Changes in the level of sister chromatid exchanges in cultured Chinese hamster cells undergoing limited proliferation

"Stationary phase ageing" of cultured Chinese hamster cells (when proliferation rate decreases and in the stationary growth phase) produces an increase in the frequency of spontaneous sister chromatid exchanges (SCE). Thiophosphamide-induced (24 h) frequency of SCE increases from 2-day to 5-day "age" and later (in the stationary phase) is practically the same. The "stationary ageing" cultured cells are suggested to be used as a model system for studying molecular-genetic age changes.     

Aphidicolin-Resistant Mutants of Mouse Lymphoma L5178y Cells with a High Incidence of Spontaneous Sister Chromatid Exchanges

Two aphidicolin-resistant cell mutants (AC 12 and AC 41) with a fourfold increase in spontaneous frequency of sister chromatid exchanges (SCEs) were obtained out of over 400 aphidicolin-resistant mutants isolated from mouse lymphoma L5178Y cells. They also exhibited three- to fourfold increases in spontaneous frequency of chromosome aberrations (CAs). To determine whether the high level of SCE frequency in AC 12 is caused by 5-bromodeoxyuridine (BrdUrd) used for visualizing SCEs, the effect of BrdUrd incorporated into DNA on SCE induction was analyzed. The SCE frequencies in AC 12 remained constant at BrdUrd incorporation levels corresponding to 2-90% substitution for thymidine in DNA. In addition, the small amount of BrdUrd incorporated into both daughter and parenteral DNA strands in AC 12 had minimal effect on SCE induction. Furthermore, AC 12 and AC 41 were slightly resistant to BrdUrd with respect to the induction of CAs, the inhibition of cell-cycle progression and the decrease in mitotic activity. These findings suggest that the high incidence of SCEs in AC 12 and AC 41 is formed by their intrinsic defects, not by the effects of BrdUrd used. The analysis of SCE frequencies in hybrid cells between these mutants and the parental L5178Y revealed that the genetic defects in AC 12 and AC 41 appear to be recessive, and that these two mutants belong to the same complementation group. Furthermore, AC 12 belonged to a different complementation group from ES 4, which was isolated previously from L5178Y as an SCE mutant with a twofold higher frequency of spontaneous SCEs. This finding indicates that at least two different genetic defects participate in the formation of the high incidence of spontaneous SCEs in mouse cells. These SCE mutants would provide valuable cell materials for studying the molecular mechanism of SCE formation.     

Effects of mitomycin C on sister chromatid exchange in normal and Bloom's syndrome cells

Bloom's syndrome lymphocytes, which are characterized by a high incidence of sister chromatid exchanges (SCE: 80.6 per cell), were treated with mitomycin C (MMC) and the effect of the chemical on SCE frequency compared with that in normal cells. Raising the concentration of MMC from 1 X 10(-9) to 1 X 10(-7) g/ml led to about 10-fold increase (61.7 SCE per cell) in the SCE frequency over the base line in normal lymphocytes (6.4 SCE per cell), though chromosome aberrations remained at a relatively low frequency. MMC caused about a two-fold rise in SCE in cells of Bloom's syndrome (128.8 SCE at 10(-9) g/ml; 139.3 SCE at 10(-8) g/ml). The frequency of chromosome aberrations in Bloom's syndrome cells at concentrations of MMC of 1 X 10(-9) and 1 X 10(-8) g/ml was 0.350 and 0.825 per cell, respectively, and low when compared to the increased number of SCE. The increased frequency of SCE in normal and Bloom's syndrome cells is in contrast to the reported findings with cells from Fanconi's anemia and xeroderma pigmentosum. The distribution of SCE in MMC-treated normal cell correlates with that of spontaneous SCE in cells of Bloom's syndrome.     

