Showing papers on "Metaphase published in 1982"

Rabl's Model of the Interphase Chromosome Arrangement Tested in Chinese Hamster Cells by Premature Chromosome Condensation and Laser-UV-Microbeam Experiments

Abstract: In 1885 Carl Rabl published his theory on the internal structure of the interphase nucleus We have tested two predictions of this theory in fibroblasts grown in vitro from a female Chinese hamster, namely (1) the Rabl-orientation of interphase chromosomes and (2) the stability of the chromosome arrangement established in telophase throughout the subsequent interphase Tests were carried out by premature chromosome condensation (PCC) and laser-UV-microirradiation of the interphase nucleus Rabl-orientation of chromosomes was observed in G1 PCCs and G2 PCCs The cell nucleus was microirradiated in G1 at one or two sites and pulse-labelled with 3H-thymidine for 2h Cells were processed for autoradiography either immediately thereafter or after an additional growth period of 10 to 60h Autoradiographs show unscheduled DNA synthesis (UDS) in the microirradiated nuclear part(s) The distribution of labelled chromatin was evaluated in autoradiographs from 1035 cells after microirradiation of a single nuclear site and from 253 cells after microirradiation of two sites After 30 to 60h postincubation the labelled regions still appeared coherent although the average size of the labelled nuclear area fr increased from 142% (0h) to 265% (60h) The relative distance dr, ie the distance between two microirradiated sites divided by the diameter of the whole nucleus, showed a slight decrease with increasing incubation time Nine metaphase figures were evaluated for UDS-label after microirradiation of the nuclear edge in G1 An average of 43 chromosomes per cell were labelled Several chromosomes showed joint labelling of both distal chromosome arms including the telomeres, while the centromeric region was free from label This label pattern is interpreted as the result of a V-shaped orientation of these particular chromosomes in the interphase nucleus with their telomeric regions close to each other at the nuclear edge Our data support the tested predictions of the Rabl-model Small time-dependent changes of the nuclear space occupied by single chromosomes and of their relative positions in the interphase nucleus seem possible, while the territorial organization of interphase chromosomes and their arrangement in general is maintained during interphase The present limitations of the methods used for this study are discussed

A relationship between replicon size and supercoiled loop domains in the eukaryotic genome.

Abstract: In interphase nuclei and metaphase chromosomes, eukaryotic chromosomal DNA is arranged in negatively supercoiled loops whose length averages several tens of kilobases (kb). Supercoiling indicates that the loops are tenaciously bound at both ends through the periodic attachment of DNA to a non-histone protein structural support, termed the nuclear matrix, skeleton or cage in interphase nuclei and the scaffold in metaphase chromosomes1–16. The looped organization has been envisaged to be important in DNA replication2,11,15,17–25 and transcription26–28. We report here a close relationship between average loop length and replicon size in different animal and plant species. Autoradiographic experiments support the hypothesis that DNA synthesis occurs at fixed sites on the nuclear matrix15,20. We also describe a modification of the looped organization occurring during early embryonic development in Xenopus laevis.

Mitosis in a cell with multiple centrioles.

Abstract: N115 mouse neuroblastoma cells possess a large number of microtubule organizing centers (MTOCs) which can be identified ultrastructurally as single centrioles. The distribution and activity of these organizing centers can be followed through all stages of the cell cycle by labeling microtubules with anti-tubulin and chromatin with the Hoechst dye, Bisbenzimid. We have found that multiple MTOCs persist and continue to organize microtubules during mitosis. They exhibit a well-defined sequence of movements, starting from a loose cluster during interphase, proceeding to a widely and evenly dispersed arrangement in prophase, gathering into small clusters and chains during prometaphase, and residing in two ring-shaped groups at the mitotic poles during metaphase and anaphase. Despite their large number of centrioles, virtually all N115 cells show a normal bipolar mitosis, but often with unequal numbers of centrioles at the two poles. Such observations bring into question the importance of the centriole in establishing bipolar division in this cell type.

The kinetochores of Caenorhabditis elegans.

