全文获取类型
收费全文 | 1684篇 |
免费 | 189篇 |
出版年
2022年 | 22篇 |
2021年 | 45篇 |
2020年 | 23篇 |
2019年 | 33篇 |
2018年 | 36篇 |
2017年 | 41篇 |
2016年 | 55篇 |
2015年 | 79篇 |
2014年 | 84篇 |
2013年 | 101篇 |
2012年 | 105篇 |
2011年 | 125篇 |
2010年 | 73篇 |
2009年 | 59篇 |
2008年 | 86篇 |
2007年 | 99篇 |
2006年 | 81篇 |
2005年 | 71篇 |
2004年 | 67篇 |
2003年 | 58篇 |
2002年 | 71篇 |
2001年 | 29篇 |
2000年 | 30篇 |
1999年 | 28篇 |
1998年 | 12篇 |
1997年 | 16篇 |
1996年 | 20篇 |
1995年 | 10篇 |
1994年 | 11篇 |
1993年 | 13篇 |
1992年 | 16篇 |
1991年 | 21篇 |
1990年 | 22篇 |
1989年 | 12篇 |
1988年 | 9篇 |
1987年 | 10篇 |
1986年 | 11篇 |
1985年 | 11篇 |
1984年 | 9篇 |
1983年 | 11篇 |
1982年 | 15篇 |
1981年 | 11篇 |
1980年 | 11篇 |
1979年 | 12篇 |
1978年 | 7篇 |
1977年 | 10篇 |
1976年 | 8篇 |
1974年 | 8篇 |
1973年 | 7篇 |
1972年 | 5篇 |
排序方式: 共有1873条查询结果,搜索用时 15 毫秒
1.
The gorgoniid Eugorgia is exclusively an eastern Pacific genus. It has a wide geographic and bathymetric range of distribution, found from California to Perú and extends down to 65 m deep. Two new species are herein described. The morphological characters were analyzed and illustrated by light and scanning electron microscopy. Eugorgia beebei
sp. n. can be distinguished by its white, ascending, sparse colony growth. Eugorgia mutabilis
sp. n. can be distinguished by its white colony that changes color after collection, and the conspicuous sharp-crested disc sclerites. From a morphological point of view the new species are related to the daniana-group, the rubens-group and the siedenburgae-group of Eugorgia; their affiliations, and the proposal of a new group are discussed. These new species increases the number of species in the genus to 15, and contribute to the knowledge of the eastern Pacific octocoral biodiversity. 相似文献
2.
3.
Theresa M. Bert Kevin J. McCarthy Hector Cruz-Lopez Steven Bogdanowicz 《Evolution; international journal of organic evolution》1996,50(2):655-671
Many investigators categorize individuals from hybrid zones to facilitate comparisons among genotypic classes (e.g., parental, F1, backcross) for comparative studies in which components of fitness or geographic variation are being analyzed. Frequently, multiple character sets representing genetically independent traits are used to classify these individuals and various methodologies are employed to combine the classifications obtained from the different character sets. We adapted the principles of total evidence and taxonomic congruence (two formalized approaches used by systematists in formulating phylogenetic hypotheses) to address the problem of discriminating hybridizing species and classifying individuals from hybrid zones. As our model, we used two morphological (coloration and morphometric) and two molecular (allozyme and mitochondrial DNA restriction-fragment-length polymorphism) character sets that differentiate two stone crab species (Menippe adina and M. mercenaria). Using principal-components analysis, we determined that combining character sets and eliminating characters or character sets that did not have large eigenvector coefficients for the principal component that best separated the two species yielded the highest level of discrimination between species and allowed us to classify a broad range of morpho-genotypes as hybrids. For the stone crabs, three diagnostic allozyme loci and five diagnostic coloration characters best separated the species. The two character sets were not completely congruent, but they agreed in their classification of 50% of the individuals from the hybrid zone and rarely strongly disagreed in their classifications. Classification discrepancies between the two character sets probably represent variation between traits in interspecific gene flow rather than intraspecific, ecologically mediated variation. Our results support the assertions of previous investigators who espoused the benefits associated with using multiple character sets to classify individuals from hybrid zones and demonstrate that, if character sets are reasonably congruent and numerically balanced, combining diagnostic characters from multiple character sets (a total-evidence approach) can enhance discriminatory power between species and facilitate the assignment of hybrid-zone individuals to genotypic classes. On the contrary, classifying hybrid-zone individuals using character sets separately (a taxonomic-congruence approach) provides the opportunity to compare levels of introgression between species and to assess reasons for discordance among the data sets. 相似文献
4.
5.