In vivo sister chromatid exchange in cells of various organs of the mouse

In vivo sister chromatid exchange (SCE) in mouse cells derived from various organs was studied by infusing BrdU from the tail vein. It was found that at BrdU concentrations ranging from 2.2–13.5 g/g/h, the SCE frequency in bone marrow cells seemed to stay at a constant level (1.5–2/cell/two cell cycles) whereas it started to rise as the BrdU dose exceeded this dose range. When BrdU within this dose range was infused continuously from the tail vein for appropriate hours to label chromosomes in various organs, the average SCE frequencies per cell were found to be 1.64 in bone marrow cells, 1.82 in spermatogonia, 1.99 in splenic cells, 2.89 in intestinal cells and 3.69 in cells from adjuvant stimulated lymph nodes. It is suggested that the spontaneous level of the in vivo SCE frequency might be about 1.5–2/cell/two cell cycles in the mouse. In cells derived from intestine and adjuvant stimulated lymph node, some unknown factors might work as a inducer of SCEs resulting in a significant increase in the SCE frequency in these organs.     

Ultrastructure of Saccharomyces cerevisiae strain AG1-7 and its responses to changes in environment

Asynchronous populations of the budding yeast Saccharomyces cerevisiae strain AG1-7 were examined by freeze-fracture electron microscopy for ultrastructural changes occurring in response to changes in the environment, specifically the following: temperature (23 or 37 degrees C); cell density (exponential, early stationary, and stationary phases); various periods of nitrogen starvation at low cell density, and return of nitrogen-starved cells to nitrogen-replete medium. This information has been gathered in preparation for ultrastructural examination of comparable responses of temperature-sensitive cell-cycle mutants. The plasma membrane was found to be particularly responsive to changes in environment. A high proportion (75%) of cells in exponential phase populations at 37 degrees C displayed paracrystalline arrays of plasma membrane particles, whereas this proportion was much lower (20%) at 23 degrees C in the same medium; plasma membrane grooves were longer at 37 than at 23 degrees C. In budded cells, the mother cell displayed paracrystalline arrays more frequently than the bud. Entry of cells into stationary phase, either through permitting population growth or by limiting nitrogen supply, resulted in increases in numbers of paracrystalline arrays and grooves. Groove depth also increased. The paracrystalline-array and groove-density responses were independent, both during entry into stationary phase and during the subsequent lag phase. Unusual groove forms appeared during stationary phase in high cell density populations, but not in low cell density nitrogen-starved populations. "Aggregate" and "geometric" tonoplast forms, previously described in strain A364A when grown under some of the conditions used here, were not found in AG1-7 under any of the conditions used here. It was demonstrated that particle-free patches can arise rapidly on the tonoplast of AG1-7 in response to temperature change from 37 to 23 degrees C. During stationary phase, spherosomes (lipid droplets) increased in size, particularly in response to nitrogen depletion. After 72 h of nitrogen starvation, about 10% of cell volume consisted of spherosomes. Changes in vacuolar content and mitochondrial form were also noted during entry into stationary phase.     

Unequal SCE is a rare event in homologous recombination between duplicated neo gene fragments in CHO cells

The frequency of sister-chromatid exchange (SCE) was studied in Chinese hamster ovary (CHO) cell lines with stable insertions of the vector pIII-14gpt which contains 2 truncated neomycin resistance (neo) gene fragments. Recombination between regions of homology in the 2 fragments can restore a functional neo gene and make the cell resistant to the antibiotic G418, a neomycin analogue. Unequal SCE would be one of several possible mechanisms for this event. The observed spontaneous rate of formation of G418-resistant subclones was approximately 6.4 x 10(-6) per cell per generation, as compared to the estimated spontaneous frequency of 3 SCE per cell per generation. Given this SCE frequency, the probability of an SCE occurring in a target site of about 1600 bp (the distance separating the homologous regions in the neo fragments) would be about 8 x 10(-7) per cell per generation, or approximately one tenth of the estimated rate of recombination. Treatment of the cells with methyl methanesulfonate (MMS, 50 x 10(-6) M) induced about 80-90 SCE per cell, corresponding to a probability of 2 x 10(-5) SCE per 1600-bp target per cell. In the same cell culture, MMS treatment induced 4-8 x 10(-4) recombination events per cell giving rise to G418 resistance. Cells treated with HN2 (up to 4 x 10(-6) M) showed a significant increase in SCEs, but no change in the frequency of G418-resistant revertants. These results suggest that the 2 pathways leading to SCE and recombination respectively are uncoupled, and only a small fraction of the recombination events, if any, are due to unequal SCE in this system.     

