首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   193篇
  免费   0篇
系统科学   2篇
教育与普及   2篇
理论与方法论   1篇
现状及发展   28篇
研究方法   29篇
综合类   126篇
自然研究   5篇
  2019年   1篇
  2018年   1篇
  2016年   2篇
  2015年   2篇
  2014年   1篇
  2012年   5篇
  2011年   10篇
  2010年   2篇
  2009年   2篇
  2008年   9篇
  2007年   14篇
  2006年   7篇
  2005年   6篇
  2004年   9篇
  2003年   15篇
  2002年   14篇
  2001年   8篇
  2000年   14篇
  1999年   12篇
  1996年   1篇
  1992年   7篇
  1991年   1篇
  1990年   2篇
  1989年   1篇
  1988年   2篇
  1987年   4篇
  1986年   2篇
  1985年   3篇
  1984年   1篇
  1983年   1篇
  1981年   1篇
  1979年   2篇
  1978年   3篇
  1977年   1篇
  1976年   2篇
  1975年   2篇
  1974年   1篇
  1972年   1篇
  1971年   1篇
  1970年   7篇
  1969年   5篇
  1967年   2篇
  1966年   4篇
  1965年   1篇
  1959年   1篇
排序方式: 共有193条查询结果,搜索用时 15 毫秒
101.
Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 st1\:*{behavior:url(#ieooui) } /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} Life forms and floral characteristics of plants at Arches National Park (desert communities), the Mt. Nebo complex, and a subalpine meadow in the Uinta Mountains (montane and subalpine communities) were compared. Characteristics observed were (1) life form, (2) longevity, (3) pollination system, (4) flower structure, (5) flower symmetry, and (6) flower color. Common families in each flora were also compared. Results showed that there is a significant overrepresentation of shrub species at Arches, and an underrepresentation of perennial forbs. Relative number of perennial forb species was significantly higher at Mt. Nebo and the subalpine meadow than at Arches National Park. Native annuals and wind-pollinated species were significantly overrepresented at Arches. Flowers with open structure that permit free access of most insects to nectar and pollen were overrepresented at Mt. Nebo and in the subalpine meadow. The distribution of flower colors also differs significantly among these ecologically contrasting floras, with yellow being best represented in the desert and white in the mountains.    相似文献   
102.
Rich RM  Collins ML  Black CM  Longstaff FA  Koch A  Benson A  Reitzel DB 《Nature》2012,482(7384):192-194
NGC 4449 is a nearby Magellanic irregular starburst galaxy with a B-band absolute magnitude of -18 and a prominent, massive, intermediate-age nucleus at a distance from Earth of 3.8?megaparsecs (ref. 3). It is wreathed in an extraordinary neutral hydrogen (H?I) complex, which includes rings, shells and a counter-rotating core, spanning ~90?kiloparsecs (kpc; refs 1, 4). NGC 4449 is relatively isolated, although an interaction with its nearest known companion--the galaxy DDO 125, some 40?kpc to the south--has been proposed as being responsible for the complexity of its H?I structure. Here we report the presence of a dwarf galaxy companion to NGC 4449, namely NGC 4449B. This companion has a V-band absolute magnitude of -13.4 and a half-light radius of 2.7?kpc, with a full extent of around 8?kpc. It is in a transient stage of tidal disruption, similar to that of the Sagittarius dwarf near the Milky Way. NGC 4449B exhibits a striking S-shaped morphology that has been predicted for disrupting galaxies but has hitherto been seen only in a dissolving globular cluster. We also detect an additional arc or disk ripple embedded in a two-component stellar halo, including a component extending twice as far as previously known, to about 20?kpc from the galaxy's centre.  相似文献   
103.
Global landscape of protein complexes in the yeast Saccharomyces cerevisiae   总被引:4,自引:0,他引:4  
Identification of protein-protein interactions often provides insight into protein function, and many cellular processes are performed by stable protein complexes. We used tandem affinity purification to process 4,562 different tagged proteins of the yeast Saccharomyces cerevisiae. Each preparation was analysed by both matrix-assisted laser desorption/ionization-time of flight mass spectrometry and liquid chromatography tandem mass spectrometry to increase coverage and accuracy. Machine learning was used to integrate the mass spectrometry scores and assign probabilities to the protein-protein interactions. Among 4,087 different proteins identified with high confidence by mass spectrometry from 2,357 successful purifications, our core data set (median precision of 0.69) comprises 7,123 protein-protein interactions involving 2,708 proteins. A Markov clustering algorithm organized these interactions into 547 protein complexes averaging 4.9 subunits per complex, about half of them absent from the MIPS database, as well as 429 additional interactions between pairs of complexes. The data (all of which are available online) will help future studies on individual proteins as well as functional genomics and systems biology.  相似文献   
104.
105.
