Showing papers on "Heterochromatin published in 1979"

Functional aspects of satellite DNA and heterochromatin.

Abstract: Publisher Summary This chapter discusses the functional aspects of satellite DNA and heterochromatin. Despite all attempts to formulate simple rules governing satellite evolution, it is now clear that each case so far analyzed has brought with it its own claims for generalization, none of which have proven sufficiently all-embracing. One initial hypothesis on satellite evolution was that satellites wax and wane with amazing rapidity in evolutionary terms, so that closely related species differ drastically in amount or type of satellite. However, improved methods of DNA sequencing have led to the suggestion that closely related species appear to modulate their satellites from a common library. Because some kinds of heterochromatin are now known to contain satellite DNA and because a large literature exists on the properties of heterochromatin, it is necessary to consider the relationships between heterochromatin and satellite DNA in some detail. The term “heterochromatin” was initially used to define chromosomes or chromosome segments which did not uncoil at mitotic telophase and so maintained a condensed or heteropycnotic state throughout interphase and into the subsequent prophase of the next division cycle.

Heterochromatin and satellite DNA in man: properties and prospects.

Abstract: In reviewing the properties of heterochromatin and satellite DNA in man, it is clear that the human genome does not readily lend itself to experimental tests of the postulated functions for satellite DNA Since the spectrum of known structural properties of vertebrate and invertebrate satellite DNAs are broadly overlapping, an alternative avenue is to experimentally manipulate the heterochromatin of an organism, and then evaluate the generality of the results When this is done in Drosophila melanogaster, the one organism where such an experimental approach is indeed possible, the results provide no support for most of the popular hypotheses concerning satellite DNA function They do, however, reveal an important effect on the meiotic system, namely that the position of crossover events can be markedly altered in the presence of heterochromatin known to be rich in satellite DNAs This effect is not peculiar to Drosophila, since supporting data are readily available from natural situations in both mammals and grasshoppers In all such cases, the effects are most easily discernible where the heterochromatic blocks are substantial in size, and non-centric in location, situations which do not apply in man The human system, however, offers other potentials The ubiquity of naturally occurring heterochromatic polymorphisms, coupled with the extreme sensitivity of the human genome to perturbation, offers some scope for assessing the possible somatic effects of alterations in the amount of satellite DNA

Satellite DNA and heterochromatin variants: the case for unequal mitotic crossing over.

Abstract: Variations of constitutive heterochromatin (heteromorphisms) appear to be a general feature of eucaryotes. A variety of molecular and cytogenetic evidence supports the hypothesis that heteromorphisms result from unequal double-strand exchanges during mitotic DNA replication. Constitutive heterochromatin consists of highly repeated DNA sequences that are not transcribed. Thus, heteromorphisms are tolerated without overt phenotypic effect. Several of the highly repeated DNAs that comprise constitutive heterochromatin have been shown to contain site-specific endonuclease recognition sequences interspersed at regular intervals dependent upon nucleosome structure. These interspersed short repeated sequences could mediate unequal crossovers, resulting in quantitative variability of constitutive heterochromatin and satellite DNA. De novo variations of constitutive heterochromatin may be useful as markers of exposure to mutagens and/or carcinogens.

Genetic control of chromosome breakage and rejoining in Drosophila melanogaster: spontaneous chromosome aberrations in X-linked mutants defective in DNA metabolism

Abstract: Eight X-linked recombination-defective meiotic mutants (representing five loci) and 12 X-linked mutagen-sensitive mutants (representing seven loci) of Drosophila melanogaster have been examined cytologically in neuroblast metaphases for their effects on the frequencies and types of spontaneous chromosome aberrations. Twelve mutants, representing five loci, significantly increase the frequency of chromosomal aberrations. The mutants at these five loci, however, differ markedly both in the types of aberrations produced and the localization of their effects along the chromosome. According to these criteria, the mutants can be assigned to four groups: (i) mutants producing almost exclusively chromatid breaks in both euchromatin and heterochromatin; (ii) mutants producing chromatid and isochromatid breaks in both euchromatin and heterochromatin; (iii) mutants producing chromatid mutants producing chromatid and isochromatid breaks clustered in the heterochromatin.

Histone gene deficiencies and position--effect variegation in Drosophila.

