CHROMOSOME LENGTH AND DNA LOOP SIZE DURING EARLY EMBRYONIC-DEVELOPMENT OF XENOPUS-LAEVIS

The looped organization of the eukaryotic genome mediated by a skeleta l framework of non-histone proteins is conserved throughout the cell c ycle. The radial loop/scaffold model envisages that the higher order a rchitecture of metaphase chromosomes relies on an axial structure arou nd which looped DNA domains are radially arranged through stable attac hment sites. In this light we investigated the relationship between th e looped organization and overall morphology of chromosomes. In develo ping Xenopus laevis embryos at gastrulation, the bulk of the loops ass ociated with histone-depleted nuclei exhibit a significant size increa se, as visualized by fluorescence microscopy of the fully extended DNA halo surrounding high salt treated, ethidium bromide stained nuclei. This implies a reduction in the number of looped domains anchored to t he supporting nucleoskeletal structure. The cytological analysis of me taphase plates from acetic acid fixed whole embryos, carried out in th e absence of drugs inducing chromosome condensation, reveals a progres sive thickening and shortening of metaphase chromosomes during develop ment. We interpret these findings as a strong indication that the size and number of DNA loops influence the thickness and length of the chr omosomes, respectively. The quantitative analysis of chromosome length distributions at different developmental stages suggests that the sho rtening is timed differently in different embryonic cells.