S phase of the cell cycle

In each cell cycle the complex structure of the chromosome must be replicated accurately. In the last few years there have been major advances in understanding eukaryotic chromosome replication. Patterns of replication origins have been mapped accurately in yeast chromosomes. Cellular replication proteins have been identified by fractionating cell extracts that replicate viral DNA templates in vitro. Cell-free systems that initiate eukaryotic DNA replication in vitro have demonstrated the importance of complex nuclear architecture in the control of DNA replication. Although the events of S phase were relatively neglected for many years, knowledge of DNA replication is now advancing rapidly in step with other phases of the cell cycle.     

细胞周期调控的分子机制

由各级调控因子组成的级联调控网络对细胞周期进程进行严格而有序的调控,是细胞正常增殖分裂的基础。概述了ＣＤＫ、Ｃｙｃｌｉ和ＣＤＩ在细胞周期调控中的作用。     

The cell cycle coming under control

An article in 30 June issue (";Dangerous' liaisons in cell biology," p. 1539), which discussed the commentary in the 2 June issue of the journal Cell by Max L. Birnstiel and Meinrad Busslinger regarding the transgenic mice experiments of Corrado Spadafora and co-workers, stated that Vienna's Institute of Molecular Pathology (IMP), for which Birnstiel and Busslinger work, was seeking patents on extensions of the Spadafora experiments. On further investigation, we have learned that this is not correct. At present no applications have been applied for or granted to IMP or its parent companies Genentech and Boehringer Ingelheim. We wish to correct the record, apologize to Dr. Birnstiel and Dr. Busslinger for this misstatement, and alert our readers to their letter, which is printed in our Letters section on page 243.     

Dynamic complex formation during the yeast cell cycle

To analyze the dynamics of protein complexes during the yeast cell cycle, we integrated data on protein interactions and gene expression. The resulting time-dependent interaction network places both periodically and constitutively expressed proteins in a temporal cell cycle context, thereby revealing previously unknown components and modules. We discovered that most complexes consist of both periodically and constitutively expressed subunits, which suggests that the former control complex activity by a mechanism of just-in-time assembly. Consistent with this, we show that additional regulation through targeted degradation and phosphorylation by Cdc28p (Cdk1) specifically affects the periodically expressed proteins.     

A cell cycle phosphoproteome of the yeast centrosome

Centrosomes organize the bipolar mitotic spindle, and centrosomal defects cause chromosome instability. Protein phosphorylation modulates centrosome function, and we provide a comprehensive map of phosphorylation on intact yeast centrosomes (18 proteins). Mass spectrometry was used to identify 297 phosphorylation sites on centrosomes from different cell cycle stages. We observed different modes of phosphoregulation via specific protein kinases, phosphorylation site clustering, and conserved phosphorylated residues. Mutating all eight cyclin-dependent kinase (Cdk)-directed sites within the core component, Spc42, resulted in lethality and reduced centrosomal assembly. Alternatively, mutation of one conserved Cdk site within γ-tubulin (Tub4-S360D) caused mitotic delay and aberrant anaphase spindle elongation. Our work establishes the extent and complexity of this prominent posttranslational modification in centrosome biology and provides specific examples of phosphorylation control in centrosome function.     

柴胡皂甙-d对人肝癌HepG2细胞增殖及细胞周期的影响

目的 探讨柴胡皂甙-d(SSd)对人肝癌HepG2细胞增殖和细胞周期的影响.方法 (1)常规培养人肝癌HepG2细胞;(2)噻唑蓝(MTT)实验观察不同质量浓度SSd处理HepG2细胞72 h后以及10mg/L的SSd作用HepG2细胞不同时间对HepG2细胞生长指数的影响;(3)流式细胞术(FCM)检测SSd处理HepG2细胞48h后细胞周期的分布.结果 SSd能明显抑制HepG2细胞的增殖,以10mg/L的SSd作用48h的抑制作用最明显.FCM检测显示SSd作用48h后实验组处于S期的细胞数较对照组明显减少(P<0.01).结论 SSd对人肝癌HepG2细胞的体外增殖具有抑制作用,并能使细胞周期中处于S期的细胞数减少.     

Toxins-antitoxins: plasmid maintenance,programmed cell death,and cell cycle arrest

Antibiotic resistance, virulence, and other plasmids in bacteria use toxin-antitoxin gene pairs to ensure their persistence during host replication. The toxin-antitoxin system eliminates plasmid-free cells that emerge as a result of segregation or replication defects and contributes to intra- and interspecies plasmid dissemination. Chromosomal homologs of toxin-antitoxin genes are widely distributed in pathogenic and other bacteria and induce reversible cell cycle arrest or programmed cell death in response to starvation or other adverse conditions. The dissection of the interaction of the toxins with intracellular targets and the elucidation of the tertiary structures of toxin-antitoxin complexes have provided exciting insights into toxin-antitoxin behavior.     

