Mitotic Prophase Research Articles

Chromosome segregation: how to open cohesin without cutting the ring?

Chromosome segregation is triggered when the ring-shaped chromosomal cohesin complex is opened by proteolytic cleavage to release pairs of sister chromatids. Even before this dramatic event in anaphase, many cohesin rings lead a dynamic life on chromosomes, and in metazoan cells a good part of them dissociate from chromosome arms during mitotic prophase. Two new papers in The EMBO Journal address how chromatin can get into and out of the cohesin ring without requiring its cleavage.

Karyotype of the Bird Blowfly, Protocalliphora falcozi Séguy, 1928 (Diptera, Calliphoridae)

The karyotype structure of Protocalliphora falcozi is described for the first time. The diploid complement comprises 2n = 12, n(male) = 5+XY. Male mitotic plates include four pairs of long (metacentric) and one pair of medium-sized (submetacentric) autosomes. The submetacentric sex chromosomes X and Y are the smallest elements of the set. An achiasmatic meiosis was determined. The heterochromosomes do not form a heterovalent because they occur separately during the first meiotic division. The examined karyotype shows a pericentromeric position of constitutive heterochromatin in all autosomes. The longer arm of the X chromosome is heterochromatic, while the Y is entirely euchromatic. The NORs are active at mitotic prophase and at early meiotic stages. Small, bright signals were observed in the centromeric regions of autosomes after DAPI staining.

Karyomorphological Studies in Some Species of Parnassia L. (Saxifragaceae s.l.) in East Asia and Intraspecific Polyploidy of P. palustris L.

Karyomorphological information is one of the most important characters for cytotaxonomy. We described karyomorphology of 14 species of Parnassia in East Asia. They had commonly the resting chromosomes of the simple chromocenter type and the mitotic prophase chromosomes of the proximal type. The somatic chromosome number of 2n=14 was shown in three species, that of 2n=18 was shown in six species, that of 2n=18 or 36 was shown in two species, that of 2n=32 was shown in one species, that of 2n=36 or 36+1∼8 s was shown in one species, and that of 2n=18, 27, 36 or 45 was shown in one species. They were commonly monomodal (gradual) decrease in length from the largest to the smallest chromosomes. However, their absolute chromosome sizes were different from each other. Average chromosome sizes were recognized as three types small (1.21 μm), medium (1.7–2.36 μm), and large (3.1–3.75 μm). Parnassia palustris showed four cytotypes that could be likely cytogeographically correlated with habitats, polyploidy, and sizes of pollen grains.

A Shared Role for RBF1 and dCAP-D3 in the Regulation of Transcription with Consequences for Innate Immunity

Previously, we discovered a conserved interaction between RB proteins and the Condensin II protein CAP-D3 that is important for ensuring uniform chromatin condensation during mitotic prophase. The Drosophila melanogaster homologs RBF1 and dCAP-D3 co-localize on non-dividing polytene chromatin, suggesting the existence of a shared, non-mitotic role for these two proteins. Here, we show that the absence of RBF1 and dCAP-D3 alters the expression of many of the same genes in larvae and adult flies. Strikingly, most of the genes affected by the loss of RBF1 and dCAP-D3 are not classic cell cycle genes but are developmentally regulated genes with tissue-specific functions and these genes tend to be located in gene clusters. Our data reveal that RBF1 and dCAP-D3 are needed in fat body cells to activate transcription of clusters of antimicrobial peptide (AMP) genes. AMPs are important for innate immunity, and loss of either dCAP-D3 or RBF1 regulation results in a decrease in the ability to clear bacteria. Interestingly, in the adult fat body, RBF1 and dCAP-D3 bind to regions flanking an AMP gene cluster both prior to and following bacterial infection. These results describe a novel, non-mitotic role for the RBF1 and dCAP-D3 proteins in activation of the Drosophila immune system and suggest dCAP-D3 has an important role at specific subsets of RBF1-dependent genes.

Karyotype Analysis of an Endemic Species Primulina tabacum (Gesneriaceae)

The cytology of an endemic species Primulina tabacum (Gesneriaceae) was studied in this paper. The chromosome numbers and karyotype were investigated for the first time. The results revealed that the interphase nuclei were categorized to be rod prochromosome type and the mitotic prophase chromosomes were classified as proximal type. The chromosome number was 2n=36. The karyotype formula was 2n=2x=36=24m (1SAT)+12sm and the karyotype is 2A type.

