Showing papers on "Heterochromatin published in 1986"
TL;DR: The amplification, deletion and transposition of different repeated DNA sequences and the spread of the mutations in populations produces a turnover of repetitive DNA during evolution, which is shown to be responsible for divergence of chromosome structure between species.
Abstract: Most higher plant genomes contain a high proportion of repeated sequences. Thus repetitive DNA is a major contributor to plant chromosome structure. The variation in total DNA content between species is due mostly to variation in repeated DNA content. Some repeats of the same family are arranged in tandem arrays, at the sites of heterochromatin. Examples from the Secale genus are described. Arrays of the same sequence are often present at many chromosomal sites. Heterochromatin often contains arrays of several unrelated sequences. The evolution of such arrays in populations is discussed. Other repeats are dispersed at many locations in the chromosomes. Many are likely to be or have evolved from transposable elements. The structures of some plant transposable elements, in particular the sequences of the terminal inverted repeats, are described. Some elements in soybean, antirrhinum and maize have the same inverted terminal repeat sequences. Other elements of maize and wheat share terminal homology with elements from yeast, Drosophila, man and mouse. The evolution of transposable elements in plant populations is discussed. The amplification, deletion and transposition of different repeated DNA sequences and the spread of the mutations in populations produces a turnover of repetitive DNA during evolution. This turnover process and the molecular mechanisms involved are discussed and shown to be responsible for divergence of chromosome structure between species. Turnover of repeated genes also occurs. The molecular processes affecting repeats imply that the older a repetitive DNA family the more likely it is to exist in different forms and in many locations within a species. Examples to support this hypothesis are provided from the Secale genus.
287 citations
TL;DR: It is suggested that karyotypic evolution has occurred but does not appear to be associated with speciation in this group of Tilapiine species.
Abstract: Seven Tilapiine species from three generaTilapia, Sarotherodon, andOreochromis were cytogenetically studied for chromosome number, chromosome morphology, and DNA content. The chromosome number 2n=44 was the same in all seven species. Arm number (NF) differences indicate the possible role of pericentric invasions in the karyotypic evolution of these species. C-banding of metaphase chromosomes shows that heterochromatin is localised around the centromere in all species ofOreochromis and Sarotherodon butT. zillii has more heterochromatin with six chromosomes having completely C-positive short arms. DNA values vary between 0.84 pq forO. macrochir and 1.21 pq forO. aureus. No heteromorphic sex chromosome pair could be found in any species. These findings suggest that karyotypic evolution has occurred but does not appear to be associated with speciation in this group.
101 citations
TL;DR: It is suggested that satellite DNA may have an impact on cell proliferation through the effect of late-replicating satellite-rich heterochromatin on the duration of S-phase of the cell cycle and even small alterations in cell cycle time may have magnified effects on organismal growth.
Abstract: The functional and evolutionary significance of highly repetitive, simple sequence (satellite) DNA is analysed by examining available information on the patterns of variation of heterochromatin and cloned satellites among newts (family Salamandridae), and particularly species of the European genus Triturus. This information is used to develop a model linking evolutionary changes in satellite DNAs and chromosome structure. In this model, satellites accumulate initially in large tandem blocks around centromeres of some or all of the chromosomes, mainly by repeated chromosomal exchanges in these regions. Centromeric blocks later become broken up and dispersed by small, random chromosome rearrangements in these regions. They are dispersed first to pericentric locations and then gradually more distally into the chromosome arms and telomeres. Dispersal of a particular satellite is accompanied by changes in sequence structure (for example, base substitutions, deletions, etc.) and a corresponding decrease in its detectability at either the molecular or cytological level. On the basis of this model, observed satellites in newt species may be classified as \`old', \`young', or of `intermediate' phylogenetic age. The functions and effects of satellite DNA and heterochromatin at the cellular and organismal levels are also discussed. It is suggested that satellite DNA may have an impact on cell proliferation through the effect of late-replicating satellite-rich heterochromatin on the duration of S-phase of the cell cycle. It is argued that even small alterations in cell cycle time due to changes in heterochromatin amount may have magnified effects on organismal growth that may be of adaptive significance.
86 citations
TL;DR: The structure of appoximately 10 kilobases of DNA originating from the subcloned DNA segments of five lambda clones from rye heterochromatin is described.
