全文获取类型
收费全文 | 18558篇 |
免费 | 79篇 |
国内免费 | 91篇 |
专业分类
系统科学 | 89篇 |
丛书文集 | 177篇 |
教育与普及 | 69篇 |
理论与方法论 | 53篇 |
现状及发展 | 8355篇 |
研究方法 | 753篇 |
综合类 | 9065篇 |
自然研究 | 167篇 |
出版年
2012年 | 283篇 |
2011年 | 427篇 |
2010年 | 155篇 |
2008年 | 335篇 |
2007年 | 362篇 |
2006年 | 340篇 |
2005年 | 367篇 |
2004年 | 379篇 |
2003年 | 343篇 |
2002年 | 330篇 |
2001年 | 500篇 |
2000年 | 535篇 |
1999年 | 358篇 |
1994年 | 318篇 |
1992年 | 292篇 |
1991年 | 241篇 |
1990年 | 289篇 |
1989年 | 269篇 |
1988年 | 277篇 |
1987年 | 295篇 |
1986年 | 300篇 |
1985年 | 357篇 |
1984年 | 255篇 |
1983年 | 271篇 |
1982年 | 225篇 |
1981年 | 217篇 |
1980年 | 254篇 |
1979年 | 531篇 |
1978年 | 442篇 |
1977年 | 409篇 |
1976年 | 337篇 |
1975年 | 407篇 |
1974年 | 509篇 |
1973年 | 453篇 |
1972年 | 462篇 |
1971年 | 587篇 |
1970年 | 639篇 |
1969年 | 559篇 |
1968年 | 561篇 |
1967年 | 483篇 |
1966年 | 427篇 |
1965年 | 308篇 |
1964年 | 139篇 |
1959年 | 169篇 |
1958年 | 324篇 |
1957年 | 258篇 |
1956年 | 207篇 |
1955年 | 204篇 |
1954年 | 200篇 |
1948年 | 172篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
942.
Hoffmann K Dreger CK Olins AL Olins DE Shultz LD Lucke B Karl H Kaps R Müller D Vayá A Aznar J Ware RE Sotelo Cruz N Lindner TH Herrmann H Reis A Sperling K 《Nature genetics》2002,31(4):410-414
Pelger-Hu?t anomaly (PHA; OMIM *169400) is an autosomal dominant disorder characterized by abnormal nuclear shape and chromatin organization in blood granulocytes. Affected individuals show hypolobulated neutrophil nuclei with coarse chromatin. Presumed homozygous individuals have ovoid neutrophil nuclei, as well as varying degrees of developmental delay, epilepsy and skeletal abnormalities. Homozygous offspring in an extinct rabbit lineage showed severe chondrodystrophy, developmental anomalies and increased pre- and postnatal mortality. Here we show, by carrying out a genome-wide linkage scan, that PHA is linked to chromosome 1q41-43. We identified four splice-site, two frameshift and two nonsense mutations in LBR, encoding the lamin B receptor. The lamin B receptor (LBR), a member of the sterol reductase family, is evolutionarily conserved and integral to the inner nuclear membrane; it targets heterochromatin and lamins to the nuclear membrane. Lymphoblastoid cells from heterozygous individuals affected with PHA show reduced expression of the lamin B receptor, and cells homozygous with respect to PHA contain only trace amounts of it. We found that expression of the lamin B receptor affects neutrophil nuclear shape and chromatin distribution in a dose-dependent manner. Our findings have implications for understanding nuclear envelope-heterochromatin interactions, the pathogenesis of Pelger-like conditions in leukemia, infection and toxic drug reactions, and the evolution of neutrophil nuclear shape. 相似文献
943.
We have characterized the human gene SLC39A4, which encodes a protein with features characteristic of a ZIP zinc transporter. The chromosomal location and expression of SLC39A4, together with mutational analysis of eight families affected with acrodermatitis enteropathica, suggest that SLC39A4 is centrally involved in the pathogenesis of this condition. 相似文献
944.
Stöck M Lamatsch DK Steinlein C Epplen JT Grosse WR Hock R Klapperstück T Lampert KP Scheer U Schmid M Schartl M 《Nature genetics》2002,30(3):325-328
Green toads are common in the Palaearctic region, where they have differentiated into several taxa. The toads exist with variable amounts of ploidy, similar to other anuran species or reptiles. In vertebrate biology, the very rare occurrence of triploidy is coupled with infertility or unisexuality, or requires the coexistence of individuals of different ploidy in a reproductive community. The reproduction of naturally occurring triploids has been reported to occur only through parthenogenesis, gynogenesis or hybridogenesis. The bisexual reproduction of pure triploids has been considered to be impossible because of the problem of equally distributing three chromosome sets in meiosis. Here we report geographically isolated populations of green toads (Bufo viridis complex) that are all-triploid and reproduce bisexually. 相似文献
945.