Ly1+ PRO-B lymphocyte clones. Phenotype, growth requirements and differentiation in vitro and in vivo. 总被引:27,自引:4,他引:23 下载免费PDF全文
Several clones obtained from the bone marrow of a BALB/c mouse were found to contain the heavy and light chain Ig genes in the germline configuration, to express Ly1 and to carry the B cell lineage markers B-220, Lyb8 and BP-1; these clones are Pgp-1+, LFA-1+, J11d+, Mac-1+ and Thy1-, Lyt2-, L3T4-, GM1.2- and Ia-. Three clones analyzed in detail (Lyd9, LyH7 and Lyb9) have receptors for interleukin (IL) 2 and IL3 as assessed with the 7D4 and CC11 monoclonal antibodies respectively. They grow in rIL3 but not in rIL2 or rIL1; both rIL4 and rIL5 also promote their proliferation, albeit to a much lesser extent than rIL3. None of the interleukins tested alone or in various combinations promoted the clones to differentiate in vitro along the B cell pathway. Treatment with 5-Azacytidine (5-Aza) induced cell surface Ia expression but not rearrangement or expression of Ig genes. However, 5-Aza-treated Lyd9, LyH7 and Lyb9 cells co-cultured with X-ray irradiated accessory cells and LPS gave rise to Ly1+, IgM+ B lymphocytes (range 14-51%) including mu + kappa + (78-93%), and mu + lambda + (9-25%) B lymphocytes. In vivo, the Lyd9, LyH7 and Lyb9 clones gave rise to IgM+ B lymphocytes (8.5-17%) including mu + kappa +, and mu + lambda +, but not to Lyt2+ or L3T4+ T lymphocytes after 4-6 weeks of transfer into Scid mice. Our results indicate that Ly1+ IgM+ cells comprise a subpopulation of B lymphocytes that is derived from IL3-responsive Ly1+ PRO-B lymphocytes. 相似文献
6.
Nitric oxide from L-arginine stimulates the soluble guanylate cyclase in adrenal glands 总被引:23,自引:0,他引:23
M Palacios R G Knowles R M Palmer S Moncada 《Biochemical and biophysical research communications》1989,165(2):802-809
The formation of nitric oxide (NO) by an L-arginine:NO synthase and its stimulation of the soluble guanylate cyclase was studied in rat whole adrenal and bovine cortex and medulla cytosol. In the presence of L-arginine, the stimulation of soluble guanylate cyclase was accompanied by the formation of citrulline and NO2-, formed from NO. The NO synthase was NADPH- and Ca(2+)-dependent and was inhibited by several L-arginine analogues. These results indicate that rat and bovine adrenal cytosol contains an L-arginine:NO synthase. 相似文献
7.
Summary Alterations of HLA class I genes were found in 3 of 12 human colon cancers. Rearrangements in HLA class I genes were observed in 2 cancers and amplification of HLA-coding genes was observed in 1 cancer. All 3 cancers were at an advanced stage. No examples of amplification or rearrangement in the HLA genes were found in 10 other tumours of diverse types. No alterations in the 2-microgubulin gene were observed in 22 human solid tumours included in this study. The association between alterations in HLA genes and proto-oncogenes in these tumours is discussed. 相似文献
8.
9.
H. Nquyen H.H. Le R.Y. Lee M. Staufenbiel J.M. Palacios G. Mengod H. Lübbert 《Neurochemistry international》1991,19(4):433-436
We have previously isolated serotonin 5-HT1C receptor cDNA clones. In contrast to most other receptors coupled to GTP binding proteins, the 5-HT1C receptor gene contains several introns in its coding region. A similar exon-intron distribution is found in the 5-HT2 receptor gene. The presence of large introns tempted us to test whether exchange of exons contributes to serotonin receptor heterogeneity. Therefore, blots with RNAs from different regions of the brain and brain slices were hybridized in situ with probes representing individual exons. We did not find any evidence for the exchange of exons which should result in an unequal distribution of hybridization signals found with the exon-specific probes. With the methodology used we should be able to see differential splicing in the form where individual exons are used alternatively resulting in mRNAs coding for different serotonin receptor types. We would not, however, see if a given exon can be modified by the alternative use of several splice-junctions. 相似文献
10.
Rita Maria Ulloa Hector Norberto Torres Claudia M. Ochatt Maria Teresa Téllez-Iñón 《Molecular and cellular biochemistry》1991,102(2):155-163
DEAE-cellulose column chromatography of Neurospora crassa soluble mycelial extracts leads to the resolution of three major protein kinase activity peaks designated PKI, PKII, and PKIII.PKII activity is stimulated by Ca2+ and Neurospora or brain calmodulin. Maximal stimulation was observed at 2 µM-free Ca2+ and 1 µg/ml of the modulator. The stimulatory effect of the Ca2+-calmodulin complex was blocked by EGTA and by some calmodulin antagonists such as phenothiazine drugs or compound 48/80.PKII phosphorylates different proteins, among which histone II-A at a low concentration and CDPKS, the synthetic peptide specific for Ca2+-calmodulin dependent protein kinases, are the best substrates. Some phosphorylation can be detected in the absence of any exogenous acceptor. PKII activity assayed in the presence of histone II-A or in the absence of exogenous phosphate acceptor (autophosphorylation) co-elute in a DEAE-cellulose column at 0.28 M NaCl. As result of the autophosphorylation reaction of the purified enzyme a main phosphorylated component of 70 kDa was resolved by SDS-polyacrylamide gel electrophoresis. It is possible that this component is an active part of this enzyme. 相似文献