Abstract: The structure of appoximately 10 kilobases of DNA originating from the subcloned DNA segments of five lambda clones from rye heterochromatin is described. The sequence data suggested that much of the variation between the 380 base pair (bp) units of the tandem arrays in hetercohromatin must have preceded the amplification events which gave rise to the arrays. Furthermore, evidence for recombination between the tandem arrays of the units (subsequent to the presumed amplification events) was obtained. A single 14-bp region which did not show any variation in sequence was found to be part of a dyad symmetry, characteristic for DNA-binding proteins. The analysis of a molecule which contained a junction between the heterochromatic "350" DNA sequence family and apparently nonheterochromatic DNA is also presented. This nonheterochromatic DNA contains a dispersed repetitive type of sequence ("5.3") which is found adjacent to a wide range of sequences as judged from genomic analyses, in situ hybridization studies ...
79 citations
TL;DR: It is shown that (i) HRDNA accumulates most probably in regions of near zero crossing over (heterochromatin), and that (ii), due to random genetic drift the effect of unequal crossover on copy numbers is stronger, the smaller the population size.
Abstract: In eukaryotic chromosomes, large blocks of satellite DNA are associated with regions of reduced meiotic recombination. No function of highly repeated, tandemly arranged DNA sequences has been identified so far at the cellular level, though the structural properties of satellite DNA are relatively well known. In studying the joint action of meiotic recombination, genetic drift and natural selection on the copy number of a family of highly repeated DNA (HRDNA), this paper looks at the structure–function debate for satellite DNA from the standpoint of molecular population genetics. It is shown that (i) HRDNA accumulates most probably in regions of near zero crossing over (heterochromatin), and that (ii), due to random genetic drift the effect of unequal crossover on copy numbers is stronger, the smaller the population size. As a consequence, highly repeated sequences are likely to persist longest (over evolutionary times) in small populations. The results are based on a fairly general class of models of unequal crossing over and natural selection which have been treated both analytically and by computer simulation.
77 citations
TL;DR: 12 different loci showing dominant suppression have been identified and their map positions determined and most of the identified loci suppressing position-effect variegation represent essential genetic funtions; only three loci represent nonessential functions.
Abstract: As a result of a genetic analysis of 63 third chromosome suppressor mutations of position-effect variegation 12 different loci showing dominant suppression have been identified and their map positions determined. A compilcation of the genetic data available for each suppressor locus is given. The strong suppressor effects of the mutations have been quantified by measurements of white variegation inwm4h/wm4h,wm4h/Y andwm4h/O flies. Mutant alleles of three loci were found in these studies to dominate over the strong enhancer effect of complete loss of the Y chromosome. Most of the identified loci suppressing position-effect variegation represent essential genetic funtions; only three loci represent nonessential functions. Mutations of two loci display recessive butyrate sensitivity and lethal interaction with the heterochromatic Y chromosome suggesting that these genes affect chromosomal condensation. Studies with deficiencies and triploids revealed that most of the loci represent haplo-abnormal suppressor functions. The use of the isolated mutant material for genetic, developmental and molecular studies of processes connected with gene inactivation in position-effect variegation is discussed.
69 citations
TL;DR: The role of intercalated heterochromatin would be to avoid the pathological consequences of gonosome-autosome translocations resulting from inactivation of the sex chromosomes in female somatic cells and male germinal cells.
Abstract: A study of sex chromosomes and synaptonemal complexes in male specimens of Gerbillus chiesmani, G. nigeriae, G. hoogstrali, and Taterillus pygargus is reported. In each of these Gerbillidae species there are two or three translocations of autosomes with X and Y chromosomes. Analysis of mitotic chromosomes consistently shows the presence of constitutive heterochromatin on the der t(X;autosome) at the X-autosome junction and on the der t(Y;autosome). Analysis of the synaptonemal complexes shows the existence of an unusual structure, lightly stained, at the X-autosome junction and at the Y-autosome junction, which is probably heterochromatic in nature, thus corresponding to the mitotic patterns. This heterochromatin separates the autosomal and gonosomal segments, which behave independently and normally. By analogy with findings from humans and other mammals, a general hypothesis is proposed on the role of intercalated heterochromatin between translocated gonosomes and autosomes. This hypothesis explains why the pathological consequences of these translocations may be very different in males and females. The role of intercalated heterochromatin would be to avoid the pathological consequences of gonosome-autosome translocations resulting from inactivation of the sex chromosomes in female somatic cells and male germinal cells.
