Chromosome replication in fibroblasts of the Syrian hamster (Mesocricetus auratus)

Replication of the heterogeneous heterochromatin of the sex chromosomes of Microtus cabrerae

We used bromosubstitution to investigate the mode of replication of different types of heterochromatin located in the sex chromosomes of Microtus cabrerae. Our results clearly show that, although the heterochromatin is late replicating, the replication timing of different types of constitutive heterochromatin is related to their banding properties: R-type heterochromatin replicates before G-type heterochromatin, and this replication is asynchronous in both sex chromosomes. Furthermore, the late replication behaviour of the inactive X may spread to its constitutive heterochromatin. In some cells, a region of the constitutive heterochromatin of the late replicating X spontaneously switches to early replication, which may be related to transcriptional activity. Replication behaviour of the constitutive heterochromatin in the Y chromosome is similar to that of the late replicating X.

BrdU pulse/reverse staining protocols for investigating chromosome replication

By using a reverse Giemsa staining procedure (TT chromatin pale, TB chromatin dark) it is possible to detect replication in metaphase chromosomes with short (approximately 10 min) 5-bromodeoxyuridine (BrdU) pulses. A pulse protocol allows us to consider the question "What is replicating at this point in time?" and we have investigated replication patterns during cycle transit in stimulated human female lymphocytes. A clear-cut demarcation between R-zone early and G-zone late was not found. Instead, whilst replication commences (with a very staggered start) in R-zones, activity soon appears to transgress band boundaries and gives rise to cells with unclassifiable patterns where chromosomes take on a mottled or reticulate appearance. Replication in R-zones dies out leaving a clear G-zone pattern persisting for the remainder of S which terminates with a very staggered finish. When pulse duration is increased (approximately 1 h) the frequency of unclassifiable cells falls and occasional "mixed-pattern" cells appear which have, within the same cell, typical R- and G-zone regions. The existence of such cells indicates that if a mid-S replication pause exists (and the absence of any mid-S wave of pale stained cells suggests that it does not) it does not make exclusive separation between dark R- and G-band zones.

Breakage of human interphase chromosomes by alpha particles and X-rays

The technique of premature chromosome condensation (PCC) was used to compare the early formation of chromosome breaks in non-cycling HF19 human diploid fibroblasts when irradiated with slow alpha particles (3.2 MeV, 128 keV micron-1) or 250 kVP X-rays. For both radiations the production of PCC breaks increased approximately linearly with dose. The production coefficient for alpha particles was 12.5 +/- 0.6 per cent per Gy and for X-rays it was 5.8 +/- 0.2 per cell per Gy. Hence, the relative biological effectiveness (RBE) of the alpha particles was 2.16 +/- 0.13. This is smaller than reported values of the RBE for the production of chromosome-type exchange aberrations by slow alpha particles. This implies that there is a difference, spatial or qualitative, in the initial breaks produced by the densely ionizing alpha particle tracks and the more sparsely ionizing electron tracks from the X-rays.

Generalized blocking in S phase by methotrexate

An attempt was made to enhance the frequency of prometaphase cells for high-resolution-banding studies in untransformed Syrian hamster fibroblasts using a typical methotrexate (MTX) block/bromodeoxyuridine release schedule. The recovery 'wave' was serially sampled and detailed sub-phase analysis made using the replication bands resulting from bromodeoxyuridine uptake. Of the 3 batches of MTX used, one (Sigma greater than 2 years old) was found to have decayed to a non-toxic compound which produced almost no measurable perturbation of the cell cycle at any concentration used. The other 2 (new Sigma and new Lederle), whilst producing mitotic index fluctuations which could be construed to indicate blocking and "synchrony", gave absolutely no evidence of any specific blocking site, but rather a general stoppage (or slowing down) during MTX treatment and continuation in exactly the same order as untreated controls upon release.

Replication kinetics of X chromosomes in fibroblasts and lymphocytes

The kinetics of replication for early and late replicating X chromosomes in karyotypically normal fibroblasts and lymphocytes was studied using terminal bromodeoxyuridine (BrdU) treatment followed by Hoechst/light/Giemsa staining. Although the order of band appearance differs between the two tissues, the programme (order and interval between band appearances) for early replicating bands (dark R-bands) is identical in the two homologues. This is probably also the case for later replicating bands (dark G-bands) though the criteria for determining mean band appearance times are less reliable for these bands when terminal BrdU treatment is used. This means that the late X has a delayed start but thereafter proceeds at the same pace as its early counterpart.

