首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   8909篇
  免费   968篇
  国内免费   2347篇
  2024年   40篇
  2023年   263篇
  2022年   344篇
  2021年   626篇
  2020年   523篇
  2019年   551篇
  2018年   477篇
  2017年   340篇
  2016年   435篇
  2015年   602篇
  2014年   680篇
  2013年   710篇
  2012年   891篇
  2011年   802篇
  2010年   548篇
  2009年   492篇
  2008年   622篇
  2007年   509篇
  2006年   463篇
  2005年   396篇
  2004年   322篇
  2003年   300篇
  2002年   268篇
  2001年   201篇
  2000年   193篇
  1999年   140篇
  1998年   75篇
  1997年   60篇
  1996年   43篇
  1995年   42篇
  1994年   36篇
  1993年   35篇
  1992年   34篇
  1991年   29篇
  1990年   11篇
  1989年   16篇
  1988年   17篇
  1987年   16篇
  1986年   10篇
  1985年   9篇
  1984年   6篇
  1983年   10篇
  1982年   6篇
  1981年   6篇
  1979年   5篇
  1978年   3篇
  1974年   2篇
  1973年   3篇
  1969年   2篇
  1967年   2篇
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
21.
用对阿特拉津(Atrazine)除草剂抗性的龙葵生物型B_(12)株系作材料,制备叶绿体DNA。B_(12)株ctDNA(叶绿体DNA)经BamHI酶解,在0.7%琼脂糖凝胶电泳上呈现24条带,其中最大的片段为18.6kb,最小的片段为1kb。用pBR322作为载体,构建B_(12)株ctDNA BamHI片段文库。通过与探针的分子杂交,从中筛选出含有编码叶绿体32kd蛋白质的阿特拉津抗性基因的克隆pSB135和含有ATP合酶α亚单位基因的克隆pSB132。  相似文献   
22.
贵州产宽叶缬草(Valeriana officinalis L.var.latifolia Miq.)油,用Finnigan4510型毛细管气相色谱/质谱/计算机联用方法进行了化学成分分析,共检出了29个成分鉴定了其中21个成分,占全精油的92.33%,主要成分为乙酸龙脑酯,α-蒎烯,莰烯,β-蒎烯,柠檬烯,乙酸葛缕酯,二氢乙酸葛缕酯等。该油芬芳,适于调配烟用香精,亦用于调节器配食用香精和化妆品香精。  相似文献   
23.
Effect of prostaglandin F2 alpha on the secretion of human prolactin   总被引:1,自引:0,他引:1  
This study examines the role of PGF2a (prostaglandin F2alpha) in increasing the secretion rate of human prolactin. 11 women (mean gestational period, 18 weeks) seeking pregnancy termination were divided into 4 groups: 1) Group 1 consisted of 6 women who received 30 mg initially of PGF2a injected intramuscularly and an additional 15 mg after 24 hours if abortion had not occured; mean induction to termination period was 38 hours; 2) Group 2 comprised of 3 women who received PGF2a (500-1500 ug) via the transcervical route at 1 to 2 hourly interval; average number of injections was 20; mean induction to termination period, 24 hours; 3) Group 3 had 2 women receiving hypertonic saline by intraamniotic injection; mean induction to termination period was 51 hours; 4) Group 4 had 4 women who served as controls; mean observation period, 20 hours. Venous blood samples were heparinized in tubes at intervals of 2 to 3 hours. A homologous radioimmunoassay using highly purified human prolactin (for iodination and standards) plus rabbit antihuman prolactin measured serum prolactin. Spikes of serum prolactin up to 550 ng/ml were observed at irregular intervals in 5 women in Group 1; the spikes were less frequent and of smaller amplitude in Groups 3 and 4. The increase in serum prolactin was dramatic and more sustained in Group 2 patients and peaked towards the end of the prostaglandin infusion. Serum prolactin of Group 2 patients were significantly higher than those of Groups 3 and 4 (p0.01). 5 of 9 women whose pregnancies were terminated by PGF2a lactated. However, there was no significant difference between the mean serum prolactin levels in women who lactated (136 ng/ml) and those who did not (120 ng/ml). Although PGF2a is not a lactogenic hormone, this study shows that PGF2a stimulates the secretion of human prolactin during second trimester pregnancy. The fact that the transcervical route caused a significant increase in serum prolactin and the intraamniotic route did not is attributed to the increased systemic absorption of PGF2a following transcervical administration. No correlation was seen between the presence or absence of lactation and the serum prolactin level following pregnancy termination with PGF2a.  相似文献   
24.
Molecular cloning of rat cytolysin   总被引:4,自引:0,他引:4  
Rat cytolysin is one of the cytolytic factors present in the cytoplasmic granules of rat NK-like cytolytic cells and purified cytolysin exhibits an apparent Mr or 70 kDa. Cytolysis produced by cytolysin occurs in the presence of Ca2+ and is accompanied by the formation of membrane lesions of 160 A diameter. We have isolated a cDNA encoding rat cytolysin from the cDNA library of a rat large granular lymphocyte (LGL) cell line, by hybridization of the rat library with a cDNA probe for mouse perforin. The amino acid sequence deduced from the nucleotide sequence of the isolated cDNA insert indicates that the mature cytolysin protein consist of 534 amino acids with a leader peptide of 20 amino acids. The protein contains two functionally important domains: the first domain is believed to contain the transmembrane channel and the second domain consists of an epidermal growth factor-type "class B" cysteine-rich region. A comparison with mouse perforin indicates that the two genes are very similar (89.9% nucleotide and 84.9% amino acid identity). Northern blot hybridization analysis indicates that cytolysin mRNA is expressed in rat lymphocytes (lymphokine-activated killer cells and LGL cells) and LGL cell lines.  相似文献   
25.