Shugoshin is a Mad1/Cdc20-like interactor of Mad2

Mammalian centromeric cohesin is protected from phosphorylation-dependent displacement in mitotic prophase by shugoshin-1 (Sgo1), while shugoshin-2 (Sgo2) protects cohesin from separase-dependent cleavage in meiosis I. In higher eukaryotes, progression and faithful execution of both mitosis and meiosis are controlled by the spindle assembly checkpoint, which delays anaphase onset until chromosomes have achieved proper attachment to microtubules. According to the so-called template model, Mad1-Mad2 complexes at unattached kinetochores instruct conformational change of soluble Mad2, thus catalysing Mad2 binding to its target Cdc20. Here, we show that human Sgo2, but not Sgo1, specifically interacts with Mad2 in a manner that strongly resembles the interactions of Mad2 with Mad1 or Cdc20. Sgo2 contains a Mad1/Cdc20-like Mad2-interaction motif and competes with Mad1 and Cdc20 for binding to Mad2. NMR and biochemical analyses show that shugoshin binding induces similar conformational changes in Mad2 as do Mad1 or Cdc20. Mad2 binding regulates fine-tuning of Sgo2's sub-centromeric localization. Mad2 binding is conserved in the only known Xenopus laevis shugoshin homologue and, compatible with a putative meiotic function, the interaction occurs in oocytes.

Cell-cycle control as a target for calcium, hormonal and developmental signals: the role of phosphorylation in the retinoblastoma-centred pathway

During the life cycle of plants, both embryogenic and post-embryogenic growth are essentially based on cell division and cell expansion that are under the control of inherited developmental programmes modified by hormonal and environmental stimuli. Considering either stimulation or inhibition of plant growth, the key role of plant hormones in the modification of cell division activities or in the initiation of differentiation is well supported by experimental data. At the same time there is only limited insight into the molecular events that provide linkage between the regulation of cell-cycle progression and hormonal and developmental control. Studies indicate that there are several alternative ways by which hormonal signalling networks can influence cell division parameters and establish functional links between regulatory pathways of cell-cycle progression and genes and protein complexes involved in organ development. An overview is given here of key components in plant cell division control as acceptors of hormonal and developmental signals during organ formation and growth. Selected examples are presented to highlight the potential role of Ca(2+)-signalling, the complex actions of auxin and cytokinins, regulation by transcription factors and alteration of retinoblastoma-related proteins by phosphorylation. Auxins and abscisic acid can directly influence expression of cyclin, cyclin-dependent kinase (CDK) genes and activities of CDK complexes. D-type cyclins are primary targets for cytokinins and over-expression of CyclinD3;1 can enhance auxin responses in roots. A set of auxin-activated genes (AXR1-ARGOS-ANT) controls cell number and organ size through modification of CyclinD3;1 gene expression. The SHORT ROOT (SHR) and SCARECROW (SCR) transcriptional factors determine root patterning by activation of the CYCD6;1 gene. Over-expression of the EBP1 gene (plant homologue of the ErbB-3 epidermal growth factor receptor-binding protein) increased biomass by auxin-dependent activation of both D- and B-type cyclins. The direct involvement of auxin-binding protein (ABP1) in the entry into the cell cycle and the regulation of leaf size and morphology is based on the transcriptional control of D-cyclins and retinoblastoma-related protein (RBR) interacting with inhibitory E2FC transcriptional factor. The central role of RBRs in cell-cycle progression is well documented by a variety of experimental approaches. Their function is phosphorylation-dependent and both RBR and phospho-RBR proteins are present in interphase and mitotic phase cells. Immunolocalization studies showed the presence of phospho-RBR protein in spots of interphase nuclei or granules in mitotic prophase cells. The Ca(2+)-dependent phosphorylation events can be accomplished by the calcium-dependent, calmodulin-independent or calmodulin-like domain protein kinases (CDPKs/CPKs) phosphorylating the CDK inhibitor protein (KRP). Dephosphorylation of the phospho-RBR protein by PP2A phosphatase is regulated by a Ca(2+)-binding subunit.