Abstract: Light microscopy of the mitotic chromosomes of Caenorhabditis elegans suggests that non-localized kinetochores are present, since the chromosomes appear as stiff rods 1 to 2 micrometers in length and lack any visible constriction. The holokinetic structure was confirmed by reconstructions of electron micrographs of dividing nuclei in serially sectioned embryos. In prophase the kinetochore appears as an amorphous projection approximately 0.18-0.2 micrometer in diameter in cross section and in longitudinal section it appears to be continuous along the chromatin. At prometaphase and metaphase the kinetochore is a convex plaque covering the poleward face of the chromosome and extending the length of the chromosome. In longitudinal section the kinetochore is a trilaminar structure with electron dense inner and outer layers of 0.02 micrometer, and an electron lucent middle layer of 0.03 micrometer. The inner layer is adjacent to a more electron dense region of chromatin. The kinetochore was also seen as a band extending the length of the chromosome in whole mount preparations of chromosomes stained with ethanolic phosphotungstic acid. Most gamma ray induced chromosome fragments segregate normally in embryonic mitoses, but some fragments display aberrant behavior. Similar behavior was seen in embryos carrying a genetically characterized free duplication. It is suggested that mitotic segregation of small fragments may be inefficient because the probability of attachment of microtubules to the kinetochore is proportional to kinetochore length.

Analysis of chromosome positions in the interphase nucleus of Chinese hamster cells by laser-UV-microirradiation experiments.

Abstract: Unsynchronized cells of an essentially diploid strain of female Chinese hamster cells derived from lung tissue (CHL) were laser-UV-microirradiated (λ=257 nm) in the nucleus either at its central part or at its periphery. After 7–9 h postincubation with 0.5 mM caffeine, chromosome preparations were made in situ. Twenty-one and 29 metaphase spreads, respectively, with partial chromosome shattering (PCS) obtained after micro-irradiation at these two nuclear sites, were Q-banded and analyzed in detail. A positive correlation was observed between the frequency of damage of chromosomes and both their DNA content and length at metaphase. No significant difference was observed between the frequencies of damage obtained for individual chromosomes at either site of microirradiation. The frequency of joint damage of homologous chromosomes was low as compared to nonhomologous ones. Considerable variation was noted in different cells in the combinations of jointly shattered chromosomes. Evidence which justifies an interpretation of these data in terms of an interphase arrangement of chromosome territories is discussed. Our data strongly argue against somatic pairing as a regular event, and suggest a considerable variability of chromosome positions in different nuclei. However, present data do not exclude the possibility of certain non-random chromosomal arrangements in CHL-nuclei. The interphase chromosome distribution revealed by these experiments is compared with centromere-centromere, centromere-center and angle analyses of metaphase spreads and the relationship between interphase and metaphase arrangements of chromosomes is discussed.

Functional autonomy of monopolar spindle and evidence for oscillatory movement in mitosis.

Abstract: The oscillations of chromosomes associated with a single spindle pole in monocentric and bipolar spindles were analysed by time-lapse cinematography in mitosis of primary cultures of lung epithelium from the newt Taricha granulosa. Chromosomes oscillate toward and away from the pole in all stages of mitosis including anaphase. The duration, velocity, and amplitude of such oscillations are the same in all stages of mitosis. The movement away from the pole in monocentric spindle is rapid enough to suggest the existence of a previously unrecognized active component in chromosome movement, presumably resulting from a pushing action of the kinetochore fiber. During prometaphase oscillations, chromosomes may approach the pole even more closely than at the end of anaphase. Together, these observations demonstrate that a monopolar spindle is sufficient to generate the forces for chromosome transport, both toward and away from the pole. The coordination of the aster/centrosome migration in prophase with the development of the kinetochore fibers determines the course of mitosis. After the breaking of the nuclear envelope in normal mitosis, aster/centrosome separation is normally followed by the rapid formation of bipolar chromosomal fibers. There are two aberrant extremes that may result from a failure in coordination between these processes: (a) A monocentric spindle will arise when aster separation does not occur, and (b) an anaphaselike prometaphase will result if the aster/centrosomal complexes are already well-separated and bipolar chromosomal fibers do not form. In the latter case, the two monopolar prometaphase half-spindles migrate apart, each containing a random number of two chromatid (metaphase) monopolar-oriented chromosomes. This random segregation of prometaphase chromosome displays many features of a standard anaphase and may be followed by a false cleavage. The process of polar separation during prometaphase occurs without any visible interzonal structures. Aster/centrosomes and monopolar spindles migrate autonomously by an unknown mechanism. There are, however, firm but transitory connections between the aster center and the kinetochores as demonstrated by the occasional synchrony of centrosome-kinetochore movement. The data suggest that aster motility is important in the progress of both prometaphase and anaphase in normal mitosis.

