全文获取类型
收费全文 | 19221篇 |
免费 | 1411篇 |
国内免费 | 1504篇 |
专业分类
22136篇 |
出版年
2024年 | 48篇 |
2023年 | 329篇 |
2022年 | 658篇 |
2021年 | 1088篇 |
2020年 | 671篇 |
2019年 | 906篇 |
2018年 | 803篇 |
2017年 | 558篇 |
2016年 | 880篇 |
2015年 | 1156篇 |
2014年 | 1462篇 |
2013年 | 1522篇 |
2012年 | 1813篇 |
2011年 | 1568篇 |
2010年 | 996篇 |
2009年 | 854篇 |
2008年 | 946篇 |
2007年 | 813篇 |
2006年 | 660篇 |
2005年 | 580篇 |
2004年 | 486篇 |
2003年 | 437篇 |
2002年 | 387篇 |
2001年 | 285篇 |
2000年 | 290篇 |
1999年 | 304篇 |
1998年 | 196篇 |
1997年 | 199篇 |
1996年 | 189篇 |
1995年 | 151篇 |
1994年 | 136篇 |
1993年 | 97篇 |
1992年 | 140篇 |
1991年 | 114篇 |
1990年 | 100篇 |
1989年 | 79篇 |
1988年 | 52篇 |
1987年 | 33篇 |
1986年 | 28篇 |
1985年 | 41篇 |
1984年 | 18篇 |
1983年 | 23篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1980年 | 6篇 |
1979年 | 5篇 |
1977年 | 3篇 |
1974年 | 2篇 |
1962年 | 1篇 |
1950年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
硒对培养人胚肝细胞Ⅲ型前胶原,羟脯氨酸合成的影响 总被引:7,自引:0,他引:7
原代培养人胚肝细胞经1.156×10 ̄(-7)mol/L硒预处理4h,加入20mmol/L四氟化碳作用20h,观察硒对其Ⅲ型前胶原(PCⅢ)和羟脯氨酸(Hyp)生成的影响。结果培养液中PCⅢ水平、细胞内Hyp含量及细胞内外丙二醛(MDA)水平均降低,与未加硒对照组比较差别有显著性(P<0.01)。而硒谷腕甘肽过氧化物酶(Se-GSH-PX)活性则较对照组显著增高(P<0.001),且PCⅢ水平与Se-GSH-P_X/MDA比值呈负相关(r=-0.9156,P<0.01)。提示硒可提高Se-GSH-P_X/MDA比值,抑制脂质过氧化激发的肝细胞胶原合成。 相似文献
52.
湖北、河南、安徽三省大别山区地理新分布植物何家庆(安徽大学生物系合肥230039)关键词大别山区,种子植物,地理新分布THENEWGEOGRAPHICALDISTRIBUTIONOFSPERMATOPHYTEINDABIESHANTHEREGIONS... 相似文献
53.
建立了测定人乳腺癌胞浆cAMP结合蛋白(cAMPb.p.)方法。综合研究了其温度、保温时间、配体浓度、稳定性等条件。cAMPb.p.的K_D值为2.90×10~(-8)mol/L.并测定了60例雌激素受体(ER)Fu性乳腺癌标本的cAMPb.p.含量。此组病人术后均接受系统的内分泌治疗,ER/cAMPbp,比值范围为7.7~362×10~(-3),ER/cAMPb.p.比值≥40×10~(-3)的五年生存率明显高于比值<40×10~(-3)组,(p<0.005).表明测定ER/cAMPb.p.比值对预测患者内分泌治疗疗效,优于单独测定ER. 相似文献
54.
