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PubMed1975

Prophase chromosome movements in living house cricket spermatocytes and their relationship to prometaphase, anaphase and granule movements.

Chromosome and granule movements in meiotic prophase and prometaphase have been studied by time-lapse cinemicrography in live spermatocytes of the house cricket, Acheta domesticus. Chromosome movements in prophase cells, up to one hour or more before breakdown of the nuclear envelope, are described. These movements are frequent but saltatory; are based mostly at chromosome ends but also at kinetochores; occur in very intimate association with the inside of the nuclear envelope; are directed towards and away from the extranuclear centres (centrioles); tend weakly to accumulate bivalents round the two centres and reach a velocity of 0.65 micron/sec. Saltatory movements in granules associated with extranuclear asters are remarkably similar to basic characteristics to the intranuclear chromosome movements. Surprisingly, the chromosome movements (and those granules) are reversably blocked by colcemid (but not lumi-colcemid), and yet occur in the apparent absence of an intranuclear envelope. However, kinetochore movements in very early prometaphase are similar in velocity and other respects to prophase movements; later prometaphase movements are clearly slower, and those of anaphase very much slower still. -The prophase movements suggest a two component model for motion: a non-microtubule, linear force producer together withrotubules with a skeletal, orientational role. Arguably, both these components are also necessary for chromosome movements in prometaphase and anaphase.

Animals↗
PubMed2002

Homolog interaction during meiotic prophase I in Arabidopsis requires the SOLO DANCERS gene encoding a novel cyclin-like protein.

Interactions between homologs in meiotic prophase I, such as recombination and synapsis, are critical for proper homolog segregation and involve the coordination of several parallel events. However, few regulatory genes have been identified; in particular, it is not clear what roles the proteins similar to the mitotic cell cycle regulators might play during meiotic prophase I. We describe here the isolation and characterization of a new Arabidopsis mutant called solo dancers that exhibits a severe defect in homolog synapsis, recombination and bivalent formation in meiotic prophase I, subsequently resulting in seemingly random chromosome distribution and formation of abnormal meiotic products. We further demonstrate that the mutation affects a meiosis-specific gene encoding a novel protein of 578 amino acid residues with up to 31% amino acid sequence identity to known cyclins in the C-terminal portion. These results argue strongly that homolog interactions during meiotic prophase I require a novel meiosis-specific cyclin in Arabidopsis.

Amino Acid Sequence↗
PubMed1990

Ooplasmic transfusion: prophase germinal vesicle oocytes made developmentally competent by microinjection of metaphase II egg cytoplasm.

Approximately one fourth of all human oocytes collected for in vitro fertilization are of immature origin. Even when these oocytes undergo nuclear maturation, fertilization, and cleavage in vitro, transfer of such embryos rarely results in pregnancy reaching delivery. We hypothesized that human embryos derived from prophase I oocytes were developmentally incompetent because they lacked a factor(s) found in in vivo matured oocytes. Using micromanipulation techniques in monkeys, we removed ooplasm from metaphase II oocytes and injected it into prophase I oocytes. After nuclear maturation, oocytes were transferred to the fallopian tube for fertilization. After ooplasmic transfusion, prophase I oocytes resulted in a delivery rate of 13%. When metaphase II ooplasm was heated or exposed to ribonuclease A before microinjection into prophase I oocytes, it lost effectiveness in conferring developmental competence.

Animals↗
PubMed1995

DNA changes involved in the formation of metaphase chromosomes, as observed in mouse duodenal crypt cells stained by osmium-ammine. I. New structures arise during the S phase and condense at prophase into "chromomeres," which fuse at prometaphase into mitotic chromosomes.