In situ hybridization at the electron microscope level: hybrid detection by autoradiography and colloidal gold

Abstract: In situ hybridization has become a standard method for localizing DNA or RNA sequences in cytological preparations. We developed two methods to extend this technique to the transmission electron microscope level using mouse satellite DNA hybridization to whole mount metaphase chromosomes as the test system. The first method devised is a direct extension of standard light microscope level using mouse satellite DNA hybridization to whole mount metaphase chromosomes as the test system. The first method devised is a direct extension of standard light microscope in situ hybridization. Radioactively labeled complementary RNA (cRNA) is hybridized to metaphase chromosomes deposited on electron microscope grids and fixed in 70 percent ethanol vapor; hybridixation site are detected by autoradiography. Specific and intense labeling of chromosomal centromeric regions is observed even after relatively short exposure times. Inerphase nuclei present in some of the metaphase chromosome preparations also show defined paatterms of satellite DNA labeling which suggests that satellite-containing regions are associate with each other during interphase. The sensitivity of this method is estimated to at least as good as that at the light microscope level while the resolution is improved at least threefold. The second method, which circumvents the use of autoradiogrphic detection, uses biotin-labeled polynucleotide probes. After hybridization of these probes, either DNA or RNA, to fixed chromosomes on grids, hybrids are detected via reaction is improved at least threefold. The second method, which circumvents the use of autoradiographic detection, uses biotin-labeled polynucleotide probes. After hybridization of these probes, either DNA or RNA, to fixed chromosomes on grids, hybrids are detected via reaction with an antibody against biotin and secondary antibody adsorbed to the surface of over centromeric heterochromatin and along the associated peripheral fibers. Labeling is on average ten times that of background binding. This method is rapid and possesses the potential to allow precise ultrastructual localization of DNA sequences in chromosomes and chromatin.

Construction and characterization of genomic libraries from specific human chromosomes

Abstract: Highly purified fractions of human chromosomes 21 and 22 were isolated from a suspension of metaphase chromosomes stained with ethidium bromide by using a fluorescence-activated cell sorter (FACS II). Two recombinant DNA libraries, representing chromosomes 21 and 22, were constructed by complete digestion of DNA from these fractions with EcoRI and insertion into the vector lambda gtWES lambda B. Twenty clones selected at random from the chromosome 22 library hybridized to EcoRI-digested human DNA, and five of these clones hybridized to single bands identical in size to the phage inserts. These five single-copy sequences and a clone coding for an 8S RNA isolated by screening the chromosome 22 library for expressed sequences were characterized in detail. Hybridization of all six clones to a panel of sorted chromosomes and hybrid cell lines confirmed the assignment of the sequences to chromosome 22. The sequences were localized to regions of chromosome 22 by hybridization to translocated chromosomes sorted from a cell line having a balanced translocation t(17;22)(p13;q11) and to hybrid cell lines containing the various portions of another translocation t(X;22)(q13;q112). Five clones reside on the long arm of chromosome 22 between q112 and pter, while one clone and an 18S rRNA gene isolated from the chromosome 22 library reside pter and g112. The construction of chromosome-specific libraries by this method has the advantage of being direct and applicable to nearly all human chromosomes and will be important in molecular analysis of human genetic diseases.