Abstract: Position–effect variegation, originally observed in insects, has since been demonstrated in plants and mammals. When cells bear a chromosomal rearrangement which juxtaposes a euchromatic gene near to heterochromatin, a fraction of the cells exhibit no expression of that gene. The resultant organism is a mosaic for activity of the rearranged gene. The degree of mosaicism can be modified; such factors as elevated developmental temperature, additional Y chromosome heterochromatin in the genome, and various modifier genes enhance the proportion of cells in which the gene is active1. Although the phenomenon has been extensively documented, the underlying molecular mechanism of position–effect variegation remains unclear. The proportion of cells exhibiting gene inactivation increases with greater proximity of the rearranged gene locus to heterochromatin. It has been proposed that the variegating gene is inactivated by a ‘spreading effect’, or limited spatial diffusion of molecules from the adjacent heterochromatin2. This explanation supposes that the juxtaposed gene locus is condensed into a transcriptionally inactive state, possibly by a phosphorylated histone protein subspecies characteristic of Drosophila heterochromatin3. Such a model leads to the prediction that an alteration in the amount of cellular histone protein would affect the expression of variegating genes. To test this prediction, we have investigated the effect of a reduction in the number of genes coding for histone protein on the degree of mosaicism observed in two variegating gene systems. Our results suggest that the amount of variegating gene expression is a function of histone gene multiplicity.

Cytological studies of heterochromatin function in the Drosophila melanogaster male: autosomal meiotic pairing

Abstract: In Drosophila melanogaster it is now documented that the different satellite DNA sequences make up the majority of the centromeric heterochromatin of all chromosomes. The most popular hypothesis on this class of DNA is that satellite DNA itself is important to the pairing processes of chromosomes. Evidence in support of such a hypothesis is, however, circumstantial. This hypothesis has been evaluated by direct cytological examination of the meiotic behaviour of heterochromatically and/or euchromatically rearranged autosomes in the male. It was found that neither substantial deletions nor rearrangements of the autosomal heterochromatin cause any disruption of meiotic pairing. Autosomal pairing depends on homologs retaining sufficient euchromatic homology. This is the first clear demonstration that the highly repeated satellite DNA sequences in the heterochromatin of the second, third and fourth chromosomes are not important in meiotic pairing, but rather that some euchromatic homology in the autosomes is essential to ensure a regular meiotic process. These results on the autosomes, when taken in conjunction with our previous studies on sex chromosome pairing, clearly indicate that satellite DNA is not crucial for male meiotic chromosome pairing of any member of the D. melanogaster genome.

Anomalous electrophoretic mobility of Drosophila phosphorylated H1 histone: is it related to the compaction of satellite DNA into heterochromatin?

Abstract: In embryonic nuclei of Drosophila virilis, 45% of the DNA is satellite, and congruent to 50% of the H1 histone is phosphorylated. In polytene salivary gland nuclei, less than 1% of the DNA is satellite, and less than 10tion. The phosphorylated H1's migrate 4% slower than the unphosphorylated H1's on SDS-acrylamide gels. The mobility difference may arise because the phosphorylated and unphosphorylated H1's have different conformations in SDS. This putative conformational difference could be essential to the compaction of satellite DNA into heterochromatin.

Types of Chromatin Organization in Plant Nuclei

Abstract: The nuclear ultrastructure of 15 angiospermal plants was studied with respect to the structural type, the proportion of condensed chromatin, the diameter of the chromatin fibers, and the DNA content. It was found that (1) at least three species-specific ultrastructural types can be distinguished among plants: the diffuse, the chromomeric and the chromonematic type: the diffuse nuclei exhibit always chromocenters, but evenly distributes, often hardly visible euchromatin, the two other types occur in sub-types, characterized by the presence or absence of chromocenters, and exhibit euchromatin of the same electron density as heterochromatin;—(2) the species-specific nuclear type is mainly determined by the nuclear DNA content (2 C value), high values favoring the chromonematic type, low values the other three types and the appearance of chromocenters;—(3) the diameters of the chromatin fibers are similar, although significantly different between certain species, in all structural types, falling into distinct classes of about 50, 100, 150, 200, 300 and 400 A. The proportion of the various fibers varies between species, and one or the other class may be absent in a given species;—(4) The proportion of chromatin that is condensed into an electron-dense state is positively correlated with the DNA content; while in chromocentric nuclei the condensed chromatin corresponds to heterochromatin at the light microscope level, it corresponds to both eu- and heterochromatin in the other types of nuclei.

