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.