全文获取类型
收费全文 | 49628篇 |
免费 | 3943篇 |
国内免费 | 3185篇 |
专业分类
56756篇 |
出版年
2024年 | 93篇 |
2023年 | 633篇 |
2022年 | 1437篇 |
2021年 | 2454篇 |
2020年 | 1556篇 |
2019年 | 1880篇 |
2018年 | 1798篇 |
2017年 | 1380篇 |
2016年 | 1988篇 |
2015年 | 2946篇 |
2014年 | 3442篇 |
2013年 | 3680篇 |
2012年 | 4348篇 |
2011年 | 4049篇 |
2010年 | 2364篇 |
2009年 | 2150篇 |
2008年 | 2460篇 |
2007年 | 2202篇 |
2006年 | 1883篇 |
2005年 | 1657篇 |
2004年 | 1392篇 |
2003年 | 1225篇 |
2002年 | 1043篇 |
2001年 | 951篇 |
2000年 | 838篇 |
1999年 | 816篇 |
1998年 | 511篇 |
1997年 | 543篇 |
1996年 | 490篇 |
1995年 | 445篇 |
1994年 | 447篇 |
1993年 | 341篇 |
1992年 | 479篇 |
1991年 | 414篇 |
1990年 | 355篇 |
1989年 | 275篇 |
1988年 | 251篇 |
1987年 | 213篇 |
1986年 | 151篇 |
1985年 | 202篇 |
1984年 | 127篇 |
1983年 | 112篇 |
1982年 | 87篇 |
1981年 | 61篇 |
1980年 | 59篇 |
1979年 | 76篇 |
1978年 | 68篇 |
1977年 | 46篇 |
1976年 | 48篇 |
1973年 | 48篇 |
排序方式: 共有10000条查询结果,搜索用时 5 毫秒
951.
952.
To explore an effective and reliable karyotyping method in Brassica crop plants,Cot-1 DNA was isolated from Brassica oleracea genome,labeled as probe with Biotin-Nick Translation Mix kit,in situ hybridized to mitotic spreads,and where specific fluorescent bands showed on each chromosome pair.25S and 5S rDNA were labeled as probes with DIG-Nick Translation Mix kit and Biotin-Nick Translation Mix kit,respectively,in situ hybridized to mitotic preparations,where 25S rDNA could be detected on two chromosome pairs and 5S rDNA on only one.Cot-1 DNA contains rDNA and chromosome sites identity between Cot-1 DNA and 25S rDNA was determined by dual-colour fluorescence in situ hybridization.All these showed that the karyotyping technique based on a combination of rDNA and Cot-1 DNA chromosome landmarks is superior to all but one.A more exact karyotype ofB.oleracea has been analyzed based on a combination of rDNA sites,Cot-1 DNA fluorescent bands,chromosome lengths and arm ratios. 相似文献
953.
954.
本文用FRAP(fluorescencerecoveryafterphotobleaching)技术,测量了静息状态和刀豆素A刺激不同时间后巨噬细胞膜磷脂、ConA受体扩散系数和荧光恢复率的变化。结果显示ConA刺激后膜磷脂和ConA受体的扩散系数和荧光恢复率均较静息状态的巨噬细胞明显降低,磷脂流动性的变化与ConA受体流动性的变化呈正相关。提示受体介导内吞导致的膜磷脂流动性的降低,可能是由于配体与细胞膜上受体结合形成配体-受体复合体,增加了受体的负荷,使受体的流动性降低,进而使膜磷脂的流动性降低。巨噬细胞内吞过程中膜磷脂和ConA受体流动性的降低,可能还与ConA刺激后巨噬细胞胞浆pH值有关。 相似文献
955.
Kun-Pin Wu Jia-Ming Chang Jun-Bo Chen Chi-Fon Chang Wen-Jin Wu Tai-Huang Huang Ting-Yi Sung Wen-Lian Hsu 《Journal of computational biology》2006,13(2):229-244
We develop an iterative relaxation algorithm called RIBRA for NMR protein backbone assignment. RIBRA applies nearest neighbor and weighted maximum independent set algorithms to solve the problem. To deal with noisy NMR spectral data, RIBRA is executed in an iterative fashion based on the quality of spectral peaks. We first produce spin system pairs using the spectral data without missing peaks, then the data group with one missing peak, and finally, the data group with two missing peaks. We test RIBRA on two real NMR datasets, hbSBD and hbLBD, and perfect BMRB data (with 902 proteins) and four synthetic BMRB data which simulate four kinds of errors. The accuracy of RIBRA on hbSBD and hbLBD are 91.4% and 83.6%, respectively. The average accuracy of RIBRA on perfect BMRB datasets is 98.28%, and 98.28%, 95.61%, 98.16%, and 96.28% on four kinds of synthetic datasets, respectively. 相似文献
956.
Yunhao Zhu Pengfei Zhao Xiaolin Wu Wei Wang Monica Scali Mauro Cresti 《Acta Physiologiae Plantarum》2011,33(4):1467-1474
The identification of proteins involved in pollen germination and tube growth is important for fundamental studies of fertility
and reproduction in flowering plants. We used 2-DE and MALDI-TOF-MS to identify differentially expressed proteins in mature
(P0) and 1-h germinated (P1) maize pollen. Among about 470 proteins separated in 2D gels, the abundances of 26 protein spots
changed (up- or down-regulation) between P0 and P1. The 13 up-regulated protein spots were mainly involved in tube wall modification,
actin cytoskeleton organization, energy metabolism, signaling, protein folding and degradation. In particular, pectin methylesterase,
inorganic pyrophosphatase, glucose-1-phosphate uridylyltransferase and rab GDP dissociation inhibitor α are highly up-regulated,
suggesting their potential role in pollen tube growth. The down-regulated 13 protein spots mainly include defense-related
proteins, pollen allergens and some metabolic enzymes. This study would contribute to the understanding of the changes in
protein expression associated with pollen tube development. 相似文献
957.