Telomeres in most immortal cells are maintained by the enzyme telomerase, allowing cells to divide indefinitely. Some telomerase-negative tumors and immortal cell lines maintain long heterogeneous telomeres by the ALT (alternative lengthening of telomeres) mechanism; such tumors are expected to be resistant to anti-telomerase drug therapies. Occasionally telomerase-negative Saccharomyces cerevisiae mutants survive, and 10% of them (type II survivors) have unstable telomeres. As in human ALT+ cells, short telomeres in yeast type II survivors lengthen abruptly; in yeast, this is dependent on the recombination proteins Rad52p and Rad50p. In human cells, ALT involves copying of sequence from a donor to a recipient telomere. We have characterized for the first time a class of complex telomere mutations seen only in ALT+ cells. The mutant telomeres are defined by the replacement of the progenitor telomere at a discrete point (fusion point) with a different telomere repeat array. Among 19 characterized fusion points, one occurred within the first six repeats of the telomere, indicating that these recombination-like events can occur anywhere within the telomere. One mutant telomere may have been involved in a secondary recombination-like mutation event, suggesting that these mutations are sporadic but ongoing in ALT+ cells. We also identified simple intra-allelic mutations at high frequency, which evidently contribute to telomere instability in ALT+ cells.  相似文献   
106.
Meiotic arrest and aneuploidy in MLH3-deficient mice   总被引:22,自引:0,他引:22  
MutL homolog 3 (Mlh3) is a member of a family of proteins conserved during evolution and having dual roles in DNA mismatch repair and meiosis. The pathway in eukaryotes consists of the DNA-binding components, which are the homologs of the bacterial MutS protein (MSH 2 6), and the MutL homologs, which bind to the MutS homologs and are essential for the repair process. Three of the six homologs of MutS that function in these processes, Msh2, Msh3 and Msh6, are involved in the mismatch repair of mutations, frameshifts and replication errors, and two others, Msh4 and Msh5, have specific roles in meiosis. Of the four MutL homologs, Mlh1, Mlh3, Pms1 and Pms2, three are involved in mismatch repair and at least two, Pms2 and Mlh1, are essential for meiotic progression in both yeast and mice. To assess the role of Mlh3 in mammalian meiosis, we have generated and characterized Mlh3(-/-) mice. Here we show that Mlh3(-/-) mice are viable but sterile. Mlh3 is required for Mlh1 binding to meiotic chromosomes and localizes to meiotic chromosomes from the mid pachynema stage of prophase I. Mlh3(-/-) spermatocytes reach metaphase before succumbing to apoptosis, but oocytes fail to complete meiosis I after fertilization. Our results show that Mlh3 has an essential and distinct role in mammalian meiosis.  相似文献   
107.
We have placed 7,600 cytogenetically defined landmarks on the draft sequence of the human genome to help with the characterization of genes altered by gross chromosomal aberrations that cause human disease. The landmarks are large-insert clones mapped to chromosome bands by fluorescence in situ hybridization. Each clone contains a sequence tag that is positioned on the genomic sequence. This genome-wide set of sequence-anchored clones allows structural and functional analyses of the genome. This resource represents the first comprehensive integration of cytogenetic, radiation hybrid, linkage and sequence maps of the human genome; provides an independent validation of the sequence map and framework for contig order and orientation; surveys the genome for large-scale duplications, which are likely to require special attention during sequence assembly; and allows a stringent assessment of sequence differences between the dark and light bands of chromosomes. It also provides insight into large-scale chromatin structure and the evolution of chromosomes and gene families and will accelerate our understanding of the molecular bases of human disease and cancer.  相似文献   
108.
M Collins  G M Rubin 《Nature》1983,303(5914):259-260
Precise excision of transposable elements in prokaryotes is a rare event which occurs at a significantly lower rate than transposition and other element-mediated events. Thus, we were intrigued by a eukaryotic transposable element which seemed capable of precise excision at high frequencies. The white-crimson (wc) mutation in Drosophila, a highly unstable allele of the X-linked eye colour locus, white, resulted from the insertion of a member of the foldback (FB) transposable element family. This mutation reverts to its parental phenotype at a frequency of greater than 1 in 10(3) X chromosomes. Characterization of these revertants by Southern blots of genomic DNA indicated that they resulted from loss of the wc insertion. Here we report the nucleotide sequence of the excision point in these revertants, and conclude that the FB element responsible for the wc mutation is capable of precise excision at high frequencies.  相似文献   
109.
SNARE-protein-mediated disease resistance at the plant cell wall   总被引:2,自引:0,他引:2  
Failure of pathogenic fungi to breach the plant cell wall constitutes a major component of immunity of non-host plant species--species outside the pathogen host range--and accounts for a proportion of aborted infection attempts on 'susceptible' host plants (basal resistance). Neither form of penetration resistance is understood at the molecular level. We developed a screen for penetration (pen) mutants of Arabidopsis, which are disabled in non-host penetration resistance against barley powdery mildew, Blumeria graminis f. sp. hordei, and we isolated the PEN1 gene. We also isolated barley ROR2 (ref. 2), which is required for basal penetration resistance against B. g. hordei. The genes encode functionally homologous syntaxins, demonstrating a mechanistic link between non-host resistance and basal penetration resistance in monocotyledons and dicotyledons. We show that resistance in barley requires a SNAP-25 (synaptosome-associated protein, molecular mass 25 kDa) homologue capable of forming a binary SNAP receptor (SNARE) complex with ROR2. Genetic control of vesicle behaviour at penetration sites, and plasma membrane location of PEN1/ROR2, is consistent with a proposed involvement of SNARE-complex-mediated exocytosis and/or homotypic vesicle fusion events in resistance. Functions associated with SNARE-dependent penetration resistance are dispensable for immunity mediated by race-specific resistance (R) genes, highlighting fundamental differences between these two resistance forms.  相似文献   
110.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号