Cytogenetic study of three edible chrysanthemum cultivars

During past decades, edible chrysanthemum (Dendranthema x grandiflorum) has become a popular dietary supplement. However, only numberable cultivars are available, cytogenetic studies especially meiotic behaviour in them remaind largely unexplored. In the present study, we analyzed the karyotype and meiotic behaviour during microsporogenesis in accessions of edible chrysanthemum. This information can be useful in cultivar improvement, as well as in elucidation of the evolution of the species. The three edible cultivars studied all had an intermediate type of interphase nucleus, and their mitotic prophase chromosomes were of the interstitial type. The chromosome number of "Baohuatangyijin" (Bn) varies from 55 to 62. "Zifengmudan" (Zn) has the karyotype 2n = 54 = 38m + + 12sm + 4st, while "Jingxingxiying" (Jg) has 2n = 55 = 38m + 15sm + 2st. Male meiosis was largely normal. Mean pairing configuration ofZn at diakinesis and metaphase I was 0.941 + + 25.07II + 0.14III + 0.63IV, while for Jg, the equivalent was 1.32I + 24.64II + 0.16III + 0.85IV + 0.05V + + 0.04VI. Chromatid separation was normal during anaphase I and anaphase II in most meiocytes. All three edible chrysanthemums appear to be allopolyploid, and the edible type is probably more primitive than the ornamental type. However, the edible type is probably more closely related to the ornamental than to the medicinal type.

A new meiosis-specific cohesin complex implicated in the cohesin code for homologous pairing.

We identify a new mammalian cohesin subunit, RAD21-like protein (RAD21L), with sequence similarity to RAD21 and REC8. RAD21L localizes along axial elements in early meiotic prophase, in a manner that is spatiotemporally different to either REC8 or RAD21. Remarkably, RAD21L and REC8 have symmetrical, mutually exclusive localization on the not-yet-synapsed homologues, implying that the cohesin patterning could provide a code for homologue recognition. RAD21 transiently localizes to axial elements after the dissociation of RAD21L and REC8 in late pachytene, a period of recombination repair. Further, we show that the removal of cohesins and synaptonemal complex during late meiotic prophase is promoted by Polo-like kinase 1, which is similar to the mitotic prophase pathway.

Chromosome studies in seven species of Allium (Alliaceae sensu stricto) in Mongolian and Russian Altai

During the course of Botanical expedition to the Mongolian and Russian Altai in 2007 and 2008, seven species of Allium were collected in eight sites and studied cytologically. Allium altaicum, A. austrosibilicum, A. senescens and A. vodopjanovae had the common chromosome number of 2n=16 (diploid), while A. nutans had that of 2n=32 (tetraploid), A. eduardii had that of 2n=16 (diploid), 17 and 32 (tetraploid) and A. rubens had 2n=16 (diploid) and 17. The common chromosome number of 2n=17 in A. eduardii and A. rubens could be aneuploidy due to karyomorphological behaviors of chromatins at the stages of resting, mitotic prophase and prometaphase. These findings of aneuploid were new to Allium,/I>. Moreover, the karyotypes of the above six diploid species were characterized by: (1) The largest chromosomes of 12.2 to 14.4 μm, and the smallest chromosomes of 5.9 to 7.4 μm; (2) Very less differences in relative lengths of the chromosomes and chromosome constitutions with respect to arm ratio not seen much differences among those chromosomes; (3) The karyotype formulas of 2n=16=12m+2sm+2st for A. altaicum; 2n=16=14m+2sm for A. austrosibilicum, A. rubens, and A. senescens; 2n=17=14m+2sm+1st for A. rubens aneuploid form; 2n=16=14m+2st for A. eduardii; 2n=17=14m+3st for A. eduardii aneuploid form; 2n=16=9m+5sm+2t for A. vodopjanovae.

Comparative karyomorphological studies in three species of Dracocepharum L. (Lamiaceae) in Altai Mountains, Russia and Mongolia

Karyomorophology at the resting nuclei, mitotic prophase and metaphase chromosomes in three species of Dracocepharum were studied. They had commonly the resting nuclei of the complex chromocenter type, the mitotic prophase chromosomes of the proximal type, and the mitotic metaphase chromosomes had 2n=12 for D. peregrinum, 2n=10 for D. nutans and 2n=24 for D. origanoides. These chromosome numbers were verified with the previous reports. The karyotype formula of three species had 2n=12=8m+4sm for D. peregrinum, 2n=10=8m+2sm for D. nutans and 2n=24=14m+10sm for D. origanoides, and chromosome sizes for D. peregrinum is a little smaller one compare with D. nutans and D. origanoides.

