Chromosome length and DNA loop size during early embryonic development of Xenopus laevis

Abstract

The looped organization of the eukaryotic genome mediated by a skeletal framework of non-histone proteins is conserved throughout the cell cycle. The radial loop/scaffold model envisages that the higher order architecture of metaphase chromosomes relies on an axial structure around which looped DNA domains are radially arranged through stable attachment sites. In this light we investigated the relationship between the looped organization and overall morphology of chromosomes. In developing Xenopus laevis embryos at gastrulation, the bulk of the loops associated with histone-depleted nuclei exhibit a significant size increase, 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 the supporting nucleoskeletal structure. The cytological analysis of metaphase plates from acetic acid fixed whole embryos, carried out in the absence of drugs inducing chromosome condensation, reveals a progressive thickening and shortening of metaphase chromosomes during development. We interpret these findings as a strong indication that the size and number of DNA loops influence the thickness and length of the chromosomes, respectively. The quantitative analysis of chromosome length distributions at different developmental stages suggests that the shortening is timed differently in different embryonic cells.

The relative ratio of condensin I to II determines chromosome shapes

“…Given the fixed volume of a chromatin fiber whose density within a chromosome has been shown to be constant from prometaphase through metaphase (Mora-Bermudez et al 2007), axial shortening would create shorter and thicker chromatids, whereas lateral compaction would make chromatids longer and thinner. In fact, it has been demonstrated that chromosomes progressively get shorter and thicker during animal development (Belmont et al 1987;Micheli et al 1993). On the basis of the ratio manipulation experiments reported here, we propose that the primary actions of condensin I and II are aimed at lateral compaction and axial shortening, respectively, and that their intricate balance acts as one of the critical determinants in shaping chromatids (Fig.…”

“…It is known that metaphase chromosomes in early embryonic cells tend to be longer and thinner compared with those in somatic cells (Belmont et al 1987;Micheli et al 1993), and the chromosomes assembled in Xenopus egg extracts recapitulate the embryonic properties (Fig. 1B,C).…”

“…The shape of metaphase chromosomes is highly characteristic, yet varies widely among different organisms (Losada and Hirano 2001). Progressive shortening and thickening of metaphase chromosomes during development have also been reported in Drosophila (Belmont et al 1987) and Xenopus laevis (Micheli et al 1993), implicating a link between chromosome shapes and developmentally regulated processes such as transcription and replication programs. While early cytologists and modern cell biologists have been describing the dynamic behavior of metaphase chromosomes for more than a century, it remains largely unknown how they might be shaped at a mechanistic level.…”

Micromechanical studies of mitotic chromosomes

“…The condensin I to condensin II ratio in egg extracts is about 5:1, while in somatic HeLa cells it is closer to 1:1 (Ono et al 2003). It has been documented that metaphase chromosomes in Xenopus embryos are twice as long as and substantially narrower than those in swimming larvae (Micheli et al 1993), the difference in condensation perhaps being due to different ratios of condensin I and II or to cohesin domain or replicon size (Marilley & Gassend-Bonnet 1989).…”

Chromatin remodelling and DNA replication: from nucleosomes to loop domains

“…In Xenopus, shortening and thickening of metaphase chromosomes from the blastula to the swimming larva stages is accompanied by a signi®cant increase in the length of DNA loops (Micheli et al, 1993). This general rearrangement of the higher order structure of chromatin re¯ects a reduction of the number of associations of MAR/SAR to the sca old and matrix.…”