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Drosophila melanogaster RecQ5, a member of the RecQ family, is expressed in early embryos. The loss of maternally-derived RecQ5 leads to spontaneous mitotic defects in syncytial embryos. We demonstrate that the mitotic defects are derived from anaphase DNA bridges. Pairs of daughter nuclei that had been linked by the bridges concurrently exited from the cycle and were eliminated by Chk2-dependent centrosome inactivation. These results suggest that the lack of RecQ5 leads to spontaneous double-stranded DNA breaks (DSBs). RecQ5 may function in the resolution of anaphase DNA bridges during mitosis or in DSB repair during interphase in syncytial Drosophila embryos.

Mutations in BLM cause Bloom's syndrome, a disorder associated with cancer predisposition and chromosomal instability. We investigated whether BLM plays a role in ensuring the faithful chromosome segregation in human cells. We show that BLM-defective cells display a higher frequency of anaphase bridges and lagging chromatin than do isogenic corrected derivatives that eptopically express the BLM protein. In normal cells undergoing mitosis, BLM protein localizes to anaphase bridges, where it colocalizes with its cellular partners, topoisomerase IIIalpha and hRMI1 (BLAP75). Using BLM staining as a marker, we have identified a class of ultrafine DNA bridges in anaphase that are surprisingly prevalent in the anaphase population of normal human cells. These so-called BLM-DNA bridges, which also stain for the PICH protein, frequently link centromeric loci, and are present at an elevated frequency in cells lacking BLM. On the basis of these results, we propose that sister-chromatid disjunction is often incomplete in human cells even after the onset of anaphase. We present a model for the action of BLM in ensuring complete sister chromatid decatenation in anaphase.

Objectives Constitutional aneuploidy occurs in at least 5% of recognised pregnancies, with apparent preferential involvement of the X chromosome and the smaller autosomes. Molecular cytogenetic investigations of cleavage-stage embryos have revealed anomalies affecting all sizes of chromosomes. The aim was to investigate the variety of anomalies arising during maternal meiosis I by analysis of unfertilised oocytes and polar bodies to gain insight into aneuploidy mechanisms. Methods Sequential FISH analysis was carried out with specific probes derived from eight chromosomes, representing all sizes. Only imbalance due to a gain of a whole chromosome or chromatid, represented by extra signals, was counted to avoid artefact. Results Data were obtained on 236 eggs from 124 patients of average age 32.5 years (range 22–44). Ten patients (average 32.6 years) had abnormal eggs. The abnormality rate for oocytes and for polar bodies was close to 4% for each. Fourteen hyperploidies were found, seven involving additional single chromatids. The abnormalities affected chromosomes 13,16,18, 21 and X but not chromosomes 1, 9 or 12. Conclusion The data provide evidence for several mechanisms leading to aneuploidy, including classical non-disjunction of whole univalents; pre-division of chromatids prior to anaphase I, leading to imbalance detected at metaphase II; gonadal mosaicism for a trisomic cell line and preferential involvement of the smaller chromosomes. Monosomy for the large autosomes is not uncommon in cleavage-stage embryos and may additionally arise from anaphase lag preferentially affecting such chromosomes. Copyright © 2003 John Wiley & Sons, Ltd.

Human embryonic aneuploidy can have a meiotic or a mitotic origin. The majority of meiotic chromosome errors arise during oogenesis. Two main aneuploidy-causing mechanisms have been defined: the first involves the nondisjunction of entire chromosomes and takes place during both meiotic divisions, whereas the second involves the premature division of a chromosome into its two sister chromatids during meiosis I, followed by their random segregation. Mitotic aneuploidy can arise as a consequence of problems such as nondisjunction, endoreduplication and anaphase lag and occurs most often during the first three divisions after fertilisation. The cleavage stage of development is characterised by the highest rates of aneuploidy, after which the incidence of cytogenetic abnormality decreases significantly. A large number of oocytes and embryos have been examined in order to define the spectrum of aneuploidies during the first few days of life and to shed light upon their origins. Various classical and molecular cytogenetic methods have been employed for this purpose, and valuable data of biological and clinical relevance have been obtained. Key Concepts: Aneuploidy is the most important genetic cause of human reproductive wastage (i.e. the principal reason for embryo implantation failure and miscarriage). The outcome of assisted reproductive treatments (e.g. in vitro fertilisation (IVF)) and natural reproductive cycles is negatively affected by aneuploidy. Most meiotically derived abnormalities arise during oogenesis. There is a strong relationship between advancing female age and increasing aneuploidy rates in oocytes. Two distinct mechanisms of oocyte chromosome malsegregation have been described, whole chromosome nondisjunction and unbalanced chromatid predivision. Post-zygotic aneuploidy usually arises during the first few mitotic divisions and leads to mosaicism in the embryo. There are three main mechanisms responsible for aneuploidy of mitotic origin: anaphase lag, endoreduplication and mitotic nondisjunction. The cleavage stage of preimplantation development is associated with the highest aneuploidy rates. The frequency of chromosome abnormalities and mosaicism declines as embryos progress to the blastocyst stage, presumably due to loss of abnormal cells or demise of affected embryos. Keywords: oocyte; preimplantation embryo; chromosome; aneuploidy; mosaicism

