Late DNA Synthesis in Heterochromatin

A search for extraterrestrial eukaryotes: physical and paleontological aspects

Physical and biochemical aspects of a proposed search for extraterrestrial eukaryotes (SETE) are considered. Such a program should approach the distinction between a primitive eukaryote and an archaebacteria. The emphasis on gene silencing suggests a possible assay suitable for a robotic investigation of eukaryoticity, so as to be able to decide whether the first steps towards eukaryogenesis have been taken in an extraterrestrial planet, or satellite. The experiment would consist of searching for cellular division and the systematic related delay in replication of heterochromatic chromosome segments. It should be noticed that the direct search for a membrane-bounded set of chromosomes does not necessarily determine eukaryotic identity, as there are prokaryotes that have membrane-bounded nucleoids. A closer look at the protein fraction of chromatin (mainly histones) does not help either, as there are some eukaryotes that may lack histones; there are also some bacteria as well as archaebacteria with histone-like proteins in their nucleoids. Comments on the recent suggestion of possible environments for a SETE program are discussed: the deep crust of Mars, and the Jovian satellite Europa, provided the existence of an ocean under its ice-covered surface is confirmed by the current Galileo mission.

Distinct cytoplasmic and nuclear fractions of Drosophila heterochromatin protein 1: their phosphorylation levels and associations with origin recognition complex proteins

The distinct structural properties of heterochromatin accommodate a diverse group of vital chromosome functions, yet we have only rudimentary molecular details of its structure. A powerful tool in the analyses of its structure in Drosophila has been a group of mutations that reverse the repressive effect of heterochromatin on the expression of a gene placed next to it ectopically. Several genes from this group are known to encode proteins enriched in heterochromatin. The best characterized of these is the heterochromatin-associated protein, HP1. HP1 has no known DNA-binding activity, hence its incorporation into heterochromatin is likely to be dependent upon other proteins. To examine HP1 interacting proteins, we isolated three distinct oligomeric species of HP1 from the cytoplasm of early Drosophila embryos and analyzed their compositions. The two larger oligomers share two properties with the fraction of HP1 that is most tightly associated with the chromatin of interphase nuclei: an underphosphorylated HP1 isoform profile and an association with subunits of the origin recognition complex (ORC). We also found that HP1 localization into heterochromatin is disrupted in mutants for the ORC2 subunit. These findings support a role for the ORC-containing oligomers in localizing HP1 into Drosophila heterochromatin that is strikingly similar to the role of ORC in recruiting the Sir1 protein to silencing nucleation sites in Saccharomyces cerevisiae.

In vitro reconstitution of Artemia satellite chromatin

We report the characterization of an in vitro chromatin assembly system derived from Artemia embryos and its application to the study of AluI-113 satellite DNA organization in nucleosomes. The system efficiently reconstitutes chromatin templates by associating DNA, core histones, and H1. The polynucleosomal complexes show physiological spacing of repeat length 190 +/- 5 base pairs, and the internucleosomal distances are modulated by energy-using activities that contribute to the dynamics of chromatin conformation. The assembly extract was used to reconstitute tandemly repeated AluI-113 sequences. The establishment of preferred histone octamer/satellite DNA interactions was observed. In vitro, AluI-113 elements dictated the same nucleosome translational localizations as found in vivo. Specific rotational constraints seem to be the central structural requirement for nucleosome association. Satellite dinucleosomes showed decreased translational mobility compared with mononucleosomes. This could be the consequence of interactions between rotationally positioned nucleosomes separated by linker DNA of uniform length. AluI-113 DNA led to weak cooperativity of nucleosome association in the proximal flanking regions, which decreased with distance. Moreover, the structural properties of satellite chromatin can spread, thus leading to a specific organization of adjacent nucleosomes.