Evolutionary Changes of DNA and Heterochromatin Amounts in the Scilla bifolia Group (Liliaceae)

Abstract: The amounts of nuclear DNA and constitutive heterochromatin have been determined in 18 species of the Scilla bifolia group. and in the less closely related. S. messeniaca and S. siberica. Together with morphological criteria these data allow to reconstruct the pathways of chromosome evolution in the S. bifolia group at the diploid level with considerable accuracy. A progressive decrease of the DNA content by a factor of 0.5 occurred during the evolution from primitive yellow-seeded (S. kladnii: 1C = 8.6 pg) to advanced black-seeded species (e.g. S. luciliae: 1C = 4.3 pg). Grey- and brown-seeded species correspond to intermediary evolutionary stages and have intermediate DNA contents. 14 of 18 species have low amounts of heterochromatin (appr. 4%) irrespective of DNA content. However, two separate evolutionary side-branches are characterized by the accumulation of C-band material (3 yellow-seeded and 1 black-seeded species). The data suggest that an incipient DNA increase due to heterochromatin addition occurred in both instances. Further speciation in the yellow-seeded branch was accompanied by a decrease of total DNA despite of an increase in C-band content.—DNA contents once more establish the close relationship of the S. nivalis subgroup with the taxa formerly classified under Chionodoxa, viz. the S. luciliae subgroup.— The systematic position of S. messeniaca (x = 9) next to the S. bifolia group, previously suggested for reasons of morphology and chromosome number, is now supported by the DNA amount (1C = 10.7 pg). S. siberica (x = 6) as a remote relative contains considerably more DNA (1C = 31.7 pg).

Condensed Interphase Chromatin in Plant and Animal Cell Nuclei: Fundamental Differences

Abstract: In electron micrographs of interphase nuclei of both plants and animals. electron-dense chromatin can be found in variable amounts. Although this condensed chromatin looks alike in all micrographs, it covers at least four different classes of chromatin: constitutive heterochromatin (which can be visualized in plants and animals e.g. by the Giemsa banding technique), facultative heterochromatin (female sex chromatin in mammals), inactivated euchromatin (in animals only), and species-specific condensed euchromatin (in plants only). Care-free interpretation of condensed chromatin as heterochromatin may cause, therefore, much confusion. The possible molecular mechanisms involved in the condensation of the various classes of chromatin, and the suitability of condensed chromatin as marker for differential DNA and RNA synthesis, respectively, are discussed.

Differential DNA Replication Involved in Transition From Juvenile to Adult Phase in Hedera helix (Araliaceae)

Abstract: Phase change from the juvenile to the adult stage in the ivy, Hedera helix, is characterized by an increase in nuclear size and DNA content, and a decrease in the proportion of heterochromatin per nucleus. Our scanning cytophotometric and electron microscopic data were compared with biochemical data, and interpreted as the expression of differential replication of respectively heterochromatin and repetitive DNA during transition from the juvenile to the adult phase. The possible causal relationships between genomic diversification and phenotypic changes are briefly discussed.

Nuclear Ultrastructure: Condensed Chromatin in Plants is Species- Specific (Karyotypical), but Not Tissue-Specific (Functional)

Abstract: In contrast to mammalian cell nuclei those of plants display nearly an identical ultrastructure in all developmental stages and tissues. This indicates that the gross organization of chromatin is species-specific, but not tissue-specific and function-dependent. The species-specific nuclear ultrastructure is determined by the basic nuclear DNA content (2 C value). The higher the DNA content, the more the euchromatin remains in the condensed state during interphase, but to a lower coiling order than the heterochromatin. Some difficulties in the interpretation of electron micrographs of cell nuclei, and the possible role of repetitive DNA sequences in the karyotypical condensation of euchromatin in plants are discussed.

