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Results for “crossover frequency”

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At least 145 records · Page 8Linked to original sources
PubMed1984

Lorcainide for high-frequency ventricular arrhythmia: preliminary results of a short-term double-blind and placebo-controlled crossover study and long-term follow-up.

Lorcainide, 100 mg twice daily was compared with placebo in 39 patients with frequent ventricular arrhythmias in a randomized double-blind crossover trial. A mean frequency of ventricular premature beats (VPBs) of at least 30 VPBs/hour was required during a drug-free period of 48 hours. Holter monitoring and a maximal symptom-limited exercise test were performed at the end of each of the 2-week double-blind treatment phase. The group averaged 350 +/- 361 (standard deviation) VPBs/hour. Lorcainide decreased the mean VPB frequency of the group by 46% (p less than 0.01), with VPB reduction beyond the expected variation in 22 of 39 patients. In 13 patients VPBs were unchanged and in 4 they increased. Eight additional patients responded during drug titration, for an overall response rate of 77% (30 of 39). Lorcainide did not significantly reduce the exercise-related VPB frequency. At 6 months 61% of patients had significant VPB suppression. Thus, lorcainide was effective in reducing the frequency and grade of spontaneous ventricular arrhythmias during short- and long-term evaluation.

Adult↗
PubMed2002

The effect of a deficiency and a deletion on recombination in chromosome 1BL in wheat.

To test two models of chiasma allocation and the distribution of crossing-over in chromosomes, genetic mapping was performed in normal, deletion and deficiency chromosome arms 1BL of wheat, Triticum aestivum L. Shortening of the chromosome arm, either by a deletion of the proximal half of the arm or by a deficiency of the terminal quarter of the arm's length, significantly reduced the frequency of multiple crossovers but did not affect the distribution of the distal, presumably the first, crossover in the arm. In the deficiency chromosome, the recombination rate in the terminal segment was much higher than that in the same segment of the complete arm. This suggests that recombination frequency is not an inherent characteristic of a segment but depends on the segment's position on the centromere-telomere axis. These observations support the classical model of chiasma distribution along the chromosome based on the point of pairing initiation, chromosome length and the positive chiasma interference. The study also demonstrates that the distribution and frequency of recombination in a chromosome segment can be manipulated. Therefore, even the segments with very low recombination frequencies could be saturated with large numbers of crossover events to produce high-density genetic maps.

Journal Article↗
PubMed2004

Collective modes and ballistic expansion of a fermi gas in the BCS-BEC crossover.

We evaluate the frequencies of collective modes and the anisotropic expansion rate of a harmonically trapped Fermi superfluid at varying coupling strengths across a Feshbach resonance driving a BCS-BEC crossover. The equations of motion for the superfluid are obtained from a microscopic mean-field expression for the compressibility and are solved within a scaling ansatz. Our results confirm nonmonotonic behavior in the crossover region and are in quantitative agreement with current measurements of the transverse breathing mode by Kinast et al. [Phys. Rev. Lett. 92, 150402 (2004)]] and of the axial breathing mode by Bartenstein et al. [Phys. Rev. Lett. 92, 203201 (2004)]].

Journal Article↗
PubMedDate not supplied

Slow evoked cortical responses to linear frequency ramps in subjects with cochlear lesions.

Slow evoked cortical potentials in response to linear frequency ramps of a continuous pure tone with a 1 kHz base frequency have been recorded from ten relatively young subjects with hearing loss of cochlear origin. At small frequency ramps, the N1-latency of their responses to the three ramp durations studied (20, 100, 500 msec) was significantly longer than those of a group with normal hearing. As the ramps are made larger, the difference between the latencies of the pathological group and the normal group becomes smaller; above a certain rate of frequency change, the latency of N1 becomes smaller in the pathological cases than in the normal group. This crossover occurs at frequency change rates around 1-3 kHz/sec. It is concluded that recording of evoked cortical responses to frequency ramps may provide an additional tool in the differential diagnosis of hearing disorders.

Acoustic Stimulation↗
PubMed1989

Are there morphologically abnormal early recombination nodules in the Drosophila melanogaster meiotic mutant mei-218?

