全文获取类型
收费全文 | 5051篇 |
免费 | 567篇 |
专业分类
5618篇 |
出版年
2022年 | 41篇 |
2021年 | 73篇 |
2020年 | 46篇 |
2019年 | 48篇 |
2018年 | 62篇 |
2017年 | 67篇 |
2016年 | 114篇 |
2015年 | 176篇 |
2014年 | 184篇 |
2013年 | 271篇 |
2012年 | 280篇 |
2011年 | 320篇 |
2010年 | 206篇 |
2009年 | 170篇 |
2008年 | 242篇 |
2007年 | 230篇 |
2006年 | 212篇 |
2005年 | 197篇 |
2004年 | 195篇 |
2003年 | 183篇 |
2002年 | 199篇 |
2001年 | 115篇 |
2000年 | 103篇 |
1999年 | 116篇 |
1998年 | 64篇 |
1997年 | 51篇 |
1996年 | 70篇 |
1995年 | 54篇 |
1994年 | 57篇 |
1993年 | 63篇 |
1992年 | 65篇 |
1991年 | 53篇 |
1990年 | 70篇 |
1989年 | 87篇 |
1988年 | 57篇 |
1987年 | 41篇 |
1986年 | 46篇 |
1985年 | 49篇 |
1984年 | 52篇 |
1983年 | 40篇 |
1982年 | 50篇 |
1981年 | 40篇 |
1980年 | 39篇 |
1979年 | 49篇 |
1978年 | 34篇 |
1977年 | 35篇 |
1976年 | 49篇 |
1975年 | 33篇 |
1974年 | 52篇 |
1973年 | 42篇 |
排序方式: 共有5618条查询结果,搜索用时 15 毫秒
131.
Peter R. Grant B. Rosemary Grant 《Evolution; international journal of organic evolution》1995,49(2):229-240
We report the natural colonization of the small Galápagos island Daphne Major by the large ground finch (Geospiza magnirostris). Immigrants of this species were present in every year of a 22-yr study, 1973–1994. Typically they arrived after a breeding season and left at the beginning of the next one. Geospiza magnirostris bred on the island for the first time in the exceptionally wet El Niño year of 1982–1983, and bred in all subsequent years except drought years. In agreement with theoretical expectations the frequency of inbreeding was unusually high. Pronounced fluctuating asymmetry in tarsus length, together with slightly reduced breeding success of inbreeding pairs, suggests a low level of inbreeding depression. Despite this, the population increased from 5 breeding individuals in 1983 to 20 breeding individuals in 1992, and probably more than twice that number in 1993, largely through recruitment of locally born birds. 相似文献
132.
133.
134.
135.
A. Jennifer Rivett Grant G. F. Mason Stuart Thomson Angela M. Pike Peter J. Savory Rachael Z. Murray 《Molecular biology reports》1995,21(1):35-41
The proteasome (multicatalytic proteinase complex) is a large multimeric complex which is found in the nucleus and cytoplasm of eukaryotic cells. It plays a major role in both ubiquitin-dependent and ubiquitin-independent nonlysosomal pathways of protein degradation. Proteasome subunits are encoded by members of the same gene family and can be divided into two groups based on their similarity to the and subunits of the simpler proteasome isolated fromThermoplasma acidophilum. Proteasomes have a cylindrical structure composed of four rings of seven subunits. The 26S form of the proteasome, which is responsible for ubiquitin-dependent proteolysis, contains additional regulatory complexes. Eukaryotic proteasomes have multiple catalytic activities which are catalysed at distinct sites. Since proteasomes are unrelated to other known proteases, there are no clues as to which are the catalytic components from sequence alignments. It has been assumed from studies with yeast mutants that -type subunits play a catalytic role. Using a radiolabelled peptidyl chloromethane inhibitor of rat liver proteasomes we have directly identified RC7 as a catalytic component. Interestingly, mutants in Prel, the yeast homologue of RC7, have already been reported to have defective chymotrypsin-like activity. These results taken together confirm a direct catalytic role for these -type subunits. Proteasome activities are sensitive to conformational changes and there are several ways in which proteasome function may be modulatedin vivo. Our recent studies have shown that in animal cells at least two proteasome subunits can undergo phosphorylation, the level of which is likely to be important for determining proteasome localization, activity or ability to form larger complexes. In addition, we have isolated two isoforms of the 26S proteinase. 相似文献
136.