X heterochromatin subdivision and cytogenetic analysis in Sciara coprophila (Diptera, Sciaridae)

Abstract: The so-called controlling element (CE), which normally programs the curious behavior of the sex chromosome in this genus, has been localized in the short right arm of the polytene X in S coprophila The localization was accomplished by use of five X-autosome translocations whose break points define three blocks of heterochromatin (“heterochromomeres”) extending from the X centromere to the very end (right) of the chromosome The behavior of the translocation chromosomes at the crucial second spermatocyte division was examined and the “precocious” chromosome identified in all five cases Then, knowing the heterochromomere make-up of each chromosome, the position of the CE could be mapped; it is located in heterochromomere H2, the same block of heterochromatin that contains 50% of the ribosomal RNA cistrons — The question of whether the CE can manipulate any centromere in the nucleus has been only partially answered It can manipulate translocation chromosomes which possess the centromere of the metacentric autosome (salivary chromosome IV) or that of the shorter rod (salivary chromosome II); but the longer rod (salivary chromosome III) whose proximal end, as seen in the polytene nucleus, is heavily laden with heterochromatin of its own, has not been brought under CE control — In one of the translocations, T23, the precocious chromosome is a very large metacentric chromosome which resembles the “peculiar” V-shaped X of S pauciseta This peculiarity is not observed in the J-shaped precocious chromosome of T29 These points are discussed

Heterogeneity of constitutive heterochromatin in somatic Syrian hamster chromosomes.

Abstract: Syrian hamster constitutive heterochromatin was analyzed for C-band distribution and for BrdU late-replication pattern. Characteristic for this species is relatively large amounts of sex-chromosome and autosomal heterochromatin. The distribution of constitutive heterochromatin was determined. The long arm of the X chromosome, the whole Y, the short arms of 8 autosomal pairs, the long arm of the smallest metacentric pair, and the centromeric regions of 12 pairs stained intensely dark on C-band preparations. In contrast to the heterochromatin in the centromeric regions, the autosomal short-arm heterochromatin has an increased susceptibility to the denaturation process, as indicated by prolonged exposure to NaOH or Ba(OH)2. Such further exposure to denaturing agents results in an intense dark stain only on the sex-chromosome heterochromatin and centromeric regions of the autosomes. The BrdU late-replication pattern demonstrated that the late-replicating regions correspond to C-bands. Centromeric regions replicate late in the S phase; however, no centromeric region is among the latest replicating segments of the complement. Centromeric and noncentromeric heterochromatin are two distinct categories of constitutive heterochromatin.

Two new B-10 translocations involved in the control of nondisjunction of the B chromosome in maize.

Abstract: A B-A translocation, TB-10(18), has been established involving breakpoints in the proximal region of the long arm of chromosome 10 and the minute short arm of the maize B chromosome. TB-10(18) differs in its nondisjunctional behavior at the second microspore division from TB-10(19), which has a breakpoint in the same region of 10 but in the heterochromatic region of the long arm of B, in the following ways: (1) Nondisjunction of the B10 chromosome of the TB-10(18) translocation occurs in the absence of the reciprocal element (10B), albeit at low frequency. (2) Presence of 10B increases the frequency of B10 nondisjunction but not to the level found for TB-10(19) and certain other translocations. (3) The frequency of B10 nondisjunction varies among closely related sublines both when 10B is present and when it is absent. It is inferred that the B10 of TB-10(18) carries all the components of B necessary for nondisjunction but that expression is weak in the absence of 10B, suggesting the existence in the B chromosome short arm of a factor influencing efficient nondisjunction.

Behaviour of sex chromosome associated satellite DNAs in somatic and germ cells in snakes.

Abstract: Sex chromosome associated satellite DNAs isolated from the snakes Elaphe radiata (sat III) (Singh et al., 1976) and Bungarus fasciatus (Elapidae) (minor satellite) are evolutionarily conserved throughout the suborder Ophidia. An autosome limited satellite DNA (B. fasciatus major satellite) is not similarly conserved. Both types of satellites have been studied by in situ hybridisation in various somatic tissues and germ cells where it has been observed that the W sex chromosome remains condensed in interphase nuclei. In growing oocytes however, the W chromosome satellite rich heterochromatin decondenses completely whilst the autosomal satellite rich regions remain condensed. Later, the cycle is reversed and the W chromosome condenses whilst the autosomal satellite regions decondense. In a primitive snake (Eryx johni johni) where the sex chromosomes are not differentiated and where there is no satellite DNA specific to them, these phenomena are absent. — The differential behaviour of autosomal and sex chromosome associated satellite DNAs is discussed in the light of gene regulation.

Underreplication of a polytene chromosome arm in the chironomid Prodiamesa olivacea.