Spontaneous mutation of leu2 in Saccharomyces cerevisiae

The data obtained indicate that spontaneous mutations in Saccharomyces cerevisiae are formed during DNA replication. With no DNA replication in the lag-period, in the stationary growth phase, spontaneous mutations are not formed in cell culture during the G1 phase of cell cycle. Experimental data show the absence of primary spontaneously occurring DNA lesion accumulation in the cell G1 phase. Spontaneous mutations of yeasts are formed in the S phase of cell cycle, apparently as DNA replication errors. It is established that the frequency of spontaneous reversions of the leu2 gene in Saccharomyces cerevisiae strain NA3-24 increases when the cells are cultivated on the culture medium with different concentrations of leucine.     

SCEs are induced by replication of BrdU-substituted DNA templates, but not by incorporation of BrdU into nascent DNA

After 3 rounds of DNA replication in the presence of BrdU, third-division metaphase cells can be scored for the frequencies of SCEs that occurred during cycles 1 and 2, and also for the frequency of SCE during cycle 3. This procedure was used to resolve the issue of SCE induction by replication of BrdU-substituted DNA templates versus induction by BrdU incorporation into nascent DNA. It was observed that third-cycle SCE frequencies in CHO are dependent upon the amount of BrdU that was present during cycles 1 and 2 and are independent of the BrdU concentration during the third cycle. It is therefore BrdU serving as a template, rather than BrdU being incorporated, that initiates the SCE event. A model is proposed that produces reasonable fits to the observed data. It also predicts a true background or spontaneous SCE frequency of 3 per cell per cycle as previously reported by Heartlein et al. (Mutation Res., 107 (1983) (103-109). The predicted single twin ratio is higher than that reported by Wolff and Perry (Exp. Cell Res., 93 (1975) 23-30), and possible explanations for this discrepancy are discussed.     

Chromosomal instability in mutagen-sensitive mutants isolated from mouse lymphoma L5178Y cells. I. Five different genes participate in the formation of baseline sister-chromatid exchanges and spontaneous chromosomal aberrations

In a search for cell mutants that show an increase or a decrease in the frequency of baseline sister-chromatid exchanges (SCEs) or spontaneous chromosomal aberrations (CAs), large numbers of mutagen-sensitive clones previously isolated from mouse lymphoma L5178Y cells were analyzed. In addition to two SCE mutants (ES 4 and AC 12) previously reported, three other mutants were identified as an SCE mutant. An ethyl methanesulfonate-sensitive mutant ES 2 and an alkylating agent-sensitive mutant MS 1 exhibited, respectively, 1.4-fold and 1.8-fold higher baseline SCE frequencies than did the parental L5178Y. In contrast, M10, which is sensitive to X-ray and 4-nitroquinoline 1-oxide, showed a reduced frequency of baseline SCEs (0.65-fold). These 5 mutants including ES 4 and AC 12 had 3--9-fold increases in spontaneous CA frequencies. Measurement of baseline SCE formation in inter-mutant hybrids revealed that M10 mutation is dominant, MS 1 and ES 4 mutations are semidominant, and ES 2 and AC 12 mutations are recessive. Because SCE frequencies in hybrids formed between pairs of 4 mutants (ES 2, MS 1, ES 4 and AC 12) were significantly lower than those in the tetraploid mutant cells, these 4 mutants probably belong to different complementation groups. Since M10 behaved dominantly with respect to SCE phenotype, it was not possible to determine by complementation test whether it belongs to a different group from the other mutants. However, the finding that M10 is complemented by other mutants for EMS sensitivity indicates that the M10 mutation is different from the other mutations. From these results, it is concluded that at least 4 different genes participate in the formation of high levels of baseline SCEs. The defects in ES 2, MS 1, ES 4, and AC 12 produce common lesions responsible for the formation of both SCEs and CAs. In contrast, the defect in M10 is associated with a high increase in spontaneous CA frequency, but conversely associated with a decrease in baseline SCE frequency. This suggests that M10 is defective in the process involved in the formation of baseline SCEs.     

