The functional organization of chromosomes and the nucleus--a special issue

The spectra of large second-step mutations are similar for two different mouse autosomes

Loss of tumor suppressor gene expression via mutations plays a critical role in cancer development, particularly when occurring in heterozygous cells. These so-called "second-step" mutational events are often large in size and arise most often from chromosome loss, mitotic recombination, or interstitial deletion. An open question in cancer research is whether different chromosomes are equally susceptible to formation of large mutations, or alternatively if the unique sequence of each chromosome will lead to chromosome-specific mutational spectra. To address this question, the spectra of second-step mutations were determined for chromosomes 8 and 11 in Aprt and Tk mutants, respectively, isolated from primary kidney clones heterozygous for both loci. The results showed that the spectra of large mutational events were essentially the same. This observation suggests that internal and external cellular environments provide the driving force for large autosomal mutational events, and that chromosome structure per se is the substrate upon which these forces act.

Chromatin organisation and nuclear architecture in growing mouse oocytes

Although the female gamete is blocked at the dictyate stage of the first meiotic prophase during the whole folliculogenesis, many important epigenetic changes occur to organise the genome to attend early embryonic development. In this paper, we will describe the results of a number of studies aimed to improve our understanding of the nuclear organization of the mouse oocyte during folliculogenesis. Using silver methods that stain NOR, centromeres and heterochromatin, as well as, the use of specific antibodies for the demonstration of centromeres, we have described the changes to the chromatin organisation and to the spatial localisation of chromocenters and centromeres during oocyte growth; these changes have been correlated to the developmental competence of the resulting antral and metaphase II (MII) oocyte.

Spatial distribution of sperm-derived chromatin in zygotes determined by fluorescence in situ hybridization

Fluorescence in situ hybridization was used to determine the spatial distribution of chromatin in zygote pronuclei. A hybrid system involving golden hamster eggs and individual human sperm permitted use of DNA probes specific for the entire human chromosome 4, for the heterochromatic region on the long arm of the human Y chromosome and for unique DNA sequences on human chromosome 19. Chromosome 4 occupied a circumscribed domain in the pronuclei, similar to findings in somatic interphases. Unlike the situation in somatic interphases, the Y heterochromatin was extended throughout the first cell cycle. Pronuclear chromatin was extended 3- to 4-fold compared to somatic interphase chromatin. The extended pronuclear chromatin conformation is likely to affect a zygote's susceptibility to environmental hazards.