Investigations on radiosensitive and radioresistant populations of Drosophila melanogaster. IV. The genetics of the relative radioresistance in stage-7 oocytes of the irradiated population RO I

The genetic system that controls the relative radioresistance in an irradiated laboratory population of Drosophila melanogaster (RÖ I) was studied. Comparisons were made between an unirradiated control population (+60, +K), the population RÖ I (after 227–333 generations of irradiation at 2100 R per generation), the sub-population RÖ I₀ (derived from RÖ I after 260 generations of irradiation and kept without irradiation for up to 74 generations), the F₁ hybrids +60/RÖ I, various homo- and heterozygous carriers of the 3 major chromosomes of RÖ I and +60, respectively, in combination with suitable balancers, and several chromosome substitution stocks of +K and RÖ I. The criteria used to assess the magnitude of radiosensitivity were dominant lethals, X-chromosome loss, and sex-linked recessive lethals induced in stage-7 oocytes at various exposure levels of X-irradiation.The data show that the radioresistance in RÖ I is controlled by a stable and homozygous genetic system. The system is semidominant. With respect to the induction of dominant lethals and sex-linked recessive lethals, the relative resistance is mainly contributed by chromosomes I and II. The effects of the two chromosomes are additive, each contributing about half the relative resistance. Resistance to the X-ray induction of X-chromosome loss is solely contributed by chromosome II.The findings suggest that at least 2 different and independent mechanisms are involved in determining the resistance of the RÖ I population.     

Studies on non-disjunction of the major autosomes in Drosophila melanogaster. I. Methodology and rate of induction by x-rays for the compound second chromosome

The induction of non-disjunction by X-irradiation of the second chromosome in stage-7 oocytes of Drosophila melanogaster has been studied by employing isochromosome stocks. This makes the quantitative recovery possible of progeny resulting from disomic and nullosomic eggs. Determination of egg hatchability has been used to correct for varying degrees of segregation in males carrying different isochromosomes. Even at exposures as low as 250 R the frequency of non-disjunction is significantly higher than in the controls. No evidence has been obtained for the existence of a threshold. In the stage-7 oocytes, the induction of non-disjunction increased linearly with radiation exposure over a range of 250–3000 R and thus seems to reflect a single-hit event. These findings could be of significance for the evaluation of genetic radiation hazards in man. In slightly younger oocyte stages the induction of disomic eggs followed dose-square kinetics. The frequency of nullosomic eggs rises exponentially with radiation exposure, presumably as a consequence of increasing chromosome loss resulting from unrestituted breaks in each of the two maternal isochromosomes. Furthermore, it was observed that the late stage-7 oocytes were more sensitivie to the induction of non-disjunction than earlier stages.     

Analysis and possible role of hyperrecombination in the termination region of the Escherichia coli chromosome.

The frequency of excisive homologous recombination has been measured at various positions along the Escherichia coli chromosome. The reporter system makes use of a lambda cI857 prophage integrated by homologous recombination within Tn5 or Tn10 transposons already installed at known positions in the E. coli chromosome. The excision frequency per cell and per generation was determined by monitoring the evolution of the relative number of temperature-resistant (cured) bacteria is a function of the age of the cultures. Excisions, due to RecA-dependent homologous exchanges, appeared to occur more frequently in the preferential termination zone for chromosome replication. The highest frequency of excision observed is compatible with a recombination event at each replication cycle in this region. On the basis of these data, we propose a model involving homologous recombination in the final steps of bacterial chromosome replication and separation.     

Homologous recombination between overlapping thymidine kinase gene fragments stably inserted into a mouse cell genome.

We have constructed a substrate to study homologous recombination between adjacent segments of chromosomal DNA. This substrate, designated lambda tk2 , consists of one completely defective and one partially defective herpes simplex virus thymidine kinase (tk) gene cloned in bacteriophage lambda DNA. The two genes have homologous 984-base-pair sequences and are separated by 3 kilobases of largely vector DNA. When lambda tk2 DNA was transferred into mouse LMtk- cells by the calcium phosphate method, rare TK+ transformants were obtained that contained many (greater than 40) copies of the unrecombined DNA. Tk- revertants, which had lost most of the copies of unrecombined DNA, were isolated from these TK+-transformed lines. Two of these Tk- lines were further studied by analysis of their reversion back to the Tk+ phenotype. They generated ca. 200 Tk+ revertants per 10(8) cells after growth in nonselecting medium for 5 days. All of these Tk+ revertants have an intact tk gene reconstructed by homologous recombination; they also retain various amounts of unrecombined lambda tk2 DNA. Southern blot analysis suggested that at least some of the recombination events involve unequal sister chromatid exchanges. We also tested three agents, mitomycin C, 12-O-tetradecanoyl-phorbol-13-acetate, and mezerein, that are thought to stimulate recombination to determine whether they affect the reversion from Tk- to Tk+. Only mitomycin C increased the number of Tk+ revertants.     

