Nuclear and cytoplasmic mitotic cycles continue in Drosophila embryos in which DNA synthesis is inhibited with aphidicolin

Universal control mechanism regulating onset of M-phase

The onset of M-phase is regulated by a mechanism common to all eukaryotic cells. Entry into M-phase is determined by activation of the p34cdc2 protein kinase which requires p34cdc2 dephosphorylation and association with cyclin.

Mutation of fission yeast cell cycle control genes abolishes dependence of mitosis on DNA replication

Entry into mitosis in fission yeast is controlled by the p34cdc2 protein kinase, which is activated by cdc25+ and inhibited by wee1+. In "wee" mutants one or the other of these controls is circumvented resulting in advancement of mitosis. We report that dependence of mitosis on DNA synthesis is lost in wee mutants in which cdc25+ control is circumvented either by mutations in cdc2+ or by overproduction of cdc25+. In contrast, dependence is maintained when the wee1+ control is bypassed. We propose that cdc25+ activity requires completion of earlier cell-cycle events such as DNA synthesis, and thus links p34cdc2 kinase activation to completion of these earlier events. Constitutive expression of cdc25+ homologs could explain why mitosis is not dependent on DNA replication in some early embryos.

Quartet: a Drosophila developmental mutation affecting chromosome separation in mitosis

The Drosophila mutation, quartet, affects development at points in the life cycle that require intense mitotic activity. Examination of embryos affected by the maternal effect of quartet has revealed defects that can be attributed to incomplete chromosome separation at mitosis. These defects include uneven spacing of nuclei, strands of DNA creating bridges between nuclei, and abnormal amounts of DNA per nucleus. Nuclei in quartet-affected embryos also have a greater-than-normal number of centrosomes. Immunofluorescent examination of the spindles in quartet-affected embryos has revealed tripolar spindles and adjacent spindles that share a common spindle pole. Finally, chromosome separation distance was measured in anaphase and telophase spindles in quartet-affected embryos and found to be blocked in anaphase. Examination of mitotic figures in quartet larvae revealed a reduced mitotic index and an elevated frequency of abnormal mitotic figures. quartet could encode a function necessary for the disengagement of chromosomes in mitosis, for kinetochore function or for function of a spindle motor. Mutations in quartet prevent the post-translational modification of three abundant proteins. These proteins may be involved in chromosome separation in mitosis.

Identification of microtubule-associated proteins in the centrosome, spindle, and kinetochore of the early Drosophila embryo

We have developed affinity chromatography methods for the isolation of microtubule-associated proteins (MAPs) from soluble cytoplasmic extracts and have used them to analyze the cytoskeleton of the early Drosophila embryo. More than 50 Drosophila embryo proteins bind to microtubule affinity columns. To begin to characterize these proteins, we have generated individual mouse polyclonal antibodies that specifically recognize 24 of them. As judged by immunofluorescence, some of the antigens localize to the mitotic spindle in the early Drosophila embryo, while others are present in centrosomes, kinetochores, subsets of microtubules, or a combination of these structures. Since 20 of the 24 antibodies stain microtubule structures, it is likely that most of the proteins that bind to our columns are associated with microtubules in vivo. Very few MAPS seem to be identically localized in the cell, indicating that the microtubule cytoskeleton is remarkably complex.

Parthenogenesis in Xenopus eggs injected with centrosomes from synchronized human lymphoid cells

In Xenopus eggs, normal development requires the participation of the centrosome provided by the sperm. Injection of foreign centrosomes purified from exponentially growing mammalian cells enables the eggs to undertake parthenogenesis. In order to know whether such a complementation required centrosomes already committed to duplication, we have prepared centrosomes from human cells synchronized at different stages of the cell cycle (G0, G1, G2). We show that the three types of centrosome possess a similar parthenogenetic activity and conclude that duplication of heterologous centrosome can be triggered in Xenopus eggs.

Checkpoints: controls that ensure the order of cell cycle events

The events of the cell cycle of most organisms are ordered into dependent pathways in which the initiation of late events is dependent on the completion of early events. In eukaryotes, for example, mitosis is dependent on the completion of DNA synthesis. Some dependencies can be relieved by mutation (mitosis may then occur before completion of DNA synthesis), suggesting that the dependency is due to a control mechanism and not an intrinsic feature of the events themselves. Control mechanisms enforcing dependency in the cell cycle are here called checkpoints. Elimination of checkpoints may result in cell death, infidelity in the distribution of chromosomes or other organelles, or increased susceptibility to environmental perturbations such as DNA damaging agents. It appears that some checkpoints are eliminated during the early embryonic development of some organisms; this fact may pose special problems for the fidelity of embryonic cell division.

Centrosomes, and not nuclei, initiate pole cell formation in Drosophila embryos

An injection of aphidicolin into early Drosophila embryos inhibits DNA synthesis and nuclear division, while centrosome replication and many other aspects of the mitotic cycle continue. If aphidicolin is injected at nuclear cycle 7-8, the normal migration of nuclei to the embryo cortex is completely inhibited. In most of these embryos, however, centrosomes continue to migrate in a coordinated manner to the cortex, where they reorganize tubulin, actin, and the overlying plasma membrane. Remarkably, the centrosomes that migrate to the posterior pole of such embryos initiate pole cell formation in the absence of nuclei. These observations demonstrate that centrosomes alone are able to direct a major reorganization of the cortical cytoskeleton when they arrive at the surface of the embryo. They also suggest that the coordinated movement of nuclei to the embryo cortex is mediated by forces acting on the centrosome rather than on the nucleus itself.