Estimation of a novel method to produce bio-oil from sewage sludge by microwave pyrolysis with the consideration of efficiency and safety 总被引:2,自引:0,他引:2
This paper presented a feasible method to produce bio-oil from sewage sludge by microwave pyrolysis. The results showed that oils derived under 400 W obtained an attractive yield (49.8 wt.%) with favorable characteristics such as high calorific value (35.0 MJ/kg), low density (929 kg/m3) and preferable chemical composition (29.5 wt.% of monoaromatics). A model to study the relationship between microwave power and mass balance of product fractions was developed, and the results indicated that the power range of the highest transforming efficiency for organics in sludge into oils was 400-600 W, the subsequent increase of power to the range of 600-800 W favored gases formation at the expense of oils, and increase of power to above 800 W led to the conversion of solids into gases, while oils remained unchanged. The analysis of sulfur and nitrogen compounds in oils showed that bio-oil should be extracted before being used as fuel. 相似文献
958.
Somatic point mutations in unrearranged immunoglobulin gene segments encoding the variable region of lambda light chains. 总被引:2,自引:1,他引:2 下载免费PDF全文
Somatic point mutations are usually found in the coding and flanking regions of functionally and aberrantly rearranged immunoglobulin variable region gene segments. Mutations in the unrearranged V gene segments of myelomas or hybridomas have not been described so far. We have cloned and sequenced unrearranged V lambda gene segments from several cell lines. There were no nucleotide changes in four unrearranged V lambda segments: one V lambda 1 from a lambda 3-producing hybridoma and one V lambda 2 from a lambda 1-producing myeloma (J558) and two V lambda 2 from a kappa-producing myeloma (P3X63). However, we found somatic mutations in the unrearranged V lambda segments from the lambda 2-producing myeloma MOPC315. The unrearranged V lambda 1 gene segment had two mutations in the coding region and the unrearranged V lambda 2 had one mutation in the 3' flanking region. We also cloned and sequenced the unrearranged J lambda and C lambda gene segments of MOPC315 and found no sequence alterations. This is consistent with the notion that the overall mutation rate is not higher in this cell line. Therefore, we suggest that the somatic hypermutation system can use unrearranged V gene segments as substrates. The extensive sequencing required for this work revealed a number of errors in the reported nucleotide sequences of the Ig lambda locus in BALB/c mice. 相似文献
959.
Nichols RA Dengler AF Nakagawa EM Bashkin M Paul BT Wu J Khan GM 《The Journal of biological chemistry》2007,282(49):36102-36111
Calcium levels in the presynaptic nerve terminal are altered by several pathways, including voltage-gated Ca(2+) channels, the Na(+)/Ca(2+) exchanger, Ca(2+)-ATPase, and the mitochondria. The influx pathway for homeostatic control of [Ca(2+)](i) in the nerve terminal has been unclear. One approach to detecting the pathway that maintains internal Ca(2+) is to test for activation of Ca(2+) influx following Ca(2+) depletion. Here, we demonstrate that a constitutive influx pathway for Ca(2+) exists in presynaptic terminals to maintain internal Ca(2+) independent of voltage-gated Ca(2+) channels and Na(+)/Ca(2+) exchange, as measured in intact isolated nerve endings from mouse cortex and in intact varicosities in a neuronal cell line using fluorescence spectroscopy and confocal imaging. The Mg(2+) and lanthanide sensitivity of the influx pathway, in addition to its pharmacological and short hairpin RNA sensitivity, and the results of immunostaining for transient receptor potential (TRP) channels indicate the involvement of TRPC channels, possibly TRPC5 and TRPC1. This constitutive Ca(2+) influx pathway likely serves to maintain synaptic function under widely varying levels of synaptic activity. 相似文献
960.
摘要:【目的】鉴定一株来自于红海榄根际土壤并具有分泌抑菌活性代谢产物的真菌菌株F12,并从其发酵液乙酸乙酯浸膏中分离抑菌活性成分。【方法】通过形态学观察以及ITS序列分析方法对菌株F12进行鉴定;利用色谱技术分离发酵液乙酸乙酯浸膏中的次生代谢产物,根据化合物的质谱、氢谱、碳谱以及理化性质确定其结构,并检测它们对细菌生长的抑制作用。【结果】菌株F12被鉴定为Aspergillus awamori strain F12;从其发酵液乙酸乙酯浸膏中分离到3种化合物:1,4-二甲氧基苯(1)、大黄素(2)和3,6-二苯甲基哌嗪-2,5-二酮(3),其中化合物1属于在本属真菌中首次报道。化合物2对金黄色葡萄球菌和枯草芽孢杆菌的生长具有明显的抑制作用,最低抑菌浓度(MIC)分别为16ng/L和32ng/L,化合物1和3对上述菌株的生长无明显的抑制活性。【结论】首次发现从红海榄根际土壤中分离到的泡盛曲霉(Aspergillus awamori)菌株F12具有合成1,4-二甲氧基苯和大黄素的能力,其中后者对微生物的生长具有明显的抑制作用。 相似文献