The correct transmission of chromosomes from mother to daughter cells is fundamental for genetic inheritance. Separation and segregation of sister chromatids in growing cells occurs in the cell cycle stage called 'anaphase'. The basic process of sister chromatid separation is similar in all eukaryotes: many gene products required are conserved. In this review, the roles of two proteins essential for the onset of anaphase in fission yeast, Cut2/securin and Cut1/separin, are discussed with regard to cell cycle regulation, and compared with the postulated roles of homologous proteins in other organisms. Securin, like mitotic cyclins, is the target of the anaphase promoting complex (APC)/cyclosome and is polyubiquitinated before destruction in a manner dependent upon the destruction sequence. The anaphase never occurs properly in the absence of securin destruction. In human cells, securin is an oncogene. Separin is a large protein (MW approximately 180 kDa), the C-terminus of which is conserved, and is thought to be inhibited by association with securin at the nonconserved N-terminus. In the budding yeast, Esp1/separin is thought to be a component of proteolysis against Scc1, an essential subunit of cohesin which is thought to link duplicated sister chromatids up to the anaphase. Whether fission yeast Cut1/separin is also implicated in proteolysis of cohesin is discussed.

We have identified new mitotic forces in crane-fly spermatocytes, separate from forces on the kinetochore, that propel chromosome arms in anaphase towards the spindle pole. In normal spermatocytes, the chromosome arms in anaphase generally trail the kinetochore to the pole. After ultraviolet-microbeam irradiation of a kinetochore spindle fibre, however, chromosome arms moved closer to the pole than the kinetochore. This poleward arm-movement occurred regardless of whether the irradiation stopped the movement of the associated chromosomes, and occurred both in chromosomes associated with the irradiated fibre and in chromosomes not associated with the irradiated fibre. Arms that moved ahead of the kinetochore continued to lead the kinetochore to the pole for the duration of anaphase. Ultraviolet-microbeam-irradiation-induced movement of arms ahead of the kinetochore is specific for irradiation of spindle fibres: irradiations of the cytoplasm outside the spindle had no effect, and irradiations of the region between spindle and mitochondrial sheath (that outlines the spindle) and irradiations of the interzonal region are much less effective than irradiations of spindle fibres in causing arms to move. We argue that in crane-fly spermatocytes forces propelling chromosome arms toward the pole are part of normal anaphase.

The transition from metaphase to anaphase was examined in primary oocytes of the Mediterranean mealmoth, Ephestia kuehniella. Isolated spindles were analysed using phase-contrast microscopy and antitubulin immunofluorescence. Metaphase I bivalents in female Lepidoptera contain material derived from the synaptonemal complexes, the elimination chromatin, which is located between the homologs. Upon the onset of anaphase I, the homologs detach from the elimination chromatin. The present observations revealed that prior to the start of anaphase A, the homologs move centripetally, i.e., toward the central spindle axis, while the elimination chromatin remains stationary. This type of movement is not directly comparable with conventional chromosome movements such as prometaphase congression, anaphase A, and anaphase B, since it occurs at right angles to the preferential orientation of the spindle MTs. A MT-based mechanism may nevertheless account for the centripetal shift of homologs in meiosis I of the moth: bundling of kinetochore MTs of different chromosomes could transfer the chromosomes toward the spindle axis. Since the homologs move along the poleward surfaces of the plate of elimination chromatin, a MT-independent force producer could exist as well at the interface between the chromosomes and the plate of elimination chromatin. © 1993 Wiley-Liss, Inc.

