Unblocking of meiotic crossing-over between the silent mating-type cassettes of fission yeast,conditioned by the recessive,pleiotropic mutant rik1

Summary The mating-type region of Schizosaccharomyces pombe consists of three subloci: the expressed cassette at mat1, and the silent cassettes at mat2-P and mat3-M. Previous work has shown that the genetically inert spacer region of 15 kb between mat2 and mat3 is completely devoid of meiotic recombination. This crossover blockage is lifted in the recessive mutant rik1. Other properties such as mating-type switching, sporulation efficiency and spore viability are also affected in this pleiotropic mutant. Presumably the wild-type rik1 product is responsible for heterochromatinization throughout the silent domain of the mating-type region.     

DNA changes involved in the formation of metaphase chromosomes,as observed in mouse duodenal crypt cells stained by osmium-ammine I. New structures arise during the S phase and condense at prophase into “chromomeres,” which fuse at prometaphase into mitotic chromosomes

Background: In the hope of understanding how chromosomes condense at mitosis, we took advantage of a subdivision of the cell cycle into 11 stages to examine the changes in DNA taking place during the stages preceding the emergence of metaphase chromosomes. Methods: To identify DNA changes, pieces of mouse duodenum were fixed in formaldehyde, and sections of the rapidly dividing cells of the crypts were stained by the osmium-ammine method, which is specific for the detection of DNA in the electron microscope. Results: Throughout the cell cycle, DNA is present in nucleofilaments composed of rows of 11-nm-wide nucleosomes. At stage I, during which the DNA-synthesizing or S phase of the cell cycle begins, some of the nucleofilaments are compacted in the heterochromatin accumulations associated with nuclear envelope and nucleoli, while the others are scattered in the nucleoplasm where they appear either “free” or “attached” to the heterochromatin. This DNA distribution is similar to that observed in the noncycling cells examined. After the beginning of the S phase, “free” nucleofilaments are seen to assemble into structures composed of compacted nucleofilaments and referred to as “aggregates”; these make their appearance at stage II and increase in size through stage III up to the end of S during stage IV. Meanwhile, the heterochromatin associated with nuclear envelope and nucleoli expands toward the nucleoplasm in the form of protrusions referred to as “bulges,” which gradually enlarge during stages III and IV, while the heterochromatin shrinks and eventually vanishes. On average, a total of 1,171 aggregates and bulges are formed in the nucleus during the S phase. At the apparition of stage V, which corresponds approximately to prophase, aggregates and bulges are rapidly gathered into an average of 288 spheroidal bodies referred to as “chromomeres.” These are connected to one another by nucleofilamentous bridges in such a way as to be lined up in rows. The formation of rows of chromomeres represents in the electron microscope the prophasic condensation observed in the light microscope. Finally, during stage VIa, which corresponds to prometaphase, the chromomeres approach one another within each row, make contact, and coalesce to become the 40 chromosomes of the mouse, which during stage VIb are organized in the equatorial plate of metaphase. Conclusions: The condensation of metaphase chromosomes occurs in three main steps. The first and longest takes place during the S phase, as nucleofilaments are assembled into aggregates, while the heterochromatin give rise to bulges. The brief second step occurs toward the beginning of prophase, when the numerous aggregates and bulges are congregated into a limited number of chromomeres, which are lined up in rows. The third step takes place during the brief prometaphase, when the chromomeres of a row coalesce into a mitotic chromosome. © 1995 Wiley-Liss, Inc.     

Das Problem der Sexchromatin-negativen Zellen

Zusammenfassung Die Faktoren, die für das Auftreten Sexchromatin-negativer weiblicher Zellen verantwortlich gemacht werden können, werden zusammenfassend dargestellt. In solchen Zellen ist keines der beiden X-chromosomen während der Interphase stärker kondensiert, d.h., es fehlt die in Sexchromatin-positiven Zellen zu beobachtende Heteropyknose eines X-Chromosoms.In somatischen Zellen steht des Fehlen der Heteropyknose in keinem Zusammenhang mit der DNS-Synthese-Periode, es ist auch nicht korreliert mit der Spätreplikation eines X-Chromosoms, die in allen Zellen, mit und ohne Sexchromatin, vorhanden ist. Die genetische Inaktivierung eines X-Chromosoms bleibt in Sexchromatin-negativen Zellen offenbar bestehen. Die in dieser Hinsicht gegen die Lyon-Hypothese vorgebrachten Einwände sind nicht aufrechtzuerhalten.. Da die genetische Inaktivität heterochromatischer Chromosomen nicht an die Heteropyknose gebunden ist, kann diese auch nicht die Ursache der Inaktivität sein.Eine gewisse Sonderstellung nehmen Oocyten und frühembryonale Zellen ein, bei denen die genetische Inaktivierung eines der beiden X-Chromosomen vermutlich nicht stattfindet, und die keine Heteropyknose eines X-Chromosoms erkennen lassen.Die Ausbildung eines Sexchromatinkörperchens kann durch bestimmte Faktoren, wie Zellcyclusstadium und Zelldichte beeinflußt werden. Die hier vorgelegten eigenen Befunde an ein-und mehrkernigen Zellen von Microtus agrestis ergaben aber, daß der Grad der Heteropyknose der Geschlechtschromosomen innerhalb eines Zellkernes unabhängig von äußeren Faktoren witgehend fixiert ist und bei der Mitose vermutlich auf die Tochterzellen weitergegeben wird.