BACKGROUND: In the hope of understanding how chromosomes condense at mitosis, we took advantage of a subdivision of the cell cycle into 11 stages to examine the changes in DNA taking place during the stages preceding the emergence of metaphase chromosomes. METHODS: To identify DNA changes, pieces of mouse duodenum were fixed in formaldehyde, and sections of the rapidly dividing cells of the crypts were stained by the osmium-ammine method, which is specific for the detection of DNA in the electron microscope. RESULTS: Throughout the cell cycle, DNA is present in nucleofilaments composed of rows of 11-nm-wide nucleosomes. At stage I, during which the DNA-synthesizing or S phase of the cell cycle begins, some of the nucleofilaments are compacted in the heterochromatin accumulations associated with nuclear envelope and nucleoli, while the others are scattered in the nucleoplasm where they appear either "free" or "attached" to the heterochromatin. This DNA distribution is similar to that observed in the noncycling cells examined. After the beginning of the S phase, "free" nucleofilaments are seen to assemble into structures composed of compacted nucleofilaments and referred to as "aggregates"; these make their appearance at stage II and increase in size through stage III up to the end of S during stage IV. Meanwhile, the heterochromatin associated with nuclear envelope and nucleoli expands toward the nucleoplasm in the form of protrusions referred to as "bulges," which gradually enlarge during stages III and IV, while the heterochromatin shrinks and eventually vanishes. On average, a total of 1,171 aggregates and bulges are formed in the nucleus during the S phase. At the apparition of stage V, which corresponds approximately to prophase, aggregates and bulges are rapidly gathered into an average of 288 spheroidal bodies referred to as "chromomeres." These are connected to one another by nucleofilamentous bridges in such a way as to be lined up in rows. The formation of rows of chromomeres represents in the electron microscope the prophasic condensation observed in the light microscope. Finally, during stage VIa, which corresponds to prometaphase, the chromomeres approach one another within each row, make contact, and coalesce to become the 40 chromosomes of the mouse, which during stage VIb are organized in the equatorial plate of metaphase. CONCLUSIONS: The condensation of metaphase chromosomes occurs in three main steps. The first and longest takes place during the S phase, as nucleofilaments are assembled into aggregates, while the heterochromatin gives rise to bulges. The brief second step occurs toward the beginning of prophase, when the numerous aggregates and bulges are congregated into a limited number of chromomeres, which are lined up in rows. The third step takes place during the brief prometaphase, when the chromomeres of a row coalesce into a mitotic chromosome.

Animals↗
PubMed2002

Meiotic prophase abnormalities and metaphase cell death in MLH1-deficient mouse spermatocytes: insights into regulation of spermatogenic progress.

The MLH1 protein is required for normal meiosis in mice and its absence leads to failure in maintenance of pairing between bivalent chromosomes, abnormal meiotic division, and ensuing sterility in both sexes. In this study, we investigated whether failure to develop foci of MLH1 protein on chromosomes in prophase would lead to elimination of prophase spermatocytes, and, if not, whether univalent chromosomes could align normally on the meiotic spindle and whether metaphase spermatocytes would be delayed and/or eliminated. In spite of the absence of MLH1 foci, no apoptosis of spermatocytes in prophase was detected. In fact, chromosomes of pachytene spermatocytes from Mlh1(-/-) mice were competent to condense metaphase chromosomes, both in vivo and in vitro. Most condensed chromosomes were univalents with spatially distinct FISH signals. Typical metaphase events, such as synaptonemal complex breakdown and the phosphorylation of Ser10 on histone H3, occurred in Mlh1(-/-) spermatocytes, suggesting that there is no inhibition of onset of meiotic metaphase in the face of massive chromosomal abnormalities. However, the condensed univalent chromosomes did not align correctly onto the spindle apparatus in the majority of Mlh1(-/-) spermatocytes. Most meiotic metaphase spermatocytes were characterized with bipolar spindles, but chromosomes radiated away from the microtubule-organizing centers in a prometaphase-like pattern rather than achieving a bipolar orientation. Apoptosis was not observed until after the onset of meiotic metaphase. Thus, spermatocytes are not eliminated in direct response to the initial meiotic defect, but are eliminated later. Taken together, these observations suggest that a spindle assembly checkpoint, rather than a recombination or chiasmata checkpoint, may be activated in response to meiotic errors, thereby ensuring elimination of chromosomally abnormal gamete precursors.