Effects of Hyperthermia on Dividing Chinese Hamster Ovary Cells and on Microtubules in Vitro

Abstract: The ultrastructure of Chinese hamster ovary cells was examined immediately after heating cells in mitosis, and the findings were compared with (a) the behavior of heated cells monitored with time lapse cinematography following heat shocks and (b) proliferative survival of individual cells followed for 7 days after heating. Treatment of dividing cells at 45.5 degrees (5 to 15 min) disassembled the spindle and disrupted both the contractile ring and the midbody-cytoplasmic bridge complex to varying degrees depending on the length of heating. The spindle did not reform upon return to 37.0 degrees. Microtubular proteins were heated in vitro to determine if their inactivation was responsible for the inability of the spindle to reform. Heat completely disassembled the intact microtubules and inactivated a proportion of the microtubular proteins in vitro; however, a fraction of the microtubular proteins from heat-disassembled microtubules still was capable of reassembly. The time lapse studies indicated that cells in division at the time of heating entered G1 without completing division. Of the resultant tetraploid cells, 88% had greater than or equal to 2 nuclei; 59% of the tetraploid cells divided 35 +/- 7 (S.D.) hr following the heat shock (control generation time, 13 +/- 2 hr), and 95% of the flattened progeny had more than one nucleus. The fate of individual cells that were in mitosis or G1 when treated at 45.5 degrees for 4.5 min was monitored for 7 days. The survival of the total heated population of cells was 19%, but the surviving cells were almost totally accounted for by the G1 cells present as contaminants in the heated population. Less than 2% (2 of 115) of the monitored cells that were heated in mitosis formed colonies. Therefore, the morphological disruption of the spindle, contractile ring, or midbody-cytoplasmic bridge complex by a heat of 45.5 degrees prevented cytokinesis, and the resultant tetraploid cells became proliferatively dead.

Vicia cytogenetic tests for environmental mutagens: A report of the U.S. environmental protection agency Gene-Tox program☆

Abstract: Vicia root-tip mitotic and pollen mother-cell meiotic tests are two major kinds of cytogenetic tests for environmental mutagens. According to the present review, 81 of 85 earlier studies used mitotic tests to determine the frequencies of chromosome or chromatid aberrations and/or sister-chromatid exchange from root-tip meristematic cells; only 4 used meiotic tests to determine the frequencies of chromosome aberration from pollen mother cells. Treatment of root-tip meristem can be done by allowing the newly germinated roots to absorb the chemical mutagens from a water solution. Pollen mother cells can be treated by spraying the solution or pipetting the liquid over the flower buds. After an appropriate recovery time, the samples are fixed and stained, and the slides are prepared for metaphase or anaphase figures for scoring aberration frequencies. Slides for meiotic tests are prepared for metaphase I and/or Anaphase I stages for scoring chromosome aberration frequencies. Results of both cytogenetic tests should be expressed in terms of number of breaks per cell or per 100 cells. Test results of 76 chemicals from 32 classes in this review indicate that the Vicia root-tip mitotic test is reliable, efficient, and relatively inexpensive. These results also reveal that antibiotics are most frequently studied, followed by alkyl sulfones, pyrimidine, and purine derivatives. Of all the agents studied through root-tip mitotic tests, about 90% gave positive responses; antibiotics (phleomycin and bleomycin) had very high mutagenicity (less than 1 ppm gave positive response).

The chromosome complement of single-pronuclear haploid mouse embryos following activation by ethanol treatment

Abstract: A technique in which mouse eggs are stimulated to develop parthenogenetically following their brief incubation in 7% ethanol in PBS is described. Very high rates of activation were achieved, and a detailed analysis presented of the class of parthenogenone which develops a single haploid pronucleus following second polar body extrusion. As preliminary studies on presumptive haploid morulae indicated that a proportion of the metaphase spreads examined had an aneuploid chromosome constitution, the incidence of aneuploidy at the first cleavage mitosis was investigated. In the control groups the level of aneuploidy was about 1%, whereas in the ethanol-treated series the incidence ranged from 13 . 6-18 . 8%. Additional pre-treatment of ethanol-activated oocytes in low osmolar medium raised the incidence of aneuploidy to 28 . 3%. Metaphase groups with 18, 19, 21 and 22 chromosomes present were observed in addition to groups with a normal complement of 20 chromosomes. The possible mode of action of ethanol in inducing parthenogenetic activation of mouse oocytes, and a high incidence of aneuploidy, is discussed in relation to previous knowledge of the action of this agent. Preliminary studies using G-banding indicate that the aneuploidy observed appears to arise as a result of non-disjunction which may involve any of the chromosomes of the complement.

