EVIDENCE FOR SISTER-STRAND CROSSING OVER IN MAIZE

Abstract

ROSSING OVER, the exchange of chromatin between homologous chro-C mosomes, is of utmost importance in genetic studies. Although little is known about the mechanism which is responsible for this exchange, a number of facts on this subject have been fairly well established: (1) Crossing over is produced by breakage and reunion of broken end's (even on BELLING'S hypothesis, breakage and reunion have to be postulated to explain 3-strand and 4-strand doubles). ( (3) Crossing over occurs at prophase of the first meiotic division. (4) Only 2 of the 4 chromatids are involved in any one crossover. ( It has generally been assumed that there is little or no sister-strand crossing over.