全文获取类型
收费全文 | 25762篇 |
免费 | 2125篇 |
国内免费 | 2124篇 |
专业分类
30011篇 |
出版年
2024年 | 68篇 |
2023年 | 357篇 |
2022年 | 814篇 |
2021年 | 1338篇 |
2020年 | 874篇 |
2019年 | 1072篇 |
2018年 | 1011篇 |
2017年 | 747篇 |
2016年 | 1069篇 |
2015年 | 1566篇 |
2014年 | 1848篇 |
2013年 | 1950篇 |
2012年 | 2390篇 |
2011年 | 2045篇 |
2010年 | 1300篇 |
2009年 | 1033篇 |
2008年 | 1404篇 |
2007年 | 1185篇 |
2006年 | 1069篇 |
2005年 | 913篇 |
2004年 | 754篇 |
2003年 | 641篇 |
2002年 | 586篇 |
2001年 | 492篇 |
2000年 | 378篇 |
1999年 | 430篇 |
1998年 | 259篇 |
1997年 | 218篇 |
1996年 | 259篇 |
1995年 | 206篇 |
1994年 | 257篇 |
1993年 | 153篇 |
1992年 | 222篇 |
1991年 | 185篇 |
1990年 | 174篇 |
1989年 | 113篇 |
1988年 | 85篇 |
1987年 | 75篇 |
1986年 | 49篇 |
1985年 | 63篇 |
1984年 | 48篇 |
1983年 | 39篇 |
1982年 | 37篇 |
1981年 | 24篇 |
1980年 | 16篇 |
1979年 | 25篇 |
1977年 | 15篇 |
1976年 | 16篇 |
1975年 | 15篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 36 毫秒
11.
12.
The effects of phosphorylation on the interaction between spectrin and ankyrin were investigated. Spectrin and ankyrin were phosphorylated using purified human erythrocyte membrane and cytosolic (casein kinase A) kinases. These two kinases have similar properties as well as activities toward spectrin and ankyrin. Both kinases catalyzed the incorporation of about 2 mol of phosphate/mol of spectrin and about 7 mol of phosphate/mol of ankyrin. These phosphates were incorporated primarily into seryl and threonyl residues of the proteins. The phosphopeptide maps of ankyrin phosphorylated by the membrane kinase and casein kinase A were identical. Binding studies indicate that ankyrin exhibits different affinities for spectrin dimers (KD = 2.5 +/- 0.9 X 10(-6) M) and tetramers (KD = 2.7 +/- 0.8 X 10(-7) M). These dissociation constants were not appreciably affected by the phosphorylation of spectrin. On the other hand, phosphorylation of ankyrin was found to significantly reduce its affinity for either phosphorylated or unphosphorylated spectrin tetramers (KD = 1.2 +/- 0.1 X 10(-6) M) but not spectrin dimers (KD = 2.5 +/- 0.4 X 10(-6) M). The same results were obtained using either the membrane kinase or casein kinase A as the phosphorylating enzyme. The above observation suggests that ankyrin phosphorylation may provide an important mechanism for the regulation of the erythrocyte membrane cytoskeletal network. 相似文献
13.
14.
15.
Successful replacement of avulsed scalp. Case report 总被引:2,自引:0,他引:2
M M Lu 《Plastic and reconstructive surgery》1969,43(3):231-234
16.
17.
Retrovirus-mediated transgenic keratin expression in cultured fibroblasts: specific domain functions in keratin stabilization and filament formation. 总被引:37,自引:0,他引:37
With retrovirus-mediated gene transfer, we used intact and deleted keratin proteins to investigate the molecular basis of intermediate filament function. Three levels of assembly show a different stringency for the involvement of individual keratin domains: protein accumulation requires the alpha helix domains; stable filament formation additionally requires both N- and C-terminal domains of either one of the two interacting keratins, suggesting that head to tail homotypic interaction is important for effective elongation; and higher order organization of the cytoplasmic network depends on correct type I-type II pairing of keratins. The presence of two distinct interaction sites along potentially different axes may explain the characteristic morphology of keratin intermediate filament networks. 相似文献
18.
