Subject - genetics, cell biology

In genetics, crossing over or chromosomal crossover is the exchange of DNA between the two homologous chromosomes in prophase I of meiosis. This recombination process increases genetic diversity because it results in formation of chromosomes with a mixture of maternal and paternal genes.

During the metaphase of mitosis or meiosis the chromosomes are in a state that can be examined through an optical microscope. In prophase I of meiosis, both of these homologous chromosomes are arranged side-by-side to form a tetrad. This allows crossing over to occur between DNA strands, which results in the formation of Holliday junctions between the single strands of DNA. When the chromosomes move away from each other as meiosis progresses, the chromatids remain attached a bit longer in places where crossing over has occurred. This structure is visible by optical microscope and is known as the chiasma (from the greek Χ).