全文获取类型
收费全文 | 67805篇 |
免费 | 282篇 |
国内免费 | 557篇 |
专业分类
系统科学 | 1251篇 |
丛书文集 | 567篇 |
教育与普及 | 283篇 |
理论与方法论 | 529篇 |
现状及发展 | 35118篇 |
研究方法 | 1470篇 |
综合类 | 27237篇 |
自然研究 | 2189篇 |
出版年
2013年 | 818篇 |
2012年 | 788篇 |
2011年 | 2939篇 |
2009年 | 630篇 |
2008年 | 998篇 |
2007年 | 1217篇 |
2006年 | 1214篇 |
2005年 | 1447篇 |
2004年 | 2609篇 |
2003年 | 2134篇 |
2002年 | 1871篇 |
2001年 | 1899篇 |
2000年 | 1404篇 |
1999年 | 1270篇 |
1998年 | 649篇 |
1997年 | 781篇 |
1994年 | 700篇 |
1993年 | 697篇 |
1992年 | 1200篇 |
1991年 | 976篇 |
1990年 | 1087篇 |
1989年 | 875篇 |
1988年 | 849篇 |
1987年 | 860篇 |
1986年 | 934篇 |
1985年 | 1164篇 |
1984年 | 943篇 |
1983年 | 814篇 |
1982年 | 867篇 |
1981年 | 899篇 |
1980年 | 1018篇 |
1979年 | 1773篇 |
1978年 | 1523篇 |
1977年 | 1483篇 |
1976年 | 1277篇 |
1975年 | 1275篇 |
1974年 | 1319篇 |
1973年 | 1434篇 |
1972年 | 1548篇 |
1971年 | 1619篇 |
1970年 | 1935篇 |
1969年 | 1620篇 |
1968年 | 1548篇 |
1967年 | 1429篇 |
1966年 | 1283篇 |
1965年 | 1000篇 |
1959年 | 564篇 |
1958年 | 868篇 |
1957年 | 667篇 |
1956年 | 546篇 |
排序方式: 共有10000条查询结果,搜索用时 25 毫秒
61.
Thyroid cancer after Chernobyl. 总被引:17,自引:0,他引:17
62.
Targeted disruption of the mouse transforming growth factor-beta 1 gene results in multifocal inflammatory disease. 总被引:149,自引:0,他引:149
M M Shull I Ormsby A B Kier S Pawlowski R J Diebold M Yin R Allen C Sidman G Proetzel D Calvin 《Nature》1992,359(6397):693-699
Transforming growth factor-beta 1 (TGF-beta 1) is a multifunctional growth factor that has profound regulatory effects on many developmental and physiological processes. Disruption of the TGF-beta 1 gene by homologous recombination in murine embryonic stem cells enables mice to be generated that carry the disrupted allele. Animals homozygous for the mutated TGF-beta 1 allele show no gross developmental abnormalities, but about 20 days after birth they succumb to a wasting syndrome accompanied by a multifocal, mixed inflammatory cell response and tissue necrosis, leading to organ failure and death. TGF-beta 1-deficient mice may be valuable models for human immune and inflammatory disorders, including autoimmune diseases, transplant rejection and graft versus host reactions. 相似文献
63.
Short alanine peptides, containing 16 or 17 residues, appear to form alpha-helices in aqueous solution. But the main spectroscopic analyses used on helical peptides (circular dichroism and nuclear magnetic resonance) cannot distinguish between an alpha-helix (in which the ith residue is hydrogen-bonded to residue i + 4; ref. 9) and the next most common peptide helix, the 3(10)-helix10 (i-->i + 3 hydrogen-bonding). To address this problem we have designed single and doubly spin-labelled analogues of alanine-based peptides in which the nitroxide spin label forms an unbranched side chain extending from the sulphur atom of a cysteine residue. Here we report the circular dichroism, Fourier-transform infrared and electron-spin resonance spectra of these peptides under helix-forming conditions. The infrared absorbance gives an amide I' band with a frequency that is substantially different from that observed for alpha-helices. The electron-spin resonance spectra of doubly labelled helices show that the ranking of distances between side chains, around a single turn (residues 4-8), is inconsistent with an alpha-helical structure. Our experiments suggest that the more likely peptide geometry is a 3(10)-helix. 相似文献
64.
