Mechanism of Crossing-over

Abstract

A NUMBER of theories have emerged from time to time attempting to visualize the exact mechanism involved during crossing-over, which is now known to be the sole agent conditioning the formation of the cytologically visible chiasma. Among these the one proposed by Darlington¹ based on torsions is the most elaborate, in the sense that it offers an explanation for the long and intricate succession of events. His hypothesis, however, implies a few assumptions for which there are formidable physical objections. The most important of these are: (1) the existence of a pairing force which brings homologues together in the zygotene of meiosis, and (2) the postulate that a break at a particular level in one chromatid under strain induces a break in a non-identical chromatid at precisely the same spot due to transfer of stress. In regard to (1), every cytologist is aware of the physical difficulties which are inherent in suggesting the existence of specific attractions operating over any thing more than extramolecular ranges. However, Delbrück's² theory of autocatalytic synthesis of poly-peptides appears to provide a physico-chemical basis for this force of attraction. In order to test the validity of the second assumption, an attempt is now made to examine the forces operating in a helical system which is in equilibrium under torsion.