The behaviour of fragile X and other aberrations during recovery from low folate conditions

Whole blood from two mentally retarded fra-X brothers was grown in low folate medium where fra-X expression was enhanced. Bromodeoxyuridine was added to mitigate the low folate conditions and metaphases were sampled sequentially, and stained for replication banding, through one cell cycle of recovery. The replication bands allowed detailed analysis of the cell cycle and the allocation of individual cells to precise sub-phases. Various classes of fra-X and all other types of chromosomal aberrations were scored in these classified cells. The fra-X does not conform in morphology to any of the known simple chromatid intrachange types, which were often present within the same cells, but the subsequent fall in frequency once bromodeoxyuridine was added closely paralleled that of the conventional aberrations. Normal folate level frequencies of fra-X are restored by the time early S-phase cells (sub-phase SkI) reach metaphase. When sub-phased cells are rearranged in true chronological sequence, there is a suggestion of a sudden fall in frequency between SkII-III (about 70% of the transit of S). This suggests that the critical point for low folate enhancement occurs in this region of the S-phase. This is somewhat earlier than the band-appearance distribution curve for Xq27 which lies within sub-phase SkIV.

On the localization of mitomycin C-induced aberrations in normal human and Fanconi's anaemia cells

This paper summarizes the results of a series of experiments with primary cultures of normal human fibroblasts and lymphocytes designed to investigate chromatid aberration 'break-point' localization after a 1-h pulse of mitomycin C. For discontinuities and interchanges, 60-70% of the inferred 'break-points' were localized to defined paracentric heterochromatin and the centromeric regions (i.e. approximately 21% by length of the normal karyotype), irrespective of 'dose', aberration frequency, sample time or cycle sub-phase as determined by replication banding. Chromatid intrachanges are non-(or negatively) localized because of an inescapable scoring bias. SCE in fibroblasts show no such localization. Cells from a number of Fanconi's anaemia subjects were examined. In poorly growing cultures, localization was as high as in normal cells but in vigorous cultures localization was reduced to approximately 30%. It is suggested that the enhanced aberration sensitivity of this syndrome could arise because non-localized aberrations, usually eliminated before division in normal cells, are allowed to reach mitosis in FA cells.

The induction of chromosome exchange aberrations by carbon ultrasoft X-rays in V79 hamster cells

V79 hamster cells in plateau (extended G1) phase were irradiated with either 250 kV ('hard') X-rays or carbon K characteristic ultrasoft X-rays under conditions minimizing cell overlap. These cells were killed most effectively by the carbon X-rays, by a factor of about 3 relative to hard X-rays, in agreement with our previous findings with cells in exponential growth. Chromosome-type aberrations were measured at 3 fixation times within the first division cycle after irradiation, and an approximately uniform sensitivity to aberration induction was found for both radiations. The combined aberration data show that carbon X-rays are 2 or more times as effective as hard X-rays, depending on dose and/or data fit. Exchange aberrations require recombination between two separate chromosomes, but they are induced efficiently by carbon X-rays with a substantial linear component to the dose-response despite the very short electron tracks (approximately less than 7 nm) that they produce in the cell. This implies either that the participating DNA helices must be lying extremely close together at the time of radiation damage, so that one track can effectively damage both helices, or that only one radiation-damaged chromosome is needed to promote an exchange event.

A model for effective pairing and recombination at meiosis based on early replicating sites (R-bands) along chromosomes

A model for meiotic pairing is proposed in which early replicating sites (R-band equivalent) along chromosomes are envisaged as sites for synaptic initiation. Only within such sites will "effective" pairing for recombination be established. Pairing in later replicating (G- and C-band equivalent) regions will be "ineffective" and will not provide for the stringent requirements of the crossover process. Exchange events might be predetermined at S-phase, and possibly at junctions between early and later replicating sequences, these being seen as vulnerable sites for breakage. Temporal shifts in replication from early to late S, are postulated to produce localized pairing disruption and lowering of crossover values as regions of chromatin shift from being effectively to ineffectively paired.

