Behavior of the chromosome core in mitosis and meiosis

Tandem Repeat-Based Probes Support the Loop Model of Pericentromere Packing

Constitutive heterochromatin is the most mysterious part of the eukaryotic genome. It forms vital chromosome regions such as the centromeric and the pericentromeric ones. The main component of heterochromatic regions are tandem repeats (TR), and their specific organization complicates assembly, annotation, and mapping of these regions. Unannotated and unmapped TR arrays are still present in database contigs. In this study, we used a set of TR in the genomes of the pig (Sus scrofa) and the Chinese hamster (Cricetulus griseus) identified with the help of bioinformatics techniques and determined the specificity of the designed probes. The signal of the 4 pig TR probes in spermatogenic cells was often ring-shaped, especially in primary spermatocytes. The rings were located in the regions relatively weakly stained with DAPI. The unique assembly of the centromeric region was traced using the hamster meiotic chromosomes. The probe specific to chromosome 5 was used. Two signals, arranged as rings, were seen at the pachytene stage, similar to those in the pig spermatogenic cells. In the spermatogenic cells of both pig and hamster, the rings appeared on the chromosomes with pericentromeric TR probes. Our observations support the loop model of the centromeric region, the size of the loops being about 50 kb.

Morphology and behaviour of silver-stained chromatid cores in mitotic chromosomes analysed by whole mount electron microscopy

Using silver staining and the whole mount electron microscopy technique of squashed chromosomes, we studied the substructural organization and behaviour of chromatid cores in mitotic chromosomes of spermatogonia of the grasshopper Oedaleus infernalis during mitosis. It was found that the formation of mitotic chromatid cores takes place during the transition from prophase to prometaphase. Each chromosome contains two compact chromatid cores which are surrounded by a halo of dispersed argyrophilic material emanating radially from the cores. In early metaphase the chromatid core usually appears as an extended, slender network running longitudinally through the entire length of the chromatid, while in late metaphase the core frequently has a spiral appearance. In addition, our results revealed the existence of interconnections between sister chromatid cores along their entire length, as a result of which sister chromatid cores appear as a single interconnected core network in mitotic metaphase chromosomes. At this stage the core occupies a lateral position in each chromatid. However, during the transition from metaphase to anaphase, the interconnections are gradually released to allow the individualization of sister chromatid cores and the segregation of chromosomes. The core comes to occupy a central position in each segregated chromatid. These findings demonstrate the presence of an intrinsic interconnected core network within metaphase chromosomes which could be involved in the maintenance and segregation of chromosomes during mitosis.

Visualizing the spatial relationships between defined DNA sequences and the axial region of extracted metaphase chromosomes

Using fluorescence in situ hybridization to extracted metaphase chromosomes, we present visual evidence that specific human DNA sequences occupy distinctive positions with respect to the axial region of chromosomes and that the DNA is organized into loops emanating from this region. In a stretch of unique DNA on chromosome 11, large loops of DNA can be traced and one specific region associated with the axial region of the chromosome. Within rDNA, nontranscribed spacer sequences are more closely apposed to the chromosome axis than are rRNA genes. Heterochromatic and euchromatic DNAs appear to be organized into loops of similar size. We could not detect loops at centromeres; most alphoid DNA appears to remain close to the axial region.

The substructural organization of the chromosome core (scaffold) in meiotic chromosomes of Trilophidia annulata

The substructural organization of chromosome cores or nonhistone proteins was studied within intact metaphase chromosomes at the second meiotic division in the grasshopper Trilophidia annulata by silver staining as well as light microscopy and whole mount electron microscopy of squash chromosomes. Our results revealed that the metaphase II chromosome contains a longitudinal, helical coiling core structure. Probably the two last organizational levels of the core packaging are achieved by helical coiling. The core structure retains the morphological characteristics of the original metaphase chromosome, surrounded by a halo of dispersed materials, which may be composed mainly of nonhistone proteins. The kinetochore is found to be connected with the chromosome core. The present findings combined with our previous observations on the helical structure of metaphase II chromosomes suggest that the folding path of the internal core structure in metaphase chromosomes is consistent with the final helical arrangement of the chromosome itself. These observations also imply that in condensed metaphase chromosomes nonhistone protein may form a compact network structure with helical appearance, which extends throughout the entire chromosome.

The telochore: a telomeric differentiation of the chromosome axis

We have analysed by means of silver staining the structure of the chromosome axis at the telomeres of meiotic chromosomes in three different grasshopper species. At metaphase I the chromatid axes run the length of the chromatids although they do not reach the chromosome ends. The axes of sister chromatids are associated and show a round differentiation at their distal ends that we have named the 'telochore'. Telochores never contact the chromosome ends: there is always some chromatin beyond them. In late metaphase I bivalents with a distal chiasma, anaphase I and metaphase II half-bivalents and anaphase II chromatids, the axes clearly possess one telochore in each chromosome end. These results seem to indicate that telochores are differentiations of the distal ends of chromatids. We discuss the possible structural significance of telochores according to the current scaffold/radial loop model of chromatin organization of eukaryotic metaphase chromosomes. Additionally, we suggest the possible functional role of the telochore as a nucleoprotein domain forming a protective cap for telomeric DNA.

