We studied chromosome movement after kinetochore microtubules were severed. Severing a kinetochore fibre in living crane-fly spermatocytes with an ultraviolet microbeam creates a kinetochore stub, a birefringent remnant of the spindle fibre connected to the kinetochore and extending only to the edge of the irradiated region. After the irradiation, anaphase chromosomes either move poleward led by their stubs or temporarily stop moving. We examined actin and/or microtubules in irradiated cells by means of confocal fluorescence microscopy or serial-section reconstructions from electron microscopy. For each cell thus examined, chromosome movement had been recorded continuously until the moment of fixation. Kinetochore microtubules were completely severed by the ultraviolet microbeam in cells in which chromosomes continued to move poleward after the irradiation: none were seen in the irradiated regions. Similarly, actin filaments normally present in kinetochore fibres were severed by the ultraviolet microbeam irradiations: the irradiated regions contained no actin filaments and only local spots of non-filamentous actin. There was no difference in irradiated regions when the associated chromosomes continued to move versus when they stopped moving. Thus, one cannot explain motion with severed kinetochore microtubules in terms of either microtubules or actin-filaments bridging the irradiated region. The data seem to negate current models for anaphase chromosome movement and support a model in which poleward chromosome movement results from forces generated within the spindle matrix that propel kinetochore fibres or kinetochore stubs poleward.

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We studied the distribution of acetylated a-tubulin in the microtubules of spermatogenic cells from the crane fly Nephroromci sufurcilis (Loew) using a monoclonal antibody specific for acetylated a-tubulin (6-1 IB-I). We found that cells in all stages of spermatogenesis contained acetylated microtubules, including primary spermatocytes, meiotic cells, spermatids, and sperm. A subset of the acetylated microtubules (those in midbodies and flagella) were resistant to cold depolymerization. Newly polymerized microtubules in nondividing cells were not acetylated for up to 15 min, indicating that acetylation lagged behind polymerization. In spindles, newly polymerized microtubules were acetylated after 5 min.Antibodies to acetylated a-tubulin selectively stained chromosome-to-pole fibers in dividing cells, but the staining appeared to decrease and taper off at the kinetochores. This observation supports the hypothesis that tubulin subunits add at the kinetochore in metaphase and that acetylation occurs subsequent to addition. Further, this taper may be useful as a marker in anaphase, to distinguish between different hypotheses of chromosome motion.

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Background: People with cerebral palsy (CP) are less physically active than the general population and, consequently, are at increased risk of preventable disease. Evidence indicates that low-moderate doses of physical activity can reduce disease risk and improve fitness and function in people with CP. Para athletes with CP typically engage in ‘performance-focused’ sports training, which is undertaken for the sole purpose of enhancing sports performance. Anecdotally, many Para athletes report that participation in performance-focused sports training confers meaningful clinical benefits which exceed those reported in the literature; however, supporting scientific evidence is lacking. The aim of this paper is to describe the protocol for an 18-month study evaluating the clinical effects of a performance-focused swimming training programme for people with CP who have high support needs. Methods: This study will use a concurrent multiple-baseline, single-case experimental design across three participants with CP who have high support needs. Each participant will complete a five-phase trial comprising: baseline (A1); training phase 1 (B1); maintenance phase 1 (A2); training phase 2 (B2); and maintenance phase 2 (A3). For each participant, measurement of swim velocity, health-related quality of life and gross motor functioning will be carried out a minimum of five times in each of the five phases. Discussion: The study described will produce Level II evidence regarding the effects of performance-focused swimming training on clinical outcomes in people with CP who have high support needs. Findings are expected to provide an indication of the potential for sport to augment outcomes in neurological rehabilitation.

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Many higher eukaryotes have dispersed repetitive DNA and multiple instances of segmental duplications. As well, many plants and lower animals are polyploids. Thus restricting reciprocal genetic exchange to truly homologous chromosomes is likely a multi-step process. We propose the following sequence of events. First the ability to form a synaptonemal complex (SC) prematurely (i.e. before homology checking/recognition) is precluded by the organization of chromosomes during premeiotic S phase. Next rough alignment is accomplished regionally by having key allelic transcription units brought to the same transcription center. Once rough alignment is accomplished, close alignment can occur in conjunction with homology checking/recognition. Successful homology checking produces changes that now permit SC formation within the region of the check. Some organisms (with challenges to true homology such as dispersed repetitive DNA and segmental duplications) may require that, for a region to be competent to form an SC, successful homology checks must occur at both ends of the region. Successful early SC formation may provide an environment in which recombination intermediates can be earmarked for resolution into crossovers. Later in prophase I SC formation can occur nonhomologously, if two unsynapsed chromosomal axes meet.

Glycerinated rabbit psoas myofibrils, F-actin, and myofibril ghosts were irradiated with ultraviolet light (UV) to investigate how UV blocks myofibril contraction. Myofibril contraction is most sensitive to 270- and 290-nm wavelength light. We irradiated I and A bands separately with 270- and 290-nm wavelength light using a UV microbeam and constructed dose-response curves for blocking sarcomere contraction. For both wavelengths, irradiations of A bands required less energy per area to block contraction than did irradiations of I bands, suggesting that the primary effects of both 270- and 290-nm wavelength light in stopping myofibril contraction are on myosin. We investigated whether the primary effect of UV in blocking I-band contraction is the depolymerization of actin by comparing the relative sensitivities of I-band contraction, F-actin depolymerization, and thin filament depolymerization to 270- and 290-nm light. We also compared the dose of UV required to depolymerize F-actin in solution with the dose needed to block I-band contraction and the dose required to alter thin filament structure in myofibril ghosts. The results confirm that UV blocks I-band contraction by depolymerizing actin. We discuss how the results might be relevant to the hypothesis that an actomyosin-based system is involved in chromosome movement.