A countdown clock in mitotic prophase: design logic for dual pathway mitosis

A countdown clock in mitotic prophase: design logic for dual pathway mitosis

Sister chromatid resolution: a cohesin releasing network and beyond

When chromosomes start to assemble in mitotic prophase, duplicated chromatids are not discernible within each chromosome. As condensation proceeds, they gradually show up, culminating in two rod-shaped structures apposed along their entire length within a metaphase chromosome. This process, known as sister chromatid resolution, is thought to be a prerequisite for rapid and synchronous separation of sister chromatids in anaphase. From a mechanistic point of view, the resolution process can be dissected into three distinct steps: (1) release of cohesin from chromosome arms; (2) formation of chromatid axes mediated by condensins; and (3) untanglement of inter-sister catenation catalyzed by topoisomerase II (topo II). In this review article, we summarize recent progress in our understanding the molecular mechanisms of sister chromatid resolution with a major focus on its first step, cohesin release. An emerging idea is that this seemingly simple step is regulated by an intricate network of positive and negative factors, including cohesin-binding proteins and mitotic kinases. Interestingly, some key factors responsible for cohesin release in early mitosis also play important roles in controlling cohesin functions during interphase. Finally, we discuss how the step of cohesin release might mechanistically be coordinated with the actions of condensins and topo II.

Ploidy variation in Trigonobalanus verticillata (Fagaceae)

Cytological studies were carried out on two populations of Trigonobalanus verticillata in Yinggeling, Hainan Province, China (YGL), and Fraser’s Hill, Malaysia (FH). In the two populations, the pattern of interphase nuclei was of the simple chromocentre type, the mitotic prophases were of the proximal interstitial type and chromosome numbers were 2n = 2x = 14 (YGL) and 2n = 6x = 42 (FH), representing diploid and hexaploid respectively. The basic chromosome number of the species was x = 7; this is a unique number in the Fagaceae and in the Fagales as well. The characteristic karyotype of the diploid population in China was 2n = 14 = 8m + 4sm + 2st. Based on the new finding of the chromosome number in this study, we clarify the previous dispute about the basic chromosome number and propose that the two Asian Trigonobalanus species have closer affinity.

Releasing cohesin from chromosome arms in early mitosis: opposing actions of Wapl–Pds5 and Sgo1

The cohesin complex establishes sister chromatid cohesion during S phase. In metazoan cells, most if not all cohesin dissociates from chromatin during mitotic prophase, leading to the formation of metaphase chromosomes with two cytologically discernible chromatids. This process, known as sister chromatid resolution, is believed to be a prerequisite for synchronous separation of sister chromatids in subsequent anaphase. To dissect this process at a mechanistic level, we set up an in vitro system. Sister chromatid resolution is severely impaired upon depletion of Wapl from Xenopus egg extracts. Exogenously added human Wapl can rescue these defects and, remarkably, it can do so in a very short time window of early mitosis. A similar set of observations is made for Pds5, a factor implicated previously in the stabilization of interphase cohesion. Characteristic amino acid motifs (the FGF motifs) in Wapl coordinate its physical and functional interactions with Pds5 and cohesin subunits. We propose that Wapl and Pds5 directly modulate conformational changes of cohesin to make it competent for dissociation from chromatin during prophase. Evidence is also presented that Sgo1 plays a hitherto underappreciated role in stabilizing cohesin along chromosome arms, which is antagonized by the mitotic kinases polo-like kinsase (Plk1) and aurora B.

Karyomorphology of three species inDipentodon(Dipentodontaceae),Perrottetia(Celastraceae), andTapiscia(Tapisciaceae) of the order Huerteales and their phylogenetic implications

Abstract The karyomorphology of three species in Dipentodon (Dipentodontaceae), Perrottetia (Celastraceae), and Tapiscia (Tapisciaceae), namely Dipentodon sinicus, Perrottetia racemosa, and Tapiscia sinensis, was investigated in the present study. Recent molecular research has discovered close relationships among these three genera, which has led to the establishment of the order Huerteales with Perrottetia being placed in Dipentodontaceae. Herein we report the chromosome numbers of D. sinicus and P. racemosa for the first time, and present their karyotype formulas as 2n= 34 = 22 sm + 12 st (D. sinicus), 2n= 20 = 11 m + 9 sm (P. racemosa), and 2n= 30 = 22 m(2SAT) + 8sm (T. sinensis). Asymmetry of their karyotypes is categorized to be Type 3B in D. sinicus, Type 2A in P. racemosa, and Type 2A in T. sinensis. Each of the species shows special cytological features. Compared with Perrottetia, Dipentodon has a different basic chromosome number, a higher karyotype asymmetry, and different karyomorphology of its interphase nuclei, mitotic prophase, and metaphase. Thus, on the basis of these results, we have reservations regarding the suggestion of placing Dipentodon and Perrottetia together in the family Dipentodontaceae.