The effect of a new PAF antagonist BN 50739 was studied on PAF-induced [3H]-serotonin release from washed rabbit platelets in vitro and on PAF-induced hypotension in vivo. BN 50739 competitively inhibited PAF-induced [3H]-serotonin release from the platelets in a dose-dependent manner. In the presence of 4, 10 and 50 nM of BN 50739, the concentration of PAF inducing 50% maximal [3H]-serotonin release from the platelets (EC50) increased from 2.15 nM to 5.10, 45.10 and 900 nM, respectively. The IC50 of BN 50739 for PAF (10 nM) induced [3H]-serotonin release was 3.67 nM. Under the same experimental condition, the IC50s of BN 50726, BN 50730, BN 50741, WEB 2086, SRI 63-441 and BN 52021 were 5.40, 4.61, 6.88, 5.98, 40.90 nM and 14.90 microM, respectively. PAF-induced hypotension in conscious rats was also inhibited dose-dependently by i.p. pretreatment of BN 50739 (3 and 10 mg/kg). PAF-induced hypotension was diminished both in magnitude and duration in rats pretreated with BN 50739. These data taken together indicate that BN 50739 is a most potent PAF antagonist in vitro and in vivo.  相似文献   
26.
Reovirus genome segment S4 codes for polypeptide sigma3, a major outer capsid component of virions and a double-stranded RNA (dsRNA)-binding protein implicated in viral cytopathogenesis. We have constructed a stable HeLa cell line (S4tTA) that produces functional sigma3 under tetracycline transactivator control. In the absence of tetracycline, S4tTA cells synthesized stable dsRNA-binding sigma3 that accumulated in the nucleus as well as in the cytoplasm. However, in induced S4tTA cells also expressing reovirus outer shell polypeptide mu1/mu1C, migration of sigma3 into the nucleus was blocked, probably as a result of formation of a complex with mu1/mu1C which was exclusively in the cytoplasm. Mutant analyses indicated a correlation between dsRNA-binding activity and nuclear entry of sigma3, suggesting an additional role(s) for this capsid protein in virus-cell interactions.  相似文献   
27.
噻替派浓度为0.1%、0.3%、0.5%时,黑胸大蠊精母细胞染色体断裂和裂隙率分别为6.3%、 10.5%和14.2%,显著地高于对卵母细胞的影响;和雄虫外周血淋巴细胞微核率呈平行关系,随微核率增多而增加。5-氟尿嘧啶浓度为0.1%、0.3%和0.5%时,卵母细胞染色体断裂和裂隙率分别为3.5%、9.8%和16.2%,和雌虫外周血淋巴细胞微核率呈平行关系,随微核率增多而增加,而对雄虫生殖细胞影响不显著。 Abstract:0.1%,0.3%,0.5% Thio-TEPA induced 6.3%,10.5% and 14.2% chromosome break or gap in spermatocyte of cockroach respectively.This was markedly higher than those in oocyte.In doses from 0.1 to 0.5 Tho-TEPA the frequency of micronucleus increased parallely with nuclear damage.0.1%,0.3%,0.5% 5-fluorouracil induced 3.5%,9.8%,16.2% chromosome break or gap in oocytes respectively.This was paralled with the frequency of micronucleus in lymphocytes of the female.5-fluorouracil showed not marked effect on spermatocyte.  相似文献   
28.
本文应用透射电镜对朱顶红成熟花粉水合、活化和萌发的动态过程中营养细胞质的结构和组成变化进行了观察。成熟花粉具质体、线粒体、内质网、高尔基体。微丝束以聚集体的形式存在。花粉活化后,细胞器的数目和结构发生显著变化:质体和线粒体的片层明显增加,内质网片层狭窄,高尔基体活跃产生小泡,脂体降解及微丝聚集体散开。花粉萌发后,细胞质中出现周质微管和被刺小泡,此期细胞器的变化不明显。微丝以纤丝状遍布整个花粉管中。  相似文献   
29.
固定化虫荧光素酶光纤传感器   总被引:1,自引:0,他引:1  
固定化虫荧光素酶光纤传感器蔡谨,王顺光,杨歧生,吉鑫松(浙江大学化工系生化教研室,杭州310027;中国科学院上海生物化学研究所,200031)关键词虫荧光素酶,ATP,固定化酶,光纤生物传感器ATP是生物体内极为重要的能量物质。如何准确快速地定量A...  相似文献   
30.
应用SDS-聚丙烯酰胺凝胶电泳及免疫转印技术对流行性出血热患才血清中免疫合物组分进行了分析。流行性出血热循环免疫复合物经SDS-PAGE分离,考马斯亮兰染色,显色主要有7条带,分子量分别为23kD,50kD,52kD,65kD,72kD,80kD及100kD。采用该病毒特异性抗血清、单克隆抗体以及人免疫球蛋白、补体成分抗血清识别,在其特环免疫复合物中可检出特异性病毒抗在及相应的免疫球状蛋白和补体成  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号