Showing papers on "Heterochromatin published in 1968"

Position-effect variegation

Abstract: Publisher Summary This chapter focuses on the cytogenetic aspects of position-effect variegation. One of the diagnostic characteristics of position-effect variegation, at least in Drosophila, is the suppression of the variegation toward wild type upon the addition of a Y chromosome and, conversely, the enhancement of the variegation toward the mutant phenotype upon the deletion of this element. Three facts are mentioned, which suggest that position-effect variegation is operating at the transcriptional, or possibly the translational, level of gene expression. These facts are the ubiquity of genes affected, the cis-trans nature of the effect and the centromere-distance effect. The phenomenon of position-effect variegation is overly blessed with the number of interesting observations that have been made. The position-effect nature of the variegation in the mouse was demonstrated by use of a translocation between the X chromosome and genophore 8 that produced brown variegation. Position-effect variegation in the mouse is always associated with rearrangements of the X, an element that is known to become heteropycnotic in one of the X's of an XX cell. This correlates well with the heterochromatic regions responsible for variegation in Drosophila .

Late DNA synthesis in heterochromatin.

Abstract: It seems to be a rule of chromosome replication that heterochromatin synthesizes its DNA at a later stage than the rest of the chromosome.

The replicative organization of DNA in polytene chromosomes of Drosophila hydei.

Abstract: Salivary-gland nuclei ofDrosophila hydei were pulse-labeledin vitro with3H-thymidine and studied autoradiographically in squash preparations. The distribution of radioactive label over the length of the polytene chromosomes was discontinuous in most of the labeled nuclei; in some nuclei the pattern of incorporation was continuous. Comparison of the various labeling patterns of homologous chromosome regions in different nuclei showed that specific replicating units are replicated in a specific order. By combining autoradiography with cytophotometry of Feulgen-stained chromosomes, it was possible to correlate thymidine labeling of specific bands with their DNA content. The resulting data indicate that during the S-period many or perhaps all of the replicating units in a salivary-gland nucleus start DNA synthesis simultaneously but complete it at different times. Furthermore, the data support the hypothesis that the chromomere is a unit of replication or replicon. The DNA content of haploid chromomeres was found to be about 5×10-4 pg for the largest bands inDrosophila hydei. From the results of H3-thymidine autoradiography and Feulgen-cytophotometry on neuroblast and anlage nuclei it was concluded that during growth of the polytenic nucleus heterochromatin is for the most part excluded from duplication. The results of DNA measurements in interbands of polytene chromosomes do not agree with a multistrand structure for the haploid chromatid. A chromosome model is proposed which is in accordance with the reported results and with current views concerning the replicative organization of chromosomes.

Fine structure analysis of a chromosome segment in Drosophila melanogaster. Analysis of x-ray-induced lethals.

Abstract: Allelism tests were performed between X-ray-induced lethals that were located on the X-chromosome of Drosophila at a region covered by the Y.ma-l + translocation. The complementation map which was constructed from the matrix of allelic combinations proved to be linear. So far 20 sections, or functional units, have been delineated. Further analysis should increase the number of sections until each section comprises one cistron. Practically all post-meiotic X-ray induced lethals in the region were aberrations. It is inferred that visible mutations induced by X-rays were deficiencies at non-essential loci. The breakage points of mutations in the segment were not distributed at random. The “hot spot” was attributed to an intercalary heterochromatic band, which by pairing with the adjacent proximal heterochromatin caused preference in breaks simultaneously at the “hot spot” and the proximal heterochromatin, and hence also caused preference for extensive aberrations. A correlation was found between male sterility and heterochromatic aberrations, but not between male sterility and X:A translocations. This kind of analysis suggests new approaches to the study of the organization of the genetic material in chromosomes and the nature of various mutational events.

Ultrastructural localization of nucleic acid synthesis in human blood cells.

Abstract: THE chromatin of the interphase cell nucleus has been divided into two types on histochemical and ultrastructural grounds. Electron microscopy has shown heterochromatin or condensed chromatin to have a structure of densely compacted fibrils, whereas euchromatin shows a looser arrangement of similar fibrils. Demarcation between these two types is sharp, but there is continuity of fibrils between them1. Microspectrophotometric measurements of Feulgen-stained nuclei have shown that heterochromatin contains DNA two to three times as concentrated as in euchromatin2. It is rendered more electron dense by the uranyl stain used in electron microscopy3. Cells which are inactive in nucleic acid synthesis usually have a pachychromatic nuclear pattern with more heterochromatin than those which are more active.