57 citations
TL;DR: Conclusions drawn from the mutants are the following: At least one half of the W serves no vital function and is dispensable, and female sex determination, if the W is involved at all, rests on the other half.
Abstract: Structural mutants of the W chromosome that are kept in laboratory strains are described using light and electron microscopic techniques. The series comprises a translocation, a deficiency, fusions, and W fragments. — Conclusions drawn from the mutants are the following: At least one half of the W serves no vital function and is dispensable. Female sex determination, if the W is involved at all, rests on the other half. The amount and form of heterochromatin in somatic interphase nuclei depends on the amount and location of the W chromosome material. The W inactivation in somatic cells does not spread to attached autosomal segments.
57 citations
TL;DR: The cloning and detailed analysis of the integrated hepatitis B virus sequences in a human hepatoma cell line and the integration of at least one at human satellite DNA sequences are reported.
Abstract: We previously reported the cloning and detailed analysis of the integrated hepatitis B virus sequences in a human hepatoma cell line. We report here the integration of at least one of hepatitis B virus at human satellite DNA sequences. The majority of the cellular sequences identified by this satellite DNA were organized as a multimeric composition of a 0.6-kilobase EcoRI fragment. This clone hybridized in situ almost exclusively to the centromeric heterochromatin of chromosomes 1 and 16 and to a lower extent to chromosome 2 and to the heterochromatic region of the Y chromosome. The immediate flanking host sequence appeared as a hierarchy of repeating units which were almost identical to a previously reported human satellite III DNA sequence.
48 citations
TL;DR: Heterochromatin is not mostly biologically inert, containing only a few widely-spaced genes, but contains information that is, however, organized differently from that in euchromatin.
47 citations
ARCO1
TL;DR: The structure, genome organization, chromosomal location, conservation across species and transcription on lampbrush chromosomes, of an AT-rich satellite DNA component of the newt, Triturus vulgaris meridionalis, is studied.
Abstract: We have studied the structure, genome organization, chromosomal location, conservation across species and transcription on lampbrush chromosomes, of an ATrich satellite DNA component of the newt, Triturus vulgaris meridionalis. The satellite (Sat G), originally isolated by gradient centrifugation, represents about 2% of the vulgaris genome and comprises a highly repetitive sequence family (HindIII family), whose monomers have been cloned. The repeat units are about 330 bp long, as measured on gels, and a cloned unit (pTvm1) is 310 bp long, as shown by sequencing. Abundant clusters of the HindIII family sequences are located within the pericentric heterochromatin (i.e. the C-bands placed at both sides of, and at a certain distance from, the centromeres) in most chromosomes. Both the sequence family and its overall pattern of chromosomal distribution are conserved within the genus Triturus, despite a few species-specific differences. The great majority of the HindIII family sequences are unexpressed on lampbrush chromosomes; they reside within pericentric, condensed segments of the chromosome axis (“loopless bars”). Only a few sequences are transcribed on some loops, suggesting that transcription promotion does not depend on the satellite sequences themselves.
TL;DR: Differing amounts of eu-and heterochromatin, DNA rearrangements during cellular development, or amplification of certain genes may give rise to the existence of nuclei which differ from one another despite their common origin from one zygotes.
Abstract: Differentiation denotes the process by which cells of multicellular organisms become different from one another. The closely related process by which nuclei become functionally and/or structurally different from one another can be called nuclear differentiation. Nuclear differentiation is a well-known, but often poorly understood phenomenon. For instance, large differences exist between gamete nuclei, e.g., sperm nuclei and somatic nuclei of the same organism, with regards to nuclear size, chromatin condensation, transcriptional activity, structure of the nuclear lamina, and many other characteristics. But differences in nuclear structure and in the organization of the genetic material also occur between different somatic nuclei. Differing amounts of eu-and heterochromatin, DNA rearrangements during cellular development, or amplification of certain genes may give rise to the existence of nuclei which differ from one another despite their common origin from one zygote nucleus.
TL;DR: It is proposed that most sequence segments within a domain may be either functionally superfluous or instrumental, depending on how many related sequences are present in the domain, and by distinguishing functional compatibility from pivotal functionality.
