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Results for “crossover (CO) formation”

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PubMed2026

Analyzing Meiosis in Maize.

Meiosis is central to sexual reproduction and the main source of genetic diversity in plants. Understanding how meiotic processes are regulated has direct relevance to agriculture. As meiotic recombination is the vehicle of plant breeding, gaining the ability to influence recombination patterns can accelerate crop improvement. Maize is a powerful model for studying plant meiosis, thanks to its large chromosomes, well-developed genetics, and the availability of diverse cytogenetic and molecular tools. Insights gained from maize studies can extend to other species. In this review, we describe a variety of approaches for examining meiosis and meiotic recombination in maize. Cytological techniques, including protein immunolocalization and fluorescence in situ hybridization (FISH), enable visualization of chromosome structure and behavior, as well as crossover (CO) formation. Chromatin immunoprecipitation (ChIP) is used in meiosis research to determine locations of recombination proteins, identify recombination sites, and elucidate chromatin features, such as histone modifications. Quantification of COs at specific genomic sites through pollen typing by droplet digital PCR allows precise high-resolution measurement of recombination rates. Combining cytology, protein localization, and molecular assays provides a multiscale picture of meiosis, linking molecular mechanisms to chromosome behavior and, ultimately, to genetic variation.

Journal Article↗

PubMed2026

Meiotic Origins of Non-Mosaic Klinefelter Syndrome (47, XXY): Mechanisms, Dimorphism, and Emerging Genetic Susceptibility.

BACKGROUND: Non-mosaic Klinefelter syndrome (47,XXY) arises from sex-specific meiotic mechanisms leading to nondisjunction during gametogenesis. In maternal cases, errors occur predominantly during meiosis I, frequently involving X chromosomes that lack crossovers or exhibit crossovers outside optimal chromosomal locations; this nondisjunction is further exacerbated by advancing maternal age and the subsequent deterioration of cohesins. Conversely, paternal 47,XXY stems primarily from the failure of obligate recombination within the pseudoautosomal region, which is critical for accurate XY segregation. OBJECTIVE: Drawing on the latest literature, this review transitions from a descriptive account of non-mosaic Klinefelter syndrome (nKS) toward a comprehensive analysis of mechanistic and genomic evidence, establishing a unified framework of sex-specific meiotic vulnerability. METHODS: We synthesized recent findings from forward genetic approaches in mouse models with data from human observational and reverse-genetic studies, delineating an updated view of the established and putative mechanisms that modulate sex chromosome segregation. RESULTS: We frame XX and XY nondisjunction as a genetically modulated process rather than a purely stochastic event. The integrated evidence supports a unified framework in which maternal and paternal mechanisms converge primarily on meiotic recombination failure but differ in timing, chromosomal context, and genetic modulation. CONCLUSIONS: From a clinical standpoint, identifying the molecular drivers of sex chromosome aneuploidy will enhance genetic counseling and risk stratification. Such insights are poised to facilitate informed reproductive decision-making and timely therapeutic support, ultimately reducing the burden of nKS comorbidities and improving patients' quality of life.

aneuploidy↗

PubMed1982

Intergenomic recombination events among pairs of defective cauliflower mosaic virus genomes in plants.

Pairs of defective cauliflower mosaic virus genomes coinoculated in plants are rescued at high frequency by homologous, intergenomic recombination events. Defective genomes may be rescued by one of at least two general types of recombination mechanisms. One mechanism appears to operate on linear DNA molecules with complementary sticky ends. Recombination of this type can be explained by the formation of mixed dimers (or concatamers) which can be resolved into normal genomes by a single crossover event. Another type of recombination takes place among viral DNA molecules which do not have complementary sticky ends. Rescue of this type yields normal viral DNA molecules which can be resolved only by the equivalent of a double crossover or internal gene conversion event between pairs of infecting molecules. Co-infection of plants with pairs of defective genomes derived from different viral isolates results in the formation of hybrid viral genomes. Analysis of these hybrids reveals that recombination can take place at different sites in the viral genome.

DNA↗

PubMed1993

Agonist-induced lipoxin A4 generation: detection by a novel lipoxin A4-ELISA.