We report on monozygotic (MZ) twins who were discordant for phenotypic sex and Ullrich-Turner syndrome (UTS). The nonviable female was hydropic with cystic hygromas, ventricular septal defect, bicuspid aortic valve, polysplenia, intestinal malrotation, and small ovaries. The male was phenotypically normal. The monochorionic, diamniotic placenta had hydropic changes limited to the UTS infant's side. Skin samples from the infants and blood from their parents were obtained for cytogenetic and molecular analysis. Karyotypes of the twins were 45,X and 46,XY. Quinacrine polymorphisms on 7 chromosomes and RFLP analysis at 8 loci showed complete identity. MZ twins discordant for phenotypic sex have been described previously. Most of these show evidence of mosaicism in a 45,X patient with a normal 46,XY cell line, and a normal 46,XY male. While the issue of mosaicism in our case cannot be fully resolved, no mosaicism was found in 50 cells analyzed cytogenetically from each culture or by PCR analysis of a Y-specific sequence. The twins probably originated from either postzygotic nondisjunction or anaphase lag, followed or accompanied by twinning. The discordant placental morphology suggests an embryonic origin of at least part of the placental mesenchymal core.

Ullrich-Turner syndrome occurred in one of a pair of female twins. The chromosome constitution of the affected twin was 45,X/46,XX and that of the normal twin 46,XX. Investigation of banded chromosomes, red cell antigens, HLA types, red cell enzymes, and serum proteins indicates monozygosity. The twins are discordant for height, pterygium colli, ovarian function, strabismus, dental eruption, external ear formation, hearing loss, and performance scores on the Wechsler Intelligence Test. All of these differences can be attributed to X monosomy in one cell line in the affected twin, presumably resulting from mitotic nondisjunction or anaphase lag early during embryonic development. Ten other pairs of apparently monozygotic twins discordant for the Ullrich-Turner syndrome have been reported previously, and the findings in these cases are reviewed.

Mitotic micronuclei were isolated from Tetrahymena thermophila and data on spindle ultrastructure were obtained from serial, transverse sections. Comparison of data from nuclei at meta- and early anaphase with data from nuclei at late anaphase showed that during anaphase, sister kinetochores move from the equator to the spindle poles, but kinetochore translocation occurs without any apparent change in either the number or length of kinetochore microtubules. This unprecedented result is ascribed significance with regard to the mechanism of kinetochore transport since there are only a limited number of ways that result could be achieved. The organization of the peripheral sheath changes during anaphase as evidenced by gaps in the sheath at late anaphase. Numerous kinetochore and non-kinetochore microtubules are located in polar regions of the spindle at late anaphase, whereas those regions contained only peripherally arranged microtubules at earlier stages. Tracking of individual kinetochore microtubules in late anaphase nuclei showed that some of them appeared to become incorporated into the peripheral sheath near the pole. At early and late anaphase, crossbridges connect adjacent microtubules throughout the spindle poleward to the kinetochores, as well as in the interzone.

During early anaphase PtK1 cells were briefly treated with the rapidly reversible microtubule (MT) poison nocodazole. This treatment abruptly stopped chromosome motion and effected a large decrease in spindle birefringence. On removal of the drug, chromosome to pole motion (anaphase A) returned, though at a lesser rate but not extent than untreated cells. In most cases elongation of the pole-pole distance (anaphase B) also occurred, at both a rate and to an extent less than in untreated cells. During the recovery period following drug arrest spindle birefringence did not return to pretreatment levels. Electron microscopic analysis of nocodazole arrested, or arrested and released, cells revealed extensive disassembly of the nonkinetochore class of MTs (nkMTs), particularly evident in the astral region. Microtubules seen in the interzone region were largely fragments of midbody precursors. Kinetochore MTs (kMTs) appeared to be unaffected by the brief drug treatment chosen for these experiments. Analysis of MT profiles seen in transverse sections of the interzone region indicated in treated and released cells approximately 60% fewer MTs. This may suggest that chromosome motion during anaphase is not dependent on interactions between kMTs and nkMTs and separation of the spindle poles can occur in the presence of disrupted interzonal MTs.

A B S T R A C T Details of mitosis in the chloromonadophycean alga Vacuolaria virescens Cienk. have been studied with the light microscope. The chromosomes are relatively large (up to 12 in length at metaphase) and so mitotic stages are readily distinguishable. Chromosomes can be recognized in interphase nuclei as fine strands of chromatin. Contraction of these chromosomes marks the beginning of mitosis and continues progressively until the transition from metaphase to anaphase. Disintegration of nucleoli is complete by late prophase and nucleolar reformation begins in telophase. Some chromosomes exhibit less densely stained regions; centromeres are also present as indicated by their differential staining and by the behavior of chromosomes at metaphase and anaphase. At anaphase progeny chromosomes move apart parallel to the division axis of the nucleus. As anaphase progresses the chromosomes fuse at the polar surface of the progeny chromosome groups. This process continues in telophase and the chromosome groups become more spherical. By the end of telophase nucleolar reformation has begun and the chromosomes have relaxed to their interphase condition.