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   34358篇
  免费   3100篇
  国内免费   3011篇
  2024年   91篇
  2023年   486篇
  2022年   1087篇
  2021年   1777篇
  2020年   1234篇
  2019年   1419篇
  2018年   1425篇
  2017年   1041篇
  2016年   1405篇
  2015年   2152篇
  2014年   2420篇
  2013年   2634篇
  2012年   2959篇
  2011年   2817篇
  2010年   1765篇
  2009年   1569篇
  2008年   1707篇
  2007年   1591篇
  2006年   1449篇
  2005年   1123篇
  2004年   1068篇
  2003年   971篇
  2002年   864篇
  2001年   664篇
  2000年   599篇
  1999年   582篇
  1998年   344篇
  1997年   282篇
  1996年   272篇
  1995年   226篇
  1994年   234篇
  1993年   160篇
  1992年   247篇
  1991年   216篇
  1990年   180篇
  1989年   153篇
  1988年   127篇
  1987年   128篇
  1986年   114篇
  1985年   122篇
  1984年   71篇
  1983年   63篇
  1982年   65篇
  1981年   46篇
  1980年   46篇
  1979年   63篇
  1978年   51篇
  1977年   52篇
  1975年   48篇
  1974年   49篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
High-performance electrophoresis chromatography (HPEC) is a recent development that features continuous-elution gel electrophoresis for isolating proteins or peptides in range of 1 to 300 microgram quantities. Column gel electrophoresis is conducted under thermostated conditions, and the field voltage can be varied within a run with a programmable power supply. Applications of this apparatus in protein purification are presented to demonstrate the utility of the (Model 230A) HPEC. These examples include on-line detection with direct analyte recovery of highly purified sample, which mimics high-performance liquid chromatography, for subsequent structure-function characterization. A method to remove salts from sodium dodecyl sulfate electrophoresed samples for subsequent sequencing or amino acid analysis is described. This desalting procedure recovers from 90%-95% of the sample and employs a low molecular weight cut-off membrane during sample centrifugation onto a polyvinylidene difluoride membrane. Subsequent washings are performed to efficiently remove salts, free amino acids and detergents that are known to interfere with sequence analysis. Sequence information such as initial recovery, repetitive yields and chromatogram comparisons are presented to demonstrate the utility of this procedure when used following isolation of sample with HPEC.  相似文献   
72.
刘绍华  程菊英   《广西植物》1990,10(4):372-375
用酸浸提、酒精沉淀法,从桔皮中提取果胶,通过正交设计分析,获得最佳得率的工艺条件。即果胶浸提液的酸度为pH2,反应时间2小时。沉淀果胶的酒精浓度为50%,其得率是12.6%。  相似文献   
73.
中国九个人群耵聍的遗传多态性   总被引:6,自引:2,他引:4  
翁自力  金锋 《人类学学报》1990,9(3):236-243
报道了九个人群的耵聍位点基因。计算表明中国各族人群在耵聍位点上的遗传分化程度非常大,固定指数F_(ST)=0.22。本文根据耵聍基因频率在我国和邻近地区的分布趋势,认为亚洲东北地区应是干型基因的起源地,目前世界上耵聍位点基因频率分布格局主要是基因扩散的结果,而非选择作用造成的。  相似文献   
74.
程观炽  金力 《遗传学报》1990,17(3):230-237
利用与β地中海贫血致病基因连锁的遗传标记进行产前诊断已成为可能,但群体中遗传标记与致病基因的不完全连锁决定了产前诊断的局限性。根据中国人群中7个遗传标记的多态性分布数据,我们计算了各遗传标记及其所有组合的可诊断率。又据各遗传标记及其组合的可诊断率,我们可以对:(1)所作出产前诊断的可诊断率进行预先评价;(2)选择适合于中国人群β地中海贫血产前诊断的最佳策略。本文提出了中国人群中选择遗传标记进行β地中海贫血产前诊断的最佳路线。同时将基因的连锁分析法与寡核苷酸法进行了比较。  相似文献   
75.
A large number of mutations were introduced into the carboxy-terminal domain of pp60c-src. The level of phosphorylation on Tyr-416 and Tyr-527, the transforming activity (as measured by focus formation on NIH 3T3 cells), kinase activity, and the ability of the mutant pp60c-src to associate with the middle-T antigen of polyomavirus were examined. The results indicate that Tyr-527 is a major carboxy-terminal element responsible for regulating pp60c-src in vivo. A good but not perfect correlation exists between lack of phosphorylation at Tyr-527 and increased phosphorylation at Tyr-416, between elevated phosphorylation on Tyr-416 and activated kinase activity, and between activated kinase activity and transforming activity. Phosphorylation of Tyr-527 was insensitive to the mutation of adjacent residues, indicating that the primary sequence only has a minor role in recognition by kinases or phosphatases which regulate it in vivo. Three mutants which have in common a modified Glu-524 residue were phosphorylated on Tyr-416 and Tyr-527 and were weakly transforming. This suggests that other mechanisms besides complete dephosphorylation of Tyr-527 can lead to increased phosphorylation of Tyr-416 and activation of the transforming activity of pp60c-src. Furthermore, the residues between Asp-518 and Pro-525 were required to form a stable complex with middle-T antigen. The proximity of these sequences to Tyr-527 suggests a model in which middle-T activates pp60c-src by binding directly to this region of the molecular and thereby preventing phosphorylation of Tyr-527. Alternatively, middle-T binding may mediate a conformational change in this region, which in turn induces an alteration in the level of phosphorylation at Tyr-527 and Tyr-416.  相似文献   
76.