Adaptor Proteins, Signal Transducing↗
PubMed1998

Dynamics of meiotic prophase I during spermatogenesis: from pairing to division.

This review focuses on recent developments in our understanding of meiotic chromosome behavior in mammalian spermatogenesis, with a special emphasis on prophase I events in the best characterized organism, the laboratory mouse. The dynamics of chromosome movement prior to pairing and synapsis of chromosomes are complex and implicate function for both centromeres and telomeres in getting homologous chromosomes together in intimate synapsis. Likely candidates for mediating pairing and recombination include a host of proteins implicated in DNA repair and recombination, which have been shown to localize to the synaptonemal complex during meiotic prophase I. Precocious induction of meiotic metaphase in cultured pachytene spermatocytes has led to new information about requirements for MPF and topoisomerase II activity during the transition from meiotic prophase to metaphase. Together, the studies reviewed here increase our understanding of how chromosomes get together with their homologous partners and how these partners subsequently come apart.

Animals↗
PubMed1987

Cytological evidence that the Sxr fragment of XY,Sxr mice pairs homologously at meiotic prophase with the proximal testis-determining region.

Self-pairing of the Y chromosome at prophase of meiosis in XY,Sxr male mice appears to take place in many cells to the exclusion of pairing between the Y and the X. This phenomenon offers an explanation for the high level of X-Y separation seen in these males at prophase of meiosis, additional separations being evident, however, when metaphase I (MI) cells are examined. A minority of prophase cells show the Y paired both autologously and with a sub-terminal region of the X which could be the normal pairing region. The balloon-like configurations observed when self-pairing occurs suggest that the distal Sxr fragment is inverted on the Y chromosome of Sxr carrier males in relation to the normal proximal testis-determining (Td)-containing region.

Animals↗
PubMed1994

Saccharomyces cerevisiae cells lacking the homologous pairing protein p175SEP1 arrest at pachytene during meiotic prophase.

Saccharomyces cerevisiae cells containing null mutations in the SEP1 gene, which encodes the homologous pairing and strand exchange protein p175SEP1, enter pachytene with a delay. They arrest uniformly at this stage of meiotic prophase, probably revealing a checkpoint in the transition from pachytene to meiosis I. At the arrest point, the cells remain largely viable and are cytologically characterized by the duplicated but unseparated spindle pole bodies of equal size and by the persistence of the synaptonemal complex, a cytological marker for pachytene. In addition, fluorescence in situ hybridization revealed that in arrested mutant cells maximal chromatin condensation and normal homolog pairing is achieved, typical for pachytene in wild type. A hallmark of meiosis is the high level of homologous recombination, which was analyzed both genetically and physically. Formation and processing of the double-strand break intermediate in meiotic recombination is achieved prior to arrest. Physical intragenic (conversion) and intergenic (crossover) products are formed just prior to, or directly at, the arrest point. Structural deficits in synaptonemal complex morphology, failure to separate spindle pole bodies, and/or defects in prophase DNA metabolism might be responsible for triggering the observed arrest. The pachytene arrest in sep1 cells is likely to be regulatory, but is clearly different from the RAD9 checkpoint in meiotic prophase, which occurs prior to the pachytene stage.

Chromosomes, Fungal↗
PubMed1995

Meiosis in the leek (Allium porrum L.) revisited. I. Prophase I pairing.

