全文获取类型
收费全文 | 11357篇 |
免费 | 993篇 |
国内免费 | 298篇 |
出版年
2024年 | 7篇 |
2023年 | 103篇 |
2022年 | 101篇 |
2021年 | 419篇 |
2020年 | 231篇 |
2019年 | 344篇 |
2018年 | 392篇 |
2017年 | 313篇 |
2016年 | 458篇 |
2015年 | 653篇 |
2014年 | 707篇 |
2013年 | 752篇 |
2012年 | 976篇 |
2011年 | 907篇 |
2010年 | 561篇 |
2009年 | 480篇 |
2008年 | 600篇 |
2007年 | 591篇 |
2006年 | 508篇 |
2005年 | 480篇 |
2004年 | 395篇 |
2003年 | 351篇 |
2002年 | 394篇 |
2001年 | 248篇 |
2000年 | 258篇 |
1999年 | 200篇 |
1998年 | 112篇 |
1997年 | 72篇 |
1996年 | 70篇 |
1995年 | 82篇 |
1994年 | 63篇 |
1993年 | 55篇 |
1992年 | 92篇 |
1991年 | 85篇 |
1990年 | 81篇 |
1989年 | 79篇 |
1988年 | 54篇 |
1987年 | 43篇 |
1986年 | 57篇 |
1985年 | 60篇 |
1984年 | 26篇 |
1983年 | 30篇 |
1982年 | 18篇 |
1981年 | 26篇 |
1980年 | 13篇 |
1979年 | 20篇 |
1978年 | 17篇 |
1977年 | 15篇 |
1976年 | 9篇 |
1972年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 546 毫秒
91.
Gary B. Quistad Quyen Nguyen Paul Bernasconi Douglas J. Leisy 《Insect biochemistry and molecular biology》1994,24(10):955-961
The potency of venom from Bracon hebetor against lepidopterous larvae has been known for over 40 years, but previous attempts to purify and characterize individual protein toxins have been largely unsuccessful. Three protein toxins were purified from venom of this small parasitic wasp and the amino acid sequences of 22–31 consecutive residues at the amino-terminus were determined. These relatively large toxins (apparent molecular mass 73 kDa) were labile under many isolation techniques, but anion-exchange chromatography allowed purification with retention of biological activity. Two purified toxins were quite insecticidal (LD50 < 0.3μg/g) when injected into six species of lepidopterous larvae. On a molar basis, one toxin (Brh-I) has the highest known biocidal activity against Heliothis virescens (LD50 = 2 pmol/g). 相似文献
92.
Qiyuan Chen Melanie Smith Tam Nguyen Darryl W. Maher Peter Hersey 《Cancer immunology, immunotherapy : CII》1994,38(6):385-393
Previous studies have shown that recognition of melanoma by cytotoxic T lymphocytes may be restricted by HLA-A1, A2 and other HLA antigens. The present study examined the cytotoxic specificity and major histocompatibility complex restriction of cloned cytotoxic T lymphocytes (CTL) isolated from a patient with the HLA phenotype A3,31 who had been immunized with a vaccine prepared from HLA-A1,3 melanoma cells. Cytotoxic assays against HLA-typed allogeneic melanoma cells indicated that cloned CTL from the patient were able to kill allogeneic melanoma cells expressing HLA-A1 but not other HLA-A1-positive cells. Studies on a representative clone indicated that proliferation and cytokine (tumour necrosis factor ) production in response to melanoma cells was also associated with HLA-A1 on melanoma cells. Response to the melanoma cells was associated with interleukin-4 (IL-4) rather than IL-2 production. The antigen recognized in the context of HLA-A1 on allogeneic melanoma cells was detected in cytotoxic assays on cells from 9 of 12 HLA-A1+ melanoma cell lines and did not appear to be the product of the MAGE-1 or-3 genes. These findings suggest that T cells can recognize melanoma antigens in the context of alloantigens and that allogeneic vaccines containing immunodominant alloantigens may generate CTL that are ineffective against autologous melanoma. The study does not, however, exclude the possibility that CTL with specificity to the latter may be activated by allogeneic vaccines, and further studies are needed to answer this question. 相似文献
93.
细叶黄芪叶肉原生质体发育早期细胞壁再生的研究 总被引:1,自引:0,他引:1
采用透射电镜术、电镜多糖细胞化学染色、细胞壁荧光染色以及香豆素抑制细胞壁再生等方法,对细叶黄芪(Astragalusm elilotoides var.tenuis)叶肉原生质体细胞壁的再生及其化学特点进行了研究。结果表明,离体培养24 小时的原生质体表面产生一些突起小泡,有时可见少量纤维组分的形成。培养3 天时这种纤维组分明显增多。至5 天时可清楚看到再生壁是由纤维和颗粒构成。六亚甲四胺银染色证明它们都是由多糖组分组成的。另外,培养36 小时的原生质体有相互粘连的现象。电镜观察、荧光染色及香豆素处理的研究表明粘连与再生壁的形成有关。根据上述观察结果,对原生质体再生壁的结构及其化学性质等问题进行了讨论 相似文献
94.