Lipoxin A4 (LXA4) possesses potent bioactions. To facilitate its detection, an enzyme-linked immunosorbent assay (ELISA) was developed that proved sensitive and selective. Quantitation by ELISA of LXA4 generated from cellular sources strongly correlated (r = 0.99) with values obtained by high-pressure liquid chromatography (HPLC). We used this LXA4-ELISA to examine parameters influencing LXA4 generation from endogenous substrates during human platelet-neutrophil (PLT-PMN) interactions in vitro. Agonist-induced LXA4 production was clearly evident at a PLT-PMN ratio of 10:1, and recombinant human granulocyte/monocyte colony stimulating factor-priming of PMN augmented LXA4 generation 5-6 fold. The chemotactic peptide formylmethionyl-leucyl-phenylalanine, platelet-derived growth factor and arachidonic acid (20:4n-6) each stimulated formation of immunoreactive LXA4 (iLXA4) in these co-incubations. The presence of iLXA4 was also evaluated in vivo in aspirin-sensitive asthmatic patients who, in a randomized, double-blind crossover design, underwent nasal lavage after they each ingested a predetermined threshold dose of aspirin or placebo. Aspirin challenge provoked statistically significant increases in iLXA4 in each patient (P < 0.005). These results validate the use of a solid-phase ELISA for detection of LXA4. Furthermore, the use of this ELISA has allowed the first documentation of iLXA4 formation in human subjects with aspirin-sensitive asthma following specific antigenic challenge.

Adult↗

PubMed1978

Lateral element cross connections of the synaptonemal complex and their relationship to chiasmata in rat spermatocytes.

Observations are presented in support of the hypothesis that at meiotic prophase a reciprocal crossover is accompanied by a crossover of the lateral elements of the synaptonemal complex, SC (Moens, 1974). Rat spermatocyte nuclei in developmental stage VII (Clermont, 1972) of the seminiferous epithelium cycle, but not pachytene nuclei in stages I to VI, were found to have SC modifications in the form of a cross connection between the lateral elements. In structure these crossover elements, CO elements, resemble the lateral element. It is found in a variety of positions, usually more or less perpendicular to the SC but also slanted or parallel along the central element or detached from the SC. Reconstructions of entire nuclei indicate an average of one such CO element per SC and a nonrandom distribution of CO elements among the SCs. Because the crossing-over of lateral elements produces a 180 degree twist or removes a 180 degree twist, the pattern of SC coiling was examined. Coiling starts in early pachytene prior to CO element formation. At stage VII one nucleus had a total of 78 coils, all counter clockwise, and another nucleus had 97 such 180 degree coils. It is noted that if SC coils are associated with the process of crossing-over, then the regulation of crossover distribution such as chiasma position interference has an explanation in the structure and behavior of the SC.

Animals↗

PubMed1999

[The contact model of crossing over: definition of conditions required for the co-orientation of homologs].

Three phenomena have been examined: (1) independence of the segregation of structurally normal homologues from the position of crossover exchange relative to the centromere; (2) independence of chromosomal rearrangement ability to disturb disjunction from its position on chromosome; and (3) dependent, predominantly distal position of crossover exchanges in nondisjunction chromosomes. All three phenomena are explained in terms of the contact model of crossing over. According to the model, disjunction of the homologues is provided by a series of local contacts between them. The maximal number of contacts per arm is constant and is determined by the size of the absolute interference zone. The co-orientating influence of the contact decreases with the movement of its site away from the centromeric region responsible for the co-orientation. The role of the contact consists of bringing the centromeric regions of the homologues together. Co-orientation success does not depend on the presence or absence of a crossover exchange at the site of the contact. The nondisjunction chromosomes are selected with regard to the absence of proximal (or close to proximal) contacts. Hence, the exchanges formed on the basis of these contacts, are co-orientation-associated. In the disjunction chromosomes, the entire set of contacts is present. Any of these can be the first to develop and generate a crossover exchange. Because of this, exchanges in the disjunction chromosomes do not demonstrate association with the co-orientation. According to the model, the distortion of the previous contact disturbs (retards or prevents) the formation of the next contact. Hence, distortion of the contact formation in the site of rearrangement, irrespective to its location, would negatively affect the formation of the contact in the proximal region, and would promote nondisjunction.

