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   2996篇
  免费   207篇
  国内免费   165篇
  3368篇
  2023年   26篇
  2022年   53篇
  2021年   52篇
  2020年   56篇
  2019年   138篇
  2018年   97篇
  2017年   77篇
  2016年   63篇
  2015年   80篇
  2014年   174篇
  2013年   232篇
  2012年   117篇
  2011年   138篇
  2010年   84篇
  2009年   120篇
  2008年   124篇
  2007年   122篇
  2006年   131篇
  2005年   100篇
  2004年   96篇
  2003年   72篇
  2002年   77篇
  2001年   57篇
  2000年   52篇
  1999年   63篇
  1998年   54篇
  1997年   36篇
  1996年   55篇
  1995年   49篇
  1994年   52篇
  1993年   38篇
  1992年   34篇
  1991年   51篇
  1990年   31篇
  1989年   48篇
  1988年   27篇
  1987年   21篇
  1985年   36篇
  1984年   52篇
  1983年   49篇
  1982年   48篇
  1981年   39篇
  1980年   48篇
  1979年   28篇
  1978年   27篇
  1977年   30篇
  1976年   26篇
  1975年   22篇
  1974年   17篇
  1973年   19篇
排序方式: 共有3368条查询结果,搜索用时 0 毫秒
991.
Long non-coding RNAs play significant roles in many biological processes. The roles of lncRNAs in Pichia pastoris remain unclear. In this work, we focused on the identification of lncRNAs in P. pastoris and exploration of their potential roles in stress response to PLA2 overexpression and methanol induction. By strand specific RNA sequencing, 208 novel long non-coding RNAs were identified and analyzed. Bioinformatic analysis showed potential trans-target genes and cis-regulated genes of 39 differential lncRNAs. Functional annotation and sequence motif analysis indicated that lncRNAs participate in pathways related to methanol degradation and production of the recombinant protein. The differential expression of lncRNAs was validated by qRT-PCR. Lastly, the potential functions of three lncRNAs were evaluated by knockdown of their expression and analysis of the expression levels of target genes. Our study identifies novel lncRNAs in P. pastoris induced during use as a bioreactor, facilitating future functional research.  相似文献   
992.
Murine sarcoma virus (MSV)-immune T cells from C57BL/6 mice respond to intact RBL-5 tumor cells with the production of leukocyte adherence inhibition factor (LAIF), which mediates an adherence inhibition response of macrophages. LAIF is elaborated by isolated Lyt-2+ cells incubated with RBL-5 cells, whereas Lyt-1+ cells elaborate a substance that enhances macrophage adherence. Spleen macrophages or peritoneal exudate macrophages from MSV-immune mice when present at concentrations of 0.1% changed the response of Lyt-1+ cells from the formation of an adherence enhancing factor to the formation of an adherence inhibiting factor. Migration inhibition factor (MIF) was formed by Lyt-1+ cells, but not by Lyt-2+ cells under identical culture conditions. Addition of either spleen macrophages from mice with progressively growing tumors or tumor-infiltrating macrophages suppressed LAIF formation by both Lyt-1+ and Lyt-2+ cells. Tumor-infiltrating macrophages elicited an adherence enhancing factor from Lyt-2+ cells when present at high concentrations. The results suggest that the extent of macrophage adherence in vitro is the outcome of an interaction of macrophages with mediators that have opposing effects.  相似文献   
993.
马鸡的分类、分布及演化关系的初步探讨   总被引:13,自引:0,他引:13  
卢欣  郑光美  顾滨源 《动物学报》1998,44(2):131-137
(1)在耳羽簇发达程度,尾羽形状,腓肠肌外支腿膜内的种子骨和第二趾收肌,雄鸡间争偶和占区行为,巢址选择,瓣生雏形态等方面,白马鸡和藏马鸡,蓝马鸡和褐钯鸡分别表现出明显不同的一致性。基于此,我们认为白马鸡和藏马鸡是原始的类型,蓝马鸡和褐马鸡有较近的亲缘关系。(2)青藏高原短暂的隆起历史和相应发生的生态环境的复杂变化,是白马鸡各亚种形态多变而不稳定的主要原因。与白马鸡相比,藏马鸡的体色特化而稳定,结合  相似文献   
994.
花胸散白蚁多型行为的初步分析   总被引:3,自引:1,他引:3  
张健华 《昆虫学报》1998,41(4):376-381
该文在花胸散白蚁Reticulitermes fukienensis Light的侦察行为和取食行为方面进行了比较详细的探讨,同时还分析了其消化系统发育与取食行为变化之间的相互关系。实验结果表明年龄是影响花胸散白蚁多型行为的一个比较重要的因素。而且发现幼蚁和工蚁的消化系统发育是与其取食行为相一致的,特别是后肠囊从幼蚁的细条消化道发育为成年工蚁的膨大囊袋这一过程是花胸散白蚁群体中存在着取食多型行为的强有力的证据。  相似文献   
995.
国人颅骨弧弦周长的测量及其性别的判别分析   总被引:1,自引:1,他引:1  
本文对青岛地区出土、已知性别的成年颅骨249个(男142,女107)进行了弧、弦、周长的测量,计算了有关指数,对额、顶、枕骨的弧长作了比较和分型,并用判别分析方法鉴定颅骨性别。  相似文献   
996.