Abstract: Certain as yet poorly defined functions of DNA appear to involve collectively domain-sized sequences. It is proposed that most sequence segments within a domain may be either functionally superfluous or instrumental, depending on how many related sequences are present in the domain. When redundant and functionally dispensable, such DNA segments presumably still have to conform to compositional or sequence-motif patterns that characterize the domain. In its relations with neighboring sequences, such DNA is required to be “polite.” Polite DNA is DNA that, without being crucially involved in function, is subject to constraints of conformity and, through its base composition, respects a function for which it is not required. This concept is developed by contrasting the distribution of specific and general functions over DNA with this distribution as found in proteins and by distinguishing functional compatibility from pivotal functionality. The sequence constraints to which heterochromatin as well as, apparently, long interspersed repetitive sequences are known to be subject seem to imply that DNA, even when it does not carry out a pivotal function, is indeed, at the very least, required to be polite.
TL;DR: The results show that the heterochromatin can be eliminated in presomatic cells during early embryogenesis at two different stages and in both cases the consequence of this process is the generation of somatic cells with a 2n=60 karyotype.
Abstract: Chromosome organization and the phenomenon of “chromatin diminution” in the two species Parascaris univalens (2n=2) and P. equorum (2n=4) were cytologically analysed by a variety of staining techniques (Quinacrine, Hoechst 33258, Chromomycin A3 and C-banding). The results show that: (1) the chromosomes of the two species differ markedly in both the location and the type of heterochromatin they contain; (2) in both species there is a strong chromosome polymorphism which, however, ranges within a basic species-specific phenotype; (3) the heterochromatin can be eliminated in presomatic cells during early embryogenesis at two different stages and in both cases the consequence of this process is the generation of somatic cells with a 2n=60 karyotype. Moreover, evidence suggesting the sterility of hybrids between the two species is provided.
TL;DR: Heterochromatin appears to be one of the few potentially variable fractions of the cyprinid genome which may have contributed to the extensive speciation exhibited by these fish in North America.
Abstract: Chromosomal heterochromatin was differentiated in two closely related spe-cies of the North American cyprinid genus Notropis. The amounts of heterochro-matin in the genomes of the two species were appreciable: 35% in N. lutrensis and 39% in N. venustus. The chromosomal distribution of heterochromatin was similar in both species: most or all chromosomes contained procentric bands, a few pos-sessed terminal and/or interstitial bands, and there was a preponderance of sub-metacentric and acrocentric chromosomes with entirely heterochromatic short arms. Several chromosomes could be putatively homologized either within or between the two species, although in general the amount and type of heterochromatin made precise homologization difficult. The total amount of heterochromatin resolved in these two species is considerably more than that found among most of the relatively few fish species studied to date. Heterochromatin appears to be one of the few potentially variable fractions of the cyprinid genome which may have contributed to the extensive speciation exhibited by these fish in North America. A few marker chromosomes, specifically the one bearing the nucleolus organizer region or NOR, may prove valuable in cyprinid taxonomy.
TL;DR: This volume reviews the question of germ line-soma differentiation from the viewpoint of three different organisms, finding that in all three organisms germ line and somatic cells are substantially distinguished in their karyotypes.
Abstract: The three reviews assembled in this volume approach the question of germ line-soma differentiation from the viewpoint of three different organisms. In all three organisms germ line and somatic cells are substantially distinguished in their karyotypes. Entire chromosomes or sections thereof are excluded from the somatic cells. This is achieved by quite different mechanisms. The divergent development of the karyotypes occurs within the germ line (as, for example, in Sciara) or during the earliest development of the organism (as, for example, in Ascaris). In ciliates an even more complex sequence of DNA rearrangements is found.
Journal Article•
TL;DR: The statistical analysis of 1,000 somatic Pairing configurations from 5-azaC-treated cells showed that the somatic pairing between the heterochromatic regions of homologous chromosomes is preferred over that between nonhomologous chromosome.
Abstract: The cytidine analog 5-azacytidine (5-azaC) induces an undercondensation of the heterochromatin in human chromosomes 1, 9, 15, 16, and Y when it is added in low concentrations to the late S-phase of growing lymphocyte cultures. In interphase nuclei, these heterochromatic regions are frequently somatically paired. The somatic pairing configurations are preserved up to metaphase stage in the 5-azaC-treated cultures and are thus susceptible to a direct microscopical examination. The statistical analysis of 1,000 somatic pairing configurations from 5-azaC-treated cells showed that the somatic pairing between the heterochromatic regions of homologous chromosomes is preferred over that between nonhomologous chromosomes.
TL;DR: The analysis of various types of heterochromatin with respect to specific satellite and nonsatellite DNA sequences suggest that the staining profiles are probably related to sequence diversity.
