Sister chromatid exchanges in isolabeling

Isolabeling observed by autoradiography in sister chromatids at the second or later metaphases after incorporation of ³H-thymidine has sometimes been ascribed to an exchange between the multiple DNA duplexes in polynemic sister chromatids. An analysis reported here on the frequency and size of isolabeled regions in chromosomes of the rat kangaroo shows that all isolabeling can be accounted for by sister chromatid exchanges coupled with the image spread that can occur in tritium autoradiographs. Hence, in this case it becomes unnecessary to postulate binemy or polynemy to explain isolabeling.     

Sister chromatid exchanges in barley

Summary A mean frequency of 20.6 sister chromatid exchanges (SCEs) per cell has been observed in a reconstructed karyotype of Hordeum vulgare by application of the FPG technique after unifilar incorporation of BrdU into chromosomes. The involvement in SCEs of the 48 segments into which the chromosome set had been subdivided was, with a single deviation, length proportional and independent of the segment's heterochromatin content. Asymmetric bands, indicative of an uneven distribution of adenine and thymidine between the DNA strands in adenine (A)-thymidine (T) rich chromosome regions, could not be detected after incubation of the cells in BrdU for one cycle of DNA replication.     

Sister chromatid exchanges in the cells of higher eukaryotes

Experimental data on the influence of culture conditions on the frequency of sister chromatid exchange (SCE) are reviewed. Evidence is provided that spontaneous SCEs exist. Various methods for the identification of spontaneous SCEs are discussed.     

Sister chromatid exchanges in Allium cepa

Differential staining of sister chromatids with Giemsa after BrdU incorporation into DNA was performed in Allium cepa L. chromosomes. A treatment solution containing 10^–7 M FdU, 10^–4 M BrdU and 10^–6 M Urd was found to ensure BrdU incorporation without affecting cell cycle duration. After several procedures before staining the slides with Giemsa had been tested, treatment with the fluorochrome compound 33258 Hoechst, exposure to UV light and heating at 55° C in 0.5×SSC, were found to be essential for good differentiation. The distribution of SCEs per chromosome agrees with the expected Poisson distribution. The mean value of SCEs per chromosome occurring when cells were exposed to the treatment solution for two consecutive rounds of replication (=5.5) was double the mean value observed when cells were exposed to the same treatment for only one round of replication (=2.8). SCEs were found to occur more frequently in those chromosome regions corresponding neither to C-bands nor to late replicating DNA-rich regions. Finally, the occurrence of SCEs involving less than the width of a chromatid is discussed.     

Sister chromatid exchanges in Vicia faba

Evolutionary loss of the Y chromosome has occurred in Climacia areolaris (Hagen) of the neuropteran family Sisyridae. The diploid set comprises 6 pairs of autosomes, plus 2 X chromosomes in the female and 1 X in the male. The Y is retained in Sisyra vicaria (Walker) of the same family: its chromosome number is 14 in both sexes including 2X chromosomes in the female and 1X plus Y in the male. Two alternative pathways for the segregation of the sex chromosomes-distance segregation and sex bivalent formation-co-exist in the latter species in a ratio of approximately 1 to 6; the possible phylogenetic significance of this feature is discussed.     

Sister chromatid exchanges in Tradescantia paludosa

A modified fluorescence-plus-Giemsa technique is described that allows differential staining of sister chromatids in root tip cells from cuttings of Tradescantia paludosa. With this staining technique, chromatids with both DNA strands unsubstituted are differentiated from chromatids containing 5-bromouracil in place of thymine in one of the strands of the DNA duplex. The baseline level of sister chromatid exchanges was shown to be dependent on the concentration of 5-bromodeoxyuridine in the treatment solution, the mean frequency being 43.5 sister chromatid exchanges per cell for the experimental protocol suggested.     

Sister chromatid exchanges in protein-energy malnutrition

Summary The frequency of sister chromatid exchanges in children suffering from severe protein-energy malnutrition was investigated by the fluorescent-plus-Giemsa method. Children suffering from kwashiorkor had significantly higher mean SCEs per circulating lymphocyte than did normal children. A small but statistically significant decrease in these levels was observed following nutritional rehabilitation.     

Fenvalerate-induced chromosome aberrations and sister chromatid exchanges in the bone marrow cells of mice in vivo

To investigate whether subjects with low-acid states are exposed to increased genetic risk with respect to controls, we evaluated mutagenicity and presence of clastogenic factors (CF) in the gastric juice of chronic atrophic gastritis and omeprazole-treated patients. Mutagenic gastric juice was found in 8/15 (53%) chronic atrophic gastritis patients, 8/11 (73%) omeprazole-treated patients, and 2/13 (15%) healthy control subjects. The mean mutagenicity ratio of omeprazole-treated patients (1.52+/-0.48/0.1 ml gastric juice) was significantly higher than those of either controls (1.07+/-0.15; P<0.01) or chronic atrophic gastritis patients (1.16+/-0.21; P<0.05). Only chronic atrophic gastritis patients showed an increased clastogenic index with respect to healthy controls (2.67+/-2.13 versus 0.38+/-0.51; P<0.001). These findings expand our knowledge of gastric disease risk factors, and indicate that there may well be a risk of mucosal DNA damage arising from the presence of mutagenic and CF in the gastric juice.     