Meiotic chromosome pairing of the tetraploid species Allium porrum, the cultivated leek, was analysed by electron microscopy of 83 surface-spread nuclei in the late zygotene to early diplotene interval of prophase I, from four different varieties. Prophase I quadrivalent frequency, at 71%, marginally but significantly exceeds the two-thirds expected on a simple random end-pairing model, suggesting that more than two autonomous pairing sites occur, in some tetrasomes at least. This pattern of synaptic behaviour is consistent with an autotetraploid status, but comparison with other tetraploids, including other Allium species, indicates that Allium porrum may be a weak segmental allopolyploid displaying limited preferential homologous pairing. The incidence of pairing partner switches (PPSs) in prophase I quadrivalents is relatively low; 90% of all analysed quadrivalents had only one or two PPSs. The positional distribution of PPSs along quadrivalents was distinctly uneven with peaks in mid-chromosome arms and reduced frequencies around centromeres and near the ends. The four different varieties of leek analysed were remarkably similar in their meiotic behaviour despite their diverse breeding history, but individual plants within varieties displayed substantial variation in quadrivalent and PPS frequencies.

Allium↗
PubMed1977

Comparative sensitivities of meiotic prophase stages in male mice to chromosome damage by acute X-and chronic gamma-irradiation.

Radiation-induced multivalents, fragments and bivalent separation were studied at metaphase I in mouse spermatocytes. These cells had been irradiated with 200 rad X-rays as spermatogonia or in different stages of prophase. Radiation sensitivity increased towards the latter end of prophase with respect to multivalents and fragments. These results were compared with protracted gamma-irradiation throughout prophase.

Animals↗
PubMed1985

Mitomycin C effects on cell cycle progression, including inhibition of very late prophase, as seen in living neuroblasts of Chortophaga viridifasciata, with some observations on mitomycin C purity.

Observations were made on living neuroblasts (Nbs) of the grasshopper (Chortophaga viridifasciata) embryo during a 4-h recovery period following 1-h in vitro exposure to 10(-8), 10(-6), and 10(-4) M mitomycin C (MMC). None of these concentrations affected the duration of mid-mitosis (prometaphase, metaphase, anaphase), but one as low as 10(-8) M causes a small reduction in the rate at which Nbs move through the remainder of the cell cycle, primarily by retarding their progress through S. As the concentration is increased there is slower movement through S and also prophase (there are no true G1 and G2 periods in the rapidly dividing Nb: 4-h cell cycle at 38 degrees C). A significant proportion of the cells exposed to 10(-4) M are blocked for 1 or more h at very late prophase, i.e., just before nuclear membrane breakdown. In such retarded prophases the chromosomes resemble c-metaphase chromosomes even though the nuclear membrane remains intact. Mass spectrometry data revealed that one lot of the MMC used contained one or more impurities.

Animals↗
PubMed1992

A predominant basic alpha-tubulin isoform present in prophase Xenopus oocyte decreases during meiotic maturation.

Xenopus oocytes are blocked in prophase of the first meiotic division. During the G2/M transition drastic changes occur both in the cytoskeletal organization and in the capacity of tubulin to polymerize. Posttranslational modification of tubulin isoforms might be one of the factors that control the dynamic properties of microtubules. We have therefore analysed, by two-dimensional polyacrylamide gel electrophoresis, the isotubulins purified from Xenopus oocytes, and we show that tubulin is resolved into at least four alpha-isoforms and four beta-isoforms. We have identified a basic alpha (alpha b)-tubulin isoform which is specific to prophase arrested oocyte and that progressively disappears during meiotic maturation; its decrease is initiated when the nuclear envelope breaks down and is controlled by the nucleus. Using 35S methionine labelled oocytes we demonstrate that the disappearance of the alpha b isotubulin results from both an arrest of its biosynthesis after maturation, and from posttranslational modification which induces a shift of this alpha-isoform to a more acidic pI. Moreover, in vitro experiments using 35S prelabelled tubulin purified from prophase oocytes show that metaphase extracts containing MPF activity are able to induce the acidification of the alpha b-isoform, suggesting that the observed posttranslational modification might be regulated by p34cdc2. However, the nature of this modification remains to be elucidated.

Animals↗
PubMed1991

Intracellular localization of MAP2-related protein (O-map) in prophase I and metaphase II oocytes of Xenopus.

