Fig 6. Model demonstrating how Drosophila might differ from other organisms in the extent of cohesin-independent connections between chromosomes.

Hypothetical cohesin-independent connections that form between chromosomes and the timing of their removal. In this model, the cohesin-independent connections that form between sister chromatids in Drosophila (potentially, DNA catenations) also form between homologs. In other organisms, extensive cohesin-independent connections between sisters and/or homologs may not form or may be resolved more quickly (homolog not pictured). As shown, cohesin-independent connections may be resolved later in the cell cycle in Drosophila than in other organisms; alternatively, it is possible that cohesin-independent connections are formed more efficiently in Drosophila, so that they are more widespread. Condensin II may antagonize inter-homolog or inter-sister interactions in a number of ways: they may remove residual cohesin, recruit topoisomerases to remove catenations between chromosomes, or, as pictured here, compact chromosomes in cis, limiting their ability to participate in trans interactions [13,64,69,71]. Further, it is possible that condensin II cooperates with condensin I during mitosis in the process of resolving sister chromatids (see S13 Fig). Note that, in the top panel representing Drosophila, all cohesin-independent linkages are resolved by metaphase, but in theory, some connections between homologs might remain in metaphase and in anaphase. Similarly, in the lower panel representing other organisms, all cohesin-independent connections between sisters have been resolved by G2, but it is possible that they remain in certain regions.