Abstract: The constitutive heterochromatin of human chromosomes is evaluated by various selective staining techniques, i.e., CBG, G-11, distamycin A plus 4,6-diamidino-2-phenylindole-2-HCl (DA/DAPI), the fluorochrome D287/170, and Giemsa staining following the treatments with restriction endonucleases AluI and HaeIII. It is suggested that the constitutive heterochromatin could be arbitrarily divided into at least seven types depending on the staining profiles expressed by different regions of C-bands. The pericentromeric C-bands of chromosomes 1, 5, 7, 9, 13-18, and 20-22 consist of more than one type of chromatin, of which chromosome 1 presents the highest degree of heterogeneity. Chromosomes 3 and 4 show relatively less consistent heterogeneous fractions in their C-bands. The C-bands of chromosomes 10, 19, and the Y do not have much heterogeneity but have characteristic patterns with other methods using restriction endonucleases. Chromosomes 2, 6, 8, 11, 12, and X have homogeneous bands stained by the CBG technique only. Among the chromosomes with smaller pericentric C-bands, chromosome 18 shows frequent heteromorphic variants for the size and position (inversions) of the AluI resistant fraction of C-band. The analysis of various types of heterochromatin with respect to specific satellite and nonsatellite DNA sequences suggest that the staining profiles are probably related to sequence diversity.
TL;DR: The differences in chiasma distribution found between each karyomorph revealed an underlying tendency for chiasmata to occupy characteristic positions in the bivalents depending on the heterochromatin distribution.
Abstract: Chiasma variation has been studied in two selected chromosomes from two species of the genus Arcyptera according to the heterochromatin distribution per bivalent. The differences in chiasma distribution found between each karyomorph revealed an underlying tendency for chiasmata to occupy characteristic positions in the bivalents depending on the heterochromatin distribution. It seems that the heterozygosity of the bivalent more than the presence of heterochromatic segments is readjusting a standard pattern of chiasma distribution. The findings of this survey are discussed in relation to the significance and role of genetical recombination in natural populations depending on the frequency of karyomorphs per populations and the pattern of chiasma distribution per individual.
TL;DR: The constitutive heterochromatin of the Indian muntjac was examined following digestion with various restriction endonucleases, as well as by selective fluorescence staining with distamycin A plus 4'-6-diamidino-2-phenylindole.
Abstract: The constitutive heterochromatin of the Indian muntjac (Muntiacus muntjak) was examined following digestion with various restriction endonucleases (AluI, HaeIII, HinfI, and MboI), as well as by selective fluorescence staining with distamycin A plus 4'-6-diamidino-2-phenylindole. Distinct areas within the C-bands were found to have characteristic staining patterns which were more conspicuous in the sex chromosomes. Two dot-like structures resistant to AluI were found in the X and Y1 chromosomes in the same position as the nucleolus organizer regions.
TL;DR: The main consequence of Robertsonian processes does not imply detectable quantitative variation in the genome portion involved in the Rb process, and the high rate of Rb exchange in the house mouse could be favoured by the simultaneous effects of undetectable losses of chromosomal material and high repetitiveness of the DNA involved.
Abstract: The pericentromeric heterochromatin of meiotic trivalents formed by the Robertsonian (Rb) chromosomes and the two homologous acrocentrics in the house mouse was evaluated by static cytophotometry after selective staining. To reveal pericentromeric heterochromatin specifically, C-banding Giemsa and Hoechst 33258 stains were utilized. Five different Rb chromosomes were investigated and none of them possessed less pericentromeric heterochromatin than the sum of the two homologous acrocentrics. Moreover the total A-T (DAPI) and DNA (PI) content was quantitatively evaluated, by flow cytometry, in G0/G1 nuclei belonging to four different Rb mouse populations, karyotypically characterized by the presence of up to nine Rb chromosomes. Again there were no significant difference, of DAPI and PI content, in the Rb populations nor between any of them and the NMRI/HAN strain with forty aerocentric chromosomes. We conclude that the main consequence of Robertsonian processes (i.e. the rapid variation of the karyotype structure) does not imply detectable quantitative variation in the genome portion involved in the Rb process. We also discuss the possibility that the high rate of Rb exchange in the house mouse could be favoured by the simultaneous effects of undetectable losses of chromosomal material, high repetitiveness of the DNA involved, the presence of the same major type of satellite DNA over each chromosome and the all acrocentric constitution of the karyotype.