Asynchrony of DNA replication and mitotic spiralization along heterochromatic portions of Chinese hamster chromosomes.

Abstract: Sequence of DNA synthesis and mitotic chromosome spiralization along heterochromatic portions of the sex (X1X2) and of some marker chromosomes in cultured Chinese hamster cells were studied, employing two methods: study of segmentation pattern caused in chromosomes with colcemid, and autoradiography with tritiated thymidine. The heterochromatic portions of all chromosomes studied were characterized by striking internal asynchrony of DNA replication. In particular, they had segments that replicated relatively early. The short arm of the X2 chromosome, heterochromatic in female somatic cells, had at least three such segments. Replication patterns of the long arms of the X1 and X2 chromosomes were different. In X1 this arm contains several segments showing relatively early replication. The long arm of X2 had no similar segments. The possible significance of the data obtained is discussed with regard to the problem of genetic inertness of heterochromatin. At the terminal stage of the S period, H3-thymidine seems to be incorporated into condensed chromatin of interphase nuclei. On the basis of the data obtained, it is proposed that during replication of heterochromatin consecutive despiralization of parts of it takes place.

Duration of Mitotic Cycle and Patterns of DNA Replication in Chromosomes of Allium Cepa

Abstract: SUMMARYValues of mitotic cycle parameters were determined in Allium cepa root-tip cells after incorporation of tritiated thymidine. Roots were grown from bulbs cultivated in darkness (21°C). The means of the values from two independent experiments were: G1 = 3 h 20 min.; S: 12 h; G2 = 3 h 40 min; mitosis = 4 h; mitotic cycle = 23 h.The pattern of DNA replication in Allium chromosomes was also investigated. Counts of silver grains over euchromatic and heterochromatic regions of metaphase chromosomes revealed an asynchronous DNA replication in heterochromatin. In metaphases from cells treated at the end of S, heterochromatic segments (i.e. juxta-centromeric and distal segments of chromosomes) were selectively labelled. On the contrary, the labelling was distributed at random in chromosomes from cells which were at the beginning of S when the precursor was given. It was concluded that DNA synthesis lasts longer in heterochromatin than in euchromatin.

Heterochromatin and genetic activity in mealy bugs. I. Compensation for inactive chromatin by increase in cell number.

Abstract: I N male coccids displaying the lecanoid system of chromosome behavior one of the haploid sets of chromosomes (the paternal contribution) heterochromatizes early in embryogeny, and in most tissues remains inactive throughout the life history of the organism (reviewed in BROWN and NUR 1964). This heterochromatin is inactive genetically (reviewed in BROWN and NUR 1964) and the inactivity is the result of repression of RNA synthesis (BERLOWITZ 1965). Female mealy bugs do not exhibit this alteration of chromosome structure. Indeed heterochromatization of the paternal set is the only known chromosomal dimorphism that distinguishes the sexes in these species. Until late in the second larval instar, male and female mealy bugs are the same size and morphologically indistinguishable; however, cytologically they are readily distinguishable. The males possess physiologically haploid tissues. and yet up to this time apparently keep up metabolically with normal, diploid females. Under these conditions one would expect to find increases either in the amount of genetic material or in the activity of the genes in male mealy bugs to compensate for the repression of a haploid set of chromosomes. Towards the end of second instar, a rather profound sexual dimorphism ensues ( NELSON-REES 1960). detected in its earliest manifestation as a color difference in males and females. In this paper experiments are described in which the amount of DNA/g in males and females was determined biochemically. Further experiments were designed to determine if the observed greater amount of DNA in males can be attributed to an increase in cell number in organs composed of cells displaying a heterochromatic set.

[The duration of DNA replication in eu- and heterochromatin in Microtus agretis].