Animals↗

PubMed1996

Metabolic pathway for poly(3-hydroxybutyrate-co-3-hydroxyvalerate) formation in Nocardia corallina: inactivation of mutB by chromosomal integration of a kanamycin resistance gene.

The gene encoding the large subunit of the methylmalonyl-coenzyme A (CoA) mutase in Nocardia corallina (mutBNc) was cloned. A 4.3-kbp BamHI fragment containing almost the entire mutBNc was identified by Southern hybridization experiments employing a digoxigenin-labeled probe deduced from mutB of Streptomyces cinnamonensis, mutBNc was interrupted by insertion of a kanamycin resistance gene block (mutB::kan or mutB::neo) and introduced into N. corallina to obtain mutB-negative strains by homologous recombination. Four of sixteen kanamycin-resistant clones occurred via double-crossover events and harbored only the interrupted mutBNc. These exhibited no growth on odd-chain fatty acids in the presence of kanamycin but exhibited wild-type growth on even-chain fatty acids, glucose, and succinate. Whereas the wild type of N. corallina accumulates a copolyester of 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV) containing more than 60 mol% 3HV from most carbon sources, mutB-negative strains accumulated poly(3HB-co-3HV) containing only 2 to 6 mol% 3HV. Methylmalonyl-CoA mutase activity was not found in these clones. Therefore, this study provides strong evidence that the majority of 3HV units in poly(3HB-co-3HV) accumulated by N. corallina are synthesized via the methylmalonyl-CoA pathway.

Bacterial Proteins↗

PubMed1990

DNA recombination during PCR.

PCR co-amplification of two distinct HIV1 tat gene sequences lead to the formation of recombinant DNA molecules. The frequency of such recombinants, up to 5.4% of all amplified molecules, could be decreased 2.7 fold by a 6 fold increase in Taq DNA polymerase elongation time. Crossover sites mapped essentially to three discrete regions suggesting specific Taq DNA polymerase pause or termination sites. PCR mediated recombination may be a problem when studying heterogeneous genetic material such as RNA viruses, multigene families, or repetitive sequences. This phenomenon can be exploited to create chimeric molecules from related sequences.

Amino Acid Sequence↗

PubMed2006

Direct PEM fuel cell using "organic chemical hydrides" with zero-CO2 emission and low-crossover.

A series of "organic chemical hydrides" such as cyclohexane, methylcyclohexane, cyclohexene, 2-propanol, and cyclohexanol were applied to the direct PEM fuel cell. High performances of the PEM fuel cell were achieved by using cyclohexane (OCV = 920 mV, PD(max) = 15 mW cm(-2)) and 2-propanol (OCV = 790 mV, PD(max) = 78 mW cm(-2)) as fuels without CO(2) emissions. The rates of fuel crossover for cyclohexane, 2-propanol, and methanol were estimated, and the rates of fuel permeation of cyclohexane and 2-propanol were lower than that of methanol. Water electrolysis and electro-reductive hydrogenation of acetone mediated by PEM were carried out and formation of 2-propanol in cathode side was observed. This system is the first example of a "rechargeable" direct fuel cell.

Cyclohexanes↗

PubMed2003

The pharmacology of cocaethylene in humans following cocaine and ethanol administration.

BACKGROUND: Concurrent use of cocaine and alcohol results in formation of a cocaine homolog and metabolite-cocaethylene. METHODS: To characterize cocaethylene pharmacology, ten paid volunteer subjects were given deuterium-labeled (d(5)) cocaine (0.3, 0.6, and 1.2 mg/kg and cocaine placebo) by a 15-min constant rate intravenous injection 1 h after a single oral dose of ethanol (1 g/kg) or ethanol and cocaine placebo using a double-blind, crossover design. Six of the same volunteers subsequently received a 1.2 mg/kg dose of cocaine alone. A small (7.5 mg) nonpharmacologically active dose of deuterium-labeled cocaethylene-d(3) was concurrently administered with the cocaine to enable calculation of absolute cocaethylene formation and clearance. Plasma and urine cocaine, cocaethylene, and benzoylecgonine concentrations, physiologic and subjective effects were measured. RESULTS: When co-administered with ethanol, 17+/-6% (mean+/-S.D.) of the cocaine was converted to cocaethylene. Cocaethylene peak plasma concentrations and AUC increased proportionally to the cocaine dose. Ethanol ingestion prior to cocaine administration decreased urine benzoylecgonine levels by 48% and increased urinary cocaethylene and ecgonine ethyl ester levels. Subjects liked and experienced more total intoxication after the combination of cocaine and ethanol than after either drug alone. CONCLUSIONS: In the presence of ethanol, the altered biotransformation of cocaine resulted in 17% of an intravenous cocaine dose being converted to cocaethylene and relatively lower urinary concentrations of benzoylecgonine.

