Metaphase

About: Metaphase is a research topic. Over the lifetime, 6925 publications have been published within this topic receiving 291590 citations. The topic is also known as: GO:0007091 & mitotic metaphase/anaphase transition.

Serine 31 phosphorylation of histone variant H3.3 is specific to regions bordering centromeres in metaphase chromosomes

Abstract: Histones are the fundamental components of the nucleosome. Physiologically relevant variation is introduced into this structure through chromatin remodeling, addition of covalent modifications, or replacement with specialized histone variants. The histone H3 family contains an evolutionary conserved variant, H3.3, which differs in sequence in only five amino acids from the canonical H3, H3.1, and was shown to play a role in the transcriptional activation of genes. Histone H3.3 contains a serine (S) to alanine (A) replacement at amino acid position 31 (S31). Here, we demonstrate by both MS and biochemical methods that this serine is phosphorylated (S31P) during mitosis in mammalian cells. In contrast to H3 S10 and H3 S28, which first become phosphorylated in prophase, H3.3 S31 phosphorylation is observed only in late prometaphase and metaphase and is absent in anaphase. Additionally, H3.3 S31P forms a speckled staining pattern on the metaphase plate, whereas H3 S10 and H3 S28 phosphorylation localizes to the outer regions of condensed DNA. Furthermore, in contrast to phosphorylated general H3, H3.3 S31P is localized in distinct chromosomal regions immediately adjacent to centromeres. These findings argue for a unique function for the phosphorylated isoform of H3.3 that is distinct from its suspected role in gene activation.

Human chromokinesin KIF4A functions in chromosome condensation and segregation

Abstract: Accurate chromosome alignment at metaphase and subsequent segregation of condensed chromosomes is a complex process involving elaborate and only partially characterized molecular machinery. Although several spindle associated molecular motors have been shown to be essential for mitotic function, only a few chromosome arm–associated motors have been described. Here, we show that human chromokinesin human HKIF4A (HKIF4A) is an essential chromosome-associated molecular motor involved in faithful chromosome segregation. HKIF4A localizes in the nucleoplasm during interphase and on condensed chromosome arms during mitosis. It accumulates in the mid-zone from late anaphase and localizes to the cytokinetic ring during cytokinesis. RNA interference–mediated depletion of HKIF4A in human cells results in defective prometaphase organization, chromosome mis-alignment at metaphase, spindle defects, and chromosome mis-segregation. HKIF4A interacts with the condensin I and II complexes and HKIF4A depletion results in chromosome hypercondensation, suggesting that HKIF4A is required for maintaining normal chromosome architecture. Our results provide functional evidence that human KIF4A is a novel component of the chromosome condensation and segregation machinery functioning in multiple steps of mitotic division.

Fragmentation and partitioning of the Golgi apparatus during mitosis in HeLa cells.

Abstract: Osmium impregnation was used to determine the number of Golgi apparatus in both interphase and mitotic HeLa cells. The number was found to increase substantially during mitosis to the point where random partitioning alone would explain the nearly equal numbers found in each daughter cell.

The kinesin Eg5 drives poleward microtubule flux in Xenopus laevis egg extract spindles.

Abstract: Although mitotic and meiotic spindles maintain a steady-state length during metaphase, their antiparallel microtubules slide toward spindle poles at a constant rate. This “poleward flux” of microtubules occurs in many organisms and may provide part of the force for chromosome segregation. We use quantitative image analysis to examine the role of the kinesin Eg5 in poleward flux in metaphase Xenopus laevis egg extract spindles. Pharmacological inhibition of Eg5 results in a dose–responsive slowing of flux, and biochemical depletion of Eg5 significantly decreases the flux rate. Our results suggest that ensembles of nonprocessive Eg5 motors drive flux in metaphase Xenopus extract spindles.

Cytoskeletal organization in fresh, aged and spontaneously activated human oocytes.

Abstract: The cytoskeleton of the human oocyte (microtubules and actin filaments) has been examined using fluorescence microscopy. In unfertilized oocytes in metaphase of the second meiotic division, microtubules were found exclusively within the spindle which was located at the periphery of the cell and oriented radially, with its long axis perpendicular to the surface membrane. The spindle was anastral and slightly pointed at each pole, the chromosomes being arranged on a metaphase plate at the equator. When treated with taxol, the oocyte spindle became astral and microtubules appeared in the cortex of the oocyte in the form of small strands or bundles. Polymerized actin was found to be present in a dense filamentous layer throughout the cortex of the unfertilized oocyte. Aged unfertilized oocytes displayed an increased incidence of disrupted or abnormal cytoskeletal organization. In parthenogenetically activated oocytes in anaphase and telophase, microtubules were again found predominantly in the spindle but in addition, cortical strands or bundles of microtubules were often present. Oocytes in late telophase sometimes showed the presence of a concentrated ring of actin in the cleavage furrow between the oocyte and the second polar body. Activated oocytes in early interphase contained a dense cortical mesh of microtubules and a midbody remnant between the oocyte and the polar body. The cytoskeletal organizations of mouse and human oocytes are compared.