell that is diploid, meaning it has two copies of eachchromosome. The parent cell undergoes one round of DNA replication followed bytwo separate cycles of nuclear division. The process results in four daughtercells that are haploid, which means they contain half the number of chromosomesof the diploid parent cell.
Meiosishas both similarities to and differences from mitosis, which is a cell divisionprocess in which a parent cell produces two identical daughter cells. Meiosisbegins following one round of DNA replication in cells in the male or femalesex organs. The process is split into meiosis I and meiosis II, and bothmeiotic divisions have multiple phases. Meiosis I is a type of cell divisionunique to germ cells, while meiosis II is similar to mitosis.
MeiosisI, the first meiotic division, begins with prophase I. During prophase I, thecomplex of DNA and protein known as chromatin condenses to form chromosomes.The pairs of replicated chromosomes are known as sister chromatids, and theyremain joined at a central point called the centromere. A large structurecalled the meiotic spindle also forms from long proteins called microtubules oneach side, or pole, of the cell. Between prophase I and metaphase I, the pairsof hom*ologous chromosome form tetrads. Within the tetrad, any pair of chromatidarms can overlap and fuse in a process called crossing-over or recombination.Recombination is a process that breaks, recombines and rejoins sections of DNAto produce new combinations of genes. In metaphase I, the hom*ologous pairs ofchromosomes align on either side of the equatorial plate. Then, in anaphase I,the spindle fibers contract and pull the hom*ologous pairs, each with twochromatids, away from each other and toward each pole of the cell. Duringtelophase I, the chromosomes are enclosed in nuclei. The cell now undergoes aprocess called cytokinesis that divides the cytoplasm of the original cell intotwo daughter cells. Each daughter cell is haploid and has only one set ofchromosomes, or half the total number of chromosomes of the original cell.
MeiosisII is a mitotic division of each of the haploid cells produced in meiosis I.During prophase II, the chromosomes condense, and a new set of spindle fibersforms. The chromosomes begin moving toward the equator of the cell. Duringmetaphase II, the centromeres of the paired chromatids align along theequatorial plate in both cells. Then in anaphase II, the chromosomes separateat the centromeres. The spindle fibers pull the separated chromosomes towardeach pole of the cell. Finally, during telophase II, the chromosomes areenclosed in nuclear membranes. Cytokinesis follows, dividing the cytoplasm ofthe two cells. At the conclusion of meiosis, there are four haploid daughtercells that go on to develop into either sperm or egg cells.