TL;DR: An unexpected plant, with a recombinant phenotype, arose from a control population where homoeologous pairing was thought to be suppressed, and it appears that only the terminal region has been changed.
Abstract: Homoeologous recombination between wheat chromosomes and the long arm of chromosome 1R of rye present in a wheat background as the wheat–rye translocation, 1DS.1RL, has been reported. During the course of this study an unexpected plant (308-17), with a recombinant phenotype, arose from a control population where homoeologous pairing was thought to be suppressed. The putative recombinant chromosome in plant 308-17 carried two seed protein loci located on 1RL (namely, the rye Glu-R1 locus and a recently discovered globulin-like marker) but appeared to lack C-banded heterochromatin. Further investigation of this apparent recombinant chromosome in progeny of 308-17 using a cloned probe for the 350 family of rye heterochromatic sequence indicated that some terminal heterochromatin was still present but in a much reduced amount. Because it appears that only the terminal region has been changed, the modified chromosome in 308-17 most likely did not arise from homoeologous recombination but rather from some form ...
TL;DR: Differences in the repair response to damage in selected genomic regions may account for some of the profound differences seen in the carcinogenic response in different tissues or when different organisms are compared.
Abstract: There have been many attempts to analyze the complex relationships between measured DNA repair levels in mammalian cells and various biological end-points such as survival, mutagenesis, and transformation. In such studies the repair proficiency has been assumed to be uniform throughout the genome and, in fact, cell population homogeneity is also generally assumed. However, it is quite likely that DNA damage in some domains of the genome is processed more efficiently than in others and that such heterogeneity in repair would result in corresponding differences in the responses seen for particular biological effects. The genome includes many different functional classes of DNA of which some are “silent” (e.g., repetitive sequences in heterochromatin, unexpressed genes) while others are active (e.g., expressed genes, elements required for the translation machinery, and various regulatory regions). The consequences of unrepaired or misrepaired damage in DNA will most certainly depend upon the precise location of the damage with respect to these functional classes. This fact is well-documented at the nucleotide sequence level in bacteria by correlations of the spectrum of particular lesions with “hot spots” for mutagenesis. However, we have obtained little information on the specificity of the mutagenic response in relation to damage and repair in mammalian systems, analyzed at the various levels of genomic organization. Differences in the repair response to damage in selected genomic regions may account for some of the profound differences seen in the carcinogenic response in different tissues or when different organisms are compared. Therefore, it may be important to understand the “fine structure” of DNA repair in mammalian genomes in order to assess carcinogenic risks.
TL;DR: Results indicate that the chromosomal instability in the MeWo cultures is mainly limited to sequences containing active NOR and centromeric heterochromatin from chromosomes #13 and #15.
TL;DR: A mouse transformant cell line containing two herpes viral thymidine kinase (tk) genes integrated in pericentromeric heterochromatin demonstrates that tk repression is associated with changes in DNA methylation and suggests that the flanking DNA may exert cis effects on tk gene expression.
Abstract: We have obtained a mouse transformant cell line containing two herpes viral thymidine kinase (tk) genes integrated in pericentromeric heterochromatin. Restriction analysis of tk- revertant and tk+ rerevertant derivatives suggest that one of the two tk genes is repressed in tk- cells, but is reactivated in tk+ rerevertants. The results of Northern analysis indicated that repression-activation is probably controlled at the transcriptional level. To examine the molecular basis for this repression, we cloned the tk gene from a tk- revertant cell line. Then, using the cloned tk gene as donor DNA to select for tk+ transformants, we found that it has a transfection efficiency indistinguishable from the viral tk gene. This indicates that repression is probably not mediated via any DNA sequence changes within the tk gene. The results of further studies by restriction analysis, azacytidine treatments, and secondary DNA transfection assays demonstrated that tk repression is associated with changes in DNA methylation. Surprisingly, derepression of the tk gene was accompanied by rearrangements in the flanking DNA. The latter result suggests that the flanking DNA may exert cis effects on tk gene expression. Additional studies with this system may provide insights into the molecular basis underlying position effects in heterochromatin.
TL;DR: Four taxa have been distinguished in the Crepis praemorsa complex and a strong correlation between altitudinal distribution of taxa and heterochromatin quantity has been found: number and distribution of constitutive hetechromatin bands.
