IMMUNOCYTOLOGY OF CHIASMATA AND CHROMOSOMAL DISJUNCTION AT MOUSE MEIOSIS

Immunocytological and in situ hybridization evidence supports the hypo thesis that at meiosis of chiasmate organisms, chromosomal disjunction and reductional segregation of sister centromeres are integrated with synaptonemal complex functions. The Mr 125,000 synaptic protein, Syn1 , present between cores of paired homologous chromosomes during pachyt ene of meiotic prophase, is lost from synaptonemal complexes coordinat ely with homolog separation at diplotene. Separation is constrained by exchanges between non-sister chromatids, the chiasmata. We show that the Mr 30,000 chromosomal core protein, Cor1, associated with sister c hromatid pairs, remains an axial component of post-pachytene chromosom es until metaphase I. We demonstrate that at this time the chromatin l oops are still attached to their cores. A reciprocal exchange event be tween two homologous non-sister chromatids is therefore immobilized by anchorage of sister chromatids to their respective cores. Cores thus contribute to the sister chromatid cohesiveness required for maintenan ce of chiasmata and proper chromosomal disjunction. Cor1 protein accum ulates in juxtaposition to pairs of sister centromeres during metaphas e I. Presumably, independent movement of sister centromeres at anaphas e I is restricted by Cor1 anchorage. That reductional separation of si ster centromeres is mediated by Cor1, is supported by the dissociation of Cor1 from separating sister centromeres at anaphase II and by its absence from mitotic anaphases.