Cyclin B1 is essential for mitosis in mouse embryos, and its nuclear export sets the time for mitosis.

There is remarkable redundancy between the Cyclin-Cdk complexes that comprise the cell cycle machinery. None of the mammalian A-, D-, or E-type cyclins are required in development until implantation, and only Cdk1 is essential for early cell divisions. Cyclin B1 is essential for development, but whether it is required for cell division is contentious.

Plk4 and Aurora A cooperate in the initiation of acentriolar spindle assembly in mammalian oocytes.

Establishing the bipolar spindle in mammalian oocytes after their prolonged arrest is crucial for meiotic fidelity and subsequent development. In contrast to somatic cells, the first meiotic spindle assembles in the absence of centriole-containing centrosomes. Ran-GTP can promote microtubule nucleation near chromatin, but additional unidentified factors are postulated for the activity of multiple acentriolar microtubule organizing centers in the oocyte.

Over-expression of Plk4 induces centrosome amplification, loss of primary cilia and associated tissue hyperplasia in the mouse.

To address the long-known relationship between supernumerary centrosomes and cancer, we have generated a transgenic mouse that permits inducible expression of the master regulator of centriole duplication, Polo-like-kinase-4 (Plk4). Over-expression of Plk4 from this transgene advances the onset of tumour formation that occurs in the absence of the tumour suppressor p53. Plk4 over-expression also leads to hyperproliferation of cells in the pancreas and skin that is enhanced in a p53 null background.

Spindle formation in the mouse embryo requires Plk4 in the absence of centrioles.

During the first five rounds of cell division in the mouse embryo, spindles assemble in the absence of centrioles. Spindle formation initiates around chromosomes, but the microtubule nucleating process remains unclear. Here we demonstrate that Plk4, a protein kinase known as a master regulator of centriole formation, is also essential for spindle assembly in the absence of centrioles.

RNA polymerase II kinetics in polo polyadenylation signal selection.

Regulated alternative polyadenylation is an important feature of gene expression, but how gene transcription rate affects this process remains to be investigated. polo is a cell-cycle gene that uses two poly(A) signals in the 3' untranslated region (UTR) to produce alternative messenger RNAs that differ in their 3'UTR length. Using a mutant Drosophila strain that has a lower transcriptional elongation rate, we show that transcription kinetics can determine alternative poly(A) site selection.

Dual role of topoisomerase II in centromere resolution and aurora B activity.

Chromosome segregation requires sister chromatid resolution. Condensins are essential for this process since they organize an axial structure where topoisomerase II can work. How sister chromatid separation is coordinated with chromosome condensation and decatenation activity remains unknown.

The condensin I subunit Barren/CAP-H is essential for the structural integrity of centromeric heterochromatin during mitosis.

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.

Could condensin scaffold the mitotic chromosome?

One of the most remarkable and yet poorly understood events during the cell cycle is how dispersed chromatin fragments are transformed into chromosomes every time cells undergo mitosis. It has been postulated that mitotic chromosomes might contain an axial scaffold that is involved in condensation but its molecules and structure have remained elusive. Recent data suggests that the condensin complex might indeed be an essential part of the scaffold that provides a platform for other proteins to localize and promote different aspects of chromosome condensation.

Condensin-dependent localisation of topoisomerase II to an axial chromosomal structure is required for sister chromatid resolution during mitosis.

Assembly of compact mitotic chromosomes and resolution of sister chromatids are two essential processes for the correct segregation of the genome during mitosis. Condensin, a five-subunit protein complex, is thought to be required for chromosome condensation. However, recent genetic analysis suggests that condensin is only essential to resolve sister chromatids.

Inferences on the evolutionary history of the Drosophila americana polymorphic X/4 fusion from patterns of polymorphism at the X-linked paralytic and elav genes.

In Drosophila there is limited evidence on the nature of evolutionary forces affecting chromosomal arrangements other than inversions. The study of the X/4 fusion polymorphism of Drosophila americana is thus of interest. Polymorphism patterns at the paralytic (para) gene, located at the base of the X chromosome, suggest that there is suppressed crossing over in this region between fusion and nonfusion chromosomes but not within fusion and nonfusion chromosomes.