Inherited variants of MYH associated with somatic G:C-->T:A mutations in colorectal tumors 总被引:5,自引:0,他引:5
Al-Tassan N Chmiel NH Maynard J Fleming N Livingston AL Williams GT Hodges AK Davies DR David SS Sampson JR Cheadle JP 《Nature genetics》2002,30(2):227-232
Inherited defects of base excision repair have not been associated with any human genetic disorder, although mutations of the genes mutM and mutY, which function in Escherichia coli base excision repair, lead to increased transversions of G:C to T:A. We have studied family N, which is affected with multiple colorectal adenomas and carcinoma but lacks an inherited mutation of the adenomatous polyposis coli gene (APC) that is associated with familial adenomatous polyposis. Here we show that 11 tumors from 3 affected siblings contain 18 somatic inactivating mutations of APC and that 15 of these mutations are G:C-->A transversions--a significantly greater proportion than is found in sporadic tumors or in tumors associated with familial adenomatous polyposis. Analysis of the human homolog of mutY, MYH, showed that the siblings were compound heterozygotes for the nonconservative missense variants Tyr165Cys and Gly382Asp. These mutations affect residues that are conserved in mutY of E. coli (Tyr82 and Gly253). Tyrosine 82 is located in the pseudo-helix-hairpin-helix (HhH) motif and is predicted to function in mismatch specificity. Assays of adenine glycosylase activity of the Tyr82Cys and Gly253Asp mutant proteins with 8-oxoG:A and G:A substrates show that their activity is reduced significantly. Our findings link the inherited variants in MYH to the pattern of somatic APC mutation in family N and implicate defective base excision repair in predisposition to tumors in humans. 相似文献
946.
Ultraviolet emissions from the magnetic footprints of Io, Ganymede and Europa on Jupiter 总被引:2,自引:0,他引:2
Clarke JT Ajello J Ballester G Ben Jaffel L Connerney J Gérard JC Gladstone GR Grodent D Pryor W Trauger J Waite JH 《Nature》2002,415(6875):997-1000
Io leaves a magnetic footprint on Jupiter's upper atmosphere that appears as a spot of ultraviolet emission that remains fixed underneath Io as Jupiter rotates. The specific physical mechanisms responsible for generating those emissions are not well understood, but in general the spot seems to arise because of an electromagnetic interaction between Jupiter's magnetic field and the plasma surrounding Io, driving currents of around 1 million amperes down through Jupiter's ionosphere. The other galilean satellites may also leave footprints, and the presence or absence of such footprints should illuminate the underlying physical mechanism by revealing the strengths of the currents linking the satellites to Jupiter. Here we report persistent, faint, far-ultraviolet emission from the jovian footprints of Ganymede and Europa. We also show that Io's magnetic footprint extends well beyond the immediate vicinity of Io's flux-tube interaction with Jupiter, and much farther than predicted theoretically; the emission persists for several hours downstream. We infer from these data that Ganymede and Europa have persistent interactions with Jupiter's magnetic field despite their thin atmospheres. 相似文献
947.
Lee J Kim H Kahng SJ Kim G Son YW Ihm J Kato H Wang ZW Okazaki T Shinohara H Kuk Y 《Nature》2002,415(6875):1005-1008
Motivated by the technical and economic difficulties in further miniaturizing silicon-based transistors with the present fabrication technologies, there is a strong effort to develop alternative electronic devices, based, for example, on single molecules. Recently, carbon nanotubes have been successfully used for nanometre-sized devices such as diodes, transistors, and random access memory cells. Such nanotube devices are usually very long compared to silicon-based transistors. Here we report a method for dividing a semiconductor nanotube into multiple quantum dots with lengths of about 10nm by inserting Gd@C82 endohedral fullerenes. The spatial modulation of the nanotube electronic bandgap is observed with a low-temperature scanning tunnelling microscope. We find that a bandgap of approximately 0.5eV is narrowed down to approximately 0.1eV at sites where endohedral metallofullerenes are inserted. This change in bandgap can be explained by local elastic strain and charge transfer at metallofullerene sites. This technique for fabricating an array of quantum dots could be used for nano-electronics and nano-optoelectronics. 相似文献
948.
Mutual synergistic folding in recruitment of CBP/p300 by p160 nuclear receptor coactivators 总被引:11,自引:0,他引:11
Demarest SJ Martinez-Yamout M Chung J Chen H Xu W Dyson HJ Evans RM Wright PE 《Nature》2002,415(6871):549-553
949.
Ostertag EM DeBerardinis RJ Goodier JL Zhang Y Yang N Gerton GL Kazazian HH 《Nature genetics》2002,32(4):655-660
The L1 retrotransposon has had an immense impact on the size and structure of the human genome through a variety of mechanisms, including insertional mutagenesis. To study retrotransposition in a living organism, we created a mouse model of human L1 retrotransposition. Here we show that L1 elements can retrotranspose in male germ cells, and that expression of a human L1 element under the control of its endogenous promoter is restricted to testis and ovary. In the mouse line with the highest level of L1 expression, we found two de novo L1 insertions in 135 offspring. Both insertions were structurally indistinguishable from natural endogenous insertions. This suggests that an individual L1 element can have substantial mutagenic potential. In addition to providing a valuable in vivo model of retrotransposition in mammals, these mice are an important step in the development of a new random mutagenesis system. 相似文献
950.
Colloids display intriguing transitions between gas, liquid, solid and liquid crystalline phases. Such phase transitions are ubiquitous in nature and have been studied for decades. However, the predictions of phase diagrams are not always realized; systems often become undercooled, supersaturated, or trapped in gel-like states. In many cases the end products strongly depend on the starting position in the phase diagram and discrepancies between predictions and actual observations are due to the intricacies of the dynamics of phase transitions. Colloid science aims to understand the underlying mechanisms of these transitions. Important advances have been made, for example, with new imaging techniques that allow direct observation of individual colloidal particles undergoing phase transitions, revealing some of the secrets of the complex pathways involved. 相似文献