THE POSSIBLE FUNCTION AND ORIGIN OF CROSSING-OVER

Crossing over to repair genetic material via exchange probably emerged at the initial stages of organic evolution! earlier than the appearan ce of the DNA molecule in its present form. In that period, nitrous ba ses existed as free molecules, although, like in double-helical DNA, t hey formed aggregates. The electrochemical characteristics of pyrimidi nes differ from one another to a greater extent than those of purines, which are almost electrically neutral. : Therefore, it is possible th at the DNA electrostatic balance is accounted forby nonspecific intera ction between pyrimidine bases. The data obtained by Nussinov et al. [ 37, 38] indicate that, in homooligomers, the arrangement of pyrimidine bases in opposite DNA strands exhibits some restrictions with respect to their mutual positions. In functioning DNA, the electrostatic bala nce of pyrimidine arrangement in complementary strands may favor rapid transitions of the molecule between an open helix and the original st ate. It is supposed that crossing over restores the electrostatically balanced spatial arrangement of pyrimidine homooligomers and, hence, t he functional activity of the entire DNA molecule.