Abstract: The genome of Prodiamesa olivacea (Diptera, Chironomidae) has a 2 C DNA content of 0.25 pg. Mitotic metaphases reveal 3 pairs of chromosomes: 2 metacentric ones and one submetacentric. The latter comprises 20.8% of total Feulgen DNA. During larval polytenization the complemental portion of the 3rd falls to 6.5%. Concomitantly the polytene 3rd chromosome is much shorter than expected. It has no constriction and is shaped like a ball sector. — Underreplication is understood as suppression of DNA syntheses mainly in the long arm of the 3rd chromosome at the first to third endoreplicative cycle. Most of the dense heterochromatin seen in the apex of the 3rd polytene element is not itself underreplicated; it conceals the underreplicated long arm of this chromosome. — In ovarian nurse cells which are closely connected with the germ line the longer heterochromatic arm of the 3rd polyneme chromosome is fully replicated. — Underreplication is discussed in the context of “DNA silencing”.

Unstable ring Y chromosome in an aspermic male.

Abstract: A ring Y chromosome was found in a male showing growth retardation and aspermia but normally developed external genitalia. The ring does not display either the characteristic brilliant Y fluorescence or the typical latereplicating heterochromatin.

Preferential occurrence of sister chromatid exchanges at heterochromatin-euchromatin junctions in the wallaby and hamster chromosomes.

Abstract: Chromosomes of two mammalian species, the white-throated wallaby and the rat-like hamster, possessed large amounts of constitutive heterochromatin which is detectable as C bands. By making use of this character, the frequency of sister chromatid exchanges (SCEs) was determined for the C band and the euchromatic regions of the chromosome. In both species, the distribution of SCEs in the euchromatin of chromosomes was found to be proportional to its metaphase length, while the number of SCEs localized in the C band regions was clearly fewer than expected on the basis of the relative length of those regions at metaphase. Many SCEs were, however, detected at the junctions between the euchromatin and the C band heterochromatin. All of these findings were consistent with previous observations on the Indian muntjac and the kangaroo rat chromosomes.

Characterization of heterochromatin in Schistocerca gregaria by C- and N-banding methods

Abstract: Mitotic and meiotic chromosomes of Schistocerca gregaria were C-, mild N- and strong N-banded. After C-banding, three out of eleven autosomes show, in addition to the centromeric C-bands, a second C-band. — The mild N-banding method produces a single N-band in each of only four chromosomes. With the exception of one N-band these mild N-bands correspond to the non-centromeric, second C-bands, indicating the heterochromatic character of at least three mild N-band regions. — The strong N-banding technique produces bands both at the C- and mild N-band positions and additionally a third band in one chromosome (M8), not present after C- or mild N-banding. — The N-bands do not correspond to the nucleolus organizer regions. Because of the mechanisms of the N-banding methods, it is concluded that the centromeric heterochromatin, as well as the non-centromeric N-band regions, contain high quantities of non-histone proteins. Presumably a specific difference exists between the non-histone proteins in the centromeric and non-centromeric N-band regions because the centromeres are banded by the strong N-banding technique, but not after mild N-banding. It is concluded that the N-band regions (two exceptions) contain a heterochromatin type which has the following features in common with the α-heterochromatin of Drosophila: C- as well as N-banding positive, high nonhistone protein content, repetitive and late replicating DNA. It is discussed whether the N-banded heterochromatin regions of Schistocerca contain that DNA fraction which is, like the Drosophila α-heterochromatin, underreplicated in polyploid nuclei.

Interchromosomal effects of heterochromatic deletions on recombination in Drosophila melanogaster.

Abstract: It is now known that partial deletions of the satellite sequences in X - chromosome heterochromatin result in a significant decrease in intrachromosoma1 recombination in the proximal region of the X chromosome of D. melanogaster (YAMAMOTO and MIKLOS 1978). It is important to ask then if the loss or gain of heterochromatin on the X also alters recombination in other chromosomes of the genome (interchromosomal effects). I have looked for such alterations by measuring recombination in chromosome 3. The results clearly indicate that the partial loss of X -chromosome heterochromatin not only decreases crossing over in the proximal region of the X chromosome itself, but also increases the frequency in chromosome 3. especially in the euchromatic regions around the centromere. Furthermore, the greater the deficiency of X heterochromatin, the higher is recombination in chromosome 3 . This finding not only provides further evidence in support of the hypothesis that heterochromatin, in this case mainly composed of satellite DNA, regulates the recombination system, but it demonstrates that when the satellite content of one chromosome of the D. melanogasier genome is altered, there is an alteration in the crossover characteristics of other chromosomes in the same complement. If the amount of satellite DNA in a genome is being continuously altered, then one can predict that the recombination system is also being continually perturbed. Thus, the changing gene Combinations produced indirectly by increases or decreases of heterochromatin are among the components available to organisms to break up or form new gene combinations upon which se!ection can act.