Adult↗

PubMed2003

Metabolism and elimination of quinine in healthy volunteers.

OBJECTIVES: The aims were to investigate: (1) The renal elimination of quinine and its metabolites 3-hydoxyquinine, 2'-quininone, (10R) and (10S)-11-dihydroxydihydroquinine and (2) the relative importance of CYP3A4, CYP1A2 and CYP2C19 for the formation of 2'-quininone, (10R) and (10S)-11-dihydroxydihydroquinine in vivo. METHODS: In a randomised three-way crossover study, nine healthy Swedish subjects received a single oral dose of quinine hydrochloride (500 mg), on three different occasions: (A) alone, (B) concomitantly with ketoconazole (100 mg twice daily for 3 days) and (C) concomitantly with fluvoxamine (25 mg twice daily for 2 days). Blood and urine samples were collected before quinine intake and up to 96 h thereafter. All samples were analysed by means of high-performance liquid chromatography. RESULTS: Co-administration with ketoconazole significantly increased the area under the plasma concentration versus time curve (AUC) of 2'-quininone, (10S)-11-dihydroxydihydroquinine, and (10R)-11-dihydroxydihydroquinine, the geometric mean ratios (90% CI) of the AUC were 1.9 (1.8, 2.0), 1.3 (1.1, 1.7) and 1.6 (1.4, 1.8), respectively. Co-administration with fluvoxamine had no significant effect on the mean AUC of any of the metabolites. A mean of 56% of the administered oral quinine dose was recovered in urine after hydrolysis with beta-glucuronidase relative to the 40% recovered before hydrolysis. CONCLUSION: Quinine is eliminated in urine mainly as unchanged drug and as 3-hydroxyquinine. The major metabolite of quinine is 3-hydroxyquinine formed by CYP3A4. There is no evidence for the involvement of CYP3A4, 1A2 or 2C19 in the formation of 2'-quininone, (10S)-11-dihydroxydihydroquinine and (10R)-11-dihydroxydihydroquinine in vivo. Glucuronidation is an important pathway for the renal elimination of quinine, mainly as direct conjugation of the drug.

Antimalarials↗

PubMed2006

Control of meiotic recombination in Arabidopsis: role of the MutL and MutS homologues.

Immunocytochemistry reveals that the Arabidopsis mismatch repair proteins AtMSH4, AtMLH3 and AtMLH1 are expressed during prophase I of meiosis. Expression of AtMSH4 precedes AtMLH3 and AtMLH1 which co-localize as foci during pachytene. Co-localization between AtMSH4 and AtMLH3 occurs, but appears transient. AtMLH3 foci are not detected in an Atmsh4 mutant. However, localization of AtMSH4 is unaffected in Atmlh3, suggesting that recombination may proceed to dHj (double Holliday junction) formation. Mean chiasma frequency in Atmsh4 is reduced to 1.55 compared with 9.86 in wild-type. In contrast with wild-type, the distribution of residual crossovers in Atmsh4 closely fits a Poisson distribution. This is consistent with a two-pathway model for meiotic crossing-over whereby most crossovers occur via an AtMSH4-dependent pathway that is subject to interference, with the remaining crossovers arising via an interference-independent pathway. Loss of AtMLH3 results in an approx. 60% reduction in crossovers. Results suggest that dHj resolution can occur, but in contrast with wild-type where most or all dHjs are directed to form crossovers, the outcome is biased in favour of a non-crossover outcome. The results are compatible with a model whereby the MutL complex maintains or imposes a dHj conformation that ensures crossover formation.