An antibody directed against rat brain microtubule-associated protein 2 (MAP2) immunoprecipitated a protein of 240 kDa from a Xenopus oocyte extract. In contrast, in Xenopus brain extract, this antibody recognizes a protein of 280 kDa. The oocyte MAP2-related protein (called O-MAP) is present in both prophase I and metaphase II-blocked oocytes as demonstrated by immunoblotting experiments; it is in vivo phosphorylated. Immunocytochemical studies using the anti-rat brain MAP2 antibody demonstrated that the O-MAP colocalizes within the cortical microtubular array present in both prophase I and metaphase II oocytes. However, O-MAP is not associated with the microtubular structures which are organized during the oocyte prophase-metaphase transition, i.e., a giant cytoplasmic network and both the meiotic spindles. O-MAP therefore appears as a microtubule-associated protein oocyte specific and may play a role in the regulation of microtubule stability and the organization of the oocyte cytoskeleton.

Animals↗
PubMed2004

Cohesin component dynamics during meiotic prophase I in mammalian oocytes.

Cohesins are chromosomal proteins that form complexes involved in the maintenance of sister chromatid cohesion during division of somatic and germ cells. Three meiosis-specific cohesin subunits have been reported in mammals, REC8, STAG3 and SMC1 beta; their expression in mouse spermatocytes has also been described. Here we studied the localization of different meiotic and mitotic cohesin components during prophase I in human and murine female germ cells. In normal and atretic human fetal oocytes, from leptotene to diplotene stages, REC8 and STAG3 colocalize in fibers. In murine oocytes, SMC1beta, SMC3 and STAG3 are localized along fibers that correspond first to the chromosome axis and then to the synaptonemal complex in pachytene. Mitotic cohesin subunit RAD21 is also found in fibers that decorate the SC during prophase I in mouse oocytes, suggesting a role for this cohesin in mammalian sister chromatid cohesion in female meiosis. We observed that, unlike human oocytes, murine synaptonemal complex protein SYCP3 localizes to nucleoli throughout prophase I stages, and centromeres cluster in discrete locations from leptotene to dictyate. At difference from meiosis in male mice, the cohesin axis is progressively lost during the first week after birth in females with a parallel destruction of the axial elements at dictyate arrest, demonstrating sexual dimorphism in sister chromatid cohesion in meiosis.

Animals↗
PubMed2001

Polo-like kinase 1 phosphorylates cyclin B1 and targets it to the nucleus during prophase.

In vertebrate cells, the nuclear entry of Cdc2-cyclin B1 (MPF) during prophase is thought to be essential for the induction and coordination of M-phase events. Phosphorylation of cyclin B1 is central to its nuclear translocation, but the kinases that are responsible remain unknown. Here we have purified a protein kinase from Xenopus M-phase extracts that phosphorylates a crucial serine residue (S147) in the middle of the nuclear export signal sequence of cyclin B1. We have identified this kinase as Plx1 (ref. 16), a Xenopus homologue of Polo-like kinase (Plk)-1. During cell-cycle progression in HeLa cells, a change in the kinase activity of endogenous Plk1 toward S147 and/or S133 correlates with a kinase activity in the cell extracts. An anti-Plk1 antibody depletes the M-phase extracts of the kinase activity toward S147 and/or S133. An anti-phospho-S147 antibody reacts specifically with cyclin B1 only during G2/M phase. A mutant cyclin B1 in which S133 and S147 are replaced by alanines remains in the cytoplasm, whereas wild-type cyclin B1 accumulates in the nucleus during prophase. Co-expression of constitutively active Plk1 stimulates nuclear entry of cyclin B1. Our results indicate that Plk1 may be involved in targeting MPF to the nucleus during prophase.

4-Butyrolactone↗
PubMed1996

Localization of mouse Rad51 and Lim15 proteins on meiotic chromosomes at late stages of prophase 1.