Early recombination nodules have been suggested to perform a role in meiotic gene conversion recombination events. The meiotic recombination-defective mutant mei-218 greatly reduces the frequency of meiotic crossover (reciprocal) recombination events and reduces the number of late recombination nodules to the same extent. However, it does not reduce the frequency of simple gene conversion events, although they are abnormal in having shorter coconversion tracts than controls. The original cytological study yielded somewhat fewer early nodules in mei-218 than in controls, although very abnormal ones might have been missed. The present study failed to identify a mei-218 specific abnormal category. However, because recombination nodules are at present recognizable only by their morphology, a definitive answer to this question must await a specific probe for recombination nodules. Moreover, the possibility remains that early nodules in mei-218 are more ephemeral than are early nodules in wild type.

Animals↗
PubMed2004

"Conflicting" motion cues to the visual and vestibular self-motion systems around 0.06 Hz evoke simulator sickness.

The basic question this research addressed was, how does simulator sickness vary with simulated motion frequency? Participants were 11 women and 19 men, 20 to 63 years of age. A visual self-motion frequency response curve was determined using a Chattecx posture platform with a VR4 head-mounted display (HMD) or a back-projected dome. That curve and one for vestibular self-motion specify a frequency range in which vestibular and visual motion stimuli could produce conflicting self-motion cues. Using a rotating chair and the HMD, a third experiment supported (p < .01) the hypothesis that conflicting cues at the frequency of maximum "crossover" between the curves (about 0.06 Hz) would be more likely to evoke simulator sickness than would conflicting cues at a higher frequency. Actual or potential applications of this work include a preliminary design guidance curve that indicates the frequency range of simulated motion that is likely to evoke simulator or virtual reality sickness; for simulators intended to operate in this frequency range, appropriate simulator sickness interventions should be considered during the design process.

Computer Simulation↗
PubMed2003

Postrecognition of interleaved melodies as an indirect measure of auditory stream formation.

Primitive processes involved in auditory stream formation are measured with indirect, objective method. A target melody interleaved with a distractor sequence is followed by a probe melody that was identical to the target or differed by 2 notes. Listeners decided whether the probe melody was present or not in the composite sequence. Interleaved melody recognition is not possible when distractor sequences have the same mean frequency and maximum contour crossover with target melodies. Performance increases with mean frequency separation and timbral dissimilarity and is unaffected by the duration of the silent interval between composite sequence and probe melody. The relation between this indirect task measuring the interleaved melody recognition boundary and direct judgments measuring the fission boundary is discussed.

Acoustic Stimulation↗
PubMed2000

The occurrence of double strand DNA breaks is not the sole condition for meiotic crossing over in Drosophila melanogaster.

Analysis of the interchromosomal effects of In(2L + 2R)Cy, In(3L + 3R)LVM and their joint effect on the frequencies of single and double crossovers in the cv-v-f region of the X chromosome as well as interference showed that both inversions, occurring separately, increased the frequency of single as well as double crossovers and the coefficient of coincidence. However, when the inversions occurred together the frequencies of single crossovers no longer increased, but the frequency of double crossovers, as well as the coefficient of coincidence did increase. These results indicate firstly that the interchromosomal effects influence some precondition of exchange, but that this precondition is not an occurrence of double strand DNA breaks. Thus, the occurrence of double strand DNA breaks is not the sole condition for crossing over in Drosophila melanogaster.

Animals↗
PubMed2002

Recombination analysis of the human minisatellite MsH42 suggests the existence of two distinct pathways for initiation and resolution of recombination at MsH42 in rat testes nuclear extracts.

