Non-random segregation of sister chromosomes in Escherichia coli

It has long been known that the 5' to 3' polarity of DNA synthesis results in both a leading and lagging strand at all replication forks. Until now, however, there has been no evidence that leading or lagging strands are spatially organized in any way within a cell. Here we show that chromosome segregation in Escherichia coli is not random but is driven in a manner that results in the leading and lagging strands being addressed to particular cellular destinations. These destinations are consistent with the known patterns of chromosome segregation. Our work demonstrates a new level of organization relating to the replication and segregation of the E. coli chromosome.     

Localization of cytoplasmic dynein to mitotic spindles and kinetochores

What is the origin of the forces generating chromosome and spindle movements in mitosis? Both microtubule dynamics and microtubule-dependent motors have been proposed as the source of these motor forces. Cytoplasmic dynein and kinesin are two soluble proteins that power membranous organelle movements on microtubules. Kinesin directs movement of organelles to the 'plus' end of microtubules, and is found at the mitotic spindle in sea urchin embryos, but not in mammalian cells. Cytoplasmic dynein translocates organelles to the 'minus' end of microtubules, and is composed of two heavy chains and several light chains. We report here that monoclonal antibodies to two of these subunits and to another polypeptide that associates with dynein localize the protein to the mitotic spindle and to the kinetochores of isolated chromosomes, suggesting that cytoplasmic dynein is important in powering movements of the spindle and chromosomes in dividing cells.     

Predicting evolutionary patterns of mammalian teeth from development

One motivation in the study of development is the discovery of mechanisms that may guide evolutionary change. Here we report how development governs relative size and number of cheek teeth, or molars, in the mouse. We constructed an inhibitory cascade model by experimentally uncovering the activator-inhibitor logic of sequential tooth development. The inhibitory cascade acts as a ratchet that determines molar size differences along the jaw, one effect being that the second molar always makes up one-third of total molar area. By using a macroevolutionary test, we demonstrate the success of the model in predicting dentition patterns found among murine rodent species with various diets, thereby providing an example of ecologically driven evolution along a developmentally favoured trajectory. In general, our work demonstrates how to construct and test developmental rules with evolutionary predictability in natural systems.     

从五四运动到安源工运看青年毛泽东世界观的转变与飞跃

青年毛泽东从五四运动到安源工运的斗争实践中,不仅发现了民众特别是工人阶级的力量,而且还找到了在马克思主义理论指导下由中国共产党根本改造中国社会的正确道路,从而实现了从一个寻求社会改良的爱国者到进行社会主义革命的救国者的飞跃,实现了世界观的根本转变。     

Size variation in chromosomes from independent cultured isolates of Plasmodium falciparum

The complexity of the life cycle of the protozoan malaria parasite Plasmodium falciparum has hindered genetic analysis; even the number of chromosomes in P. falciparum is uncertain. The blood stages of rodent malaria parasites are haploid and hybridization with cloned complementary DNAs similarly suggests a haploid genome in P. falciparum blood stages (ref. 4 and our unpublished results). A novel approach to karyoptic and linkage analysis in P. falciparum has been provided recently by the technique of pulsed-field gradient (PFG) gel electrophoresis, which allows the fractionation of DNA molecules of 30-3,000 kilobases (kb), a range including the sizes of intact chromosomal DNA molecules from eukaryotes such as yeast and trypanosomatids. We describe here the fractionation by PFG electrophoresis of chromosomal DNA molecules from P. falciparum into at least seven discrete species which vary in size by up to 20% between different isolates. Several genes for P. faciparum antigens which contain repetitive sequences are located on different chromosomes. Surprisingly, two of the chromosomes seem to contain the same sequences.     

生命哲学视域下的阳光体育运动

通过对生命化教育理念的诠释,把体育教学划归到生命化教育轨迹之中,同时,把学生的体质健康教育从知识层面提升到生命层面,拓展与深化阳光体育运动的价值,有利于摆脱应试教育、考核达标等对阳光体育运动的束缚,消融其延伸影响,为阳光体育运动的深入开展开拓新的视域。     

安源路矿工人运动与农民运动相结合?

工人运动与农民运动相结合,是安源路矿工人运动的主要特点。安源路矿工人运动基本上不是单独和孤立地进行,而是一开始就孕育着团结农民和其他民众共同反抗的因素,逐步形成了工人阶级领导的、以工农联盟为基础的统一战线。     

Derivation of pluripotent epiblast stem cells from mammalian embryos

Although the first mouse embryonic stem (ES) cell lines were derived 25 years ago using feeder-layer-based blastocyst cultures, subsequent efforts to extend the approach to other mammals, including both laboratory and domestic species, have been relatively unsuccessful. The most notable exceptions were the derivation of non-human primate ES cell lines followed shortly thereafter by their derivation of human ES cells. Despite the apparent common origin and the similar pluripotency of mouse and human embryonic stem cells, recent studies have revealed that they use different signalling pathways to maintain their pluripotent status. Mouse ES cells depend on leukaemia inhibitory factor and bone morphogenetic protein, whereas their human counterparts rely on activin (INHBA)/nodal (NODAL) and fibroblast growth factor (FGF). Here we show that pluripotent stem cells can be derived from the late epiblast layer of post-implantation mouse and rat embryos using chemically defined, activin-containing culture medium that is sufficient for long-term maintenance of human embryonic stem cells. Our results demonstrate that activin/Nodal signalling has an evolutionarily conserved role in the derivation and the maintenance of pluripotency in these novel stem cells. Epiblast stem cells provide a valuable experimental system for determining whether distinctions between mouse and human embryonic stem cells reflect species differences or diverse temporal origins.     

运动、生命、美感--从中国线描绘画谈起

线条,是中国画的生命.中国传统绘画,以线作为造型的基本手段,在平面上表现物体的空间关系、运动形态和内部结构.其绘画特点生动简练概括传神.中国线描绘画就是这样以整体和谐的运动赋予对象以新的生命,以自己民族独特的表现力和美感,自立于世界绘画艺术之林.     

Genetic segregation and the maintenance of sexual reproduction

Sexual reproduction confronts evolutionary biology with a paradox: other things being equal, an asexual (all-female) population will have twice the reproductive potential of a competing sexual population and therefore should rapidly drive the sexual population to extinction. Thus, the persistence of sexual reproduction in most life forms implies a compensatory advantage to sexual reproduction. Work on this problem has emphasized the evolutionary advantages produced by the genetic recombination that accompanies sexual reproduction. Here we show that genetic segregation produces an advantage to sexual reproduction even in the absence of an advantage from recombination. Segregation in a diploid sexual population allows selection to carry a single advantageous mutation to a homozygous state, whereas two separate mutations are required in a parthenogenetic population. The complete fixation of advantageous mutations is thus delayed in a heterozygous state in asexual populations. Calculation of the selective load incurred suggests that it may offset the intrinsic twofold reproductive advantage of asexual reproduction and maintain sexual reproduction in diploid populations.     

Homologous analysis on centromeric region sequences of A and B chromosomes from rye by FISH

Four centromeric segments from B chromosomes (Bs) of rye have been microdis-sected and amplified by linker adapter PCR (LA-PCR). The PCR products ranged from 100 to 2 000 bp. Fluorescence in situ hybridization (FISH) experiment has been carried out using PCR products as the probe, which was labeled by DIG-11-dUTP. The result confirms that these PCR products from Bs centromeric region are homologous with that of A chromosomes (As) in rye. It also proves that Bs are originated from As.