Position effect variegation in Drosophila is associated with an altered chromatin structure

A euchromatic gene placed in the vicinity of heterochromatin by a chromosomal rearrangement generally exhibits position effect variegation (PEV), a clonally inherited pattern showing gene expression in some somatic cells but not in others. The mechanism responsible for this loss of gene expression is investigated here using fly lines carrying a P element containing the Drosophila melanogaster white and hsp26 genes. Following mobilization of the P element, a screen for variegation of white expression recovered inserts at pericentric, telomeric, and fourth chromosome regions. Previously identified suppressors of PEV suppressed white variegation of pericentric and fourth chromosome inserts but not telomeric inserts on the second and third chromosomes. This implies a difference in the mechanism for gene repression at telomeres. Heat shock-induced hsp26 expression was reduced from pericentric and fourth chromosome inserts but not from telomeric inserts. Chromatin structure analysis revealed that the variegating inserts showed a reduction in accessibility to restriction enzyme digestion in the hsp26 regulatory region in isolated nuclei. Micrococcal nuclease digests showed that pericentric inserts were packaged in a more regular nucleosome array than that observed for euchromatic inserts. These data suggest that altered chromatin packaging plays a role in PEV.

Characterization of a small supernumerary ring X chromosome by fluorescence in situ hybridization

We report on a male with mild learning disabilities who has a supernumerary marker chromosome. The marker chromosome was defined by fluorescence in situ hybridization as a ring X chromosome with breakpoints in the juxacentromeric region. Replication studies suggest that the ring X is late-replicating. However XIST, a gene in the X inactivation centre interval which is expressed exclusively from the inactive X chromosome, is not present on the marker, nor is it expressed in the patient's cells. These results are discussed with respect to karyotype-phenotype correlations and X inactivation.

W-heterochromatin of chicken; its unusual DNA components, late replication, and chromatin structure

About 65% of DNA in the chicken W chromosome has been shown to consist of XhoI and EcoRI family repetitive sequences. These sequences showed remarkable delay in the electrophoretic mobility at low temperature on a polyacrylamide gel. Three dimensional structures of the 0.7-kb XhoI and the 1.2-kb EcoRI family repeating units were estimated to be irregular solenoids using a computer program based on wedge angles of all the 16 dinucleotide steps. Fluorescence in situ hybridization demonstrated that these two family sequences were localized in a major heterochromatic body in an interphase nucleus. Incorporation of bromodeoxyuridine into the W chromosome in the synchronous culture of MSB-1 cells occurred about 1 h later than the peak of S phase. The chromatin structure formed along XhoI and EcoRI family sequences was suggested to be different from the total chromatin or chromatin containing the beta-actin gene sequence in that the linker DNA lengths of the former were significantly longer. Fractionation of the HaeIII-digested MSB-1 nuclei yielded a chromatin fraction in which XhoI family sequences were partially enriched. Several DNA-binding proteins showing higher affinity for the XhoI family sequence were present in this fraction.

BUdR as an S-phase marker for quantitative studies of cytokinetic behaviour in the murine cerebral ventricular zone

BUdR incorporation into replicating DNA, detected immunohistochemically, is used as an S-phase marker in the proliferative cell populations of the cerebral wall of the mouse embryo on the 14th gestational day (E14). The analysis initiates a series of studies concerned with the cytokinetic behaviour and cell output of proliferative populations involved in neocortical histogenesis. On E14 there are two periventricular proliferative zones in the cerebral wall. These are the ventricular and subventricular zones. The ventricular zone is a pseudostratified epithelium. DNA replication occurs with the cell nucleus in the outer zone of the epithelium and mitoses at the ventricular surface. Prior applications of BUdR for studies of cytogenesis in the CNS have been extended in two principal ways: (1) basic fuchsin was used as counterstain for BUdR-negative nuclei and (2) labelling indices were determined separately in strata or bins, 10 microns in height, through the full depth of the ventricular zone and overlying cerebral wall. It was established that a single injection of 50 micrograms g-1 into the pregnant dam was associated with labelling of 100% of nuclei in S-phase over an interval extending from 15 min to at least 2.0 h after injection. The zone where nuclei are undergoing S-phase (S-phase zone) extends through the outer four bins of the ventricular zone. The method has high quantitative reproducibility with an SE for labelling indices in bins within the S-phase zone less than 10% of the average values. Evidence is provided that BUdR incorporation is initiated with the nucleus in the outer aspect of the S-phase zone. The efficiency of incorporation of the marker is reduced as nuclei near the end of DNA replication and move to the inner aspect of the S-phase zone.