Arabidopsis↗

PubMed2006

Balancing the checks: surveillance of chromosomal exchange during meiosis.

During meiosis, numerous DSBs (double-strand breaks) are induced along the genome which are processed via several steps into crossovers. Crossovers ensure the faithful segregation of homologous chromosomes during meiosis I. Although required for faithful chromosome segregation, DSBs pose a severe hazard to genome integrity. Chromosome segregation in the presence of persisting DSBs can result in loss or missegregation of entire chromosome arms and in the formation of aneuploid gametes, conditions frequently associated with birth defects, still births and cancer susceptibility in offspring. Co-ordination between chromosomal exchange and meiotic cell-cycle progression is achieved via a surveillance mechanism commonly referred to as the recombination checkpoint. Both components of the mitotic DNA damage checkpoint as well as meiosis-specific functions contribute to this highly conserved surveillance system.

Chromosomes, Fungal↗

PubMed2001

Effect of metformin on fatty acid and glucose metabolism in freshly isolated hepatocytes and on specific gene expression in cultured hepatocytes.

The short-term effect of metformin on fatty acid and glucose metabolism was studied in freshly incubated hepatocytes from 24-hr starved rats. Metformin (5 or 50 mM) had no effect on oleate or octanoate oxidation rates (CO(2)+ acid-soluble products), whatever the concentration used. Similarly, metformin had no effect on oleate esterification (triglycerides and phospholipid synthesis) regardless of whether the hepatocytes were isolated from starved (low esterification rates) or fed rats (high esterification rates). In contrast, metformin markedly reduced the rates of glucose production from lactate/pyruvate, alanine, dihydroxyacetone, and galactose. Using crossover plot experiments, it was shown that the main effect of metformin on hepatic gluconeogenesis was located upstream of the formation of dihydroxyacetone phosphate. Increasing the time of exposure to metformin (24 hr instead of 1 hr) led to significant changes in the expression of genes involved in glucose and fatty acid metabolism. Indeed, when hepatocytes were cultured in the presence of 50 to 500 microM metformin, the expression of genes encoding regulatory proteins of fatty acid oxidation (carnitine palmitoyltransferase I), ketogenesis (mitochondrial hydroxymethylgltaryl-CoA synthase), and gluconeogenesis (glucose 6-phosphatase, phosphoenolpyruvate carboxykinase) was decreased by 30 to 60%, whereas expression of genes encoding regulatory proteins involved in glycolysis (glucokinase and liver-type pyruvate kinase) was increased by 250%. In conclusion, this work suggests that metformin could reduce hepatic glucose production through short-term (metabolic) and long-term (genic) effects.

Animals↗

PubMed1983

Specific mismatch correction in bacteriophage lambda crosses by very short patch repair.

In crosses under rec+, red+, gam+ conditions, mutation am6 in the cI (repressor) gene of bacteriophage lambda recombines with other cI mutations much more frequently than predicted by the physical distances involved. In four-factor crosses of am6 with mutations located 22-60 base pairs to the left, cI+ recombinants that are expected to require three crossovers (triple recombinants) are more frequent than recombinants that require only one crossover. However, when am6 is crossed with large insertions in cI, which may be expected to interfere with the formation of heteroduplexes by branch migration, the frequency of cI+ triple recombinants is very low. In addition, cI+ recombinants in crosses between am6 and adjacent mutations have a high probability of retaining the flanking markers of the am6 parent. These findings suggest that am6 is particularly susceptible to mismatch repair in heteroduplexes spanning cI. A large fraction of such heteroduplexes are presumed to be the result of branch migration from crossovers occurring at some distance from am6. The absence of co-repair when am6 is crossed with adjacent cI mutations indicates that most repair tracts extend no farther than about 20 bp to either side of the mismatch. The am6 mutation arose in the glutamine codon in a CCAGG sequence, in which the central cytosines are methylated in K12 strains. Their location in methylated sequences may make certain amber mutations susceptible to a specific very short patch (VSP) repair.

Bacteriophage lambda↗

PubMed2005

Scope and mechanistic study of the ruthenium-catalyzed ortho-C-H bond activation and cyclization reactions of arylamines with terminal alkynes.