Cytogenetic replication studies with short thymidine pulses in bromodeoxyuridine-substituted chromosomes of different mouse cell lines

In normal diploid fibroblasts of the mouse, 3T3-SV-3T3-, and Meth A-cells, the chromosome replication patterns were studied by a bromodeoxyuridine (BrdU)-labelling technique. SV-3T3 is a subline of 3T3 transformed by SV 40 and Meth A is a permanent cell line from Balb c transformed by methylcholanthrene. The use of 1 h thymidine pulses permits high resolution of the S-phase after partial synchronization of the cells at G1/S in an otherwise BrdU-substituted S-phase. It could be shown that the autosomal heterochromatin of the mouse (Mus musculus) starts replication during the early S-phase (R-band replication), continues while R-band chromatin finishes, and still replicates when G-band chromatin starts. The heterochromatin finishes before the majority of G-bands have been replicated. There is no fundamental difference in the course of chromosome replication between the different cell lines studied here. It is concluded that there are no obligate changes in the course of the S-phase linked to the process of transformation.

Characterization of chromosome replication during S-phase with bromodeoxyuridine labelling in Chinese hamster ovary and HeLa cells

Chinese hamster ovary (CHO) and HeLa cells were successively pulse labelled at 1-h intervals after the cultures were synchronized at the end of G1 (monitored by flow cytometry). The metaphases analysed afterwards showed R-type replication patterns after 1-h pulses during the early S-phase (SE; from h 1-5 after release) and replication of G- and C-bands in late S-phase (SL: from h 6-8 after release). The transition from SE to SL is abrupt, constituting a sudden switch of replication between different types of chromatin as has been described for human lymphocytes. The differences between these two cell lines and earlier results reported on a V79 Chinese hamster cell line and on normal diploid human and Chinese hamster fibroblasts are discussed.

Some observations on the localization of mitomycin C-induced aberrations in human lymphocytes

Actively cycling human lymphocytes were treated with mitomycin C for 1 h (1.4 micrograms/ml) and then grown in medium containing 10 micrograms/ml bromodeoxyuridine. Serial 5-h colcemid accumulation samples were taken up to 35 h and the air-dried methaphase spreads stained for replication banding. A complete cell-cycle subphasing analysis was made, and classified cells scored for all categories of chromatid-type aberrations and their location. In spite of the high dose which produced massive delay and cycle perturbation, there was no evidence for selective lethality of early-S cells, in fact such cells were in excess. Extreme localization of aberrations to late-replicating (mostly centromeric) regions was found at all subphases and in pre-S cells. This rules out 'localization by default' as an explanation for the observed preferential occurrence of 'break points' in these regions. The frequency of incomplete intrachanges, low in late S, rises dramatically in early S to become maximal in pre-S cells.

Uninterrupted DNA synthesis during S-phase in untransformed diploid hamster fibroblasts

Untransformed Syrian hamster fibroblasts in exponential growth were exposed to a pulse of [3H]-thymidine for 5 min, followed immediately by bromodeoxyuridine, and serial samples were taken up to 16 h. Preparations were autoradiographed and stained for replication banding. No cell with replication bands was found without significant [3H]-thymidine uptake, although the extent of uptake varied between sub-phases of S. Thus there is no indication of a total cessation of synthesis at any period during S-phase.

Subdivision of S-phase and its use for comparative purposes in cultured human cells

Intranuclear DNA synthesis and concomitant chromosome duplication occur during a discrete period of the cell cycle termed S-phase. Using replication-banding and serial time sampling in asynchronous cell populations, it is possible to subdivide the S-phase into four or five chronological compartments termed "subphases". This paper discusses methods for analysing the sampling data to obtain the average duration of these subphases and the positions within S of the borders between them. Such information not only allows a more detailed analysis of the cell cycle, but also provides parameters which can be used for rigorous comparisons of cell populations from different sources and experimental conditions. Examples are given of application of the method to normal and chromosomally abnormal primary human fibroblasts and lymphocytes growing in short-term in vitro culture.

Cytological subdivision of the S phase of human cells in asynchronous culture

A method is described for subdividing S phase cells in asynchronous cell cultures on the basis of replication band patterns produced in chromosomes by bromodeoxyuridine incorporation. The criteria used for cell classification are objective, requiring the presence or absence of specific bands on particular chromosomes, and therefore lead to subdivisions amenable to quantitative analysis and for comparative purposes. Two schemes are given: key 1, based on bands in chromosomes 2 and 5, leads to five sub-phases; and key 4, based on bands in chromosomes 3 and 4, leads to four sub-phases. The order of the sub-phases, though not their relative durations, is identical in the six primary cell cultures (four fibroblast and two lymphocyte) tested. The technique provides for a detailed study of the programme of chromosome replication in normal and abnormal cells which, in time, should produce new criteria for diagnostic purposes.

