Crossing over is the exchange of genetic information between homologous chromosomes during prophase I of meiosis I. Genetic recombination gives rise to allelic diversity in the newly formed daughter cells. In humans, crossing over produces genetically distinct haploid egg and sperm cells that undergo fertilization to produce unique offspring. Before cell division starts, the germ cell’s chromosome(s) undergo duplication in the S phase of the cell cycle. As the cells enter prophase I, duplicated chromosomes condense and form two sister chromatids (identical copies of the original chromosome) joined by the centromere. Next, the homologous chromosomes pair up and align the same gene segment from the maternal and paternal chromosomes, forming a synapse. A protein complex called the synaptonemal complex is formed that holds these homologs together. As crossing over proceeds, random pieces of DNA are swapped between the homologs, producing new combinations of alleles via homologous recombination. The 'chiasmata' mark the areas where the crossover of genetic information has occurred. As the synaptonemal complex begins to dissolve, the chiasma holds the homologous chromosomes together until recombination is completed and chromosomes are segregated correctly into the daughter cells.
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