Crossing Over Multiple

Complex and multiple crossing is the process of exchange of sections between homologous chromosomes during the process of meiosis and mitosis. Complex forms of crossings are produced directly during the differentiation of maternal and paternal germ cells, and multiple forms are produced during the life cycle of the organism (that is, the process of its development).

Complex - one pair of homologous chromosomes exchanges sections in two stages, occurring in different DNA strands. Between two homologous chromosomes there is an unbroken common region called the chiasma. The exchange occurs through this chiasm and is carried out by the forces of electrostatic repulsion or attraction of such areas when the chiasm rotates around its axis. When exchanging bivalents diverge, it is the chiasm that serves as the bridge connecting sister chromatids before they transform into single chromosomes. The divergence therefore consists of two steps: in the first, the chiasm is rotated by 90º in a plane perpendicular to the long axis of the bivalent, allowing the corresponding sections to chemically coincide; in the second, it is rotated another 90° so that the coincident sections take a position determined by chemical specificity. The second turn is necessary because in close complementary regions the same complementarity for adenine at the end of one strand may be for thymine at the beginning of another. Since the complementary regions are located at substantially aligned distances, the rotation will not lead to the development of a new incorrect topology of the DNA molecules, and the two resulting bivalent states will have the same orientation of the parts of the chromosomes corresponding to the new division of the maternal and paternal halves. Each of the chromosomes will then have one of the usual + and - states during division, then producing 2n4c gametes.

Multiple crosses - an exchange occurs between three or more chromosomes in each of which several probes of different origin are fused