Studies on mutagen-sensitive strains of Drosophila melanogaster I. The enhanced X-ray sensitivity of a mutant strain (ebony) which is UV-sensitive and also deficient in photorepair

Yegorova and colleagues (1978) showed that a mutant strain of Drosophila melanogaster (ebony) was more sensitive to UV-induced killing of embryos and also less proficient in photoreactivating (PR) ability than a wild-type (Canton-S) strain and that the genes governing UV sensitivity and PR ability were different and presumably located on the autosomes. The experiments reported in the present paper were designed to compare the patterns of sensitivity of these 2 strains and their hybrids to X-irradiation. The sensitivity of the larvae to the killing effects of X-irradiation, and of male and female germ-cell stages to the X-ray induction of genetic damage was studied.It was found that the larvae of the ebony strain are more sensitive to X-ray-induced killing than those of the Canton-S strain. The frequencies of radiation-induced dominant lethals and sex-linked recessive lethals are higher in spermatozoa sampled from ebony males than in those of Canton-S males. In spermatozoa sampled from hybrid males, the yields of dominant lethals are no higher than in those sampled from Canton-S males and do not seem to depend on the origin of the X-chromosome. There are no statistically significant differences between the ebony and Canton-S strains in the sensitivity of their spermatozoa to the induction of autosomal translocations.Stage-7 oocytes sampled from ebony females are more sensitive to the X-ray induction of dominant lethality than are those from Canton-S females; oocytes sampled from hybrid females manifest a level of sensitivity that is significantly lower than that in either parental strain. The frequencies of X-chromosome losses induced in in this germ-cell stage are significantly lower in ebony than in Canton-S females at least at the exposure level of 3000 R at which 3 experiments were carried out. There are no measurable differences in the amount of dominant lethality induced in stage-14 oocytes of ebony, Canton-S and hybrid females.When X-irradiated Berlin-K males are mated to ebony or Canton-S females, the yields of dominant lethals are higher when ebony females are used, showing that there is a “maternal effect” for this kind of damage. Such a maternal effect is also found for sex-linked recessive lethals (irradiated Muller-5 males mated to ebony or Canton-S females). However, when irradiated ring-X-chromosome-carrying males are mated to ebony or Canton-S females, the frequencies of paternal sex-chromosome losses (scored as XO males) are lower when ebony females are used.These results have been interpreted on the assumption that the ebony strain is homozygous for recessive, autosomal genes that confer increased radiosensitivity and that the Canton-S strain carries the normal, wild-type alleles for these genes. The higher yields of dominant and recessive lethals in mature spermatozoa and of dominant lethals in stage-7 oocytes are a consequence of an enhanced sensitivity to the mutagenic (in particular, to the chromosome-breaking) effects of X-irradiation and/or of defective repair of radiation-induced genetic damage. The lower yield of XO males from irradiated stage-7 oocytes of ebony females is probably a consequence of a defect in the repair of chromosome-breakage effects, resulting in the conversion of potential X losses in females into dominant lethals. The “maternal effects” for dominant lethals, sex-linked recessive lethals and for the loss of ring-X chromosomes are assumed to have a common causal basis, namely, a defective repair of chromosome-breakage events in the females of the ebony strain.     

A physical assay for detection of early meiotic recombination intermediates in Saccharomyces cerevisiae

In most eukaryotic organisms, recombination events leading to exchanges between homologous chromosomes link the homologs in a manner that allows their proper attachment to the meiotic spindle. In the yeast Saccharomyces cerevisiae these exchanges are initiated in early prophase as double-strand breaks in the DNA. These breaks are processed through a series of intermediates to yield mature crossovers late in prophase. The following experiments were designed to monitor the appearance of the earliest recombinant DNA strands formed in this process. A polymerase chain reaction assay was devised that allows the detection of recombinant strands at a known initiation site for meiotic recombination. The time and rate of appearance of recombinant strands was found to coincide with commitment to recombination, demonstrating that DNA strands bearing sequences from both parental chromosomes are rapidly formed after the initiation of meiotic recombination.     