Sister chromatid exchanges induced in cultured mammalian cells by chromate

Chromate compounds induced sister chromatoid exchanges (SCEs) and chromosome aberrations in cultured mammalian cells. Similar increases in SCE frequency were observed in human fibroblasts exposed to the compounds K2Cr2O7 and K2CrO4. Marked increases in SCE frequency in cells exposed to chromate for a 48-h period were detected at concentrations between 10(-7) and 10(-6) M. Chromosome aberrations (primarily chromatid breaks) were also produced in human cells exposed to K2CrO4 at concentrations between 8 . 10(-7) and 3 . 10(-6) M. K2CrO4, but not the trivalent compound CrCl3, induced SCEs in Chinese hamster ovary (CHO) cells at low concentrations.     

Low doses of alpha particles do not induce sister chromatid exchanges in bystander Chinese hamster cells defective in homologous recombination

We reported previously that the homologous recombinational repair (HRR)-deficient Chinese hamster mutant cell line irs3 (deficient in the Rad51 paralog Rad51C) showed only a 50% spontaneous frequency of sister chromatid exchange (SCE) as compared to parental wild-type V79 cells. Furthermore, when irradiated with very low doses of alpha particles, SCEs were not induced in irs3 cells, as compared to a prominent bystander effect observed in V79 cells [H. Nagasawa, Y. Peng, P.F. Wilson, Y.C. Lio, D.J. Chen, J.S. Bedford, J.B. Little, Role of homologous recombination in the alpha-particle-induced bystander effect for sister chromatid exchanges and chromosomal aberrations, Radiat. Res. 164 (2005) 141-147]. In the present study, we examined additional Chinese hamster cell lines deficient in the Rad51 paralogs Rad51C, Rad51D, Xrcc2, and Xrcc3 as well as another essential HRR protein, Brca2. Spontaneous SCE frequencies in non-irradiated wild-type cell lines CHO, AA8 and V79 were 0.33SCE/chromosome, whereas two Rad51C-deficient cell lines showed only 0.16SCE/chromosome. Spontaneous SCE frequencies in cell lines defective in Rad51D, Xrcc2, Xrcc3, and Brca2 ranged from 0.23 to 0.33SCE/chromosome, 0-30% lower than wild-type cells. SCEs were induced significantly 20-50% above spontaneous levels in wild-type cells exposed to a mean dose of 1.3mGy of alpha particles (<1% of nuclei traversed by an alpha particle). However, induction of SCEs above spontaneous levels was minimal or absent after alpha-particle irradiation in all of the HRR-deficient cell lines. These data suggest that Brca2 and the Rad51 paralogs contribute to DNA damage repair processes induced in bystander cells (presumably oxidative damage repair in S-phase cells) following irradiation with very low doses of alpha particles.     