Protease-induced infectivity of hepatitis B virus for a human hepatoblastoma cell line. 总被引:12,自引:4,他引:8 下载免费PDF全文
The human hepatoblastoma cell line HepG2 produces and secretes hepatitis B virus (HBV) after transfection of cloned HBV DNA. Intact virions do not infect these cells, although they attach to the surface of the HepG2 cell through binding sites in the pre-S1 domain. Entry of enveloped virions into the cell often requires proteolytic cleavage of a viral surface protein that is involved in fusion between the cell membrane and the viral envelope. Recently, we observed pre-S-independent, nonspecific binding between hepatitis B surface (HBs) particles and HepG2 cells after treatment of HBs antigen particles with V8 protease, which cleaves next to a putative fusion sequence. Chymotrypsin removed this fusion sequence and did not induce binding. In this study, we postulate that lack of a suitable fusion-activating protease was the reason why the HepG2 cells were not susceptible to HBV. To test this hypothesis, virions were partially purified from the plasma of HBV carriers and treated with either staphylococcal V8 or porcine chymotrypsin protease. Protease-digested virus lost reactivity with pre-S2-specific antibody but remained morphologically intact as determined by electron microscopy. After separation from the proteases, virions were incubated with HepG2 cells at pH 5.5. Cultures inoculated with either intact or chymotrypsin-digested virus did not contain detectable levels of intracellular HBV DNA at any time following infection. However, in cultures inoculated with V8-digested virions, HBV-specific products, including covalently closed circular DNA, viral RNA, and viral pre-S2 antigen, could be detected in a time-dependent manner following infection. Immunofluorescence analysis revealed that 10 to 30% of the infected HepG2 cells produced HBV antigen. Persistent secretion of virus by the infected HepG2 cells lasted at least 14 days and was maintained during several reseeding steps. The results show that V8-digested HBV can productively infect tissue cultures of HepG2 cells. It is suggested that proteolysis-dependent exposure of a fusion domain within the envelope protein of HBV is necessary during natural infection. 相似文献
19.
Characterization of a calcium/calmodulin-dependent protein kinase homolog from maize roots showing light-regulated gravitropism 总被引:14,自引:0,他引:14
Roots of many species respond to gravity (gravitropism) and grow downward only if illuminated. This light-regulated root gravitropism is phytochrome-dependent, mediated by calcium, and inhibited by KN-93, a specific inhibitor of calcium/calmodulin-dependent protein kinase II (CaMK II). A cDNA encoding MCK1, a maize homolog of mammalian CaMK, has been isolated from roots of maize (Zea mays L.). The MCK1 gene is expressed in root tips, the site of perception for both light and gravity. Using the [35S]CaM gel-overlay assay we showed that calmodulin-binding activity of the MCK1 is abolished by 50 M KN-93, but binding is not affected by 5 M KN-93, paralleling physiological findings that light-regulated root gravitropism is inhibited by 50 M KN-93, but not by 5 M KN-93. KN-93 inhibits light-regulated gravitropism by interrupting transduction of the light signal, not light perception, suggesting that MCK1 may play a role in transducing light. This is the first report suggesting a physiological function for a CaMK homolog in light signal transduction.Abbreviations CaM
calmodulin
- CaMK (II)
Ca2+/calmodulin-dependent protein kinase (II)
- CBP
CaM-binding protein
- CDPK
Ca2+-dependent protein kinase
- MCK1
maize homolog of mamalian CaMK
This work is supported by the National Aeronautics and Space Administration grant No: NAGW 238. 相似文献
20.
采用大鼠海马脑片体外缺血模型观察钙离子和蛋白激酶C(PKC)对神经元胞外谷氨酸(GLU)堆积的影响,结果显示:海马脑片在体外“缺血”10min,GLU在胞外的浓度增加4倍(从32±4升高到113±10pmol/(min.mgPr).n=6).N型钙通道拮抗剂蝙蝠葛苏林碱(DSL)或无钙培养液均能有效抑制这种浓度的升高(P<0.01).提示缺血10min引发的GLU浓度升高是受Ca2+内流调控的.当脑片在缺血状况下孵育30min,DSL只部分抑制这种GLU堆积,而无钙培养液则无影响,但这额外的GLU堆积可被PKC抑制剂H-7完全阻断,而被PKC激动剂PDB所加强;且不受钙调蛋白抑制剂Calmdazolium和8-溴-cAMP影响.提示缺血30min,胞外GLU的堆积受钙内流和PKC双重调控。 相似文献