We have previously described a GC-rich human minisatellite, termed MsH42, which exists in two allelic forms, long and short. Here, we have identified a third allele of medium length and localized the MsH42 locus in the chromosome 15q25.1 inside an intron belonging to a gene of unknown function. The recombinogenic potential of the three alleles was assayed in vitro incubating pBR322-based constructs containing two copies of the minisatellite MsH42 with its flanking sequences, in the presence of rat testes nuclear extracts. This assay system was configured to monitor only reciprocal exchange type events and not gene conversion. All MsH42 allelic sequences enhanced intramolecular homologous recombination promoting high rates (approximately 76%) of equal crossover, the long allele showing the highest recombinogenic activity. Removal of the MsH42 long allele flanking sequences, which are identical in the three alleles, provoked a decrease in the enhancement of recombination and in the frequency of equal crossovers, suggesting that these sequences are important for the recombinogenic activity and for the correct pairing between homologous sequences. The occurrence of some complex recombination events within the minisatellite MsH42 suggests the existence of processes related to polymerase slippage and unwinding with reinvasion during the repair synthesis. Our findings point toward the existence of two distinct biochemical pathways for initiation and resolution of recombination at the minisatellite MsH42. Finally, the in vitro recombination system employed in this study could provide an approach to dissect processes of repetitive DNA instability and recombination.

Alleles↗
PubMed2000

High resolution analysis of haplotype diversity and meiotic crossover in the human TAP2 recombination hotspot.

Little is known about the nature of recombination hotspots in the human genome and the relationship between crossover activity and patterns of linkage disequilibrium. We have therefore used both haplotype analysis and direct detection of crossovers in sperm to characterize a putative recombination hotspot in the TAP2 gene within the class II region of the MHC. Haplotype diversity provided evidence for a localized hotspot within intron 2 of this gene. Sperm DNA typing using allele-specific PCR primers to selectively amplify recombinant TAP2 molecules revealed a highly localized meiotic crossover hotspot approximately 1.2 kb long, unusually abundant in sequence polymorphisms and flanked by DNA much less active in recombination. Sperm crossover appeared to be fully reciprocal, and almost all crossover products were simple, involving a single exchange between adjacent heterozygous markers. This hotspot appears to be much more active in female than male meiosis. No primary sequence similarities could be found between any of the very few well defined crossover hotspots in the human genome, all of which show recombinationally active domains 1-2 kb long. Direct comparison of recombination frequency and haplotype diversity in TAP2 showed that linkage disequilibrium measures were a poor predictor of crossover frequency in this region, with non-recombining markers sometimes in free association and with examples of pairs of markers spanning the recombination hotspot showing substantial or even absolute linkage disequilibrium.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMedDate not supplied

Genetic transformation of mycobacteria by homologous recombination.

Mycobacteria are highly potent adjuvants; therefore, expression of foreign genes in mycobacteria provides a delivery system to induce strong immune responses against foreign proteins. In this study we report transformation of Mycobacterium smegmatis by homologous recombination using pUC19-based plasmid vectors with pyrF gene of M. smegmatis (pY6001) or pyrF gene disrupted by introducing the aminoglycoside phosphotransferase (aph) gene (pY6002). Both of these plasmids were used to transform the host cells by electroporation. The transformation and selection conditions were optimized with respect to cell number, stage of cell growth, DNA concentration, postelectroporation incubation time, and kanamycin concentration. With the plasmid Y6002, the transformation was usually a result of single crossover (class I transformants) and only 5% transformants were generated by double crossover (class II transformants). The double crossover led to the replacement of wild-type pyrF gene with the aph-disrupted pyrF gene. The gene replacement could also occur by resolution of the class I transformants into class II, but at a very low frequency. Further experiments were done to determine if the wild-type genotype could be rescued by retransformation with pY6001. Similar transformation efficiencies, as reported above, were obtained, but the frequency of double crossover increased to 35%. This transformation strategy provides a way by which the mycobacteria transformed with foreign genes will not require drug selection, a trait preferred to develop recombinant vaccines.

Blotting, Southern↗
PubMed2006

Glass-specific behavior in the damping of acousticlike vibrations.

High frequency sound is observed in lithium diborate glass, Li2O-2B2O3, using Brillouin scattering of light and x rays. The sound attenuation exhibits a nontrivial dependence on the wave vector, with a remarkably rapid increase towards a Ioffe-Regel crossover as the frequency approaches the boson peak from below. An analysis of literature results reveals that the boson-peak frequency is closely related with a Ioffe-Regel limit for sound in many glasses. We conjecture that this relation, specific to glassy materials, might be rather common among them.

Journal Article↗
PubMed1985

A double-crossover study comparing conventional ventilation with high frequency ventilation in a patient with tracheoesophageal fistula.