A study of heterochromatin in Drosophila nasuta by the 5-bromodeoxyuridine-giemsa staining technique

Abstract: Larval brain ganglia of Drosophila nasuta were cultured in vitro in the presence of 5-bromodeoxyuridine for 1 or 5 h at 24° C and the air-dried chromosome preparations stained by the Hoechst 33258-Giemsa technique to reveal bromodeoxyuridine induced sister chromatid differentiation. In 1 h as well as 5 h preparations, 10–15% of well spread metaphase plates show a sister chromatid differentiation in only C-band heterochromatin regions of different chromosomes. We infer that this sister chromatid differentiation in all heterochromatic regions is seen after bromodeoxyuridine incorporation for only one replication cycle and is related to the presence of asymmetric A-T rich satellite sequences in all the C-band regions of D. nasuta karyotype.

Inhibition of condensation of human Y chromosome by the fluorochrome Hoechst 33258 in a mouse-human cell hybrid.

Abstract: The fluorochrome Hoechst 33258 which binds preferentially to A-T base pairs, drastically inhibits the condensation of A-T-rich centromeric heterochromatin regions in mouse cell lines. The condensation of all other regions of these chromosomes is also inhibited to some extent. The human Y chromosome contains a large heterochromatic region, which is also rich in A-T base pairs. This chromosome is not affected by Hoechst 33258 in human leukocyte cell cultures. On the other hand, condensation of the multiple copies of human Y chromosome in the mouse-human cell hybrid RH-28Y-23 is inhibited and the chromosomes appear distorted in Hoechst 33258-treated cells.

Identification of sex chromosomes and characterization of the heterochromatin in culiseta longiareolata (macquart, 1838)

Abstract: The Culicidae family has a general karyotype 2n = 6, with sex chromosomes morphologically distinguishable only in Anopheles (Anophelinae subfamily); in this genus, in fact, the two pairs of autosomes are similar in length and shape while sex chromosomes are identifiable one from the other and as distinguished from autosomes in both homoand heterogametic sex (Kitzmiller, 1963; Baker & Aslamkhan, 1969; Chowdaiah et al., 1971). On the contrary, all members of the Culicinae subfamily so far investigated have indistinguishable sex chromosomes (Breland, 1961; Baker & Aslamkhan, 1969). However, Newton et al. (1974) were able to distinguish the X from the Y chromosome by using C-banding in Aedes aegypti. In this article, we show the identification of sex chromosomes in Culiseta longiareolata (Diptera: Culicidae) and specifically the characterization of heterochromatic zones by means of banding techniques already employed for mammal and plant metaphase chromosomes (Caspersson et al., 1969; Arrighi & Hsu, 1971) and applied successfully to Diptera as well (Holmquist, 1975; Fraccaro et al., 1976; Pimpinelli et al., 1976).

The grasshopper X chromosome. I. States of condensation and the nuclear envelope at G1, S and G2 of premeiotic interphase and at early meiotic prophase.

Abstract: The sub-stages of spermatocyte interphase (G1, S and G2) have been identified in the grasshopper Brachystola magna using E.M. autoradiography and serial thin sectioning techniques. The X chromosome occupies a nuclear envelope bound compartment separate from an autosome compartment during G1 and S. At G2 the X compartment is resolved by coalescence of the membranes enveloping the X chromosome and autosome compartments.--At G1 and S, the compartmentalized X chromatin is laced with nuclear membrane material. This X chromatin associated membrane decreases in amount as the cell passes through G2 and enters early meiotic prophase. There are at least 2 and possibly 3 states of condensation of the heterochromatic X during premeiotic interphase and early meiotic prophase correlated with the presence or absence of membrane material associated with the chromatin.--The X chromatin replicates asynchronously with autosomal euchromatin and synchronously with autosomal heterochromatin associated with nucleoli. The X chromatin replication appears to be associated with the nuclear membrane.--The observations indicate that the nuclear membrane is involved with X chromosome condensation and may be implicated in asynchronous X chromosome replication as well.