Seasonal variation in egg production and batch fecundity of European hake Merluccius merluccius (L.) in the Bay of Biscay

Sexually mature female hake Merluccius merluccius with hydrated ovaries were sampled on a monthly basis in the Bay of Biscay, from May 1996 to October 1997 and from March to April 1998. The batch fecundity was positively related to total length. The relative batch fecundity ( F _Brel) varied significantly among months and years, but not between areas, i.e. International Council for the Exploration of the Sea Divisions VIIIa and VIIIb within the Bay of Biscay. Two levels of F _Brel were found in 1997: the highest between January and April (mean ± s . e . 167 ± 5 eggs g⁻¹) and the lowest from May to October (112 ± 3 eggs g⁻¹). Population condition factor and gonado-somatic indices ( I _G) followed the expected trend in relation to the monthly changes in F _Brel during 1997. The F _Brel variation between years was 9% for 1996–1997 and 28% for 1997–1998, and the difference of the I _G was c. 14 and 36%, respectively. Population relative egg production varied from a high value in January to March (985 eggs g⁻¹) to a low egg production between April and October 1997 (445 eggs g⁻¹).     

Topological requirements for homologous recombination among DNA molecules transfected into mammalian cells.

Cultured animal cells rearrange foreign DNA very efficiently by homologous recombination. The individual steps that constitute the mechanism(s) of homologous recombination in transfected DNA are as yet undefined. In this study, we examined the topological requirements by using the genome of simian virus 40 (SV40) as a probe. By assaying homologous recombination between defective SV40 genomes after transfection into CV1 monkey cells, we showed that linear molecules are preferred substrates for homologous exchanges, exchanges are distributed around the SV40 genome, and the frequency of exchange is not diminished significantly by the presence of short stretches of non-SV40 DNA at the ends. These observations are considered in relation to current models of homologous recombination in mammalian cells, and a new model is proposed. The function of somatic cell recombination is discussed.     

Recombination of bacteriophage lambda in recD mutants of Escherichia coli

RecBCD enzyme is centrally important in homologous recombination in Escherichia coli and is the source of ExoV activity. Null alleles of either the recB or the recC genes, which encode the B and C subunits, respectively, manifest no recombination and none of the nuclease functions characteristic of the holoenzyme. Loss of the D subunit, by a recD mutation, likewise results in loss of ExoV activity. However, mutants lacking the D subunit are competent for homologous recombination. We report that the distribution of exchanges along the chromosome of Red-Gam-phage lambda is strikingly altered by recD null mutations in the host. When lambda DNA replication is blocked, recombination in recD mutant strains is high near lambda's right end. In contrast, recombination in isogenic recD+ strains is approximately uniform along lambda unless the lambda chromosome contains a chi sequence. Recombination in recD mutant strains is focused toward the site of action of a type II restriction enzyme acting in vivo on lambda. The distribution of exchanges in isogenic recD+ strains is scarcely altered by the restriction enzyme (unless the phage contains an otherwise silent chi). The distribution of exchanges in recD mutants is strongly affected by lambda DNA replication. The distribution of exchanges on lambda growing in rec+ cells is not influenced by DNA replication. The exchange distribution along lambda in recD mutant cells is independent of chi in a variety of conditions. Recombination in rec+ cells is chi influenced. Recombination in recD mutants depends on recC function, occurs in strains deleted for rac prophage, and is independent of recJ, which is known to be required for lambda recombination via the RecF pathway. We entertain two models for recombination in recD mutants: (i) recombination in recD mutants may proceed via double-chain break--repair, as it does in lambda's Red pathway and E. coli's RecE pathway; (ii) the RecBC enzyme, missing its D subunit, is equivalent to the wild-type, RecBCD, enzyme after that enzyme has been activated by a chi sequence.     