Scaffold morphology in histone-depleted HeLa metaphase chromosomes

In a study of HeLa metaphase chromosomes depleted of histones with 2M NaCl and spread with cytochrome c, two new types of images of chromosome scaffolds have been observed in the electron microscope. In the first type, scaffolds are very large and fibrous but still display the shape typical of metaphase chromosomes. The regularity and lack of distortion in these scaffolds, despite their openness and seeming fragility, support the notion that the underlying scaffold structure is an interconnected network of fibers. In the second type, fibrous regions and dense regions are juxtaposed in the same chromosome scaffold. These micrographs suggest that the dense appearance of some previously observed scaffolds may be the result of incomplete adherence to the cytochrome c monolayer, leading to collapse and aggregation during dehydration.

Metaphase chromosome structure. Involvement of topoisomerase II

SCI is a prominent, 170,000 Mr, non-histone protein of HeLa metaphase chromosomes. This protein binds DNA and was previously identified as one of the major structural components of the residual scaffold structure obtained by differential protein extraction from isolated chromosomes. The metaphase scaffold maintains chromosomal DNA in an organized, looped conformation. We have prepared a polyclonal antibody against the SC1 protein. Immunolocalization studies by both fluorescence and electron microscopy allowed identification of the scaffold structure in gently expanded chromosomes. The micrographs show an immunopositive reaction going through the kinetochore along a central, axial region that extends the length of each chromatid. Some micrographs of histone-depleted chromosomes provide evidence of the substructural organization of the scaffold; the scaffold appears to consist of an assembly of foci, which in places form a zig-zag or coiled arrangement. We present several lines of evidence that establish the identity of SC1 as topoisomerase II. Considering the enzymic nature of this protein, it is remarkable that it represents 1% to 2% of the total mitotic chromosomal protein. About 60% to 80% of topoisomerase II partitions into the scaffold structure as prepared from isolated chromosomes, and we find approximately three copies per average 70,000-base loop. This supports the proposed structural role of the scaffold in the organization of the mitotic chromosome. The dual enzymic and apparent structural function of topoisomerase II (SC1) and its location at or near the base of chromatin loops allows speculation as to its involvement in the long-range control of chromatin structure.

Silver staining the chromosome scaffold

Cytological silver-staining procedures reveal the presence of a "core" running along the chromatid axes of isolated HeLa mitotic chromosomes. In this communication we examine the relationship between this "core" and the nonhistone chromosome scaffolding, isolated and characterized in previous publications from this laboratory. When chromosomes on coverslips were subjected to the steps used for scaffold isolation in vitro and subsequently stained with silver, the characteristic "core" staining was unaffected. Control experiments suggested that the "core" does not contain large amounts of DNA. When scaffolds were isolated in vitro, centrifuged onto electron microscope grids, and stained with silver, they were found to stain selectively under conditions where specific "core" staining was observed in intact chromosomes. These results suggest that the nonhistone scaffolding is the principal target of the silver stain in chromosomes.

Demonstration of kinetochores and centrioles in spermatocytes of two species of cockroaches by silver staining

Light microscopy following silver staining of spermatocytes of German and Madagascar hissing cockroaches demonstrated: (1) the localization of a kinetochore in each autosomal synaptonemal complex during pachytene, and (2) visualization of centrioles in different stages of meiotic prophase. The presence of a "hairpin-like" twist and the nucleolus organizer region in the X-chromosome was observed only in the German cockroach.

Higher order metaphase chromosome structure: evidence for metalloprotein interactions

One level of DNA organization in metaphase chromosomes is brought about by a scaffolding structure that is stabilized by metalloprotein interactions. Fast-sedimenting, histone-depleted structures (4000-7000 S), derived from metaphase chromosomes by extraction of the histones, are dissociated by metal chelators or by thiol reagents. The chromosomal (scaffolding) proteins responsible for constraining the DNA in this fast-sedimenting form are solubilized under the same conditions. Chromosomes isolated in a metal-depleted form, which generate slow-sedimenting, histone-depleted structures, can be specifically and reversibly stabilized by Cu2+, but not by Mn2+, Co2+, Zn2+ or Hg2+. Metal-depleted chromosomes can also be stabilized by Ca2+ (at 37 degrees C), but this effect is less specific than that of Cu2+. The scaffolding protein pattern that is reproducibly generated following treatment with Cu2+ is composed primarily of two high molecular weight proteins--Sc1 and Sc2 (170,000 and 135,000 daltons). The identification of this simple protein pattern has depended upon the development of new chromosome isolation methods that are highly effective in eliminating cytoskeletal contamination.