Olivier Bornet Gérard Lancelot Luc Chanteloup Nguyen T. Thuong Jean-Marie Beau 《Journal of biomolecular NMR》1994,4(4):575-580
Summary We present NMR studies of an intramolecular triple helix, the three strands of which have been linked by a hexaethylene glycol chain. To overcome the generally encountered difficulties of assignment in the homopyrimidine strands, the carbon C1 of the pyrimidines were selectively 13C-enriched. Assignments of the aromatic and sugar protons were obtained from NOESY-HMQC and TOCSY-HMQC spectra. We show that the recognition of a DNA duplex by a third strand via triplex formation is easily carried out in solution by observing the changes of the 1H1–13C1 connectivities as a function of pH. Furthermore, the conformation of the sugars has been found to be C2-endo, on the basis of the coupling constant values directly measured in an HSQC spectrum. 相似文献
95.
Zn对细胞保护作用机理的研究 总被引:16,自引:0,他引:16
应用扫描质子微探针和同步辐射x荧光分析技术测定了细胞中元素的分布和组成,为确定Zn是细胞结构成分提供了直接的实验依据.用上述核技术结合有关生化指标,分析测定了正常和损伤细胞(脂质过氧化损伤)中Fe,Zn和丙二醛、SH基含量变化的相互关系.实验结果表明,当细胞发生脂质过氧化损伤时,Fe含量和丙二醛含量同步增高,而Zn含量和SH基量则降低.给细胞补充Zn后,提高了细胞质膜中的Zn含量,SH基量也随之增加,同时丙二醛量降低.提示Zn保护细胞完整性的作用机理之一是控制脂质过氧化作用.Zn可保护膜蛋白的SH基,减少和阻止被Fe所催化的过氧化反应. 相似文献
96.
在细叶黄芪叶肉原生质体发育早期,细胞器的变化较大。离体培养4h后,线粒体的嵴和基质物质开始增加。培养3—5天后,线粒体的数量增加5倍以上,此时可见大部分线粒体围绕细胞核分布。在培养24h后,高尔基体开始发育,它们主要分布在细胞质周边区域。多糖细胞化学染色表明,高尔基体内沉积着大量嗜银物质。培养1天后,粗面内质网开始发育。培养3天时,部分叶绿体边缘出现一些空隙结构。随着叶绿体内膜结构的消失,淀粉粒增大,叶绿体逐渐转变为造粉质体。 相似文献
97.
Characterization of the traC determinant of the Enterococcus faecalis hemolysin-bacteriocin plasmid pAD1: binding of sex pheromone. 总被引:19,自引:13,他引:6 下载免费PDF全文
pAD1, a conjugative, 60-kb, hemolysin-bacteriocin plasmid in Enterococcus faecalis, encodes a mating response to a small peptide sex pheromone, cAD1, secreted by potential recipient bacteria. A gene, traC, encoding a 60.7-kDa protein with a typical amino terminal signal peptide, was identified within a region that appears to encode a product that binds to exogenous pheromone. A cloned segment of DNA containing traC resulted in specific binding of cells to synthetic cAD1. The putative traC product has strong similarity to a product of the E. faecalis plasmid pCF10 as well as oligopeptide binding proteins of Escherichia coli, Salmonella typhimurium, and Bacillus subtilis. 相似文献
98.
Increased levels of a 65-kDa stress protein (Msp65) were observed in group B Neisseria meningitidis grown under stationary-growth conditions. Electron microscopy showed two apposing rings of seven subunits, a structure typical of Escherichia coli GroEL. Msp65 was not found in either the periplasmic space or the outer membrane. Several important differences between the GroEL analogs of N. meningitidis and Neisseria gonorrhoeae are discussed. 相似文献
99.
Nguyen T. Buu 《Journal of neurochemistry》1993,61(4):1557-1560
Abstract: The neurotoxin N -methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes, via its metabolite 1-methyl-4-phenylpyridinium (MPP+ ), parkinsonism in humans, monkeys, and mice but not in rats. When incubated with mouse brain homogenates, [3 H]-MPP+ is recovered in relatively large concentrations in the brain cell nucleus. Although isolated cell nuclei from rat and mouse brain contain uptake systems for dopamine (DA), only brain cell nuclei from mice avidly take up [3 H]MPP+ . This nuclear uptake is ATP dependent and can be blocked by ouabain and Nethylmaleimide. It is not, however, affected by neuronal and vesicular blockers such as benztropine. mazindol, and reserpine. Selective uptake of MPP+ into brain cell nuclei may provide a new avenue for future investigation into the complex modes of action of the neurotoxin MPTP. 相似文献
100.
Monoclonal antibodies against the muscle-specific N-terminus of dystrophin: characterization of dystrophin in a muscular dystrophy patient with a frameshift deletion of exons 3-7. 总被引:2,自引:2,他引:0 下载免费PDF全文
T T Le T M Nguyen D R Love T R Helliwell K E Davies G E Morris 《American journal of human genetics》1993,53(1):131-139