Sister chromatid exchanges in the active and inactive X chromosomes

Lymphocyte cultures from female cattle in two geographic locations were used to determine the frequency of occurrence of sister chromatid exchanges on the active and inactive X chromosomes. The distribution of 129 exchanges among the two X chromosomes was not significantly different from a 11 ratio.     

Sister chromatid exchanges induced in human lymphocyte chromosomes by trifluralin, atrazine and simazine

Many epidemiological and experimental "in vivo" studies have proved in recent years the carcinogenic properties of herbicides. In order to evaluate the "in vitro" action on the human DNA of Trifluralin, Atrazine and Simazine (active principles of herbicides Treflan and Fogard S respectively) the authors have studied the rates of SCE in cultures of human lymphocytes exposed to different concentrations of a solution 1 ppm of the substances. Trifluralin and Simazine, but not Atrazine, increase SCE per cell, with statistical significance, in the cultures with the highest concentrations of these substances. (SCE per cell: Trifluralin 5.27 +/- 1.38, Simazine 5.09 +/- 1.19, Control 3.51 +/- 1.14).     

Sister chromatid exchanges and lack of isolabeling in Chinese hamster chromosomes

Chinese hamster CHO and Don cells were synchronized in mitosis with or without a Colcemid treatment, labeled with ³H-TdR through an entire cell cycle, and were synchronized again each time they entered mitosis. Over four cycles without label, only heterolabeling patterns were observed, and the frequency of sister chromatid exchanges (SCE's) per chromosome per cycle (0.23 or 0.02/μm) was constant. After the labeling cycle, during which the total SCE frequency was also 0.23, a disproportionately high SCE frequency at the centromeres, about 0.15, decreased dramatically to about 0.03, while the SCE frequency in the arms increased from about 0.08 to 0.20. In accord with reports in the literature, the constant SCE frequency of 0.23 appeared to be the saturation frequency resulting from the ³H disintegrations. It was postulated that during the labeling cycle, the limited number of SCE's occurred preferentially at the centromere because ³H disintegrations occurring in the daughter strand of DNA produced alterations which were preferentially repaired by a recombinational SCE mechanism. Implicit in this hypothesis is the assumption that SCE's result from the transmutation or recoil effect of the ³H disintegration and that there is a unique association between the kinetochore and the daughter strand of DNA.     

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.     

Chromatin structure of active and inactive human X chromosomes

Nuclei from a variety of human cell lines and tissues were digested with gradually increasing levels of DNase I. The DNA was then purified, treated with restriction enzymes and subjected to Southern blot hybridization using a cloned cDNA probe to 3-phosphoglycerate kinase (PGK) a housekeeping enzyme. At relatively high levels of DNase I, a specific, slightly sensitive site in chromatin sequences encoding PGK was observed in all of the cell types examined. This slightly sensitive site resides on the active X-chromosome since cell lines with increased numbers of inactive X-chromosomes do not show an increase in the region of chromatin which is sensitive. Except for this restricted region of enhanced sensitivity on the active X-chromosome, the data suggest that, for PGK encoding sequences, chromatin configurations on the active and inactive X-chromosomes are similar.     

Sister chromatid exchanges and heterochromatin

Summary The inter- and intrachromosomal distribution patterns of SCEs obtained with or without mutagen treatment are reviewed and compared, with each other as to their relation to heterochromatin and with the distribution patterns of chromatid aberrations that occurred either spontaneously in chromosomes of repair-defective human syndromes or after treatment with the mutagens (BrdU, ethylalcohol, DMBA, TMBA, maleic hydrazide, MMS, MMC). The conclusions are: No general rule is detectable for nonrandom involvement of heterochromatin in spontaneous SCEs. Mutagen-induced SCEs show the same or very similar distribution patterns as the spontaneous ones and are in no case as preferentially located as chromatid aberrations (which involve mainly the junctions between eu- and heterochromatin or other special regions). Therefore, a specific mutagen sensitivity of heterochromatincontaining chromosome regions as observed for chromatid aberrations does not exist (or is less pronounced) for SCEs. This supports the inference that different mechanisms underlie the origins of the two phenomena.     

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 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.     

