Ribosomal ITS sequences and plant phylogenetic inference.

▪ Abstract The separation of sister chromatids at the metaphase to anaphase transition is one of the most dramatic of all cellular events and is a crucial aspect of all sexual and asexual reproduction. The molecular basis for this process has until recently remained obscure. New research has identified proteins that hold sisters together while they are aligned on the metaphase plate. It has also shed insight into the mechanisms that dissolve sister chromatid cohesion during both mitosis and meiosis. These findings promise to provide insights into defects in chromosome segregation that occur in cancer cells and into the pathological pathways by which aneuploidy arises during meiosis.

Disseminating the Genome: Joining, Resolving, and Separating Sister Chromatids During Mitosis and Meiosis

Higher plants are recognized as excellent genetic models to detect environmental mutagens and are frequently used in monitoring studies. Among the plant species, Alium cepa has been used to evaluate DNA damages, such as chromosome aberrations and disturbances in the mitotic cycle. Employing the A. cepa as a test system to detect mutagens dates back to the 40s. It has been used to this day to assess a great number of chemical agents, which contributes to its increasing application in environmental monitoring. The A. cepa is characterized as a low cost test. It is easily handled and has advantages over other short-term tests that require previous preparations of tested samples, as well as the addition of exogenous metabolic system. Higher plants, even showing low concentrations of oxidase enzymes and a limitation in the substrate specification in relation to other organism groups, present consistent results that may serve as a warning to other biological systems, since the target is DNA, common to all organisms. The A. cepa test also enables the evaluation of different endpoints. Among the endpoints, chromosome aberrations have been the most used one to detect genotoxicity along the years. The mitotic index and some nuclear abnormalities are used to evaluate citotoxicity and analyze micronucleus to verify mutagenicity of different chemicals. Moreover, the A. cepa test system provides important information to evaluate action mechanisms of an agent about its effects on the genetic material (clastogenic and/or aneugenic effects). In the face of all the advantages that the A. cepa test system offers, it has been widely used to assess the impacts caused by xenobiotics, characterizing an important tool for environmental monitoring studies, where satisfactory results have been reported.

Allium cepa test in environmental monitoring: a review on its application.

Un ménage à quatre: the molecular biology of chromosome segregation in meiosis.

TL1A Is a TNF-like Ligand for DR3 and TR6/DcR3 and Functions as a T Cell Costimulator

This study provides evidence that nuclear and chromosome remodelling has taken place in sugarcane, a vegetative crop with a complex genome derived from interspecific hybridizations between Saccharum officinarum and S. spontaneum. Detailed knowledge on the chromosomal compositions of the three clones analysed was acquired. (1) All hybrid cultivars were found to be aneuploid, affecting both parental genomes (having chromosomes in addition to full genomes), with chromosome numbers from 2n=102-106 in My5514 and up to 2n=113-117 in C236-51. (2) Comparative in situ hybridization showed that about 16% of these chromosomes are inherited from S. spontaneum and less than 5% are recombinant or translocated chromosomes containing sequences of both S. officinarum and S. spontaneum. (3) Differences between the observed DNA contents (estimated by flow cytometry) and those expected from the number of chromosomes, allowed the introgression of additional S. spontaneum or S. officinarum DNA pieces into the B42231 and C236-51 cultivars to be estimated. (4) Size heterogeneity between S. officinarum homologous chromosomes carrying the 18S-5.8S-25S and 5S ribosomal genes (identified by FISH with pTa71 and pTa794, respectively) confirms remodelling occurred by chromosomal interchange events, at least in these homologous chromosomes. (5) Simultaneous visualization of nucleoli and NORs showed that all 18S-5.8S-25S loci were potentially functional in the three clones, independent of their origin and size.

Genome remodelling in three modern S. officinarumxS. spontaneum sugarcane cultivars.

The discovery that phosphorylation of selected proteins by cyclin-dependent kinases is the engine which makes the cycle run provides a new image of the control of proliferation and of its deregulation. The high conservation of this machinery in the different eukaryotic organisms emphasizes its early origin and its importance for life. It also makes the extrapolation of findings between different species feasible. The control of proliferation relies basically on accelerating and braking mechanisms which act on the engine driving the cycle. This review particularly stresses the importance of checkpoint or tumor suppressor pathways as transduction systems of negative signals which may induce a cycle braking operation. They prevent any important cycle transition, as the initiation of proliferation, that of replication, mitosis, etc., until the DNA and other cellular conditions make such a progression safe. These checkpoint pathways are able to recognize and transduce signals about the adequacy of initiating or continuing proliferation for a cell at a particular time, under a particular set of external and internal conditions. Crucial components of these pathways are proteins encoded by some of the checkpoint genes that evaluate the final balance of mitogenic and antimitogenic pathways reaching them and, if the balance is negative, they prevent temporarily cycle inititation or its progression by inhibiting the corresponding cyclin-dependent kinases. On the other hand, when the balance becomes positive, they allow the activation of the cyclin-dependent kinases. Uncontrolled cell proliferation associated with cancer always depends on the functional abrogation of at least one of the checkpoint pathways. The checkpoint or tumor suppressor protein p53 is one of the proteins in them, and mutations in the gene encoding it are present in more than half of all human tumours. The review touches new pharmacological strategies which have been opened by the discovery of portions of some of the signal transduction cascades involved in the transient brake of cell proliferation. Restoration of checkpoint pathways either prevents further proliferation of cells with damaged genome until repair is over or, alternatively, the dismantling of these checkpoints induce those cells to commit suicide (apoptosis). The fact that both restoration and dismantling of checkpoint pathways sensitive to DNA damage have not disturbing effects on any other proliferating cell with undamaged DNA makes these selective strategies promissing.

