SummaryCohesin is a highly conserved multisubunit complex that holds sister chromatids together in mitotic cells. At the metaphase to anaphase transition, proteolytic cleavage of the α kleisin subunit (Rad21) by separase causes cohesin's dissociation from chromosomes and triggers sister-chromatid disjunction. To investigate cohesin's function in postmitotic cells, where it is widely expressed, we have created fruit flies whose Rad21 can be cleaved by TEV protease. Cleavage causes precocious separation of sister chromatids and massive chromosome missegregation in proliferating cells, but not disaggregation of polytene chromosomes in salivary glands. Crucially, cleavage in postmitotic neurons is lethal. In mushroom-body neurons, it causes defects in axon pruning, whereas in cholinergic neurons it causes highly abnormal larval locomotion. These data demonstrate essential roles for cohesin in nondividing cells and also introduce a powerful tool by which to investigate protein function in metazoa.

The metaphase-anaphase transition is orchestrated through proteolysis of numerous proteins by an ubiquitin protein ligase called the Anaphase-promoting complex or cyclosome (APC/C) 1 . A crucial aspect of this process is sister chromatid separation, which is thought to be mediated by separase, a thiol protease activated by the APC/C. Separase cleaves cohesin, a ring-shaped complex that entraps sister DNAs 2, 3 . It is a matter of debate whether cohesin-independent forces also contribute to sister chromatid cohesion [4][5][6] . Using 4D-live-cell imaging of Drosophila syncytial embryos blocked in metaphase (via APC/C inhibition) we show that artificial cohesin cleavage 7 is sufficient to trigger chromosome disjunction. This is nevertheless insufficient for correct chromosome segregation. Kinetochore-microtubule attachments are rapidly destabilized by the loss of tension caused by cohesin cleavage in the presence of high Cyclin-dependent kinase 1 (Cdk1) activity, as occurs when the APC/C cannot destroy mitotic cyclins. Metaphase chromosomes undergo a bona-fide anaphase when cohesin cleavage is combined with Cdk1 inhibition. We conclude that only two key events, opening of cohesin rings and down regulation of Cdk1, are sufficient to drive proper segregation of chromosomes in anaphase.Correct attachment of sister DNAs to microtubules during mitosis depends on a force that actively holds them together. It is a matter of ongoing debate whether cohesin-independent forces (be they sister DNA concatenation [8][9][10][11] or proteinaceous linkages provided by ORC 12 or condensin 13 ) contribute to sister chromatid cohesion in metaphase cells [4][5][6] . If cohesin alone resists spindle forces prior to anaphase, then artificial opening the cohesin ring by an exogenous protease should trigger sister chromatid disjunction in the absence of separase activity. If, on the other hand, cohesin were just one of several mechanisms holding sisters together, then cleavage should not suffice. This key experiment has so far only been performed in yeast, where cleavage of cohesin's α-kleisin subunit by the Tobacco Etch virus protease (TEV) in cells arrested in metaphase initiates disjunction of most sister DNAs but not those of repetitive rDNAs within the nucleolus 14,15 . It is nevertheless argued that the larger chromosomes of animal and plant cells, or indeed other fungi, have very different properties. To address this, we used strains of the fruit fly D. melanogaster whose α-kleisin subunit Rad21 has been replaced by a version containing TEV cleavage sites (Rad21 TEV ) 7 . TEV protease injection causes catastrophic mitotic failure in syncytial embryos using Rad21 TEV ( Supplementary Information, Fig. S1). When injected before or during early stages of mitosis, it causes sister chromatids to disjoin as soon as the nuclear envelope breaks down. This precocious loss of sister chromatid cohesion is accompanied by aCorrespondence and requests for materials should be addressed to K.N. (kim.nasmyth@bioch.ox.ac.uk prometaphase delay...

O artigo analisa a relação da resiliência com eventos de vida desfavoráveis e fatores de proteção. A amostra do estudo foi de 997 adolescentes escolares da rede pública de ensino de São Gonçalo/RJ. Como medida de resiliência utilizou-se a Escala de Resiliência desenvolvida por Wagnild e Young (1993). Para medir eventos de vida foram utilizadas escalas de violência física (Straus, 1979) e psicológica (Pitzner & Drummond, 1997), itens de violência na escola e na localidade, violência entre irmãos e entre pais, e violência sexual entre outros. Como fatores de proteção utilizou-se: Escala de Apoio Social de Shebourne e Stewart (conforme citado por Chor, Grip, Lopes & Farstein, 2001), Escala de Auto-Estima (Rosemberg, 1989), itens abordando supervisão familiar, relacionamento com amigos e professores. Os eventos de vida negativos não apresentaram relação com a resiliência, enquanto os fatores de proteção mostraram-se todos correlacionados com o constructo.

During cell division, chromatin undergoes structural changes essential to ensure faithful segregation of the genome. Condensins, abundant components of mitotic chromosomes, are known to form two different complexes, condensins I and II. To further examine the role of condensin I in chromosome structure and in particular in centromere organization, we depleted from S2 cells the Drosophila CAP-H homologue Barren, a subunit exclusively associated with condensin I. In the absence of Barren/CAP-H the condensin core subunits DmSMC4/2 still associate with chromatin, while the other condensin I non-structural maintenance of chromosomes family proteins do not. Immunofluorescence and in vivo analysis of Barren/CAP-H-depleted cells showed that mitotic chromosomes are able to condense but fail to resolve sister chromatids. Additionally, Barren/CAP-H-depleted cells show chromosome congression defects that do not appear to be due to abnormal kinetochore-microtubule interaction. Instead, the centromeric and pericentromeric heterochromatin of Barren/CAP-H-depleted chromosomes shows structural problems. After bipolar attachment, the centromeric heterochromatin organized in the absence of Barren/CAP-H cannot withstand the forces exerted by the mitotic spindle and undergoes irreversible distortion. Taken together, our data suggest that the condensin I complex is required not only to promote sister chromatid resolution but also to maintain the structural integrity of centromeric heterochromatin during mitosis.