Influence of low doses of BrdU and estimation of spontaneous SCE in CHO chromosomes: three-way differential staining and an immunoperoxidase method

The influence of low doses of 5-bromodeoxyuridine (BrdU) on the occurrence of sister chromatid exchanges (SCEs) during the first cell cycle, when unsubstituted DNA templates replicate in the presence of the halogenated nucleoside (SCE1) has been assessed in third mitosis (M3) Chinese hamster ovary (CHO) cells showing three-way differential (TWD) staining. In addition, lower concentrations of BrdU, not detectable by Giemsa staining, have been tested by a high resolution immunoperoxidase method (anti-BrdU monoclonal antibody) and SCEs were scored in second mitosis (M2) cells. Our findings was a dose-response curve for SCE1 that allows an estimated mean spontaneous yield of 1.32/cell per cell cycle by extrapolation to zero concentration of BrdU. On the other hand, when the total SCE frequency corresponding to the first and second rounds of replication (SCE1+SCE2) found in M3 chromosomes was compared with the yield of SCEs scored in M2 cells grown in BrdU at doses lower than 1 M no further reduction was achieved. This seems to indicate that SCEs can occur spontaneously in this cell line, though the estimated frequency is higher than that reported in vivo.by S. Wolff     

Regularities in formation of sister chromatid exchanges (SCE) in three generations after irradiation

Frequencies of sister chromatid exchanges (SCE) induction in 3 generations of Chinese hamster cells (clone 237-7) after gamma-irradiation with a dose of 3 Gy have been studied. It is shown that the frequency of SCE significantly increases only in the 1st postirradiation cycle; it does not differ from the control in the 2nd and 3nd cycles. Irradiation induces production of cells with levels of SCE, which have practically never been discovered in non-irradiated population. These cells are eliminated from the population during the next division. Furthermore, it is stated that SCE can be produced in the same locus of chromosome during successive cell divisions. These results permit to conclude that the mechanism of formation of radiation-induced SCE differs from that in normal non-irradiated population.     

Biomarkers of DNA damage in marine mammals.

Certain environmental contaminants found in marine mammals have been shown to cause DNA damage and cancer. The micronuclei (MN), sister chromatid exchange (SCE) and/or chromosome aberration (CA) assays were used to assess baseline (spontaneous) levels of DNA damage in blood lymphocytes of individuals of the relatively healthy and lightly contaminated Arctic beluga whale (Delphinapterus leucas), Sarasota Bay, FL, bottlenose dolphin (Tursiops truncatus) and Northwestern Atlantic grey (Halichoerus grypus) and harp (Phoca groenlandicus) seal populations. MN cell (MNC) frequencies ranged between 2 and 14/1000 binucleated (BN) cells and were statistically similar between species. In bottlenose dolphins, MNC frequency was correlated with age and was significantly higher in females than in males. No intraspecific variation in MNC frequency was found in beluga whales. Intraspecific variation was not tested in seals due to the small sample size. Frequencies of SCEs and total CAs, excluding gaps, ranged, respectively, between 1 and 15 SCE(s)/per cell and 4-6 CAs/100 cells in beluga whales. SCE and CA frequencies did not vary with age or sex in beluga whales. The MN, SCE and CA assays were found to be practical tools for the detection of DNA damage in marine mammals and could be used in the future to compare DNA damage between relatively lightly and highly contaminated populations.     

Induction of sister-chromatid exchanges in Chinese hamster ovary cells by thiol and hydrazine compoudns

Cysteine, cysteamine and glutathione all induce sister-chromatid exchanges (SCEs) in Chinese hamster ovary (CHO) cells when applied to cell cultures at concentrations between 10(-4) and 10(-2) M. Acute exposure of cells th thiol compound for a period of 2--3 h resulted in a unique dose--response relationship in each instance. This consisted of two peak SCE frequencies, one at either extreme of the concentration range. Each peak corresponded to a 2--3-fold increase over the spontaneous level. A chronic exposure of 24 h, in contrast, resulted in a dose--response relationship consisting of a single peak SCE frequency (representing a 4--5-fold increase over the spontaneous level) at a concentration of approx. 4 x 10(-4) M. The effect of Cu2+ ions included in the medium at a concentration of 10(-5) M was to increase the toxicity and, at some concentrations, the SCE levels occurring after either acute or chronic exposure to thiols. Hydrazine and its derivatives, dimethylhydrazine and isonicotinic acid hydrazide (isoniazid), as well as hydrogen peroxide, also induce SCEs in CHO cells. A 2--3-fold increase over the spontaneous level was observed, depending upon the particular treatment protocol applied. SCE yields after 3 h treatment with dimethylhydrazine and isoniazid were increased if Mn2+, but not Cu2+, was included in the tissue culture medium at a concentration of 10(-5) M. SCE yields after a 24-h treatment with dimethylhydrazine in which Mn2+ was present in, and absent from, the medium were similar. Catalase was observed to reduce the SCE levels resulting from treatment with hydrogen peroxide, dimethylhydrazine and isoniazid. The effect of catalase upon SCEs induced by dimethylhydrazine and isoniazid in the presence of Mn2+ was more evident than when Mn2+ was not included in the culture medium. The significance of these results with respect to the possible active chemical species produced and the mutagenic/carcinogenic risk associated with thiol and hydraizine compounds is discussed.     