Fenvalerate-induced chromosome aberrations and sister chromatid exchanges in the bone marrow cells of mice in vivo

Fenvalerate, a synthetic pyrethroid insecticide, is commonly used in agriculture and other domestic applications due to its high insecticidal activity and low mammalian-, avian- and phyto-toxicities. However, the genotoxic effect of fenvalerate is highly equivocal. In the present study the genotoxic effects of fenvalerate was evaluated using structural chromosome aberration (CA) and sister chromatid exchange (SCE) assays in mice. Out of the three doses (5, 10 and 20 mg/kg) tested, statistically significant increase in CA was found following intra peritoneal (i.p.) treatment of 20 mg/kg of fenvalerate for 24 h (P<0.01) and 48 h (P<0.05) only. Neither the acute doses of 5 and 10 mg/kg, nor the sub-acute dose (5×4 mg/kg) of fenvalerate could induce any significant effect. All the three acute doses induced significant increase in the frequency of SCEs (P<0.01) in the bone marrow cells, which showed a significant dose-response correlation (r=0.9541, P<0.05). With certain reservations to possible impurities, from the present findings technical grade fenvalerate may be considered as a weak clastogen and a potent inducer of SCEs in mice.     

Sister chromatid exchanges induced by cyclophosphamide in V79 cells cultured in diffusion chambers in mice

Chinese hamster cells V79 were cultured in diffusion chambers (DC) and implanted into mice. An exponential growth was observed from the 2nd to 4th day after implantation. The maximum growth was reached on the 6th day. After that, cell growth and viable cell counts decreased. Three days after implantation of DC with V79 cells, the hosts received 6 hourly injections of 0.2 ml of 5-bromodeoxyuridine (BUdR) solution at concentrations of 0.125 to 1.0 x 10(-2) M. DC were removed for chromosome and sister-chromatid exchanges (SCE) analyses 24 h after the first BUdR injection. The frequency of metaphases with differentially stained chromatids, with aberrations, and the number of SCE per cell increased with BUdR dose. The frequency of metaphases with differentially stained chromatids was also positively correlated with the duration of BUdR exposure or the number of hourly injections of BUdR-solution. The effects of cyclophosphamide (CY) in V79 cells in DC in mice were studied. Injections of CY at 2.5, 5, 10 and 15 microgram per gram of body weight to the hosts caused an increase in the number of SCE per cell in a linear manner. The results from this study indicate that V79 cells cultured in DC in mice may provide a potential test system for mutagenicity.     

Sister chromatid exchanges and inhibition of DNA synthesis in irradiated human cells

X-ray irradiation inhibits DNA synthesis and enhances the frequency of sister chromatid exchanges (SCE) in normal human lymphocytes. On the contrary, cells from patients with Down's syndrome, Xeroderma pigmentosum (form II) and progeria, characterized by radioresistant DNA synthesis, do not show such an increase in SCE frequency. We suggest that radiation-induced SCE frequency is a result of inhibition of DNA replication, rather than a direct damage of chromosomes by ionizing radiation. It is in agreement with Painter's /13/ hypothesis according to which SCE are formed due to asynchronous completion of replication in contiguous replicon clusters. So, probability of SCE formation is the more the lower is rate of replication. Thus, the extent of radiation damage cannot be measured directly by the SCE frequency.     

Sister chromatid exchanges in betel and tobacco chewers

The incidence of sister-chromatid exchange (SCE) was investigated in the lymphocyte chromosomes of betel and tobacco chewers. Betel chewers and betel-with-tobacco chewers showed higher yields of SCE than normal controls. Higher frequencies of SCE were also observed in individuals who chewed more than 10 betel leaves, or betel leaves-with-tobacco, per day, compared to people who chewed less than 10 betel leaves, or betel leaves-with-tobacco, per day, respectively. Subjects who had chewed betel leaves and betel leaves + tobacco for more than 10 years showed an elevated frequency of SCE.     

Sister chromatid exchanges occur in G2-irradiated cells

DNA double-strand breaks (DSBs) are arguably the most important lesions induced by ionizing radiation (IR) since unrepaired or mis-repaired DSBs can lead to chromosomal aberrations and cell death. The two major pathways to repair IR-induced DSBs are non-homologous end-joining (NHEJ) and homologous recombination (HR). Perhaps surprisingly, NHEJ represents the predominant pathway in the G1 and G2 phases of the cell cycle, but HR also contributes and repairs a subset of IR-induced DSBs in G2. Following S-phase-dependent genotoxins, HR events give rise to sister chromatid exchanges (SCEs), which can be detected cytogenetically in mitosis. Here, we describe that HR occurring in G2-irradiated cells also generates SCEs in ~50% of HR events. Since HR of IR-induced DSBs in G2 is a slow process, SCE formation in G2-irradiated cells requires several hours. During this time, irradiated S-phase cells can also reach mitosis, which has contributed to the widely held belief that SCEs form only during S phase. We describe procedures to measure SCEs exclusively in G2-irradiated cells and provide evidence that following IR cells do not need to progress through S phase in order to form SCEs.     

