CROSSING-OVER
Crossing-over is exchange of material between two homologous chromosomes, which takes place during the long-term prophase of the first division of meiosis. Crossing-over is the basis for genetic recombination. In every pair of human homologous chromosomes crossing-over takes place in the average in two or three spots. During crossing-over rupture of the DNA double helix in one mother chromatid and one father chromatid occurs. Then the resulting sections reunite "crosswise".
One of the varieties of crossing-over, unequal crossing-over, is a recombination event, when recombination points are situated not in identical loci of the two parent DNA molecules (non-sister chromatids). In this case chromatids exchange unequal-sized segments.