本文研究了在不同pH值下吐温—80.Tritonx—100.SLS三种表面活性剂四—(对-三甲铵苯基)卟啉Soret带的影响。 相似文献
65.
生物序列分析 总被引:1,自引:0,他引:1
这次演讲将回顾近 1 0多年来应用某些随机模型于生物序列分析的研究工作 .这些模型本身有很长的历史 ,可追溯到 3 0多年以前 ,尽管从那时起 ,这些模型已经产生了很多新的变种 .在生物序列分析中模型的作用是归总那些涉及到在生物信息学中已知的模体 (motif)或域 (domain)的信息 ,并且提供一种工具在另一序列片段中寻找模体或域的实例 (instance) .我们将逐步介绍模体模型 ,从非常简单的 ,非随机情况开始 ,进而是更复杂的情况 ,直至近来的关于模体的剖面隐马氏模型 .第二个例子是来自利用一个或两个物种的序列数据进行基因发现 ,其中广义隐马氏模型或广义成对 (pair)隐马氏模型已被证实非常有效 相似文献
66.
The failing heart. 总被引:19,自引:0,他引:19
Cardiomyopathies are disorders affecting heart muscle that usually result in inadequate pumping of the heart. They are the most common cause of heart failure and each year kill more than 10,000 people in the United States. In recent years, there have been breakthroughs in understanding the molecular mechanisms involved in this group of conditions, with knowledge of the genetic basis for cardiomyopathies perhaps seeing the largest advance, enabling clinicians to devise improved diagnostic strategies and preparing the stage for new therapies. 相似文献
67.
A calcium sensor in the sodium channel modulates cardiac excitability. 总被引:11,自引:0,他引:11
Hanno L Tan Sabina Kupershmidt Rong Zhang Svetlana Stepanovic Dan M Roden Arthur A M Wilde Mark E Anderson Jeffrey R Balser 《Nature》2002,415(6870):442-447
Sodium channels are principal molecular determinants responsible for myocardial conduction and maintenance of the cardiac rhythm. Calcium ions (Ca2+) have a fundamental role in the coupling of cardiac myocyte excitation and contraction, yet mechanisms whereby intracellular Ca2+ may directly modulate Na channel function have yet to be identified. Here we show that calmodulin (CaM), a ubiquitous Ca2+-sensing protein, binds to the carboxy-terminal 'IQ' domain of the human cardiac Na channel (hH1) in a Ca2+-dependent manner. This binding interaction significantly enhances slow inactivation-a channel-gating process linked to life-threatening idiopathic ventricular arrhythmias. Mutations targeted to the IQ domain disrupted CaM binding and eliminated Ca2+/CaM-dependent slow inactivation, whereas the gating effects of Ca2+/CaM were restored by intracellular application of a peptide modelled after the IQ domain. A naturally occurring mutation (A1924T) in the IQ domain altered hH1 function in a manner characteristic of the Brugada arrhythmia syndrome, but at the same time inhibited slow inactivation induced by Ca2+/CaM, yielding a clinically benign (arrhythmia free) phenotype. 相似文献
68.
感应电会对人体造成伤害。从电磁感应和静电感应原理入手,分析了产生感应电的过程,并就如何防止感应电的危害提出了防护对策,有一定实用价值。 相似文献
69.
从炉排横向配风不均、风室压火、煤层厚度阻力及锅炉燃烧等方面分析了链条锅炉存在的问题,并针对存在的问题提出了相应的解决办法。 相似文献
70.
N. Ait-Langomazino R. Sellier G. Jouquet M. Trescinski 《Cellular and molecular life sciences : CMLS》1991,47(6):533-539
Summary A blown bitumen Mexphalte R 90/40 with a high content of saturated hydrocarbons was degraded by several microorganisms to the same extent. In batch cultures ofSaccharomycopsis lipolytica, maximal biodegradation was estimated to be about 9% w/w, 3.2·10–3 g/cm2 and 3.1·10–3 cm of degraded bitumen. The Mexphalte R 90/40 degradation rate was closely coupled to biofilm formation. The microbial activity concerned predominantly the oxidation of saturated hydrocarbons. A direct distillation bitumen 80/100 with a low content of saturated hydrocarbons and a high content of aromatic hydrocarbons and resins was more resistant to biodegradation. 相似文献