Condensin facilitates sister chromatid separation in actively transcribed DNA regions by relieving the obstructive effect of transcription

The evolutionarily conserved protein complex, condensin, is central to chromosome dynamics, including mitotic chromosome condensation and segregation. Genome-wide localization of condensin is correlated with transcriptional activity; however, the significance of condensin accumulation in transcribed regions remains unclear. Here, we demonstrate that condensin relieves the obstructive effect of mitotic transcription on sister chromatid separation in fission yeast, Schizosaccharomyces pombe. Time-lapse visualization of sister chromatid DNA separation revealed that mutant condensin causes delayed segregation specifically at mitotically transcribed, condensin-bound gene locus, ecm33+. Contrarily, the delay was abolished by transcriptional shut-off of the actively transcribed gene. We also showed that delayed separation at a heat shock-inducible gene locus, ssa1+, in condensin mutants was significantly alleviated by deletion of the gene. Since condensin has ability to remove ssDNA-binding proteins and RNA from unwound ssDNAs or DNA-RNA hybrids in vitro, we propose a model that condensin-mediated removal of mitotic transcripts from chromosomal DNA is the primary mechanism of sister chromatid separation.

Polo kinase Cdc5 associates with centromeres to facilitate the removal of centromeric cohesin during mitosis

Sister chromatid cohesion is essential for tension-sensing mechanisms that monitor bipolar attachment of replicated chromatids in metaphase. Cohesion is mediated by the association of cohesins along the length of sister chromatid arms. In contrast, centromeric cohesin generates intrastrand cohesion and sister centromeres, while highly cohesin enriched, are separated by >800 nm at metaphase in yeast. Removal of cohesin is necessary for sister chromatid separation during anaphase, and this is regulated by evolutionarily conserved polo-like kinase (Cdc5 in yeast, Plk1 in humans). Here we address how high levels of cohesins at centromeric chromatin are removed. Cdc5 associates with centromeric chromatin and cohesin-associated regions. Maximum enrichment of Cdc5 in centromeric chromatin occurs during the metaphase-to-anaphase transition and coincides with the removal of chromosome-associated cohesin. Cdc5 interacts with cohesin in vivo, and cohesin is required for association of Cdc5 at centromeric chromatin. Cohesin removal from centromeric chromatin requires Cdc5 but removal at distal chromosomal arm sites does not. Our results define a novel role for Cdc5 in regulating removal of centromeric cohesins and faithful chromosome segregation.

Centromere detection of human metaphase chromosome images using a candidate based method

Accurate detection of the human metaphase chromosome centromere is a critical element of cytogenetic diagnostic techniques, including chromosome enumeration, karyotyping and radiation biodosimetry. Existing centromere detection methods tends to perform poorly in the presence of irregular boundaries, shape variations and premature sister chromatid separation. We present a centromere detection algorithm that uses a novel contour partitioning technique to generate centromere candidates followed by a machine learning approach to select the best candidate that enhances the detection accuracy. The contour partitioning technique evaluates various combinations of salient points along the chromosome boundary using a novel feature set and is able to identify telomere regions as well as detect and correct for sister chromatid separation. This partitioning is used to generate a set of centromere candidates which are then evaluated based on a second set of proposed features. The proposed algorithm outperforms previously published algorithms and is shown to do so with a larger set of chromosome images. A highlight of the proposed algorithm is the ability to rank this set of centromere candidates and create a centromere confidence metric which may be used in post-detection analysis. When tested with a larger metaphase chromosome database consisting of 1400 chromosomes collected from 40 metaphase cell images, the proposed algorithm was able to accurately localize 1220 centromere locations yielding a detection accuracy of 87%.

Centromere Detection of Human Metaphase Chromosome Images using a Candidate Based Method

Accurate detection of the human metaphase chromosome centromere is an critical element of cytogenetic diagnostic techniques, including chromosome enumeration, karyotyping and radiation biodosimetry. Existing image processing methods can perform poorly in the presence of irregular boundaries, shape variations and premature sister chromatid separation, which can adversely affect centromere localization. We present a centromere detection algorithm that uses a novel profile thickness measurement technique on irregular chromosome structures defined by contour partitioning. Our algorithm generates a set of centromere candidates which are then evaluated based on a set of features derived from images of chromosomes. Our method also partitions the chromosome contour to isolate its telomere regions and then detects and corrects for sister chromatid separation. When tested with a chromosome database consisting of 1400 chromosomes collected from 40 metaphase cell images, the candidate based centromere detection algorithm was able to accurately localize 1220 centromere locations yielding a detection accuracy of 87%. We also introduce a Candidate Based Centromere Confidence (CBCC) metric which indicates an approximate confidence value of a given centromere detection and can be readily extended into other candidate related detection problems.