Active genes are sensitive to deoxyribonuclease I during metaphase.

Abstract: The active exogenous murine leukemia virus sequences of mouse cells growing in culture are preferentially digested by deoxyribonuclease I in metaphase chromosomes. As determined by nuclear nick translation, all of the gene sequences of these cells active during interphase are in a deoxyribonuclease I-sensitive conformation during metaphase. This method of nick translation can therefore be used to label chromosomes in situ in order to visualize the active regions of the genome.

Dynamics of spindle microtubule organization: kinetochore fiber microtubules of plant endosperm.

Abstract: Organization of kinetochore fiber microtubules (MTs) throughout mitosis in the endosperm of Haemanthus katherinae Bak. has been analysed using serial section reconstruction from electron micrographs. Accurate and complete studies have required careful analysis of individual MTs in precisely oriented serial sections through many (45) preselected cells. Kinetochore MTs (kMTs) and non-kinetochore MTs (nkMTs) intermingle within the fiber throughout division, undergoing characteristic, time-dependent, organizational changes. The number of kMTs increases progressively throughout the kinetochore during prometaphase-metaphase. Prometaphase chromosomes which were probably moving toward the pole at the time of fixation have unequally developed kinetochores associated with many nkMTs. The greatest numbers of kMTs (74-109/kinetochore), kinetochore cross-sectional area, and kMT central density all occur at metaphase. Throughout anaphase and telophase there is a decrease in the number of kMTs and, in the kinetochore cross-sectional area, an increased obliquity of kMTs and increased numbers of short MTs near the kinetochore. Delayed kinetochores possess more kMTs than do kinetochores near the poles, but fewer kMTs than chromosomes which have moved equivalent distances in other cells. The frequency of C-shaped proximal MT terminations within kinetochores is highest at early prometaphase and midtelophase, falling to zero at midanaphase. Therefore, in Haemanthus, MTs are probably lost from the periphery of the kinetochore during anaphase in a manner which is related to both time and position of the chromosome along the spindle axis. The complex, time-dependent organization of MTs in the kinetochore region strongly suggests that chromosome movement is accompanied by continual MT rearrangement and/or assembly/disassembly.

Taxol-induced anaphase reversal: evidence that elongating microtubules can exert a pushing force in living cells.

Abstract: The effects of taxol on mitosis in Haemanthus endosperm were studied. Immuno-gold staining was used to visualize microtubules; observations on microtubule arrangements were correlated with studies in vivo. Mitosis is slowed down, but not arrested, by taxol over a wide range of concentrations. Taxol promotes the formation of abundant new microtubules and lateral association within and between microtubule arrays (spindle fibers). This leads to a pronounced reorganization of the spindle, especially at the polar regions. Chromosome arms may be pushed toward the equator in metaphase. Anaphase chromosomes, with their kinetochores still pointing to the poles, move backward before resuming their poleward migration. During anaphase, the interzone is depleted of microtubules and trailing chromosome arms are stretched and often torn apart by rapidly elongating polar microtubules. Fragments are transported away from the poles, apparently "riding" on the tips of microtubules. This provides evidence of "pushing" by elongating microtubules. The desynchronization of anaphase, often observed as one of the first effects of taxol, indicates that the anchorage of different kinetochore fibers varies. The data draw attention to modifications of spindle structure due to increased microtubule lateral associations and to the role of this process in spindle integrity and chromosome movement.

High-resolution scanning electron microscopy of human metaphase chromosomes

Abstract: Human metaphase chromosomes, prepared for light microscopy were examined by scanning electron microscopy. Use of an osmium impregnation technique eliminated the need for sputter-coating and allowed high-resolution visualization of uncoated specimens. Chromosomes were of three-dimensional cylindrical profile, with well-defined chromatids and centromeres. Prior to Giemsa-banding a smooth surface morphology was observed. Relaxation of chromosome integrity by Giemsa-banding pretreatment allowed resolution of several orders of chromosome structure not previously demonstrated by scanning electron microscopy. The observed organization of the chromatin fibres allowed parallels to be drawn with the radial loop model of chromosome construction as described by Marsden and Laemmli.