S. He Z.-H. Yu C. E. Vallejos S. A. Mackenzie 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1995,90(7-8):1056-1062
The Fr gene in common bean, Phaseolus vulgaris L., is a unique gene for the study of plant nuclear-mitochondrial interactions because it appears to directly influence plant mitochondrial genome structure, resulting in the restoration of pollen fertility in cytoplasmic male sterile plants. This gene action is distinct from other pollen fertility restoration systems characterized to date. As a first step towards the map-based cloning of this unusual nuclear gene, we identified RAPD markers linked to Fr using bulked segregant analysis of near-isogenic lines. Using DNA gel blot hybridization, we localized the identified RAPD markers to a linkage group on the common bean RFLP map and constructed a linkage map of the Fr region using both RAPD markers and RFLP markers. Analysis of the mode of Fr action with the aid of identified Fr-linked DNA markers indicated that Fr functions in a semidominant fashion, showing dosage effect in controlling the dynamics of a heteroplasmic mitochondrial population. We also present our observations on the developmental distinctions, crucial in the accurate mapping of the Fr gene, between spontaneous cytoplasmic reversion and Fr-driven fertility restoration, two phenomena that are phenotypically indistinguishable. 相似文献
55.
Pollen Fertility Restoration by Nuclear Gene Fr in Cms Bean: Nuclear-Directed Alteration of a Mitochondrial Population 总被引:4,自引:0,他引:4
下载免费PDF全文
![点击此处可从《Genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Two nuclear genes, Fr and Fr2, have been identified that restore pollen fertility to cytoplasmic male sterile (CMS) common bean (Phaseolus vulgaris L.) by apparently distinct mechanisms. Whereas Fr2 appears to suppress the expression of a male sterility associated mitochondrial sequence (designated pvs), Fr restores pollen fertility by causing the elimination of this unusual mitochondrial DNA segment. To further investigate the mechanism of Fr action, Fr and Fr2 were cointroduced into the nucleus of a bean line containing the sterility inducing cytoplasm. When the effect of pvs was suppressed by Fr2, the presence of Fr no longer directed the elimination of the mitochondrial pvs sequence. This result suggests that the Fr function is dependent on proper expression of the pvs sequence. To evaluate the temporal and spatial patterns of Fr action, we undertook a polymerase chain reaction-based approach to trace the fate of the pvs sequence in different tissues of F(2) and F(3) fertile-restored plants derived from a genetic cross between a cytoplasmic male sterile line of common bean, CMS-Sprite (frfr), and fertility restorer line R351 (FrFr). We demonstrate that the Fr-directed disappearance of pvs sequence occurs during flower development. Elimination of the pvs sequence from developing megaspores results in permanent fertility restoration in the following generations. Genetic analysis demonstrated that permanent fertility restoration, that is, the complete elimination of pvs from reproductive tissues requires two doses of the Fr allele or the absence of fr in F(2) individuals. The effect of Fr was reversible until full fertility was achieved. On the basis of these results, we propose a model for the mechanism of pvs elimination by the Fr gene and discuss the dynamics of pvs-containing mitochondrial transmission in the presence of the Fr gene. 相似文献
56.
本文采用Y染色体特异的性别决定基因(Sry)作为新的细胞遗传标志,通过PCR技术来追踪观察造血干细胞的增殖与分化性能。该方法具有简便、灵敏和特异等优点。雌性受体小鼠输注雄鼠骨髓细胞和13天脾结节(CFU-S13)细胞后,Sry PCR测试受体小鼠的CFU-S结果表明,它们均为供体来源的XY细胞。用Sry PCR骨髓细胞和骨髓中脾结节生成细胞(CPU-S)的长期重建造血能力,结果表明,在存活雌性小鼠 相似文献
57.
58.