Melanin content and hydroperoxide metabolism in human melanoma cells

Human melanoma cells were grown to exponential and stationary phases showing melanin contents of 4.2 +/- 0.3 and 11.3 +/- 0.6 micrograms/10(6) cells, respectively. The cells were separated in four subpopulations by a Percoll gradient; the subpopulation of density 1.07 (g/ml) was the most enriched in pigmented cells and produced 28 and 58% of the cells in exponential and stationary phases, respectively. Melanoma cells had similar superoxide dismutase and glutathione peroxidase activities in exponential and stationary phases. Moreover melanoma cells exhibited a higher catalase activity in the stationary phase: whole homogenate and cytosol activities were 7.0 +/- 0.3 and 10.8 +/- 0.6 U/mg protein, whereas in exponential phase the activities were 4.9 +/- 0.1 and 7.6 +/- 0.3 U/mg protein for whole homogenate and cytosol, respectively. The intracellular H2O2 steady-state concentration was 3.3 +/- 0.2 and 2.1 +/- 0.2 microM H2O2 for exponential and stationary phases, respectively. The spontaneous chemiluminescence of the two culture phases was 169 +/- 27 cps/10(6) cells (exponential) and 78 +/- 24 cps/10(6) cells (stationary). The cytotoxicity of H2O2 generated extracellularly by glucose oxidase was determined after 60 min of exposure. IC50 values for exponential and stationary cell cultures were 0.9 and 2.4 mU/ml of glucose oxidase, respectively. The increased catalase activities in the stationary phase as compared with the exponential phase are consistent with the decreased intracellular H2O2, with the decreased spontaneous chemiluminescence, and with the increased resistance to exogenous H2O2.     

Sister-chromatid exchanges in a special form of xeroderma pigmentosum (form II)

The frequency of sister-chromatid exchanges (SCE) was studied in peripheral blood lymphocytes from a xeroderma pigmentosum (form II, XPII) patient. The cells were irradiated with UV or X-rays. In some experiments novobiocin (NB), inhibitor of topoisomerase II, or caffeine (CA), inhibitor of DNA repair were added to the cultures. The level of spontaneous SCE in the patient's lymphocytes was found to be significantly increased in comparison to that in the cells from normal donors. The inhibitors and UV-light caused a rise in the frequency of SCE in the cells taken from normal donors and except for NB, in the lymphocytes from the patient XPII. X-Rays did not increase SCE frequency in normal lymphocytes and lowered it in the patient's cells. SCE frequency rose when inhibitors of DNA replication and repair were used in combination with mutagens.     