BACKGROUND: In meiosis, eukaryotic chromosomes show a series of morphological changes, during which chromosomes synapse and recombine. To understand the mechanisms of the morphological changes and recombination of chromosomes, we examined stage-specific localization of the Rad51 and Lim15 proteins on the chromosomes in meiotic prophase 1. These proteins are homologous with the RecA protein and have general properties of searching and pairing of homologous DNA sequences. We used mouse chromosomes whose small sizes allow us to identify the locations of these proteins on the entire structures of the chromosomes. RESULTS: In the leptotene and zygotene stages, the Rad51 protein was present on chromatin loops of mouse testis chromosomes then the protein left the loops. In the pachytene stage, the Rad51 protein was present almost exclusively along the core of the synaptonemal complexes (SC). When the stage proceeded to diplotene, the protein was present in the synaptic regions of chromosomes, in particular, in the chiasma regions. The protein was not present on separated homologous SC cores. On the other hand, the Lim15 protein that was found on chromatin loops in early prophase 1, was present almost exclusively at both ends of the SC cores throughout the late stages of prophase 1. CONCLUSION: The Rad51 and Lim15 proteins are present in chromatin loops when chromosomes form SC. The proteins may promote pairing of homologous DNA sequences that would lead formation of SC. The Rad51 protein in the SC cores may be involved in chiasma formation in late stages. The Lim15 protein, instead, may be involved in recombination in the telomeric region or in cohesion of sister chromatids for segregation.

Adenosine Triphosphatases↗
PubMed1994

Dynamics of chromosome organization and pairing during meiotic prophase in fission yeast.

Interactions between homologous chromosomes (pairing, recombination) are of central importance for meiosis. We studied entire chromosomes and defined chromosomal subregions in synchronous meiotic cultures of Schizosaccharomyces pombe by fluorescence in situ hybridization. Probes of different complexity were applied to spread nuclei, to delineate whole chromosomes, to visualize repeated sequences of centromeres, telomeres, and ribosomal DNA, and to study unique sequences of different chromosomal regions. In diploid nuclei, homologous chromosomes share a joint territory even before entry into meiosis. The centromeres of all chromosomes are clustered in vegetative and meiotic prophase cells, whereas the telomeres cluster near the nucleolus early in meiosis and maintain this configuration throughout meiotic prophase. Telomeres and centromeres appear to play crucial roles for chromosome organization and pairing, both in vegetative cells and during meiosis. Homologous pairing of unique sequences shows regional differences and is most frequent near centromeres and telomeres. Multiple homologous interactions are formed independently of each other. Pairing increases during meiosis, but not all chromosomal regions become closely paired in every meiosis. There is no detectable axial compaction of chromosomes in meiotic prophase. S. pombe does not form mature synaptonemal complexes, but axial element-like structures (linear elements), which were analyzed in parallel. Their appearance coincides with pairing of interstitial chromosomal regions. Axial elements may define minimal structures required for efficient pairing and recombination of meiotic chromosomes.

Cell Nucleus↗
PubMed1999

Human cyclin A is required for mitosis until mid prophase.

We have used microinjection and time-lapse video microscopy to study the role of cyclin A in mitosis. We have injected purified, active cyclin A/cyclin-dependent kinase 2 (CDK2) into synchronized cells at specific points in the cell cycle and assayed its effect on cell division. We find that cyclin A/CDK2 will drive G2 phase cells into mitosis within 30 min of microinjection, up to 4 h before control cells enter mitosis. Often this premature mitosis is abnormal; the chromosomes do not completely condense and daughter cells fuse. Remarkably, microinjecting cyclin A/CDK2 into S phase cells has no effect on progress through the following G2 phase or mitosis. In complementary experiments we have microinjected the amino terminus of p21(Cip1/Waf1/Sdi1) (p21N) into cells to inhibit cyclin A/CDK2 activity. We find that p21N will prevent S phase or G2 phase cells from entering mitosis, and will cause early prophase cells to return to interphase. These results suggest that cyclin A/CDK2 is a rate-limiting component required for entry into mitosis, and for progress through mitosis until late prophase. They also suggest that cyclin A/CDK2 may be the target of the recently described prophase checkpoint.

CDC2-CDC28 Kinases↗