Rotavirus RNA polymerase requires the core shell protein to synthesize the double-stranded RNA genome. 总被引:11,自引:6,他引:5
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Rotavirus cores contain the double-stranded RNA (dsRNA) genome, RNA polymerase VP1, and guanylyltransferase VP3 and are enclosed within a lattice formed by the RNA-binding protein VP2. Analysis of baculovirus-expressed core-like particles (CLPs) has shown that VP1 and VP2 assemble into the simplest core-like structures with replicase activity and that VP1, but not VP3, is essential for replicase activity. To further define the role of VP1 and VP2 in the synthesis of dsRNA from viral mRNA, recombinant baculoviruses containing gene 1 (rBVg1) and gene 2 (rBVg2) of SA11 rotavirus were generated and used to express recombinant VP1 (rVP1) and rVP2, respectively. After purification, the proteins were assayed individually and together for the ability to catalyze the synthesis of dsRNA in a cell-free replication system. The results showed that dsRNA was synthesized only in assays containing rVP1 and rVP2, thus establishing that both proteins are essential for replicase activity. Even in assays containing a primer-linked mRNA template, neither rVP1 nor rVP2 alone directed RNA synthesis. Characterization of the cis-acting replication signals in mRNA recognized by the replicase of rVP1 and rVP2 showed that they were the same as those recognized by the replicase of virion-derived cores, thus excluding a role for VP3 in recognition of the mRNA template by the replicase. Analysis of RNA-protein interactions indicated that the mRNA template binds strongly to VP2 in replicase assays but that the majority of the dsRNA product neither is packaged nor stably associates with VP2. The results of replicase assays performed with mutant VP2 containing a deletion in its RNA-binding domain suggests that the essential role for VP2 in replication is linked to the protein's ability to bind the mRNA template for minus-strand synthesis. 相似文献
59.
帕里红景天的化学成分研究 总被引:10,自引:0,他引:10
从帕里红景天根茎的石油醚和乙醇提取部分共分得14种结晶性化合物,经光谱分析和化学反应,分别鉴定为二十二醇、二十六酸、十九醇、β-谷甾醇、二十九醇、红景天甙、麦芽糖、棉皮素-8-葡萄糖甙、胡萝卜甙、酪醇、咖啡酸、没食子酸、形花内酯和新化合物帕里甙。 相似文献
60.
He Liu H. Peter Spielmann Nikolai B. Ulyanov David E. Wemmer Thomas L. James 《Journal of biomolecular NMR》1995,6(4):390-402
Summary The effect of experimental and integration errors on the calculations in interproton distances from NOE intensities is examined. It is shown that NOE intensity errors can have a large impact on the distances determined. When multiple spin (spin diffusion) effects are significant, the calculated distances are often underestimated, even when using a complete relaxation matrix analysis. In this case, the bias of distances to smaller values is due to the random errors in the NOE intensities. We show here that accurate upper and lower bounds of the distances can be obtained if the intensity errors are properly accounted for in the complete relaxation matrix calculations, specifically the MARDIGRAS algorithm. The basic MARDIGRAS algorithm has been previously described [Borgias, B.A. and James, T.L. (1990) J. Magn. Reson., 87, 475–487]. It has been shown to provide reasonably good interproton distance bounds, but experimental errors can compromise the quality of the resulting restraints, especially for weak cross peaks. In a new approach introduced here, termed RANDMARDI (random error MARDIGRAS), errors due to random noise and integration errors are mimicked by the addition of random numbers from within a specified range to each input intensity. Interproton distances are then calculated for the modified intensity set using MARDIGRAS. The distribution of distances that define the upper and lower distance bounds is obtained by using N randomly modified intensity sets. RANDMARDI has been used in the solution structure determination of the interstrand cross-link (XL) formed between 4-hydroxymethyl-4,5,8-trimethylpsoralen (HMT) and the DNA oligomer d(5-GCGTACGC-3)2 [Spielmann, H.P. et al. (1995) Biochemistry, 34, 12937–12953]. RANDMARDI generates accurate distance bounds from the experimental NOESY cross-peak intensities for the fixed (known) interproton distances in XL. This provides an independent internal check for the ability of RANDMARDI to accurately fit the experimental data. The XL structure determined using RANDMARDI-generated restrains is in good agreement with other biophysical data that indicate that there is no bend introduced into the DNA by the cross-link. In contrast, isolated spin-pair approximation calculations give distance restraints that, when applied in a restrained molecular dynamics protocol, produce a bent structure.Abbreviations NOE
nuclear Overhauser effect
- SD
standard deviation
- HMT
4-hydroxymethyl-4,5,8-trimethylpsoralen
- XL
psoralen-DNA interstrand cross-link 相似文献