Figure 5.—

Model for the function of boundary sites and the effects of translocations on synapsis and DSB formation. (A) Prior to SC formation, homologs are aligned along their lengths at <0.5 μm. (B) In wild type, a change in chromosome structure initiates from the boundary sites. This could be SC formation although this has not been shown. The mapped boundary sites are located at cytological locations 85A-D, 91A-93D, and 95B-97C and the telomeres (T) are indicated. (C) The boundary sites are mapped by determining where crossing over is suppressed in a series of translocations. In a T(2;3)C287 heterozygote (blue; chromosome 2 portion of translocation not shown), crossing over is reduced in the cu-e but not in the e-ca regions. We propose that efficient DSB formation requires the continuity of a structure between the boundary sites. It is not known, however, if this structure is an SC component(s). Due to the translocation break, this structure cannot be continuous and SC formation cannot be completed.