Dynamic organization of DNA replication in mammalian cell nuclei: spatially and temporally defined replication of chromosome-specific alpha-satellite DNA sequences

Five distinct patterns of DNA replication have been identified during S-phase in asynchronous and synchronous cultures of mammalian cells by conventional fluorescence microscopy, confocal laser scanning microscopy, and immunoelectron microscopy. During early S-phase, replicating DNA (as identified by 5-bromodeoxyuridine incorporation) appears to be distributed at sites throughout the nucleoplasm, excluding the nucleolus. In CHO cells, this pattern of replication peaks at 30 min into S-phase and is consistent with the localization of euchromatin. As S-phase continues, replication of euchromatin decreases and the peripheral regions of heterochromatin begin to replicate. This pattern of replication peaks at 2 h into S-phase. At 5 h, perinucleolar chromatin as well as peripheral areas of heterochromatin peak in replication. 7 h into S-phase interconnecting patches of electron-dense chromatin replicate. At the end of S-phase (9 h), replication occurs at a few large regions of electron-dense chromatin. Similar or identical patterns have been identified in a variety of mammalian cell types. The replication of specific chromosomal regions within the context of the BrdU-labeling patterns has been examined on an hourly basis in synchronized HeLa cells. Double labeling of DNA replication sites and chromosome-specific alpha-satellite DNA sequences indicates that the alpha-satellite DNA replicates during mid S-phase (characterized by the third pattern of replication) in a variety of human cell types. Our data demonstrates that specific DNA sequences replicate at spatially and temporally defined points during the cell cycle and supports a spatially dynamic model of DNA replication.

Towards the molecular bases of polymerase dynamics

One aspect of the strong relationship that is known to exist between the processes of DNA replication and transcription is manifest in the coupling of the rates of movement of the replication fork (rf) and RNA polymerase (rt). We address two issues concerning the largely unexplored area of polymerase dynamics: (i) The validity of an approximate kinematic formula linking rf and rt suggested by experiments in which transcription is inhibited in some prokaryotes with the antibiotic streptolydigin, and (ii) What are the molecular bases of the kinematic formula? An analysis of the available data suggests possible molecular bases for polymerase dynamics. In particular, we are led to a hypothesis: In active chromatin rt may depend on the length (lambda t) of the transcript of the primary messenger RNA (pre-mRNA). This new effect is subject to experimental verification. We discuss possible experiments that may be performed in order to test this prediction.

Observations regarding DNA replication sites in human cells in vivo following infusions of iododeoxyuridine and bromodeoxyuridine

In studies using bromodeoxyuridine (BrdUrd) and/or iododeoxyuridine (IdUrd) to label S phase cells in cancer patients, several unique observations were made regarding DNA replication sites and the organization of newly synthesized DNA in post-mitotic cells. While the majority of tumour specimens removed at the end of infusions demonstrated concentration of replication sites around the nuclear membrane, biopsies obtained in leukaemic patients 1 week later demonstrated several distinct patterns of labelling. For example, one, two or all lobes of granulocytes were labelled. Scavenger macrophages bearing labelled leukaemic cells in their cytoplasm were also seen. Sequential IdUrd/BrdUrd labelling of solid tumours showed various patterns of nuclear/nucleolar/membrane labelling, allowing more precise localization of early versus late replication sites.

Increased sister-chromatid exchanges (SCEs) and chromosomal fragilities by BrdU in a human mutant B-lymphoblastoid cell line

From an X-irradiated human B-lymphoblastoid cell line (CCRF-SB), we have isolated a unique mutant clone (CCRF-SB-T1) which reveals high frequencies of sister-chromatid exchanges (SCEs) and chromosomal fragilities in the C-band regions of chromosomes Nos. 1, 9 and 16, when exposed to high concentrations of bromodeoxyuridine (BrdU). A clear BrdU dose-dependent increase of SCEs (9.6 SCEs/cell at 0.05 mM, 40 SCEs/cell at 0.37 mM on average) in this mutant was observed. Relative contributions of nucleoside and a thymidine (dT) analog of BrdU to high SCEs were studied, since an unusual SCE response to BrdU led us to suspect the significance of BrdU incorporation into DNA and dT pool disturbances. Addition of deoxycytidine (dC), dT or both dC and dT causes an increase of SCEs. On the other hand, deoxyadenosine (dA) and deoxyguanosine (dG) did not have significant effects on SCEs in SB-T1 cells. These results suggest that disturbances of pyrimidine-nucleotide synthesis, including gross imbalance of nucleotide pools, play a pivotal role in the high SCE induction of SB-T1 cells by BrdU.