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The problem of sex chromatin negative cells

Summary The factors possibly responsible for the occurrence of sexchromatin-negative female cells are reviewed. In such cells, neither of the two X-chromosomes is condensed during interphase; thus the heteropyknosis of one X-chromosome observed in sexchromatinpositive cells is absent.In somatic cells, the absence of heteropyknosis is not related to the DNA-synthesisperiod and is not correlated with the late replication of one of the X-chromosomes which occurs in all cells, with and without sex chromatin.The genetic inactivation of one X-chromosome seems to persist in sexchromatin-negative cells. Thus in this respect the objections to the Lyon hypothesis cannot be maintained. Since the genetic inactivity of heterochromatic chromosomes is not due to heteropyknosis, the latter cannot be the cause of their inactivity.Oocytes and early embryonic cells fall within a special category, in that the genetic inactivation of one of the two X-chromosomes presumably does not occur, also no heteropyknosis of one of the X-chromosomes can be observed.The formation of a sexchromatin body can be influenced by certain factors such as phase of the cell cycle and cell density. The findings in respect of single-and multi-nucleated cells of Microtus agrestis, here reported, show however that the degree of heteropyknosis of the sex chromosomes within a cell-nucleus is to a large extent fixed and independent of extrinsic factors. It is presumably transmitted through mitosis to the daughter cells.

Herrn Prof. Dr. G. Wolf-Heidegger zum 60. Geburtstag in Dankbarkeit und Verehrung gewidmet.     

Remodeling of heterochromatin induced by heavy metals in extreme old age

The levels of chromosome instability and heat absorption of chromatin have been studied in cultured lymphocytes derived from blood of 80–93- and 18–30-year-old individuals, under the effect of heavy metal Cu(II) and Cd(II) salts. The analysis of the results obtained indicates that 50 μM Cu(II) induced a significantly higher level of cells with chromosome aberrations in old donors (13.8 ± 1.5% vs control, 3.8 ± 1.7%), whereas treatment with 100 μM Cd(II) did not induce any changes in the background index. Analysis of the lymphocyte melting curves showed that Cu(II) ions caused more effective condensation of heterochromatin in old healthy individuals compared with young donors, which was expressed by the increase of the T _m of elderly chromatin by ~3°C compared with the norm. Treatment of lymphocyte chromatin of old individuals with 100 μM Cd(II) caused decondensation (deheterochromatinization) of both the facultative and constitutive domains of heterochromatin. The deheterochromatinization T _m was decreased by ~3–3.5°C compared with the T _m observed for young individuals. Thus, the chromatin of cultured lymphocytes from the old-aged individuals underwent modification under the influence of copper and cadmium salts. Cu(II) caused additional heterochromatinization of heterochromatin, and Cd(II) caused deheterochromatinization of facultative and constitutive heterochromatin. Our data may be important as new information on the remodeling of constitutive and facultative heterochromatin induced by heavy metals in aging, aging pathology, and pathology linked with metal ions.     

9号染色体异染色质区的变异分析

目的分析疑有染色体异常个体的9号染色体异染色质区的变异。方法采集疑有染色体异常的3075名个体静脉血，其中男性1653例，女性1422例，以1515名正常人作对照，培养其淋巴细胞进行染色体核型分析。结果疑有染色体异常个体的9号染色体异染色质区的变异率为5．56％，而正常对照组9号染色体异染色质区的变异率仅为1．32％，9号染色体异染色质区的变异包括9qh＋、9qh-及inv（9）。结论疑有染色体异常个体的9号染色体异染色质区的变异率高于正常人4．2倍（P＜0．01），这说明9号染色体异染色质区的变异可能参与一些染色体病的发生。     

