Science Collected Works - Theses

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Subject: Actin ×

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    Interaction of actin and microtubules during spatially organised cell division in green plants
    McIntosh, Kirsten (University of Melbourne, 2002)
    I investigated cytokinesis in three species of green algae: Spirogyra, Coleochaete scutata and C. nitellarum. Spirogyra initiates cytokinesis with an actin-cleavage furrow. At telophase, the edge of this furrow impinges upon the spindle fibres, transforming them into a small phragmoplast. Cell plate formation completes the cross-wall. These two sequential mechanisms cannot replace each other and a successful interaction between the microtubules of the phragmoplast and the actin of the cleavage furrow is crucial for completion of cytokinesis. Both the radial and circumferential divisions of C. scutata use a phragmoplast during cytokinesis in the apparent absence of an actin system of cleavage. The circumferential division forms a wall ingrowth which appears to be produced by the phragmoplast. Radial cytokinesis involves a phragmoplast and centrifugal expansion of a cell plate. In C. nitellarum, transverse and branch divisions use a phragmoplast which can equally well form a typical cell plate or a wall ingrowth. Neither of these divisions uses an actin cleavage furrow. Mitosis in these green algae was also investigated. High concentrations of anti-actin drugs caused abnormalities during mitosis. In Spirogyra, these abnormalities included an apparent skip from prometaphase to anaphase (i.e., bypassing metaphase) and an abnormal elongation of the spindle. These drugs caused a reversible block at metaphase in C. scutata, and a prolonged prometaphase, and sometimes a prolonged metaphase, in C. nitellarum. Collectively, these observations suggest that actin is involved in mitosis. In addition, in C. nitellarum an anti-microtubule drug caused persistent oscillations of the chromosomes after collapse of the metaphase plate. These continued movements of the chromosomes suggest that they may attach to a second motility system (not using microtubules) during mitosis. Finally, I investigated the curious phenomenon of cytoplasmic rotation in the hair cells of C. scutata and C. nitellarum. An actin-myosin system appears to operate during rotation, and in C. scutata microtubules may also be involved.
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    Actin is involved in chromosomal attachment to the spindle in the green alga Oedogonium
    Sampson, Karina (University of Melbourne, 2001)
    Lightmicroscopy, flourescence labelling and ultrastructural techniques were used, along with various anti-actin, anti-myosin and anti-microtubular drugs, to study mitosis in the green alga Oedogonium, with the aim of defining the role played by actin. Cytochalasin D and J, Latrunculin A and Butanedione Monoxime (BDM) all caused aberrations in mitosis. Cytochalasin D rapidly and reversibly blocked mitosis in these cells. During cytochalasin D or J -treatment, chromosomes appeared unable to attach or maintain attachment to the spindle, chromosomes moving about the spindle in a haphazard way. With longer treatments with Cytochalasin D or J, mitosis became increasingly abnormal; spindles often underwent slow and excessive elongation. In cells treated with Cytochalasin J or BDM, chromosome movement was often reduced to a jostling or a waving of the chromosome arms. When cells were treated with latrunculin A, division appeared almost normal except that chromatids never separated evenly to the poles. My ultrastructural studies revealed that cytochalasin D-treatment caused kinetochores to appear slightly flattened with fewer number of MTs associated with them. Actin, as defined by its reactivity with phallacidin, was found to be present in the form of small tufts at the kinetochores. This location coincides with the filamentous material detected ultrastructurally within the same cells, by Schibler and Pickett-Heaps (1980), and Pickett-Heaps and Carpenter (1993). The staining of this material was affected by anti-actin drugs, but not by the anti-microtubular drug, oryzalin. To extend this work further, I decided to do a preliminary investigation of another mitotic system, the pennate diatom Pinnularia. Mitotic cells from prometaphase onwards, displayed actin staining in the region occupied by the central spindle and the poles. Cells in prometaphase and anaphase also showed a haze of actin staining in the region of the chromosomes. My experiments (and those of other researchers, for example: Czaban and Forer, 1994; Snyder and Cohen, 1995) have led me to suspect that actin is a vital component of the spindle apparatus, possibly contributing to the correct attachment of chromosomes to the spindle.
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    Actin is involved in chromosomal attachment to the spindle in the green alga Oedogonium
    Sampson, Karina (University of Melbourne, 2001)
    Lightmicroscopy, flourescence labelling and ultrastructural techniques were used, along with various anti-actin, anti-myosin and anti-microtubular drugs, to study mitosis in the green alga Oedogonium, with the aim of defining the role played by actin. Cytochalasin D and J, Latrunculin A and Butanedione Monoxime (BDM) all caused aberrations in mitosis. Cytochalasin D rapidly and reversibly blocked mitosis in these cells. During cytochalasin D or J -treatment, chromosomes appeared unable to attach or maintain attachment to the spindle, chromosomes moving about the spindle in a haphazard way. With longer treatments with Cytochalasin D or J, mitosis became increasingly abnormal; spindles often underwent slow and excessive elongation. In cells treated with Cytochalasin J or BDM, chromosome movement was often reduced to a jostling or a waving of the chromosome arms. When cells were treated with latrunculin A, division appeared almost normal except that chromatids never separated evenly to the poles. My ultrastructural studies revealed that cytochalasin D-treatment caused kinetochores to appear slightly flattened with fewer number of MTs associated with them. Actin, as defined by its reactivity with phallacidin, was found to be present in the form of small tufts at the kinetochores. This location coincides with the filamentous material detected ultrastructurally within the same cells, by Schibler and Pickett-Heaps (1980), and Pickett-Heaps and Carpenter (1993). The staining of this material was affected by anti-actin drugs, but not by the anti-microtubular drug, oryzalin. To extend this work further, I decided to do a preliminary investigation of another mitotic system, the pennate diatom Pinnularia. Mitotic cells from prometaphase onwards, displayed actin staining in the region occupied by the central spindle and the poles. Cells in prometaphase and anaphase also showed a haze of actin staining in the region of the chromosomes. My experiments (and those of other researchers, for example: Czaban and Forer, 1994; Snyder and Cohen, 1995) have led me to suspect that actin is a vital component of the spindle apparatus, possibly contributing to the correct attachment of chromosomes to the spindle.