Detailed cell cycle analysis in human lymphocytes; application to gamma-irradiated cells

A method based on BrdU incorporation for analyzing in detail the kinetics of the cell cycle is described. The S phase has been subdivided into five subphases, each recognizable by their BrdU incorporation pattern at metaphase. The method can be useful for the study of abnormal cell cycles, and may have particular application in mutagenesis studies concerning the various subphases of the S phase, without using synchronization techniques. An application of the method is described, showing that gamma-irradiation, during the course of the S phase, leads to a lack of cells which were in early S phase at the time of irradiation. This finding can be related either to a higher lethality at this stage of the cell cycle or to a delay in completion of DNA replication after irradiation.

Replication variants of the human inactive X chromosome. II. Frequency and replication rate relative to the other chromosomes of the complement

Replication variants of the inactive X chromosome were investigated in lymphocytes from six donors by means of terminal BrdU or thymidine incorporation. There were interindividual differences in the incidence of particular variants. In endoreduplicated and tetraploid cells both allocyclic X chromosomes showed the same replication sequence. The Xp22 band of the allocyclic X chromosome seemed to replicate later than the homologous material in some cells. Initiation time of DNA synthesis within the inactive X chromosome was found to be stable; termination time, however, varied greatly relative to the other chromosomes. Early completion of replication within the heterochromatic X chromosome could be demonstrated preferentially for the Xq25-27 terminal sequence, but other variants expressed the phenomenon also. A variable replication rate of the inactive X chromosome is believed to be responsible for its asynchronous, independent replication. The biological significance of the phenomenon is discussed with respect to cell differentiation.

The disparity between homologous chromosomes during DNA replication

When the patterns of replicating chromosome bands of homologous chromosomes within a diploid cell during DNA synthesis phase are compared, the frequency of disparity (i.e. a band present on only one homologue) is less than expected on the basis of chance. This could be taken as implying some "link" between homologues which constrains their programmes of replication to keep in step. This paper develops a model showing that the observed disparities can be accommodated within a framework of homologue independence. Differences between cells in the time of appearance of bands lead in any sample to the summation of an infinitude of binomial distributions and hence to over-dispersion. The model fits observed data for bands replicating in euchromatic and heterochromatic chromosome regions obtained from Syrian hamster fibroblast cells growing in vitro.

Early-replication DNA patterns in a derivative chromosome in a Syrian hamster with only 42 chromosomes

From crosses within a 2n=43 line of Syrian hamsters (Mesocricetus auratus) lacking one derivative (der 11) of an 11;20 reciprocal translocation we have obtained homozygotes with only 42 chromosomes. These animals are homozygous deficient (nullisomic) for the centromere and short arm of chromosome 11 and for the bulk of the long arm of chromosome 20. -During cytogenetic studies, we investigated the frequency patterns of early-replicating bands in the surviving derivative (der 20) at two cytologically defined sub-phases of S using short-term fibroblast cultures. These patterns were compared with those observed in the component, untranslocated arms in normal 2n=44 cells at the same two sub-phases. -Very close agreement was found, indicating that neither the nullisomy, nor the new arm combination has interfered detectably with the pattern or programme of early band replication.

Cell-specific variation of X chromosome replication in the Syrian hamster (Mesocricetus auratus)

The sub-division of S-phase in Syrian hamsters, on the basis of Brd/U/Hoechst 33258/Giemsa banding, has allowed a quantitative comparison of the replication of individual chromosome bands within defined subphases of S. This analysis has shown that in hamsters, as has been reported in humans, there are distinct patterns of early replication in vitro in the early X, the late X in fibroblasts, and the late X in lymphocytes. In addition, it has been possible to show that, although the pattern of replication of the late X in fibroblasts differs from that in lymphocytes, the time in S at which bands first appear on this chromosome is the same in the two cell types. No significant heterogeneity can be ascribed to differences between individuals, adult or embryonic sources, culture media, or time of exposure to BrdU. The absence of any detectable heterogeneity in the replication band frequencies in autosomal heterochromatic arms suggests that the cell-specific variability of the late-replicating X is a feature of facultative rather than constitutive heterochromatin.