/pdf/cell-proliferation-and-cancer-3z842vj0s2.pdf

Cell proliferation and cancer

When DNA topoisomerase II (topo II) activity is inhibited with a non-DNA-damaging topo II inhibitor (ICRF-193), mammalian cells become checkpoint arrested in G2-phase. In this study, we analyzed chromosome structure in cells that bypassed this checkpoint. We observed a novel type of chromosome aberration, which we call Ω-figures. These are entangled chromosome regions that indicate the persistence of catenations between nonhomologous sequences. The number of Ω- figures per cell increased sharply as cells evaded the transient block imposed by the topo II-dependent checkpoint, and the presence of caffeine (a checkpoint-evading agent) potentiated this increase. Thus, the removal of nonreplicative catenations, a process that promotes chromosome individualization in G2, may be monitored by the topo II-dependent checkpoint in mammals.

Premitotic chromosome individualization in mammalian cells depends on topoisomerase II activity.

SUMMARY Applications of 5 x 10- M Zn(II) and 3 x 10- M Cd(II) depressed but did not block root growth in Allium cepa L. Both heavy metals were indirect clastogens at these moderate concentrations, for they induced the appearance of chromosomal bridges, but no breaks were detected in the first ana-telophases. On the other hand, Zn and Cd respectively potentiated six and eleven times the direct clastogenic effect of the 13-irradiation produced by [3H]thymidine incorporation into DNA. The chromosomal breaks at mitosis were more abundant when [3H] was incorporated into early replicating DNA. On the other hand, when the incorporation took place in the last half of interphase, the subsequent G2 as well as the prophase were extended (1 7-fold for Zn and 2 7-fold for Cd), and the Sexcess lesions were repaired before anaphase. The whole cycle experienced 1 5 and 2 0-fold increases in the presence of Zn and Cd in relation to that in control meristems (15 h).

The genotoxicity of Zn(II) and Cd(II) in Allium cepa root meristematic cells

The present report deals with the functional relationships among protein complexes which, when mutated, are responsible for four human syndromes displaying cancer proneness, and whose cells are deficient in DNA double-strand break (DSB) repair. In some of them, the cells are also unable to activate the proper checkpoint, while in the others an unduly override of the checkpoint-induced arrest occurs. As a consequence, all these patients display genome instability. In ataxia-telangiectasia, the mutated protein (ATM) is a kinase, which acts as a transducer of DNA damage signalling. The defective protein in the ataxia-telangiectasia-like disorder is a DNase (the Mre11 nuclease) that in vivo produces single-strand tails at both sides of DSBs. Mre11 is always present with the Rad50 ATPase in a protein machine: the nuclease complex. In mammals, this complex also contains nibrin, the protein mutated in the Nijmegen syndrome. Nibrin confers new abilities to the nuclease complex, and can also bind to BRCA1 (one of the two proteins mutated in familial breast cancer). BRCA1 has a central motif that binds with high affinity to cruciform DNA, a structure present in places where the DNA loops are anchored to the chromosomal axis or scaffold. The BRCA1 x cruciform DNA complex should be released to allow the nuclease complex to work in DNA recombinational repair of DSBs. BRCA1 also acts as a scaffold for the assembly of ATPases such as Rad51, responsible for the somatic homologous recombination. Loss of the BRCA1 gene prevents cell survival after exposure to cross-linkers. The BRCA1-RING domain is an E3-ubiquitin ligase. It can mono-ubiquitinate the FANCD2 protein, mutated in one of the Fanconi anemia complementation groups, to regulate it. Finally, during DNA replication, the nuclease complex and its activating ATM kinase are integrated in the BRCA1-associated surveillance complex (BASC) that contains, among others, enzymes required for mismatch excision repair. In short, the proteins missing in these syndromes have in common their BRCA1-mediated assembly into multimeric machines responsible for the surveillance of DNA replication, DSB recombinational repair, and the removal of DNA cross-links.

/pdf/human-syndromes-with-genomic-instability-and-multiprotein-52p8cz4tzm.pdf

C. de la Torre

Papers

Genome remodelling in three modern S. officinarumxS. spontaneum sugarcane cultivars.

Cell proliferation and cancer

Premitotic chromosome individualization in mammalian cells depends on topoisomerase II activity.

The genotoxicity of Zn(II) and Cd(II) in Allium cepa root meristematic cells

Human syndromes with genomic instability and multiprotein machines that repair DNA double-strand breaks.