Increases in the intracellular Ca2+ concentration in pancreatic islets, resulting from the Ca2+ mobilization from the intracellular source through the ryanodine receptor, are essential for insulin secretion by glucose. Cyclic ADP-ribose, a potent Ca2+ mobilizing second messenger synthesized from NAD+ by CD38, regulates the opening of ryanodine receptor. A novel ryanodine receptor mRNA (the islet-type ryanodine receptor) was found to be generated from the type 2 ryanodine receptor gene by the alternative splicing of exons 4 and 75. The islet-type ryanodine receptor mRNA is expressed in a variety of tissues such as pancreatic islets, cerebrum, cerebellum, and other neuro-endocrine cells, whereas the authentic type 2 ryanodine receptor mRNA (the heart-type ryanodine receptor) was found to be generated using GG/AG splicing of intron 75 and is expressed in the heart and the blood vessel. The islet-type ryanodine receptor caused a greater increase in the Ca2+ release by caffeine when expressed in HEK293 cells pre-treated with cyclic ADP-ribose, suggesting that the novel ryanodine receptor is an intracellular target for the CD38-cyclic ADP-ribose signal system in mammalian cells and that the tissue-specific alternative splicing of type 2 ryanodine receptor mRNA plays an important role in the functioning of the cyclic ADP-ribose-sensitive Ca2+ release.  相似文献   
997.
998.
We calculate thermal fluctuational base pair opening probability and the drug binding constant of a daunomycin-bound Poly d(CGTA) · Poly d(TACG) at temperatures from room temperature to its melting temperature. For comparison we also carry out a calculation on a drug-free DNA with the same sequence. Our calculations are carried out by means of a statistical approach using microscopic structures and established force fields and with cooperative effects incorporated into the algorithm. Both hydrogen bond disruption probabilities and drug unstacking probability are determined self-consistently. These probabilities are then used to determine temperature dependent base pair opening probabilities and the drug binding constant. The calculated base pair opening probabilities and drug binding constant are found to be in fair agreement with experiments carried out at room temperature. Our calculation shows cooperative base pair disruption and drug dissociation at certain critical temperatures close to the observed melting temperatures for similar helices. We find that the temperature dependence of the drug binding constant fits well to the van't Hoff relation, in agreement with observations. Our calculation indicates the occurrence of a premelting transition in the drug-bound DNA helix. Some comments are made about this premelting transition.  相似文献   
999.
Mechanosensory lateral line units recorded from the medulla (medial octavolateralis nucleus) and midbrain (torus semicircularis) of the bottom dwelling catfish Ancistrus sp. responded to water movements caused by an object that passed the fish laterally. In terms of peak spike rate or total number of spikes elicited responses increased with object speed and sometimes showed saturation (Figs. 7, 14). At sequentially greater distances the responses of most medullary lateral line units decayed with object distance (Fig. 11). Units tuned to a certain object speed or distance were not found. The signed directionality index of most lateral line units was between –50 and +50, i.e. these units were not or only slightly sensitive to the direction of object motion (Figs. 10, 17). However, some units were highly directionally sensitive in that the main features of the response histograms and/or peak spike rates clearly depended on the direction of object movement (e.g. Fig. 9C, D and Fig. 16). Midbrain lateral line units of Ancistrus may receive input from more than one sensory modality. All bimodal lateral line units were OR units, i.e., the units were reliably driven by a unimodal stimulus of either modality. Units which receive bimodal input may show an extended speed range (e.g. Fig. 18).Abbreviations MON medial octavolateralis nucleus - MSR mean spike rate - PSR peak spike rate - p-p peak-to-peak - SDI signed directionality index  相似文献   
1000.
Metal ion and substrate binding to carbamate kinase from Streptococcus faecalis was studied by nuclear magnetic resonance (NMR) and electron paramagnetic resonance using Mn2+ as the paramagnetic probe. The enzyme binds Mn2+ weakly (KD = 0.45 ± 0.05 mm) with a stoichiometry of one per two subunits. However, in the presence of nucleotides, tighter binding of Mn2+ was observed with KD = 44 ± 4 μm in the presence of ADP and KD = 23 ± 4 μm with ATP present. Proton relaxation rate enhancement studies were conducted on water molecules interacting with ternary enzyme-Mn2+-nucleotide and binary enzyme-Mn2+ complexes. Mn2+ bound to carbamate kinase enhances the proton relaxation rate of water giving a binary enhancement value of ?b = 9.3 ± 0.4. When enzyme-Mn2+ was titrated with ADP or ATP, a bell-shaped titration curve was obtained typical of many other enzyme-Mn2+-nucleotide ternary complexes. Computer fits to the titration data gave ternary enhancement values of ?tADP = 14 ± 1 and ?tATP = 19 ± 1. The dissociation constants for Mn-ADP and Mn-ATP binding to carbamate kinase were also obtained from these data analyses and are K1 = 2.5 ± 0.5 μm and K1 = 50 ± 8 μm, respectively. Therefore, these data demonstrate the formation of a ternary enzyme-metal-nucleotide bridge complex at the nucleotide substrate site of carbamate kinase. Distance measurements were conducted by NMR techniques with 13C-enriched carbamate and demonstrate that carbamate is 4–8 Å from enzyme-bound Mn2+. Thus carbamate binds near the metal-nucleotide substrate site of carbamate kinase.  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