Abstract: SUMMARYFour taxa have been distinguished in the Crepis praemorsa complex. These are four species (C. praemorsa s. st., C. froelichiana, C. incarnata and C. dinarica) well differenciated from geografical, ecological and genetical points of view. The karyological characteristics of each of them have been pointed out: number and distribution of constitutive hetechromatin bands. Moreover, by its telomeric distribution heterochromatin might influence the regular course of meiosis. A strong correlation between altitudinal distribution of taxa and heterochromatin quantity has been found. Then, heterochromatin might be important in adaptation to difficult ecological conditions.
TL;DR: The chromosomal distribution of distamycin A/DAPI-, D287/170-, quinacrine-, and chromomycin A3-positive heterochromatic regions, as well as the nucleolus organizer regions, is described and compared with the karyotypes of other hominoid species.
Abstract: The chromosomes of the gorilla were extensively studied with various staining techniques labeling the different classes of heterochromatin. The chromosomal distribution of distamycin A/DAPI-, D287/170-, quinacrine-, and chromomycin A3-positive heterochromatic regions, as well as the nucleolus organizer regions, is described and compared with the karyotypes of other hominoid species. Lymphocyte cultures were treated with low doses of 5-azacytidine during the last hours of culture. This cytidine analog induces distinct undercondensation in 37 heterochromatic regions in the 24 gorilla chromosomes. The 5-azacytidine-induced undercondensations are localized not only in most of the distamycin A/DAPI-bright heterochromatic regions but also in many telomeric C-bands of the chromosomes. Furthermore, 5-azacytidine preserves the somatic pairing between heterochromatic regions from the interphase nuclei into the metaphase stage. The homeologies and differences in the chromosomal localization of the various classes of heterochromatin, 5-azacytidine-sensitive regions, 5-methylcytosine-rich DNA sequences, and satellite DNAs in the gorilla, chimpanzee, orangutan, and man are discussed.
TL;DR: The results obtained indicate that most heterochromatic regions are methylated, even though the frequency of methylation is highly variable and sometimes low, and suggests that methylation does not occur in all classes of repetitive DNA.
TL;DR: The hypothesis concerning the involvement of constitutive heterochromatin of chromosome 1 in malignant disease supports the hypothesis that chromosomes 1 and 9 are affected by each other in patients with solid tumors.
Abstract: Constitutive heterochromatin of chromosomes 1, 9, and 16 was studied in 101 patients with solid tumors and 85 controls. Lymphocyte cultures were used for performing C-banded chromosome preparations. Two homologous chromosomes were regarded as heteromorphic when there was a 25% difference between their C-band size or when they fell into different classes according to the method of Patil and Lubs (1977). A statistically significant difference between patients and controls was found in chromosome 1 heteromorphism. No statistical difference between patients and controls was found in the heteromorphism of chromosomes 9 and 16. The frequencies with which pericentric inversions of the heterochromatin in chromosomes 1 and 9 occurred in cancer patients were 9.9% and 12.9%, respectively. Patients displaying this type of polymorphism usually showed an increased rate of chromosome associations. The most frequent associations were found between heterochromatic regions of chromosomes 1 and 9 and between the chromosome 9 heterochromatin and D acrocentrics. These results support the hypothesis concerning the involvement of constitutive heterochromatin of chromosome 1 in malignant disease.
TL;DR: A study of the Caledia captiva family showed that sequence variation in this family is equivalent (14% to 16%) at all levels of taxonomic comparison from within a population to between species as discussed by the authors.
Abstract: A highly repeated family of sequences from the grasshopper Caledia captiva shows a dispersed distribution at the cytological level. Members of this 185 bp sequence family are not restricted to C-band heterochromatin, but rather are distributed in regions which appear as euchromatin in C-banded chromosomes. Sequence variation in this family is equivalent (14%–16%) at all levels of taxonomic comparison from within a population to between species. However, contiguous repeats demonstrate a much lower level of variation (9%). These, and other data, indicate that the concept of sequence “homogeneity” within a family of highly repeated sequences must be qualified with respect to the extensive variation between members of a given family. Comparison of the data for the 185 bp family with those from a study of a second highly repeated family, from the same taxon, demonstrates divergent patterns of evolution. Thus, the 185 bp repeats show much greater sequence variation, as well as a seemingly random pattern of incorporation of base pair alterations. The factors which may contribute to the observed pattern of variation include the time since the sequence family originated, its cytological distribution, the frequency of unequal crossing over and gene conversion and natural selection.