Anaphase aberrations: a measure of genotoxicity in mutagen-treated fish cells.

Abstract: Rainbow trout gonad cells (RTG-2) were cultured for various lengths of time in the presence of several classes of known mutagenic chemicals and several related compounds that possessed no known mutagenic/carcinogenic activity. During the course of exposure the cells were examined for the presence of abnormalities in the chromosome arrangement of anaphase figures during mitosis. Untreated and solvent-treated (dimethylsulfoxide-treated) cells exhibited a background abnormality rate of 12% with only minor chromosomal defects being observed. This was also true for those cells exposed to naphthol and anthracene, two chemicals with no proven mutagenic or carcinogenic activity. Conversely, significant increases in the frequency of anaphase aberrations were produced in cells treated with N-methyl-N'-nitro-N-nitrosoguanidine, benzo(a)pyrene, 9-aminoacridine and mitomycin-C. These abnormalities were also far more complex and extensive than those observed in the control and nonmutagen-treated cells. Many species of fish have extremely small and numerous chromosomes, making resolution of chromosome defects such as sister chromatid exchange and deletions more difficult than in most mammalian diploid cells, which generally have larger and fewer chromosomes. Examination of cells during anaphase eliminates the need to observe each chromosome separately as well as the need to produce well-spread metaphase chromosomes. Since the sensitivity of anaphase aberrations tomore » known mutagenic/carcinogenic compounds appears to be quite high in trout cells and since hundreds of suitable cells are available for analysis, this may be an appropriate alternative or addition to some of the more standard chromosome macrolesion tests developed in mammalian systems.« less

Endomitosis in human trophoblast.

Abstract: Endopolyploidy, which arises through the duplication of DNA without accompanying nuclear division, occurs in large numbers of lower and higher plants and animals, including the best known, the salivary gland nuclei of Drosophila. Endomitosis is one of the processes leading to endopolyploidy, in which the stages of mitosis (prophase, metaphase, anaphase) take place inside the nuclear membrane and without spindle formation. In mammals, endomitosis has been observed in the trophoblast of the placenta of the mouse, rat and rabbit. This is the first report of endomitosis in a normal human tissue, the trophoblast of first trimester human placenta.

Localization of mitotic factors on metaphase chromosomes

Abstract: The objective of this study was to determine whether the mitotic factors of HeLa cells, which induce meiotic maturation, i.e. germinal vesicle breakdown (GVBD) and chromosome condensation, when injected into fully grown Xenopus laevis oocytes, were localized in the cytoplasm or associated with the metaphase chromosomes. Cytoplasmic extracts were prepared by lysing mitotic HeLa cells in low-salt hypotonic buffer and separating the chromosomes by centrifugation. Th mitotic factors bound to chromosomes were extracted with high-salt (0.2 M-NaCl) buffer. Both the cytoplasmic and chromosomal protein fractions were evaluated for their maturation-promoting activity (MPA) in the Xenopus oocytes. The results of this study indicate that both the cytoplasmic and chromosomal fractions are identical in many respects, including their ability to induce GVBD, but the specific activity of the chromosomal fraction was at least threefold greater than that of the cytoplasmic fraction. These data suggest that a major portion of the mitotic factors is localized on the metaphase chromosomes. This association does not appear to be due to adventitious binding of mitotic proteins to chromosomes during the extraction procedures. Furthermore, when extracts were prepared in a similar way from early- and mid-G2-phase HeLa cells, only the nuclear extracts had MPA and no activity was found in the cytoplasmic fraction. Both the cytoplasmic and nuclear extracts of late-G2 cells exhibited MPA. These data support the conclusion that the mitotic factors become preferentially bound to chromatin as soon as they are synthesized, and as the cell synthesizes more of these factors in preparation for mitosis, increasing amounts of them are retained in the cytoplasm.

A simple reproducible method for prometaphase chromosome analysis.