Sister chromatid exchanges in lymphocytes of nuclear medicine physicians

OBJECTIVE: The aim of this study was to assess whether occupational exposure to chronic, low doses of Iodine 131 (I-131) and Technetium 99m (Tc-99m) may lead to genotoxicity. Medical personnel occupied in nuclear medicine departments are occupationally exposed to low doses of I-131 and Tc-99m. The determination of the frequency of sister chromatid exchanges (SCEs) and of cells with a high frequency of SCEs (HFC) is considered to be a sensitive indicator for detecting genotoxic potential of mutagenic and carcinogenic agents. Therefore, we examined peripheral lymphocytes from nuclear medicine physicians for the presence of both SCE and HFC. METHODS: Sixteen exposed nuclear medicine physicians (non-smokers) were compared to 16 physicians (non-smokers) who had not been exposed to chemical or physical mutagens in their usual working environment at the same hospital. RESULTS: A statistically significant difference was found between SCE frequencies and HFC percentages measured in lymphocytes from the exposed and control groups. CONCLUSIONS: The present observation on the effect of chronic low doses of I-131 and Tc-99m indicates the possibility of genotoxic implications of this type of occupational exposure. Hence, the personnel who work in nuclear medicine departments should carefully apply the radiation protection procedures and should minimize, as low as possible, radiation exposure to avoid possible genotoxic effects.     

Sister chromatid exchanges in garlic (Allium sativum L.) callus cells

A technique is described for differential staining of sister chromatids and the study of sister chromatid exchanges (SCEs) in garlic (Allium sativum L.) callus cells. BrdU incorporation into newly synthesized DNA was ensured by culturing calli on medium containing 100 M BrdU+0.01 M FudR+1 M Urd. SCEs were visualized by FPG staining technique and their frequency was analysed. Mean frequency of SCEs in callus cells was higher than that in meristem root-tip cells. Using the same staining method, cell cycle time of callus cells was analysed. It was found that it ranges from 48 to 132 hrs. The method described represents a new approach in the study of genetic instability of plant cells cultured in vitro.Abbreviations BrdU 5-bromo-2-deoxyuridine - 2,4-D 2,4-dichlorophenoxyacetic acid - FPG fluorescent-plus-Giemsa - FudR 5-fluoro-2-deoxyuridine - SCE sister chromatid exchange - SSC 0.15 M NaCl + 0.015 M Na-citrate - T thymidine-containing strand of the DNA duplex - B 5-bromo-2-deoxyuridine-containing strand of the DNA duplex - Urd uridine     

Sister chromatid exchanges in cultured peripheral lymphocytes from twins

Summary Sister chromatid exchange points (SCE points) on individual chromosomes were studied in cultured lymphocytes from 11 monozygotic (MZ) and nine dizygotic (DZ) same-sexed pairs by means of sequential Q-banding and BUdR-Giemsa techniques. No statistically significant variation between unrelated individuals with respect to SCE points on specific chromosomes was found. Intrapair differences in the number of SCE points on specific chromosomes were not significantly smaller between MZ twin partners as compared with DZ partners. The results suggest that genetic factors do not play any major role in the frequency and distribution of SCE in normal subjects.     

Effect of pretreatment with cysteamine on gamma-radiation-induced sister chromatid exchanges in mouse bone marrow cells in vivo

The effect of pretreatment with cysteamine on gamma-radiation-induced sister chromatid exchanges (SCEs) and on the mitotic index and average generation time was determined. Groups of mice were treated in one of the following regimens: (1) irradiated, (2) treated with cysteamine and irradiated, (3) treated with cysteamine only, or (4) left untreated. Intraperitoneal administration of cysteamine preceding gamma-radiation exposure protected against SCE induction. However, radioprotection was not reflected by change in the mitotic index or in the average generation time. The results suggest that, under the experimental conditions of this study, the SCEs are caused by free radicals produced by gamma radiation, but not the additional damage indices measured.     

Sister chromatid exchanges in Drosophila melanogaster cell lines in vitro

The frequency of sister chromatid exchanges (SCEs) in two cell lines of Drosophila melanogaster with different karyotypes (XX and XY) was determined, considering (1) the distribution of SCEs within each chromosome, with reference to eu- and heterochromatin and (2) the distribution of SCEs in different chromosomes. A comparison was made between chromosome pairs within each karyotype and between the two different karyotypes. The following results were obtained. The SCEs are not randomly distributed along chromosomes, since exchanges were never observed in heterochromatin. SCEs are more frequent in XY than in XX cells; moreover, in both cell types there exists a significantly higher frequency of SCEs in the X chromosome than in the autosomes. These findings are discussed in relation to chromosome aberrations and mitotic recombination.