Time of replication of yeast centromeres and telomeres

The time of replication of centromeres and telomeres of the yeast S. cerevisiae was determined by performing Meselson-Stahl experiments with synchronized cells. The nine centromeres examined become hybrid in density early in S phase, eliminating the possibility that a delay in the replication of centromeres until mitosis is responsible for sister chromatid adherence and proper chromosome segregation at anaphase. The conserved sequence element Y', present at most telomeres, replicates late in S phase, as do the unique sequences adjacent to five specific telomeres. The early and late replication times of these structural elements may be either essential for their proper function or a consequence of some architectural feature of the chromosome.

A model for the spatio-temporal organization of DNA replication in mammalian cells

The spatio-temporal organization of chromosomal DNA replication was analyzed using a model based on a "DNA unit" (or decondensation unit) hypothesis. The model is an extension of the fork movement theory of Huberman & Riggs (1968) and can account for a partially deterministic and partially stochastic order of DNA replication in chromosomes. It presumes that each chromosome is composed of DNA units that are arranged in sequence and that are replicated in parallel. A deterministic wave of chromatin decondensation propagates along the DNA unit continuously and progressively providing a field for the random activation of replication origin. Assignment of replication times to DNA compartments by a Monte Carlo method was programmed based on the model and the program was used to stimulate DNA synthesis rate curves that can be measured by the method of Dolbeare et al. (1983, 1985). The shape of the curve is shown to constrain possible parameter values of the model, which include the rate of fork movement, the fraction of chromatin that is decondensed at the start of S-phase, the initial number of origins activated, the rate at which new origins are activated, etc. The chromosomal organization that controls the molecular level of DNA replication is briefly reviewed and its relevance to the model is also discussed.

Yeast centromeres

Significant progress has been made toward understanding the roles played by conserved centromere DNA sequences in both mitotic and meiotic chromosome segregation. We are just beginning to formulate a picture of what a yeast kinetochore actually looks like and what components other than CEN DNA are necessary for function. In the next few years some of the genes encoding structural components of the kinetochore, and perhaps some involved in regulation of kinetochore function, will be cloned. Work is already in progress to isolate and characterize the proteins necessary for the assembly, maintenance and function of this amazing biological structure.

Chromosomal localization of the Epstein-Barr virus (EBV) genome in Bloom's syndrome B-lymphoblastoid cell lines transformed with EBV

The localization of the Epstein-Barr virus (EBV) genome in chromosomes of human B-lymphoblastoid cell lines (LCLs) transformed with EBV, and the effect of EBV DNA on the level of sister chromatid exchange (SCE) in Bloom's syndrome (BS) B-LCLs, were examined with chromosomal in situ hybridization techniques using a 3H-EBV DNA probe. EBV DNA was detected in chromosomes 1-5 and 13-15 at specific G band regions in BS as well as in normal B-LCLs, regardless of SCE. Several chromosomal sites (1p31, 1q31, 4q22-24, 5q21, 13q21, 14q21) carrying EBV DNA seemed to be very characteristic in normal as well as in BS B-LCLs. There was no statistically significant difference in silver grain counts due to EBV DNA and their distribution in different chromosomes or groups among normal and BS B-LCLs with normal and high SCE. These findings strongly indicate that EBV infection did not introduce a correcting factor for BS SCE.

Distribution of histone variants in the sea urchin chromatin fractions obtained by selective micrococcal nuclease digestion

Chromatin fractions differing in their transcriptional activity were isolated by selective micrococcal nuclease digestion of nuclei from sea urchin embryos (Strongylocentrotus droebachiensis) at the gastrula and pluteus stage. The electrophoretic analysis of the chromatin proteins at the gastrula stage showed that a soluble, transcriptionally active fraction of chromatin was enriched with early variants of histones H1 and H2A. The early and late variants of histone H2A at the pluteus stage were distributed randomly between chromatin fractions. However, the content of both variants of histone H1 was essentially decreased in the soluble transcriptionally active fraction of chromatin.