An exonuclease I-sensitive DNA repair pathway in Deinococcus radiodurans: a major determinant of radiation resistance

Deinococcus radiodurans R1 recovering from acute dose of gamma radiation shows a biphasic mechanism of DNA double-strand break repair. The possible involvement of microsequence homology-dependent, or non-homologous end joining type mechanisms during initial period followed by RecA-dependent homologous recombination pathways has been suggested for the reconstruction of complete genomes in this microbe. We have exploited the known roles of exonuclease I in DNA recombination to elucidate the nature of recombination involved in DNA double-strand break repair during post-irradiation recovery of D. radiodurans. Transgenic Deinococcus cells expressing exonuclease I functions of Escherichia coli showed significant reduction in gamma radiation radioresistance, while the resistance to far-UV and hydrogen peroxide remained unaffected. The overexpression of E. coli exonuclease I in Deinococcus inhibited DNA double-strand break repair. Such cells exhibited normal post-irradiation expression kinetics of RecA, PprA and single-stranded DNA-binding proteins but lacked the divalent cation manganese [(Mn(II)]-dependent protection from gamma radiation. The results strongly suggest that 3' (rho) 5' single-stranded DNA ends constitute an important component in recombination pathway involved in DNA double-strand break repair and that absence of sbcB from deinococcal genome may significantly aid its extreme radioresistance phenotype.     

The effect of tularemia vaccine on the radioresistance of white rats exposed to x-irradiation

A single epicutaneous vaccination of Wistar rats with tularemic live vaccine 15 days before X-irradiation with doses of 6.0, 8.0 and 2.0 + 6.0 Gy was shown to increase their radioresistance. With higher doses (up to 8.0 Gy) the effect of the vaccine was less pronounced.     

Repair of site-specific double-strand breaks in a mammalian chromosome by homologous and illegitimate recombination.

In mammalian cells, chromosomal double-strand breaks are efficiently repaired, yet little is known about the relative contributions of homologous recombination and illegitimate recombination in the repair process. In this study, we used a loss-of-function assay to assess the repair of double-strand breaks by homologous and illegitimate recombination. We have used a hamster cell line engineered by gene targeting to contain a tandem duplication of the native adenine phosphoribosyltransferase (APRT) gene with an I-SceI recognition site in the otherwise wild-type APRT+ copy of the gene. Site-specific double-strand breaks were induced by intracellular expression of I-SceI, a rare-cutting endonuclease from the yeast Saccharomyces cerevisiae. I-SceI cleavage stimulated homologous recombination about 100-fold; however, illegitimate recombination was stimulated more than 1,000-fold. These results suggest that illegitimate recombination is an important competing pathway with homologous recombination for chromosomal double-strand break repair in mammalian cells.     

Homologous and illegitimate recombination in developing Xenopus oocytes and eggs.

Exogenous DNA is efficiently recombined when injected into the nuclei of Xenopus laevis oocytes. This reaction proceeds by a homologous resection-annealing mechanism which depends on the activity of a 5'-->3' exonuclease. Two possible functions for this recombination activity have been proposed: it may be a remnant of an early process in oogenesis, such as meiotic recombination or amplification of genes coding for rRNA, or it may reflect materials stored for embryogenesis. To test these hypotheses, recombination capabilities were examined with oocytes at various developmental stages. Late-stage oocytes performed only homologous recombination, whereas the smallest oocytes ligated the restriction ends of the injected DNA but supported no homologous recombination. This transition from ligation to recombination activity was also seen in nuclear extracts from these same stages. Exonuclease activity was measured in the nuclear extracts and found to be low in early stages and then to increase in parallel with recombination capacity in later stages. The accumulation of exonuclease and recombination activities during oogenesis suggests that they are stored for embryogenesis and are not present for oocyte-specific functions. Eggs were also tested and found to catalyze homologous recombination, ligation, and illegitimate recombination. Retention of homologous recombination in eggs is consistent with an embryonic function for the resection-annealing mechanism. The observation of all three reactions in eggs suggests that multiple pathways are available for the repair of double-strand breaks during the extremely rapid cleavage stages after fertilization.     

Effect of the red imported fire ant on cotton aphid population density and predation of bollworm and beet armyworm eggs