The cationic ruthenium hydride complex [(PCy(3))(2)(CO)(CH(3)CN)(2)RuH](+)BF(4)(-) was found to be a highly effective catalyst for the C-H bond activation reaction of arylamines and terminal alkynes. The regioselective catalytic synthesis of substituted quinoline and quinoxaline derivatives was achieved from the ortho-C-H bond activation reaction of arylamines and terminal alkynes by using the catalyst Ru(3)(CO)(12)/HBF(4).OEt(2). The normal isotope effect (k(CH)/k(CD) = 2.5) was observed for the reaction of C(6)H(5)NH(2) and C(6)D(5)NH(2) with propyne. A highly negative Hammett value (rho = -4.4) was obtained from the correlation of the relative rates from a series of meta-substituted anilines, m-XC(6)H(4)NH(2), with sigma(p) in the presence of Ru(3)(CO)(12)/HBF(4).OEt(2) (3 mol % Ru, 1:3 molar ratio). The deuterium labeling studies from the reactions of both indoline and acyclic arylamines with DCCPh showed that the alkyne C-H bond activation step is reversible. The crossover experiment from the reaction of 1-(2-amino-1-phenyl)pyrrole with DCCPh and HCCC(6)H(4)-p-OMe led to preferential deuterium incorporation to the phenyl-substituted quinoline product. A mechanism involving rate-determining ortho-C-H bond activation and intramolecular C-N bond formation steps via an unsaturated cationic ruthenium acetylide complex has been proposed.

Alkynes↗

PubMed2003

Effects of adrenomedullin on ocular hemodynamic parameters in the choroid and the ophthalmic artery.

PURPOSE: Adrenomedullin acts as a vasodilator and may play a role in inflammatory processes in the eye. This study was designed to determine whether nitric oxide formation is involved in the response to adrenomedullin in the ocular vasculature in vivo. METHODS: The effects of systemic intravenous adrenomedullin (3.2-16.0 pmol/[kg. min])) on choroidal blood flow were assessed by measurement of fundus pulsation amplitude and laser Doppler flow in the macula, and on blood flow in the ophthalmic artery by ultrasound Doppler flow in pilot studies (n = 7). Subsequently, in a double-blind randomized placebo-controlled crossover study in eight healthy male subjects the effects of 12.8 pmol/(kg. min) adrenomedullin on ocular and systemic hemodynamics were investigated. Adrenomedullin was co-infused with the nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine (3 mg/kg bolus and 30 micro g/[kg. min] continuous intravenous infusion) or vehicle control on separate study days. RESULTS: Adrenomedullin dose dependently increased choroidal blood flow and flow velocity in the ophthalmic artery. N(G)-monomethyl-L-arginine reduced the effect of adrenomedullin on fundus pulsation amplitude, but did not alter the flow response in the ophthalmic artery. Systemic hemodynamics were unaffected by adrenomedullin infusion. CONCLUSIONS: Ocular blood flow is sensitive to changes in adrenomedullin concentrations. The acute vasodilator effects of adrenomedullin are nitric oxide-dependent in the choroid, but not in the ophthalmic artery.

Adrenomedullin↗

PubMed2002

Complexities in ETS-domain transcription factor function and regulation: lessons from the TCF (ternary complex factor) subfamily. The Colworth Medal Lecture.

The ETS-domain transcription factor family can be divided into a series of subfamilies. Elk-1 represents the founding member of the ternary complex factor (TCF) subfamily. By focusing on the TCF subfamily, we can demonstrate the complexities that exist in the function and regulation of ETS-domain transcription factors. This article focuses on Elk-1 in detail and summarizes the functions of other TCFs. The key themes covered include the domain structure of the TCFs, the mechanisms of complex formation with serum response factor, regulation of TCFs by mitogen-activated protein kinase cascades, and transcriptional regulatory properties of the TCFs. Finally, the emerging role of the TCFs in vivo is discussed. A picture is developing indicating that, while these proteins exhibit significant sequence and functional conservation, key differences in their structure and regulation are being identified which may relate to unique functions of these proteins in vivo.

Amino Acid Sequence↗