Dosage compensation in mammals: why does a gene on the inactive X yield less product than one on the active X?

An expressed gene on the inactive mammalian X chromosome yields less product than the same gene on the active X. Characteristics of the inactive X which might be responsible for this are late replication, chromatin clumping, and altered patterns of DNA methylation. If an expressed gene on the inactive X is not replicated until late in S, it will be present in two copies for a shorter fraction of the cell cycle than its early replicating homologue and therefore yield less product. Alternatively, transcription may be slowed by a microenvironment of highly condensed chromatin or by an abnormal pattern of methylation of the DNA template. Experiments are proposed by which to test these and related hypotheses.

Differential transcription of early and late-replicating DNA in human cells

Several studies suggest that most of the functional genes of mammalian cells are contained in the early-replicating sequences of DNA. There is little direct evidence, however, to support this view. To determine whether the extent of transcription of different DNA sequences is related to the order in which these sequences are replicated, I have selectively labelled early- and late-replicating DNA with [3H]thymidine ([3H[TdR) and determined the extent to which the 3H-labelled non-repeated sequences of each DNA preparation are hybridized by increasing concentrations of RNA from exponentially growing KB cells. The fraction of DNA hybridized at infinite RNA concentration was then estimated by a plotting method which linearizes the data. Selective labelling of DNA was achieved by synchronizing a culture of KB cells by a double block of DNA synthesis and then labelling portions of the culture 0-3 h (early) or 6-9 h (late) after release of the cells from the second block. At all RNA concentrations tested, the fraction of early-replicating DNA hybridized was significantly greater than that of late-replicating DNA. At infinite RNA concentration the value for early-replicating DNA was 3-4 times as great as that of late-replicating DNA. If it is assumed that the fraction of DNA hybridized at infinite RNA concentration is proportional to the fraction of DNA which is transcribed, it can be concluded that 3-4 times as much early-replicating DNA is transcribed as late-replicating DNA in exponentially growing KB cells.

Chromosome replication in normal and transformed human lymphocytes

We analyzed the late-replication patterns of human B-lymphocyte chromosomes before and after transformation by Epstein-Barr virus. There were no statistically significant differences between normal cells and transformed cells derived from the same male individual; therefore, the order of termination of chromosome replication was unchanged by transformation. We also examined the replication patterns of T lymphocytes from the same donor and found no differences between normal B and T cells.

The effect of added H1 histone and polylysine on DNA synthesis and cell division of cultured mammalian cells

Addition of H1 histone or polylysine (10 microgram/ml) to cultured Friend erythroleukemia cells or to two mouse lymphoma cell lines (el-4 and S-49) increased levels of cell division in these cultures. There is a stimulation of incorporation of labeled thymidine into DNA in cultures containing H1 histone and polylysine. DNA fiber autoradiographic experiments revealed that replicon size is decreased in the cells cultured with H1 histone and polylysine at later periods of culture.

Liver chromatin fractions in Mus and Akodon. The concept of constitutive heterochromatin

The liver chromatin from Mus musculus and Akodon molinae was separated in 8 fractions by differential centrifugation. Like fractions from both species showed approximately similar contents of DNA, equivalent ratios of histone to non-histone proteins, corresponding template activities and equal amounts of positive C-banded material. On the other hand, heavy chromatin fractions of Mus were highly enriched in satellite DNA whereas no satellite DNA was found in Akodon chromatin. Heavy chromatin fractions isolated by differential sedimentation have been usually homologued with the constitutive heterochromatin. The properties of the constitutive chromatin are discussed and the validity of the foregoing concept is challenged. It is proposed to define the constitutive heterochromatin as those chromatin regions comprising highly repeated DNA sequences clustered in restricted areas of chromosomes and not transcribed (satellite DNA).