Abstract: The duration of DNA replication of eu- and heterochromatin in kidney epithelial cell cultures of female Microtus agrestis was determined with combined H3-thymidine pulse labelling and cytophotometric determination of Feulgen DNA. The average duration of the total cell cycle was 23.3 hrs, with a G1 period of 14.6 hrs, S period of 5 hrs, G2 period of 2.7 hrs, and mitosis of 1 hr. The replication time of eu- and heterochromatin was determined by the frequency of the different labelling patterns after pulse labelling. The time sequence of the labelling patterns was ascertained by DNA measurements. During the S period, euchromatin replicates at first alone for 3 hrs (60% of the length of S) and 1 hr (19.3%) together with heterochromatin. During the last hour (20.7%), only heterochromatic regions replicate. The sex chromatin part of the one X chromosome starts synthesis 20 minutes (7.3% of S) before the remainder of the heterochromatic X material and ends 30 minutes (9.7% of S) prior to the termination of the S period. Replication of euchromatin takes about 80% of the duration of the total S period, whereas that of heterochromatin takes only 40%.

Dauer der DNS-Replikation vonEu- und Heterochromatin bei Microtus agrestis *

Abstract: The duration of DNA replication of eu- and heterochromatin in kidney epithelial cell cultures of female Microtus agrestis was determined with combined Ha-thymidine pulse labelling and cytophotometric determination of Feulgen DNA. The average duration of the total cell cycle was 23.3 hrs, with a G 1 period of 14.6 hrs, S period of 5 hrs, G2 period of 2.7 hrs, and mitosis of 1 hr. The replication time of eu- and heterochromatin was determined by the frequency of the different labelling patterns after pulse labelling. The time sequence of the labelling patterns was ascertained by DA measurements. During the S period, euchromatin replicates at first alone for 3 hrs (60% of the length of S) and 1 hr (19.3%) together with heterochromatin. During the last hour (20.7%), only heterochromatic regions replicate. The sex chromatin part of the one X chromosome starts synthesis 20 minutes (7.3% of S) before the remainder of the heterochromatic X material and ends 30 minutes (9.7 % of S) prior to the termination of the S period. Replication of euchromatin takes about 80% of the duration of the total S period, whereas that of heterochromatin takes only 40 %.

The replication of human heterochromatin in serial culture.

Abstract: The human C group chromosomes late to start replication in asynchronous and in FUdR synchronized cell lines are X chromosomes. These same chromosomes are also heterochromatic during interphase. During metaphase these allocyclic Xs cannot be identified simply by metaphase position or morphology and show a wide range of measurements for arm ratio, centromere index and total length. Replication starts in the short arm and extends over the entire chromosome during the 2nd and 3rd hr of S until by the 4th hr distinction from other C group chromosomes cannot be made by means of the labeling pattern. When the allocyclic X chromosomes start replication the pattern of H3TdR label over interphase sex chromatin and non-specific heterochromatin shifts from unlabeled to labeled in FUdR synchronized human cell lines. The overall time required for replication of the allocyclic X is less than that for the other chromosomes in both asynchronous and FUdR treated cells. A hypothesis is presented for a direct relation between the delay of onset of replication in heterochromatin and its degree of interphase condensation.

Distribution of heterochromatin in the chromosomes of Haplopappus gracilis.

Abstract: Several approaches were employed to study the distribution of heterochromatin in root tip chromosomes of Haplopappus gracilis. Cold treatment and pretreatment in an aqueous solution of 8-hydroxyquinoline revealed achromatic gaps in metaphase chromosomes. Cold treatment also permitted the demonstration of positive heteropycnosis in prophase chromosomes. Further support for the identification of heterochromatic segments was provided by a study of the localization of chromosome aberrations induced by maleic hydrazide and an analysis of the pattern of DNA synthesis in chromosomes of root tip cells. Seven of the ten regions that were preferentially broken by maleic hydrazide also reacted differentially to cold treatment or to pretreatment with 8-hydroxyquinoline. A good correlation was found between regions that completed DNA replication late in the DNA-synnhetic period and segments that were shown to be heterochromatic by the other techniques.

Colchicine-induced supercontraction of chromosomes and heterochromatic segments in vicia faba

Abstract: The question whether the degree of chromosome condensation of heterochromatic region differs from that of euchromatic region in the process of colchicine-induced supercontraction was studied using Vicia faba root-tips. The results obtained did not indicate the differential chromosome condensation between heterochromatin and euchromatin.