We have isolated and analyzed eight strict maternal effect mutations identifying four genes, par-1, par-2, par-3, and par-4, required for cytoplasmic localization in early embryos of the nematode C. elegans. Mutations in these genes lead to defects in cleavage patterns, timing of cleavages, and localization of germ line-specific P granules. Four mutations in par-1 and par-4 are fully expressed maternal effect lethal mutations; all embryos from mothers homozygous for these mutations arrest as amorphous masses of differentiated cells but are specifically lacking intestinal cells. Four mutations in par-2, par-3, and par-4 are incompletely expressed maternal effect lethal mutations and are also grandchildless; some embryos from homozygous mothers survive and grow to become infertile adults due to absence of functional germ cells. We propose that all of these defects result from the failure of a maternally encoded system for intracellular localization in early embryos.  相似文献   
77.
D S Wang  S W Li  C Q Zeng  R X Cheng  S B Xue 《Cytometry》1988,9(4):368-373
Using DAPI, rabbit antitubulin antibody, FITC-labeled goat anti-rabbit IgG, and TRITC-phalloidin to stain individual cells, the microspectrophotometric analysis showed that three markers that represent the nucleus, microtubules (MT), and microfilaments (MF), respectively, could be recognized in individual cells without interference. The phase of the cell cycle was determined by DNA content. We found that in Indian muntjac (IM) cells, the amount of tubulin in G2 and M phases was about twice as much as that in G1 phase. In G2 cells, the cytoplasmic microtubule complex (CMTC) became denser than in G1 cells. The cytoplasmic MT extent in basically the same orientation as MF bundles in interphase. The regions where the MT is denser also have a denser MF distribution.  相似文献   
78.
79.
本文利用含有抗核基质自发抗体的硬皮人血清,以小鼠艾氏腹水癌细胞为材料,用间接免疫荧光染色的方法,追踪了对应核基质抗原在细胞周期中分布的变化。结果显示,在末期和间期之间存在一个核基质抗原从细胞质向细胞核内转移的过程。由于这一过程是通过核膜进行的,从而提示核基质结构可能有解聚和再聚合的行为。用酶化学结合间接免疫荧光染色的方法,初步研究了抗原的化学性质。染色形态的比较研究显示所用血清中可能含有不同于以前发现的、抗新的核基质抗原的自发抗体。  相似文献   
80.
The alpha-like globin gene cluster in rabbits contains embryonic zeta- globin genes, an adult alpha-globin gene, and theta-globin genes of undetermined function. The basic arrangement of genes, deduced from analysis of cloned DNA fragments, is 5'-zeta 0-zeta 1-alpha 1-theta 1- zeta 2-zeta 3-theta 2-3'. However, the pattern of restriction fragments containing zeta- and theta-globin genes varies among individual rabbits. Analysis of BamHI fragments of genomic DNA from 24 New Zealand white rabbits revealed eight different patterns of fragments containing zeta-globin genes. The large BamHI fragments containing genes zeta 0 and zeta 1 are polymorphic in length, whereas a 1.9-kb fragment containing the zeta 2 gene and the 3.5-kb fragment containing the zeta 3 gene do not vary in size. In contrast to this constancy in the size of the restriction fragments, the copy number of the zeta 2 and zeta 3 genes does vary among different rabbits. No length polymorphism was detected in the BamHI fragments containing the theta-globin genes, but again the copy number varies for restriction fragments containing the theta 2 gene. The alpha 1- and theta 1-globin genes are located in a nonpolymorphic 7.2-kb BamHI fragment. The combined data from hybridization with both zeta and theta probes shows that the BamHI cleavage pattern does not vary within the region 5'-alpha 1-theta 1- zeta 2-zeta 3-theta 2-3', but the pattern genomic blot-hybridization patterns for the progeny of parental rabbits with different zeta-globin gene patterns shows that the polymorphic patterns are inherited in a Mendelian fashion. Two different haplotypes have been mapped based on the genomic blot-hybridization data. The variation in the alpha-like globin gene cluster in the rabbit population results both from differences in the copy number of the duplication block containing the zeta-zeta-theta gene set and from the presence or absence of polymorphic BamHI sites.   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