The yeast cell cycle gene CDC34 encodes a ubiquitin-conjugating enzyme

Mutants in the gene CDC34 of the yeast Saccharomyces cerevisiae are defective in the transition from G1 to the S phase of the cell cycle. This gene was cloned and shown to encode a 295-residue protein that has substantial sequence similarity to the product of the yeast RAD6 gene. The RAD6 gene is required for a variety of cellular functions including DNA repair and was recently shown to encode a ubiquitin-conjugating enzyme. When produced in Escherichia coli, the CDC34 gene product catalyzed the covalent attachment of ubiquitin to histones H2A and H2B in vitro, demonstrating that the CDC34 protein is another distinct member of the family of ubiquitin-conjugating enzymes. The cell cycle function of CDC34 is thus likely to be mediated by the ubiquitin-conjugating activity of its product.     

Checkpoints: controls that ensure the order of cell cycle events

The events of the cell cycle of most organisms are ordered into dependent pathways in which the initiation of late events is dependent on the completion of early events. In eukaryotes, for example, mitosis is dependent on the completion of DNA synthesis. Some dependencies can be relieved by mutation (mitosis may then occur before completion of DNA synthesis), suggesting that the dependency is due to a control mechanism and not an intrinsic feature of the events themselves. Control mechanisms enforcing dependency in the cell cycle are here called checkpoints. Elimination of checkpoints may result in cell death, infidelity in the distribution of chromosomes or other organelles, or increased susceptibility to environmental perturbations such as DNA damaging agents. It appears that some checkpoints are eliminated during the early embryonic development of some organisms; this fact may pose special problems for the fidelity of embryonic cell division.     

某豆科植物粗提物(JA1)对人肝癌细胞株端粒酶活性的抑制和细胞周期的改变

采用单管法一步完成端粒重复序列扩增法检测人肝癌细胞株,经不同浓度JA1及不同时间的作用前后端粒酶活性的变化,并采用流式细胞仪分析细胞周期的变化。结果显示,JA1可显著抑制人肝癌细胞端粒酶活性,而且这种抑制效果有剂量依赖性和时间依赖性。流式细胞仪分析细胞周期的变化表明,端粒酶活性被抑制后,肝癌细胞被阻滞在G2/M。同时,在检测标本中显示有明显的DNA低含量颗粒(“亚G1期”峰),表明肝癌细胞凋亡的存在。     

转染抑癌基因PTEN对人乳腺癌ZR-75-1细胞的细胞周期和增殖能力的影响

目的：观察PTEN（phosphatase and tensin homology deleted on chromosome ten，第10号染色体上磷酸酶和张力蛋白同源缺失的基因）对人乳腺癌细胞ZR-75-1细胞增殖和细胞周期的影响。方法；利用脂质体介导法将携带有野生型和突变型PTEN cDNA的真核表达载体pBP—wt—PTEN和pBP—G129R—PTEN导八人乳腺癌ZR-75-1细胞（质粒转染成功后实验分为C—WT—PTEN组、CG129R—PTEN组和未转染质粒组即对照组）后，以RT—PCR、Western blot分析目的基因的表达，并采用MTT法和流式细胞术检测细胞增殖和细胞周期。结果：C—WT—PTEN组、C—G129R—PTEN组细胞PTEN mRNA及PTEN蛋白出现明显的高表达。C—WT—PTEN组细胞生长的抑制率可高达42．7％，与对照组比较．差异有显著性（P〈0．05）。但C—G129R—PTEN组细胞生长的抑制率与对照组比较，差异无显著性（P〉0．05）。流式细胞术显示C—WT—PTEN组细胞周期从G1期到S期已发生抑制。结论：野生型PTEN可依赖其磷酸酶活性抑制肿瘤细胞的增殖,并最终诱导细胞凋亡。     

Rapid flow cytometric analysis of the cell cycle in intact plant tissues

Mechanical chopping of plant tissues in the presence of mithramycin released intact nuclei representative of the cells within the tissues. The amount of nuclear DNA in the homogenates of monocotyledonous and dicotyledonous plants was accurately and rapidly determined by flow microfluorometry, and the distribution of nuclei involved in the cell cycle was charted for tissues selected from different physical locations or developmental stages.     

A voltage-dependent chloride current linked to the cell cycle in ascidian embryos

A voltage-dependent chloride current has been found in early ascidian embryos that is a minor conductance in the oocyte and in interphase blastomeres but that increases transiently in amplitude by more than tenfold during each cell division. Repeated cycles in the density of this chloride current could be recorded for up to 6 hours (four cycles) in cleavage-arrested embryos, whether they were activated by sperm or calcium ionophore. These data suggest that there is a direct link between the cell cycle clock and the properties of this channel, a link that results in pronounced cyclical changes in the electrical properties of early blastomeres.     

口虾蛄繁殖周期及生殖细胞发育的研究

对大连市黑石礁海域口虾蛄Oratosquilla oratoria卵巢性腺指数的周年变化进行测定,对雌雄个体的生殖细胞发育进行了组织学观察.结果表明:3月,随着水温的升高,口虾蛄繁殖个体卵巢发育逐渐成熟,性腺指数上升,至6月达到最大值,7月,随着繁殖个体的排卵,性腺指数下降,至9月出现全年的最小值;口虾蛄1龄即可性成熟,每年产卵1次;雄性生殖细胞发育分为精原细胞期、初级精母细胞期、次级精母细胞期、精细胞期和精子期5个时期,雌性生殖细胞发育分为卵原细胞期、卵黄形成前期、卵黄形成期和成熟期4个时期.