Condensation of chromosomes during mitosis is required for their segregation into daughter cells but must be reversed to allow for postmitotic functions. In this issue of Developmental Cell, Magalska et al. (2014) show that the ATPases RuvBL1/2 drive postmitotic chromatin decondensation, demonstrating that this is an active process.

In the original version of the manuscript, the following text was included in the third paragraph of the "RNA processing machinery promotes kinetochore assembly" section of the Results and discussion: "Our results are in contrast to previous studies reporting that a-amanitin treatment decreased CENP-C centromere localization (Chan et al., 2012; Quénet and Dalal, 2014)." After publication, the authors realized that the characterization of this related work was inaccurate. The new text should read: "Our results are in contrast to previous studies reporting that a-amanitin treatment decreased CENP-C and CENP-A centromere localization (Chan et al., 2012; Quénet and Dalal, 2014)." In addition, the penultimate sentence of the same paragraph included the following text: "However, the RNAi machinery has been shown to trigger transcriptional gene silencing in vertebrates through siRNA-guided H3K9 di- and trimethylation, leading to the recruitment of HP1 proteins (Alló et al., 2009; Ameyar-Zazoua et al., 2012), which abolished the function of a human artificial chromosome centromere (Nakano et al., 2003)." One citation was inaccurate. The reference Nakano et al., 2003 was removed from the reference list and the citation in the text updated. The text should read: "However, the RNAi machinery has been shown to trigger transcriptional gene silencing in vertebrates through siRNA-guided H3K9 di- and trimethylation, leading to the recruitment of HP1 proteins (Alló et al., 2009; Ameyar-Zazoua et al., 2012), which abolished the function of a human artificial chromosome centromere (Nakano et al., 2008)." The HTML and PDF versions of the article have been corrected. The error remains only in the print version. The authors apologize for the error.

Transcription at the centromere of chromosomes plays an important role in kinetochore assembly in many eukaryotes, and noncoding RNAs contribute to activation of the mitotic kinase Aurora B. However, little is known about how mitotic RNA processing contributes to spindle assembly. We found that inhibition of transcription initiation or RNA splicing, but not translation, leads to spindle defects in Xenopus egg extracts. Spliceosome inhibition resulted in the accumulation of high molecular weight centromeric transcripts, concomitant with decreased recruitment of the centromere and kinetochore proteins CENP-A, CENP-C, and NDC80 to mitotic chromosomes. In addition, blocking transcript synthesis or processing during mitosis caused accumulation of MCAK, a microtubule depolymerase, on the spindle, indicating misregulation of Aurora B. These findings suggest that co-transcriptional recruitment of the RNA processing machinery to nascent mitotic transcripts is an important step in kinetochore and spindle assembly and challenge the idea that RNA processing is globally repressed during mitosis.

Cell division in all eukaryotes depends on function of the spindle, a microtubule-based structure that segregates chromosomes to generate daughter cells in mitosis or haploid gametes in meiosis. Spindle size adapts to changes in cell size and shape, which vary dramatically across species and within a multicellular organism, but the nature of scaling events and their underlying mechanisms are poorly understood. Cell size variations are most pronounced in early animal development, as egg diameters range from tens of microns up to millimeters across animal phyla, and decrease several orders of magnitude during rapid reductive divisions. During early embryogenesis in the model organisms X. laevis and C. elegans, the spindle scales with cell size [1, 2], a phenomenon regulated by molecules that modulate microtubule dynamics [3-6], as well as by limiting cytoplasmic volume [7, 8]. However, it is not known to what extent spindle scaling is conserved across organisms and among different cell types. Here we show that in a range of metazoan phyla, mitotic spindle length decreased with cell size across an ∼30-fold difference in zygote size. Maximum spindle length varied, but linear spindle scaling occurred similarly in all species once embryonic cell diameter reduced to 140 μm. In contrast, we find that the female meiotic spindle does not scale as closely to egg size, adopting a more uniform size across species that most likely reflects its specialized function. Our analysis reveals that spindle morphometrics change abruptly, within one cell cycle, at the transition from meiosis to mitosis in most animals.

CRISPR-based technologies have emerged as powerful tools to alter genomes and mark chromosomal loci, but an inexpensive method for generating large numbers of RNA guides for whole genome screening and labeling is lacking. Using a method that permits library construction from any source of DNA, we generated guide libraries that label repetitive loci or a single chromosomal locus in Xenopus egg extracts and show that a complex library can target the E. coli genome at high frequency.

Imaging datasets are rich in quantitative information. However, few cell biologists possess the tools necessary to analyze them. Here, we present a large dataset of Xenopusextract spindle images together with an analysis pipeline designed to assess spindle morphology across a range of experimental conditions. Our analysis of different spindle types illustrates how kinetochore microtubules amplify spindle microtubule density. Extract mixing experiments reveal that some spindle features titrate, while others undergo switch-like transitions, and multivariate analysis shows the pleiotropic morphological effects of modulating the levels of TPX2, a key spindle assembly factor. We also apply our pipeline to analyze nuclear morphology in human cell culture, showing the general utility of the segmentation approach. Our analyses provide new insight into the diversity of spindle types and suggest areas for future study. The approaches outlined can be applied by other researchers studying spindle morphology and adapted with minimal modification to other experimental systems.