Cytogenetics of the parthenogenetic grasshopper Warramaba (formerly Moraba) virgo and its bisexual relatives. VI. DNA replication patterns of the chromosomes

The distribution of late-replicating segments along the chromosomes of five clones of W. virgo is described. Some, but not all of these segments correspond to C-bands. In general, the "autoradiographic profiles" (histograms of linear grain density along the length of chromosomes labeled with tritiated thymidine in late S-phase) show strong resemblances throughout the five clones. However, some significant differences exist, and these are particularly marked in the case of the Boulder clone, which is anomalous in many other respects. --A similar study has also been carried out on the two bisexual species of Warramaba ("P169" and "P196") that gave rise, by hybridization more than half a million years ago, to the parthenogenetic W. virgo. In the case of P169, the autoradiographic profiles of the three large chromosomes (X + A, B + 5, CD) which it has contributed to the W. virgo karyotype are extremely similar to those of the corresponding chromosomes in the virgo clones we have studied. In the case of P196 there is likewise, in most instances, a close resemblance of the autoradiographic profiles of the AB, X1 and CD chromosomes to those of the same chromosomes in the virgo clones, but that of the X1 shows no particular resemblance to the anomalous profile of the X1 in the Boulder clone, in which the X1 has undergone a structural reorganisation. The autoradiographic profile of the P196 CD chromosome does, however, show a much closer resemblance to that of the corresponding chromosome in the Boulder clone than to those of the CD196 in the other four virgo clones studied. These investigations confirm the considerable evolutionary stability of DNA replication patterns.

G1 and S phase mammalian cells synthesize histones at equivalent rates

To determine the effect of cell cycle position on protein synthesis, synchronized cell populations were metabolically labeled and the synthesis of the basic proteins, including histones, was examined by two-dimensional gel electrophoresis. Exponentially growing S49 mouse lymphoma or Chinese hamster ovary (CHO) cells were separated into G1 and S phase populations by centrifugal elutriation, selective mitotic detachment, fluorescence-activated cell sorting, or a combination of these, and pulse-labeled with radiolabeled amino acids. The histone proteins, both free and chromatin-bound, were completely resolved from some 300 other basic polypeptides in whole-cell lysates by a modification of the NEPHGE technique of O'Farrell, Goodman and O'Farrell (1977). Comparisons of matched autoradiograms from samples of G1 and S phase labeled cells revealed an equivalent rate of histone synthesis through the cell cycle of both S49 and CHO cells. Nuclei isolated from G1 phase S49 cells that were pulse-labeled containing between 13 and 15% of the newly synthesized nucleosomal histones present in S phase nuclei. Nuclei prepared from G1 phase cells that were pulse-labeled and then chased for 5 hr contained more than 90% of the labeled nucleosomal histones present in whole-cell lysates. It therefore seems likely tha differential alterations in the rate of histone synthesis do not occur to a significant degree as cells proceed through the cycle, but the association of newly synthesized histones with DNA takes place after the onset of DNA replication.

Cell cycle phase specificity of hydroxyurea and its effects on the cell kinetics in embryonic spinal cord

Pregnant mice (NMRI) received an intravenous injection of 500 mg/kg of hydroxyurea (HU) on day 10 of gestation. The animals were killed 1, 1.5, 2, 2.5, 4, 6, 8 or 10 h after application of the drug and the embryos removed. Forty-five min. prior to sacrifice, the animals were given an intraperitoneal injection of 5 microCi/g 3H-thymidine (specific activity 5 Ci/mmol). From autoradiographs of the embryos the percentage of mitoses, necroses and labelled cells of the spinal cord was determined at the various time intervals. It was observed that HU inhibited DNA-synthesis in all S-phase cells for more than 4 hours. All cells damaged by HU did not start DNA-synthesis again and became necrotic. Only cells entering the S-phase after the influence of HU could replicate their DNA in a normal manner. It is assumed that HU possibly blocks cells in the G2-phase and in the mitosis for a certain period of time.