Abstract: A method is described for the analysis of chromosomes in prophase and early metaphase. It involves culturing the lymphocytes in medium RPMI-1640, supplemented with 10% autologous plasma instead of fetal bovine serum. Living cells are treated with actinomycin D and colcemid for 1 h prior to harvest and harvested early at 65 h of incubation, using a hypotonic solution formulated by Ohnuki (1968). The method has been tested on several hundred clinical samples on a routine basis. On average, 30% of the dividing cells were in prometaphase.

Ultrastructure of cell division in the unicellular red alga Porphyridium purpureum

Abstract: Mitosis in the unicellular red alga Porphyridium purpureum was studied with the electron microscope. During early prophase two bipartite nucleus associated organelles (NAOs) are seen in a region that will become one of the division poles. The division axis is established by the migration of one NAO. Microbodies are associated with the poles throughout the mitotic cycle. At prometaphase the nuclear envelope (NE) subjacent to each NAO forms a nuclear pocket which breaks down or opens to form a large gap. Concomitant with polar gap formation the large NAO portion proximal to the NE disperses whereas the smaller distal NAO portion remains throughout subsequent mitotic stages. At metaphase a plate arrangement of chromatin is seen and indistinct kinetochores are associated with a single microtubule. Chromatin moves to the poles followed by pronounced interzonal midpiece (IZM) elongation. After IZM abscission the nuclei migrate to opposite ends of the elongating cell. Cytokinesis occurs by means of an ingrowing ...

Expression of genetic damage induced by alkylating agents in germ cells of female mice

Abstract: The purpose of the present experiments was to analyse the frequencies of meiotic non-disjunction and structural aberrations by comparative cytogenetic investigations in unfertilized mII oocytes and first-cleavage metaphases after pre-ovulatory treatment of female mice with alkylating agents. We also present data on the expression of both types of aberration during embryogenesis in terms of dominant lethal effects. Trenimon (TR, 1 mg/kg) induced meiotic non-disjunction, but no structural aberrations were detected at metaphase II. On the contrary, at first-cleavage metaphase, TR revealed a strong clastogenic effect. A dose of 0.25 mg TR/kg increased the frequency of cells with structural aberrations to 51.79%. Mainly chromatid and a few isochromatid aberrations were found. These results support the observations previously made (see Brewen and Preston, 1979; Obe and Beek, 1979) that an intervening round of DNA replication is necessary for a TR-induced DNA lesion to be transformed into a structural aberration. The frequency of aberrant eggs in toto analysed at first cleavage (63.39%) can be quantitatively correlated to the rate of embryonic mortality (55.17%) as measured in the dominant lethal assay at the first day after treatment. We also present data on the effects of cyclophosphamide (CYC) on the first meiotic division. CYC (150 mg/kg) enhanced the incidence of meiotic non-disjunction only slightly, but induced a high frequency of dominant lethal effects (58.94%) at the first day after application.

An ordered arrangement of chromosomes in the somatic nucleus of common wheat Triticum aestivum L.: II. Spatial relationships between chromosomes of different genomes

Abstract: Spatial relationships between chromosomes of the same genome, both homologous and non-homologous, were studied in root-tip cells of common wheat, Triticum aestivurn (2n = 6x = 42). Mean distance between members of all the 21 homologous pairs (seven in each of the three genomes) and of 45 out of the 63 possible non-homologous combinations of two (21 in each genome) were determined. To minimize disruption of nuclear chromosomal arrangement, the cells were pretreated with cold temperature either in tap water or in a physiological medium (White solution) and distances between cytologically marked chromosomes were measured at metaphase. Comparison of distances for homologues with those for non-homologues indicated clearly that, within each genome, the homologous chromosomes were significantly closer to one another than were the non-homologues. Distances between homologues were similar in all three genomes, as were distances between non-homologues. The data are consistent with the hypothesis that the chromosomes of each genome of common wheat are arranged in the somatic nucleus in a highly specific ordered pattern. In this hypothetical arrangement, homologous chromosomes are closely associated, while the nonhomologues occupy definite positions with respect to one another. The universality of the phenomenon and its cellular mechanism and biological significance are discussed.