The effects of the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae), on cotton aphid, Aphis gossypii Glover, populations and its predation of bollworm, Helicoverpa zea (Boddie), and beet armyworm, Spodoptera exigua (Hübner), (both Lepidoptera: Noctuidae) eggs were evaluated in cotton under field conditions during 2001 and 2002 in central and northern Texas. In central Texas, cotton aphid populations were approximately 5.5 times greater and predation of sentinel bollworm eggs 2 times greater in the presence of S. invicta versus in its absence, although aphid populations did not reach economic levels. Most predation of beet armyworm egg masses, measured via direct nocturnal observations, was due to S. invicta (68%) and cotton fleahopper, Pseudatomoscelis seriatus (Reuter) (21%), where S. invicta was present, and by the mite Abrolophus sp. (52%), spiders (13%), and minute pirate bug (Orius sp.) (13%) where S. invicta was absent. Predation of sentinel bollworm eggs and beet armyworm egg masses was approximately 1.5 and 4.1 times greater, respectively, in the presence of S. invicta versus in their absence. In the presence of S. invicta, the relative frequencies of minute pirate bug and cotton fleahopper were higher, and of S. invicta and native ants lower in beat bucket samples compared with their relative frequencies in nocturnal observations of predation upon beet armyworm egg masses. In the absence of S. invicta seven of eight predators sampled were similarly represented in beat bucket samples and nocturnal observations of beet armyworm egg mass predation, whereas minute pirate bug occurred at a higher frequency in beat bucket samples relative to nocturnal observations. These observations suggested that the relative frequencies of minute pirate bug, cotton fleahopper, S. invicta and native ants in beat bucket samples do not closely reflect the frequency with which these predators prey on noctuid eggs. Overall, the results of this study show that although S. invicta may promote aphid populations early in the growing season, it is an important predator of bollworm and beet armyworm eggs later in the season.     

Differentiation between wheat chromosomes 4B and 4D.

A linkage map based on homoeologous recombination, induced by the absence of the Ph1 locus, between chromosome 4D of Triticum aestivum L. (genomes AABBDD) and chromosome 4B of T. turgidum L. (genomes AABB) was compared with a linkage map of chromosome 4Am of T. monococcum L. and a consensus map of chromosomes 4B and 4D of T. aestivum based on homologous recombination. The 4D/4B homoeologous map was only one-third the length of the homologous maps and all intervals were reduced relative to the 4B-4D consensus map. After the homoeologous map was corrected for this overall reduction in recombination, the distribution of recombination in the short arm was similar in both types of maps. In the long arm, homoeologous recombination declined disproportionally in the distal to proximal direction. This gradient was shown to be largely caused by severe segregation distortion reflecting selection against 4D genetic material. The segregation distortion had a maximum that coincided with the centromere and likely had a polygenic cause. Chromosomes 4D and 4B were colinear and recombination between them occurred in almost all intervals where homologous recombination occurred. These findings suggest that these chromosomes are not differentiated structurally and that the differentiation is not segmental. In the presence of Ph1, metaphase I chromosome pairing between chromosomes composed of homologous and differentiated regions correlated with the lengths of the homologous regions. No compensatory allocation of crossovers into the homologous regions was detected. In this respect, the present results are in dramatic contrast with the crossover allocation into the pseudoautosomal region in the mammalian male meiosis.     

P+QA- and P+QB- charge recombinations in Rhodopseudomonas viridis chromatophores and in reaction centers reconstituted in phosphatidylcholine liposomes. Existence of two conformational states of the reaction centers and effects of pH and o-phenanthroline

The P+QA- and P+QB- charge recombination decay kinetics were studied in reaction centers from Rhodopseudomonas viridis reconstituted in phosphatidylcholine bilayer vesicles (proteoliposomes) and in chromatophores. P represents the primary electron donor, a dimer of bacteriochlorophyll; QA and QB are the primary and secondary stable quinone electron acceptors, respectively. In agreement with recent findings for reaction centers isolated in detergent [Sebban, P., & Wraight, C.A. (1989) Biochim. Biophys. Acta 974, 54-65] the P+QA- decay kinetics were biphasic (kfast and kslow). Arrhenius plots of the kinetics were linear, in agreement with the hypothesis of a thermally activated process (probably via P+I-; I is the first electron acceptor, a bacteriopheophytin) for the P+QA- charge recombination. Similar activation free energies (delta G) for this process were found in chromatophores and in proteoliposomes. Significant pH dependences of kfast and kslow were observed in chromtophores and in proteoliposomes. In the pH range 5.5-11, the pH titration curves of kfast and kslow were interpreted in terms of the existence of three protonable groups, situated between I- and QA-, which modulate the free energy difference between P+I- and P+QA-. In proteoliposomes, a marked effect of o-phenanthroline was observed on two of the three pKs, shifting one of them by more than 2 pH units. On the basis of recent structural data, we suggest a possible interpretation for this effect, which is much smaller in Rhodobacter sphaeroides. The decay kinetics of P+QB- were also biphasic. Marked pH dependences of the rate constants and of the relative proportions of both phases were also detected for these decays. The major conclusion of this work comes from the biphasicity of the P+QB- decay kinetics. We had suggested previously that biphasicity of the P+QA- charge recombination in Rps. viridis comes from nonequilibrium between protonation states of the reaction centers due to comparable rates of the protonation events and charge recombination. This hypothesis does not hold since the P+QB- decays occur on a time scale (tau approximately 300 ms at pH 8) much longer than protonation events. This leads to the conclusion that kfast and kslow (for both P+QA- and P+QB-) are related to conformational states of the reaction centers, existing before the flash. In addition, the fast and slow decays of P+QB- are related to those measured for P+QA-, via the calculations of the QA-QB in equilibrium QAQB- apparent equilibrium constants, K2.(ABSTRACT TRUNCATED AT 400 WORDS)     