Influence of cyclin type and dose on mitotic entry and progression in the early Drosophila embryo

Cyclins are key cell cycle regulators, yet few analyses test their role in timing the events that they regulate. We used RNA interference and real-time visualization in embryos to define the events regulated by each of the three mitotic cyclins of Drosophila melanogaster, CycA, CycB, and CycB3. Each individual and pairwise knockdown results in distinct mitotic phenotypes. For example, mitosis without metaphase occurs upon knockdown of CycA and CycB. To separate the role of cyclin levels from the influences of cyclin type, we knocked down two cyclins and reduced the gene dose of the one remaining cyclin. This reduction did not prolong interphase but instead interrupted mitotic progression. Mitotic prophase chromosomes formed, centrosomes divided, and nuclei exited mitosis without executing later events. This prompt but curtailed mitosis shows that accumulation of cyclin function does not directly time mitotic entry in these early embryonic cycles and that cyclin function can be sufficient for some mitotic events although inadequate for others.

Condensation of chromosomes during mitotic prophase. Whole mount electron microscopy of cerebral ganglion cells of Drosophila.

Condensation of chromosomes during mitotic prophase was studied by means of whole mount electron microscopy on cerebral ganglion cells of Drosophila melanogaster after short treatment with alkaliurea. In interphase and early mitotic prophase nuclei some core-like structures, i.e. interchromomeric axial fibers, were observed. During later prophase the chromatid axis seems to be polynemic in most of the cells. Later on the primary axes of chromatids disappear more and more due to supercoiling and folding of chromatin fibrils, and through the shortening of the whole linear organization of chromosomes. Some traces of subsequent coiling orders can be locally observed in the chromosome arms. The largest gyres of metaphase chromosomes are especially clear in whole mount preparations.

Giemsa C-banding and silver-staining for cytological studies in Pisum

Technical hints are given for Giemsa C-banding of the Pisum chromosomes and for silver-staining of NORs and nucleoli. C-band positive heterochromatin appears as pairs of centromeric dots on each of the seven chromosomes and as heavily stained C-bands at the secondary constrictions in the long arms of chromosomes 4 and 7. In the latter arm, there is also an intercalary proximally situated C-band. The centromeric dots are attached to chromosomal spindle fibres, which supports the supposition that they are kinetochores. The C-blocks of the SAT regions are also detectable at metaphase I of meiosis and hence facilitate the interpretation of configurations in structural hybrids involving the SAT chromosomes. In mitotic interphase nuclei, the SAT blocks appear as four chromocentres, and in the giant nuclei of the suspensor, as four endochromocentres. Ag-NORs coincide with the C-bands at the SAT regions. Moreover, the centromeric dots are stained by silver nitrate. In Pisum, nucleoli may be formed from the centromeric regions of the chromosomes. At late mitotic prophase (diminishing stage of nucleolar development), the nucleoli sometimes appear as paired heavily silver-stained bodies budding out from the chromosomes. At meiotic prophase, darkly stained intranucleolar bodies occur in slides prepared according to a suitable silver-staining technique.

Chromosome characters of two varieties of Ellisiophyllum punnatum (Scrophulariaceae)

Chromosome characters of Ellisiophyllum pinnatum var. typicum (=var. pinnatum) and E. pinnatum var. reptans, were studied. These two varieties had common characters of the resting, mitotic prophase and metaphase chromosomes; complex chromocenter types of the resting chromosomes, interstitial types of the mitotic prophase chromosomes, and the chromosome number of 2n=18 at mitotic metaphase. This species could be diploid if the basic chromosome number x=9 was accepted. The common chromosome number of 2n=18 for the two taxa was reported here for the first time. Their karyotypes were commonly mono-modal decrease in length from the longest to the shortest chromosomes. Ellisiophyllum pinnatum var. reptans had slightly larger chromosomes than E. pinnatum var. typicum had. The chromosome complement in centromeric positions of E. pinnatum var. typicum was m+sm and that of E. pinnatum var. reptans was m+sm+st.