A model for chromosome structure during the mitotic and meiotic cell cycles

The chromosome scaffold model in which loops of chromatin are attached to a central, coiled chromosome core (scaffold) is the current paradigm for chr omosome structure. Here we present a modified version of the chromosome sca ffold model to describe chromosome structure and behavior through the mitot ic and meiotic cell cycles. We suggest that a salient feature of chromosome structure is established during DNA replication when sister loops of DNA e xtend in opposite directions from replication sites on nuclear matrix stran ds. This orientation is maintained into prophase when the nuclear matrix st rand is converted into two closely associated sister chromatid cores with s ister DNA loops extending in opposite directions. We propose that chromatid cores are contractile and show, using a physical model, that contraction o f cores during late prophase can result in coiled chromatids. Coiling accou nts for the majority of chromosome shortening that is needed to separate si ster chromatids within the confines of a cell. In early prophase I of meios is, the orientation of sister DNA loops in opposite directions from axial e lements assures that DNA loops interact preferentially with homologous DNA loops rather than with sister DNA loops. In this context, we propose a bar code model for homologous presynaptic chromosome alignment that involves we ak paranemic interactions of homologous DNA loops. Opposite orientation of sister loops also suppresses crossing over between sister chromatids in fav or of crossing over between homologous non-sister chromatids. After crossin g over is completed in pachytene and the synaptonemal complex breaks down i n early diplotene (= diffuse stage), new contractile cores are laid down al ong each chromatid. These chromatid cores are comparable to the chromatid c ores in mitotic prophase chromosomes. As an aside, we propose that leptoten e through early diplotene represent the 'missing' G2 period of the premeiot ic interphase. The new chromosome cores, along with sister chromatid cohesi on, stabilize chiasmata. Contraction of cores in late diplotene causes chro mosomes to coil in a configuration that encourages subsequent syntelic orie ntation of sister kinetochores and amphitelic orientation of homologous kin etochore pairs on the spindle at metaphase I.