Effects of inhibitors of DNA synthesis on spontaneous and ultraviolet light-induced sister-chromatid exchanges in Chinese hamster cells

Effects of inhibitors of DNA synthesis on spontaneous and ultraviolet light (UV)-induced sister-chromatid exchanges (SCE) were examined in a Chinese hamster cell line, V79 B-1. The inhibitors used were hydroxyurea (HU), 1-beta-D-arabinofuranosylcytosine (ara-C), aphidicolin (APC), 2',3'-dideoxythymidine triphosphate (ddTTP), neocarzinostatin (NCS), novobiocin (NB) and cycloheximide (CHX). HU, ara-C, and APC increased spontaneous SCE frequency, and had a synergistic effect on UV-induced SCE frequency. DdTTP, NCS and NB failed to show any statistically significant effect on either spontaneous or UV-induced SCE frequencies, though NCS and NB did slightly increase both spontaneous and UV-induced SCE frequencies. On the contrary, CHX decreased spontaneous SCE frequency, and more drastically, also UV-induced SCE frequency. These results are interpreted with respect to the replicating fork of DNA, a structure postulated to be involved in the formation of spontaneous and UV-induced SCE. A new model for SCE formation is proposed.     

Spontaneous level of sister chromosome exchanges and their distribution in human cells]

Blood of practically healthy donors of both sexes (27 females and 23 males) was cultured under the standard conditions during 96 hours. Bromodeoxyuridine (BUdR) was added at the final concentration of 10 mkg/ml 28 hours before harvesting. The slides were stained with acridine orange and Giemsa for differential staining of chromatids. In each culture sister chromatid exchanges (SCE) were analysed in 50 cells, and the part of cells undergoing the first, second and third mitoses at the time of harvesting, was calculated. According to the mean number of SCE per cell, the distribution of individuals was consistent with the normal law, the mean being 6.525 and standard deviation--0.956. A significant heterogeneity in the speed of cell cycle of cultures was observed. The coefficient of variation for the part of cells undergoing the first mitosis was 50%, for the cells in the second mitosis--15%, and for the cells in the third mitosis--154%. Correlation analysis showed a positive dependence of the mean level of SCF upon the age of a donor and upon the part of cells in the second mitosis in this individual. No reliable correlation of the SCE level with the donor's sex was observed. The distribution of cells, obtained from the culture of one individual, was best approximated by beta-distribution, and the distribution of cells obtained from the cultures of different individuals--by gamma-distribution. In both there was obtained a satisfactory approximation by Pearson's distribution of the 1 type, and significant deviations were found from the normal, Poison's and the negative binomial distribution. The conditions were found of similarity of empirical distribution of SCE in cells to the normal one. For that, it is not the value of SCE for a separate cell that should be used as a unit of measurement, but the mean from the values of frequencies for 5-10 cells. Hence, it was shown that for the evaluation of the mean frequency of SCE with the precision of 1 exchange in separate individuals it is necessary to analyse 40 cells, and to observe the 15% increase of spontaneous SCE level under the action of deleterious factors--8 individuals are enough to analyse.     

The correlation between the frequency of sister chromatid exchanges and the kinetics of human lymphocyte proliferation: the dependence on the sex of the donor

Sister chromatid exchanges (SCE) and average generation time (AGT) were studied in lymphocytes from 35 donors (23 females and 12 males). A higher SCE frequency was found in lymphocytes from females than from males. Smoking increased SCE frequency in lymphocytes of males, but not of females. No differences in AGTs between males and females were found. Partial correlation coefficients between SCE frequency, AGT values, donor's age and smoking were determined. A statistically significant correlation (r = 0.650, P less than 0.01) between SCE frequency and AGT was found in lymphocytes obtained from females. In lymphocytes from males statistically significant partial correlation coefficients were detected between SCE frequency and AGT (r = -0.696, P less than 0.05), SCE frequency and donor's age (r = 0.770, P less than 0.01), SCE frequency and smoking intensity (r = 0.697, P less than 0.01), AGT value and donor's age (r = 0.882, P less than 0.01), and AGT value and smoking (r = 0.634, P less than 0.05). Thus, considerable differences in number of indices between males and females exist. The present observations together with other studies (D'Souza et al., 1988) suggest that considerations for population monitoring using cytogenetic techniques (ICPEMC Publication No 14) may be supplemented with the recommendation to use (whenever it possible) only males as donors in population studies.