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.     

Evidence that sister chromatid exchanges and chromatid breaks are two independent events

The relative frequencies of sister chromatid exchanges (SCE) and chromatid breaks in BrdU (5-bromodeoxyuridine) — sensitive site (lq22 lq23) in Chinese hamster cells after BrdU incorporation were studied. The results show that chromatid breaks do not follow the exchange hypothesis and provide evidence that chromatid breaks and SCEs are two independent events despite some common features.     

Sister chromatid cohesion in mitosis

Sister chromatid cohesion is essential for accurate chromosome segregation during the cell cycle. Newly identified structural proteins are required for sister chromatid cohesion and there may be a link in some organisms between the processes of cohesion and condensation. Proteins that induce and regulate the separation of sister chromatids have also been recently identified.     

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 exchange in newborns

Summary Siter chromatid exchanges (SCE) were analyzed in the cord and postnatal blood of controlled groups of low and high birth weight infants to detect possible associations between abnormal birth weight and SCE frequency. Structural chromosome aberration rates had previously been evaluated for all infants, and possible correlations between aberration and SCE rates were sought.No correlation was found between neonatal or postnatal SCE frequency and birthweight, nor was there evidence of association of chromosome aberration rates with SCE frequency. In all groups of infants, however, mean postnatal SCE frequencies were significantly lower than mean neonatal SCE frequencies.     

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.     

In vivo sister chromatid differentiation and base line sister chromatid exchanges in Channa punctatus.

An in vivo system for differentially stained sister chromatids by incorporating 5' Bromo 2' deoxyuridine at two consecutive round of DNA replication has been developed in C. punctatus. The base line developed frequency of sister chromatid exchanges (SCEs) was found to be 0.038 SCE/chromosome. This low baseline frequency of SCEs could be useful in detecting genotoxicity of pollutants in aquatic medium.     

Sister chromatid exchange in cigarette smokers

Summary The incidence of sister chromatid exchanges in smokers and nonsmokers was investigated. There was no difference in the SCE rate between smokers and nonsmokers, nor was there any difference between heavy (>10 per day) and light (<10 per day) smokers.     

The mechanism of sister chromatid cohesion

Each of our cells inherit their genetic information in the form of chromosomes from a mother cell. In order that we obtain the full genetic complement, cells need to ensure that replicated chromosomes are accurately split and distributed during cell division. Mistakes in this process lead to aneuploidies, cells with supernumerous or missing chromosomes. Most aneuploid human embryos are not viable, and if they are, they develop severe birth defects. Aneuploidies later in human life are frequently found associated with the development of malignant cancer. DNA replication during S-phase is linked to segregation of the sister copies in mitosis by sister chromatid cohesion. A chromosomal protein complex, cohesin, holds replicated sister DNA strands together after their synthesis. This allows pairs of replication products to be recognised by the spindle apparatus in mitosis for segregation into opposite direction. At anaphase onset, cohesin is destroyed by a site-specific protease, separase. Here I review what we have learned about the molecular mechanism of sister chromatid cohesion. Cohesin forms a large proteinaceous ring that may hold sister chromatids by encircling and topological trapping. To understand how cohesin links newly synthesised replication products, biochemical assays to study the enzymology of cohesin will be required.     

Random single chromatid and nonrandom double chromatid type segregation of human acrocentric chromosomes in BrdU-labeled mitoses

Chromatid segregation was analyzed using satellite association of 5-bromodeoxyuridine (BrdU) differentially stained acrocentric chromosomes of human leukocytes. Data were classified into cis and trans configurations in second and third division cycles. It was found that single chromatid types have random segregation (1:1) while nonrandom segregation was noted for double chromatid types. The nonrandom segregation hypothesis of earlier investigators needs to be reexamined.     

Mechanism of formation of chromatid exchanges

Chinese hamster cells with different patterns of distribution of 5-bromodeoxyuridine (BrdUrd) between chromosome subunits were subjected, during the G2 stage, to UV irradiation, which only produced breaks in BrdUrd substituted DNA. The frequency of chromatid and subchromatid interchanges as well as isochromatid aberrations was estimated. It was found that only BrdUrd containing chromatids were involved into aberrations; this result challenges the so called "molecular theory" for aberration production proposed by Leenhouts and Chadwick. A very small increase of the aberration yield in chromosomes without BrdUrd may be connected with the action of UV on the frequency of recombination. The observed frequency of interchanges was not proportional to the BrdUrd content in chromosomes and depended on the time of its incorporation: more exchanges were induced in the chromatids incorporating BrdUrd during the last round of replication. These regularities may be connected with some molecular peculiarities of chromosome structure and function.     

Statistical evaluation of sister chromatid exchanges

Summary Log-linear models are fitted to sister chromatid exchange (SCE) scores in order to test the significance of the differences in SCE scores observed between individuals or between experimental treatments. The analysis is performed at the level of chromosome groups. In each single test all measurements from all chromosome groups, both from the control and from the experimental sets, are utilized. By proceeding in this way full use is made of all the available information on the SCE scores at the level of chromosome groups and the shortcomings of the classical Student-t and chi-square tests are avoided.This work was supported by a grant Geconcerteerde Acties from the Belgian Government.     

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.