Chromatin Organization and Repetitive DNA in Anacyclus and Anthemis (Asteraceae)

Abstract: Genera and species of Anthemideae can be distinguished on the basis of their nuclear ultrastructure (pattern of chromatin condensation, amount of heterochromatin) and their DNA composition (proportion of repetitive DNA). Anthemis species exhibit chromomeric to chromonematic nuclei with some small, but well distinguishable chromocenters. Anacyclus species show highly chromonematic nuclei with a cap-like distribution of the chromonemata; heterochromatin cannot be distinguished in ultrathin sections of Anacyclus nuclei. The melting point of nuclear DNA is similar in all species studied (about 85 °C, corresponding to about 38% GC). The fractions of repetitive DNA classes are different in all species, indicating a rapid diversification of the genome composition in Anthemideae. There are rather complex relationships between the species-specific degree of chromatin condensation, amount of heterochromatin, nuclear DNA content (2C value), and proportion of highly and intermediately repetitive DNA sequences.—It is suggested that nuclear ultrastructure and DNA composition may be used to characterize species and genera in sophisticated systematic and evolutionary studies.

Studies in heterochromatin DNA: accessibility of late replicating heterochromatin DNA in chromatin to micrococcal nuclease digestion.

Abstract: Heterochromatin DNA in cactus mouse (Peromyscus eremicus) replicates in the late S phase of cell cycle. A method of obtaining cells which contain DNA preferentially labeled at heterochromatic areas by a pulse-labeling of late replicating DNA is described. When the nuclei of P. eremicus cells containing radioactively labeled DNA in heterochromatin were digested with micrococcal nuclease and the resultant nucleosomal DNA was separated by gel electrophoresis, it was found that the repeat length of nucleosomal DNA in the heterochromatin DNA is not different from that of the bulk of the genomic DNA. Furthermore, there was no significant difference in the accessibility to digestion by micrococcal nuclease between the late replicating heterochromatin DNA and the total DNA under our digestion conditions. Two dimensional gel electrophoresis patterns of nucleosomal DNAs isolated from micrococcal nuclease digested nuclei from P. eremicus, P. collatus, and P. crinitus cells in culture were very similar. Cytogenetic data showed that these three species are different in heterochromatin but similar in euchromatin.

Complete deficiency of constitutive heterochromatin on a human chromosome-9

Abstract: In cultured amniotic fluid cells a mediocentric chromosome 9 appeared to be completely deficient in constitutive heterochromatin when stained with distamycin A and DAPI. In addition, this deficient chromosome was found in a blood cell culture from the father. Both the father and the child after birth were phenotypically normal. Evidently, a considerable heterozygotic deficit of chromosome 9 heterochromatin can be tolerated without affecting the phenotype. The heterochromatin defect was also shown by G11-staining. Distamycin A-DAPI staining is highly reproducible and is recommended as a fluorescent alternative to often less successful G11-methods for the detection of heteromorphism of chromosome 9.

Molecular cytogenetics of the Equidae. I. Purification and cytological localization of a (G + C)-rich satellite DNA from Equus przewalskii.

Abstract: A (G + C)-rich density satellite DNA (ϱ = 1.713 gm/cc) has been purified from splenic DNA of Przewalski's horse, Equus przewalskii, by successive equilibrium density gradient centrifugations. The purified satellite, which may comprise as much as 29% of the total DNA, renatures rapidly; however, analyses of native, single-stranded, and reassociated molecules by analytical ultracentrifugation and melting properties suggests that some sequence heterogeniety exists in the 1.713 gm/cc satellite. Complementary RNA (cRNA) transcribed from the satellite DNA has been utilized for in situ hybridization studies with E. przewalskii metaphase chromosomes previously identified by quinacrine-banding. These studies establish that sequences complementary to the 1.713 g/cc satellite are greatly enriched in the centromeres of some, but not all, chromosomes. The differential distribution of satellite DNA sequences over heterochromatic regions allows discrimination of three classes of heterochromatin and serves to define three types of pericentromeric regions in the karyotype of this endangered equine species. Additionally, apparent polymorphism in concentrations of satellite DNA sequences between homologs in the same karyotype is noted.