Perturbation of growth and differentiation of Friend murine erythroleukemia cells by 5-bromodeoxyuridine incorporation in early S phase

Cultured Friend murine erythroleukemia cells (Friend cells) are induced to undergo erythroid differentiation when grown in the presence of dimethylsulfoxide (DMSO) and other compounds. The effects of unifilar substitution of bromouracil (BU) for thymidine in the DNA (BU-DNA) of Friend cells were examined. Cells were grown in the presence of 5-bromodeoxy-uridine (BrdU) for one generation, then centrifuged and resuspended in medium containing DMSO without BrdU. These cells exhibited a delay in the appearance of heme-producing, benzidine-reative (B+) cells and a decreased rate of cell proliferation in comparison to the control not containing BU-DNA. A transient inhibition of entry into S phase was observed when control cells or cells containing BU-DNA were grown in the presence of DMSO) for 10 to 20 hours. This transient inhibition was increased in the BrdU culture. Thus BU-substitution in Friend cells alters other cellular functions in addition to erythroid differentiation. The rate of increase in the percent of cells committed to differentiate (those forming B+ colonies in plasma clots) was similar in the BrdU and control cultures until 40 to 50 hours. After this time, a delay in the appearance of committed cells was observed in the BrdU culture. The effect of BrdU on the appearance of B+ cells was more pronounced and occurred earlier than its effect on the rate of commitment. Therefore, the delay in the appearance of B+ cells in the BrdU culture was due primarily to perturbation of post-commitment events such as the accumulation of hemoglobin. We also examined the effect on growth and differentiation after BrdU was incorporated during different intervals of S phase in cells synchronized by centrifugal elutriation or by double thymidine block and hydroxyurea treatment. The delay in the appearance of B+ cells and inhibition of cell proliferation were only observed when BrdU was incorporated in the first half of S phase. BrdU (10 muM) had no effect on growth or differentiation when present during late S or G1 and G2. These results, using two very different methods to achieve cell synchrony, indicate that the effects of BrdU on growth and differentiation described above are due to its incorporation into DNA sequences replicating during early S.

Ordered replication of DNA sequences: synthesis of mouse satellite and adjacent main band sequences

The replication of mouse satellite DNA was delayed when synchronized 3T3 cells were exposed to low concentrations of hydroxyurea during S phase, It appears that the onset of satellite replication is not a time dependent event, but instead requires that a certain amount of main band DNA be synthesized first. Using hydroxyapatite chromatography and S1 nuclease digestion, a procedure was developed to quantitate the synthesis of both satellite and neighboring main band sequences. The replication kinetics of satellite determined by this method agree with previous estimates. Main band sequences adjacent to satellite appear to replicate in concert with satellite DNA. The results are discussed and related to the limitations of the techniques utilized.

Replication of the genes coding for 5 S RNA in synchronized HeLa cells

The duplication of the genes coding for 5-S RNA has been followed during the S phase of HeLa cell cultures synchronized by mitotic detechment. Replication was analyzed by exposing the cultures to bromodeoxyuridine (BrdUrd) for 3 h at successive intervals and analyzing the DNA product in CsCl density gradients. DNA containing the 5-S genes was detected in the gradients by molecular hybridization using purified 125I-labeled 5 S RNA. In addition, as an internal marker of S phase chronology, late DNA replication was followed by examining the incorporation of [3H]thymidine into repetitive DNA (C0t less than 0.01 fraction). The results indicated that greater than 85% of the cells in the culture were synchronously dividing and that greater than 80% of the 5-S genes replicate in the first half of S phase, whereas the rapidly reassociating fraction of the DNA replicates in the second half of S phase.

Investigation of the effect of hydroxyurea on the cell cycle and the development of necrosis in the embryonic CNS of mice

On day 10 of gestation pregnant mice (strain NMRI) were given an intravenous injection of 500 mg/kg Hydroxyurea (HU). Simultaneously either 5 muCi/g or 10 muCi/g 3H-thymidine (3H-Tdr; specific activity 5Ci/mmol) was administered to the animals. At various times after treatment embryos and electron microscopy. Two hours after administration of HU condensations of the chromatin structure could be detected electron microscopically in some cells. Thirty minutes later the nucleolus became smaller and denser, the cytoplasm shrank, and the cell organelles moved closer together. Three hours after application of the drug break-down of the involved cells set in. As in the autoradiograms about 98% of the counted necroses were labelled, and since labelled thymidine is almost exclusively incorporated during the S-phase, it can be stated the HU influences only metabolic processes which take place during the S-phase. From the morphological findings it can be concluded that in the case of the S-phase-specific metabolic pathway, which is influenced by HU, we are primarily dealing with DNA synthesis.