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 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

The relationship between chromosomal aberrations and sister chromatid exchanges (SCE's) after treatment of Vicia faba root tips with thiotepa, caffeine and 8-ethoxycaffeine (EOC) was studied by using a modified fluorescent plus Giemsa (EPG) technique. At concentrations which had little effect on the frequency of chromosomal aberrations, thiotepa strongly increased the frequency of SCE's, provided that the chromosomes were allowed to replicate between treatment and fixation. Frequently, the size of the exchanged material was smaller than the diameter of the chromatid. Post-treatments with caffeine of roots previously exposed to thiotepa strongly increased the frequency of aberrations, but had little effect on the frequency of SCE's. In contrast to thiotepa, EOC caused only a slight increase in the frequency of SCE's even at concentrations which produced a high frequency of chromosomal aberrations. Thus, there was not a close correlation between SCE's and chromosomal aberrations. Single-strand exchanges between the DNA double helices in sister chromatids were not detected.     

Sister chromatid exchanges and chromatid interchanges in bloom's syndrome.

A comparison is made between the incidences of sister chromatid exchanges (SCE) per chromosome and group of chromosomes and breakage, visible at metaphase like open gaps, breaks, and breaks involved in chromatid interchange formation (CI) in Bloom's syndrome. It can be shown that the two levels of breakage SCE and CI are not correlated as to the locations. The discussion deals with possible interpretations of preferential breakage and reunion at certain homologous chromosomes and the difficulties today to understand SCEs.     

Sister chromatid exchanges in Down syndromes and normal human beings.

The BrdU-Giemsa method was used to analyze the frequency of SCEs in a group of five Down syndromes and in a group of five normal human beings. In total 25 second mitoses were scored for SCEs in each individual. Although Down syndromes exhibited a tendency to have higher rates of exchange than normal human beings the analysis of variance showed that these differences were not statistically significant. On the other hand, significant differences were observed in the rates of SCEs between individuals within each group. This variability may reflect inter-individual differences in the efficiency of the mechanisms involved in the production of exchanges. The frequency of SCEs in blood cultures in probably the average of the rates exhibited by two or more lymphocyte sub-populations with different sensitivities to BrdU. Hence, the variability in the rate of exchange between different cultures of the same individual probably arises by changes in the percentage of cells in the second mitosis deriving from each lymphocyte sub-population.     

Sister chromatid exchanges in human leukocyte chromosomes: Spontaneous and induced frequencies in early- and late-proliferating cells in vitro

Summary Human leukocyte cultures were pulse-treated with the trifunctional alkylating mutagen trenimon in a final concentration of 10^-7 M for 15–20 h after culture start, i.e., in the G₁ phase of the cell cycle. At 24 h after culture start bromodeoxyuridine (BUdR) was added to the trenimon-treated cultures and to several untreated cultures running in parallel. The series treated with BUdR only and the series treated with BUdR+trenimon were each used to prepare two cultures at different culture times. Mitoses were collected during consecutive intervals of 12 h from 30 h up to 102 h after culture initiation by colcemid. For all preparation times (42 h, 54 h, 66 h, 78 h, 90 h, and 102 h) the frequencies of first, second, and third and further mitoses were determined in the BUdR- and in the BUdR+trenimon-treated series. In the trenimon-treated series a clear cell cycle delay was detected as compared with the normal distribution of different types of mitoses found in series treated with BUdR only. Spontaneous and trenimon-induced sister chromatid exchange (SCE) frequencies were determined in second mitoses occurring at 66 h, 78 h, 90 h, and 102 h after culture start. For all these preparation times about six SCE per metaphase were consistently found in BUdR-treated, and about 19 SCE per metaphase in BUdR+trenimon-treated series, indicating a homogeneous sensitivity of early- and late-proliferating cells with respect to the induction of SCE.This paper is dedicated to Prof. B. Erich Wolf on the occasion of his 70th birthday     

Sister chromatid exchanges induced by sarcolysine in human lymphocytes in vitro

The effect of three concentrations of sarcolysine (0.5 micrograms/ml, 1 microgram/ml and 2 micrograms/ml) on the sister chromatid exchanges (SCE) was investigated in human lymphocytes in vitro. A dose related increase in SCEs frequencies was observed after sarcolysine administration and also a delayed development of cell cycle has been induced by the two last concentrations. The variation range of SCEs per cell was dose-dependent and it was considered to represent the acquired genetic instability induced by the drug.     

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.