Governing equations of motion for the otolith organs and their response to a step change in velocity of the skull 总被引:2,自引:0,他引:2
The otolith organs are the linear motion sensors of the inner ear. They comprise an overdamped second-order system and respond to gravity and skull acceleration. The governing equations of motion which describe the relative displacement of the mass with respect to the skull are developed. When these equations are non-dimensionalized they indicate that the elastic term is almost negligible with respect to the viscous and inertial terms. For a step change in skull velocity an analytic solution is given for the elastic term equal to zero and numeric solutions are given for small values of the elastic term. 相似文献
137.
Isolation of the fibrinogen-binding region of platelet thrombospondin 总被引:11,自引:0,他引:11
V M Dixit G A Grant W A Frazier S A Santoro 《Biochemical and biophysical research communications》1984,119(3):1075-1081
Purified platelet thrombospondin binds to immobilized fibrinogen if both Ca++ and Mg++ are present. Digestion of the purified molecule with thermolysin results in a limited number of discrete proteolytic fragments. When such digests are subjected to affinity chromatography on immobilized fibrinogen, only the fragments with Mr of 120,000 and 140,000 are specifically bound and subsequently eluted by the addition of EDTA to the column buffer. Examination by SDS-PAGE under both reducing and nonreducing conditions reveals that the fibrinogen-binding domain is derived from the region of the thrombospondin molecule containing the interchain disulfide bonds. The requirement for Ca++ and Mg++ for optimal binding to fibrinogen is also manifest by the Mr 120,000/140,000 thermolytic fragments. 相似文献
138.
The involvement of polyamines in cytokinesis has been examined in eggs of the amphibian X. laevis. Microinjection of spermine or spermidine into unfertilized eggs induced a shortening of the first division cycle and early formation of cleavage-like constrictions. Eggs were activated by injection and developed furrows about 45 min later, whereas the first division normally occurred around 120 min after activation. In terms of concentration, spermine was slightly more effective than spermidine, but putrescine had no influence on the division cycle. 相似文献
139.
Transformation of Neurospora crassa with recombinant plasmids containing the cloned glutamate dehydrogenase (am) gene: evidence for autonomous replication of the transforming plasmid. 总被引:10,自引:6,他引:4 下载免费PDF全文
D M Grant A M Lambowitz J A Rambosek J A Kinsey 《Molecular and cellular biology》1984,4(10):2041-2051
We have characterized Neurospora crassa transformants obtained with plasmid pJR2, which consists of the Neurospora glutamate dehydrogenase (am) gene cloned in pUC8 and an am132 host strain which contains a deletion encompassing the cloned fragment. Every one of 33 transformants tested showed extreme meiotic instability: less than 1 or 2% am+ progeny were obtained in initial or successive backcrosses between am+ transformants and am132 or in intercrosses between am+ progeny. Furthermore, am+ progeny from backcrosses gave a high proportion of auxotrophic (am) mitotic segregants during vegetative growth. These results indicate that the am+ character is not stably integrated into chromosomal DNA in any of the transformants tested. Nuclear DNAs from six transformants were analyzed by Southern hybridization. All six transformants contained sequences homologous to pJR2. Four showed restriction fragments expected for unmodified pJR2, but most showed additional bands. Southern blots of undigested DNAs showed that the plasmid sequences are present predominantly in high-molecular-weight form (larger than 20 kilobases). Southern blots showed that auxotrophic (am) progeny from a backcross to am132 had lost restriction bands corresponding to free plasmid but retained additional bands, apparently integrated into chromosomal DNA in a nonfunctional manner. Considered together, these results are most reasonably interpreted as follows: recombinant plasmids containing the am+ gene can replicate autonomously in N. crassa, the free plasmids are present in oligomeric or modified form or both, and plasmid sequences also integrate at multiple sites in the deletion host but in a nonfunctional manner. An alternate interpretation--that tandem repeats of the plasmid are integrated into chromosomal DNA but eliminated during meiosis--cannot be completely excluded. However, stable integration of the am gene can be obtained under a variety of other conditions, viz., using the am gene cloned in a phage lambda vector (J. A. Kinsey and J. A. Rambosek, Mol. Cell. Biol. 4:117-122, 1984), using derivatives of pJR2, or using pJR2 to transform a frameshift mutant. 相似文献
140.
Jan E Grant 《Plant Cell, Tissue and Organ Culture》1984,3(2):169-173
Cotyledonary tissue from immature embryos of Glycine canescens was induced to callus and then form embryo-like structures. These structures could be cultured into whole plants and grown in soil. 相似文献