DNA changes involved in the formation of metaphase chromosomes,as observed in mouse duodenal crypt cells stained by osmium-ammine II. Tracing nascent dna by bromodeoxyuridine into structures arising during the S phase

Background. Adult cetacean males, like non-mammalian vertebrates and other testicond mammals, have intra-abdominal testes. There is no evidence of a processus vaginalis in them. Testicondia in cetaceans is considered secondary as they are judged, evolutionarily, the descendants of terrestrial mammals (ungulates) with testis descent. A possible argument in support of the latter contention would be that cetacean fetuses develop gubernacula which are the primordia of the processus vaginalis and other structures associated with testis descent in other placental mammals. the present study intended to analyse cetacean fetuses in this respect. Methods. Serial sections of 25 fetuses (total body length between 39.5 and 160 mm) of 4 cetacean species (Delphinus delphis, Phocoena phocoena, Eschrichtius robustus, Physeter catodon) were examined with special attention to the presence or absence of structures homologous to the gubernaculum of other placental mammals (rats and humans). Results. Gubernacular primordia were observed in fetuses from about the time of onset of sexual differentiation. Their shape and anatomical relationship with the surrounding structures were similar as those in mammals with testis descent. The gubernaculum in males developed into a large mass of dense connective tissue in the ventral-caudal abdominal region at the site of the insertion of the mesonephric inguinal ligament and associated to the tip of the internal abdominal oblique muscle. No (or only very little) development of a processus vaginalis was noticed. Conclusions. The results demonstrate initial emergence of mammalianlike gubernacular primordia in cetacean fetuses without their further development to elaborate structures required for testis descent. The findings support the view that cetaceans are secondarily testicond. It is suggested that (1) absence of the pelvic girdle together with (2) the development of structures in and beyond the caudal abdominal region, particularly the caudal hypaxial musculature, precludes the outgrowth, into caudal direction, of hollow organs (such as the processus vaginalis) from the abdominal cavity. © 1995 Wiley-Liss, Inc.     

Theoretical mechanisms for synthesis of carcinogen-induced embryonic proteins: XVII. Heterochromatin mechanisms

A mechanism for induced embryonic gene expression via a process of deheterochromatization using a model carcinogen has been derived. First ethionine becomes activated to S-adenosyl-L-ethionine which inhibits the methylation of nicotinamide, a resulting product of polyADP-ribose polymerase. This causes hyporibosylated nucleosome core histones which normally would base pair by virtue of the adenine moieties with thymidine-rich regions of DNA, being the precursor of heterochromatin. Thus in the anomalous state a deheterochromatized condition of embryonic genes would be created. Possibly embryonic genes are dispersed in AT-rich regions potentially capable of becoming hyperspiralized by this process. Repressable embryonic genes would not be inactivated. It was also noted that hyomethylated non-histone chromatin proteins cause an extension of the nucleosome chanins which would also favor the above situation. This mechanism explains our experimental findings of the relatively rapid reversal of ethionine induced alpha-fetoprotein levels by methionine. The process of heterochromatization is hypothesized to be induced by short (pentanucleotides) moities of poly (ADP-ribose), formed on core histones, that hydrogen bond to thymidine rich inter Nu body DNA.     

Contribution of AT-, GC-, and methylated cytidine-rich DNA to chromatin composition in Malpighian tubule cell nuclei of Panstrongylus megistus (Hemiptera, Reduviidae)

The Malpighian tubule cell nuclei of male Panstrongylus megistus, a vector of Chagas disease, contain one chromocenter, which is composed solely of the Y chromosome. Considering that different chromosomes contribute to the composition of chromocenters in different triatomini species, the aim of this study was to determine the contribution of AT-, GC-, and methylated cytidine-rich DNA in the chromocenter as well as in euchromatin of Malpighian tubule cell nuclei of P. megistus in comparison with published data for Triatoma infestans. Staining with 4′,6-diamidino-2-phenylindole/actinomycin D and chromomycin A₃/distamycin, immunodetection of 5-methylcytidine and AgNOR test were used. The results revealed AT-rich/GC-poor DNA in the male chromocenter, but equally distributed AT and GC DNA sequences in male and female euchromatin, like in T. infestans. Accumulation of argyrophilic proteins encircling the chromocenter did not always correlate with that of GC-rich DNA. Methylated DNA identified by immunodetection was found sparsely distributed in the euchromatin of both sexes and at some points around the chromocenter edge, but it could not be considered responsible for chromatin condensation in the chromocenter, like in T. infestans. However, unlike in T. infestans, no correlation between the chromocenter AT-rich DNA and nucleolus organizing region (NOR) DNA was found in P. megistus.