Genetic and physical mapping of homoeologous recombination points involving wheat chromosome 2B and rye chromosome 2R.

Wide hybrids have been used in generating genetic maps of many plant species. In this study, genetic and physical mapping was performed on ph1b-induced recombinants of rye chromosome 2R in wheat (Triticum aestivum L.). All recombinants were single breakpoint translocations. Recombination 2RS-2BS was absent from the terminal and the pericentric regions and was distributed randomly along an intercalary segment covering approximately 65% of the arm's length. Such a distribution probably resulted from structural differences at the telomeres of 2RS and wheat 2BS arm that disrupted telomeric initiation of pairing. Recombination 2RL-2BL was confined to the terminal 25% of the arm's length. A genetic map of homoeologous recombination 2R-2B was generated using relative recombination frequencies and aligned with maps of chromosomes 2B and 2R based on homologous recombination. The alignment of the short arms showed a shift of homoeologous recombination toward the centromere. On the long arms, the distribution of homoeologous recombination was the same as that of homologous recombination in the distal halves of the maps, but the absence of multiple crossovers in homoeologous recombination eliminated the proximal half of the map. The results confirm that homoeologous recombination in wheat is based on single exchanges per arm, indicate that the distribution of these single homoeologous exchanges is similar to the distribution of the first (distal) crossovers in homologues, and suggest that successive crossovers in an arm generate specific portions of genetic maps. A difference in the distribution of recombination between the short and long arms indicates that the distal crossover localization in wheat is not dictated by a restricted distribution of DNA sequences capable of recombination but by the pattern of pairing initiation, and that can be affected by structural differences. Restriction of homoeologous recombination to single crossovers in the distal part of the genetic map complicates chromosome engineering efforts targeting genes in the proximal map regions.     

Effect of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TTPA) upon membrane ionic exchanges in sea urchin eggs

The effect of TPA (12-O-tetradecanoylphorbol-13-acetate) upon ionic exchanges was investigated in eggs of the sea urchin Arbacia lixula. Ouabain-sensitive 86Rb uptake and amiloride-sensitive 24Na influx were dramatically stimulated after TPA addition, indicating an enhancement of total ionic permeabilities. Stimulation by TPA of both Na+/H+ and Na+/K+ exchanges was canceled by amiloride, suggesting that activation of protein kinase C elicits, via Na+/H+ activity, stimulation of the sodium pump. However, TPA did not stimulate sodium pump activity and Na+/H+ exchange at the same rate as fertilization, probably because of an absence of calcium-dependent events. Further fertilization of TPA-pretreated eggs triggered an enhancement of sodium pump activity when the TPA treatment duration did not exceed 10 min. It is suggested that TPA activates preexisting transporting mechanisms in plasma membranes of unfertilized eggs (Na+ pump, Na+/H+ exchange) without eliciting corresponding regulatory mechanisms (Na+ stat, pH stat).     

The role of homologous recombination repair in the formation of chromosome aberrations

The repair of DNA double strand breaks by homologous recombination can occur by at least two pathways: a Rad51-dependent pathway that is predominantly error free, and a Rad51-independent pathway (single strand annealing, SSA) that is error prone. In theory, chromosome exchanges can result from (mis)repair by either pathway. Both repair pathways will involve a search for homologous sequence, leading to co-localization of chromatin. Genes involved in homologous recombination repair (HRR) have now been successfully knocked out in mice and the role of HRR in the formation of chromosome exchanges, particularly after ionising radiation, is discussed in the light of new evidence.