Respiratory distress, from severe gastric aspiration pneumonitis and abdominal distention in the patient with tracheoesophageal fistula frequently requires mechanical ventilatory support. Bulk flow ventilation can lead to enlargement of the fistulous tract, elevation of gastric intraluminal pressures, raised airway pressures with hemodynamic instability, and retained secretions. We report a case of tracheoesophageal fistula, secondary to perforation of a squamous cell carcinoma of the esophagus, with temporary improvement in gas exchange on high frequency ventilation after failing on a conventional ventilator. The patient initially failed to improve on an Engstrom ventilator (Engstrom-Gambro, Inc., Barrington, IL) at 13 l/minute ventilation. Instituting high frequency jet ventilation with a VS 600 Jet Ventilator (Instrument Development Corporation, Pittsburgh, PA) at initial settings of 35 psi, rate 150, inspiratory time 40%, FiO2 0.8 and 12 cm H2O positive end expiratory pressure (PEEP), provided incremental improvement in gas exchange and oxygenation up to 26 cm H2O PEEP. However, in view of progressive multi-organ failure we terminated the jet ventilation after 48 h and returned the patient to conventional ventilation. We were unable to provide life-sustaining ventilation and oxygenation with either an Engstrom ventilator at 13 l/-minute ventilation or an MA-1 ventilator (Puritan-Bennett, Kansas City, MO) at a tidal volume of 800 cc and a ventilator rate of 30. Terminal respiratory failure occurred. Based on the period of improvement using high frequency jet ventilation, we believe this mode of ventilatory support is beneficial in the management of tracheoesophageal fistula.

Aged↗
PubMed1989

Yeast intrachromosomal recombination: long gene conversion tracts are preferentially associated with reciprocal exchange and require the RAD1 and RAD3 gene products.

A yeast intrachromosomal recombination system based on an inverted repeat has been designed to examine mitotic gene conversion tract length and the association of crossing over with gene conversion as a function of the conversion tract length. Short conversion tracts are found to be preferentially noncrossover while conversion tracts longer than 1.16 kb show a 50% association with crossover. Mutation in the excision repair gene RAD1 leads to a reduction in conversion tracts of at least 1.16 kb and a reduction in crossovers associated with conversion, regardless of the length of the conversion tract. Mutation in the excision repair gene RAD3, which encodes a DNA helicase, also leads to a reduction in conversion tracts of at least 1.16 kb, but has no effect on the frequency of associated crossovers. The roles of RAD1 and RAD3 in recombination are discussed.

Chromosome Inversion↗
PubMed1989

The effect of cisapride on defaecation in normal human subjects--lack of effect on faecal excretion of water, fat, and bile acids.

In order to elucidate whether or not the increased stool frequency that occurs during cisapride treatment is a result of malabsorption of water, fat, and bile acids, 12 healthy volunteers were dosed with either tablets of placebo q.d.s. or tablets of 10 mg cisapride q.d.s. during two periods of 5 days in a double-blind, crossover study. Stool frequency, stool consistency, and side-effects were recorded each day. Total faecal mass, faecal water content, and faecal excretion of fat and bile acids were determined during the last 72 h of each study period. Mean daily stool frequency was 18.8% higher during cisapride [1.68 +/- 0.12 (S.E.M.)] administration than during placebo (1.42 +/- 0.12); P = 0.038. The stool consistency score increased by 11.8% towards softer stools during cisapride dosing (N.S.). There were no significant differences in total faecal mass (placebo 399.4 g/72 h; cisapride; 414.5 g/72 h), faecal water content (placebo; 75.6%: cisapride 76.2%), or faecal excretion of fat (placebo; 12.7 g/72 h: cisapride; 11.6 g/72 h) and total bile acids (placebo; 2212 mumol/72 h: cisapride; 2261 mumol/72 h). The side-effects reported during placebo were constipation (n = 3), and during cisapride meteorism (n = 4) and increased appetite (n = 2). The increased stool frequency during cisapride treatment is not caused by malabsorption of water, fat, or bile acids, but seems to be the consequence of a direct motor effect.

Adult↗