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   21966篇
  免费   2435篇
  国内免费   3篇
  2021年   263篇
  2018年   223篇
  2017年   210篇
  2016年   405篇
  2015年   615篇
  2014年   673篇
  2013年   898篇
  2012年   1055篇
  2011年   1000篇
  2010年   758篇
  2009年   690篇
  2008年   983篇
  2007年   1028篇
  2006年   897篇
  2005年   907篇
  2004年   891篇
  2003年   904篇
  2002年   784篇
  2001年   502篇
  2000年   472篇
  1999年   446篇
  1998年   296篇
  1997年   258篇
  1996年   234篇
  1995年   220篇
  1994年   221篇
  1993年   209篇
  1992年   401篇
  1991年   385篇
  1990年   388篇
  1989年   334篇
  1988年   332篇
  1987年   325篇
  1986年   300篇
  1985年   320篇
  1984年   328篇
  1983年   299篇
  1982年   260篇
  1981年   219篇
  1980年   218篇
  1979年   260篇
  1978年   250篇
  1977年   223篇
  1976年   217篇
  1975年   197篇
  1974年   234篇
  1973年   225篇
  1972年   194篇
  1971年   173篇
  1970年   160篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
In a recent study, we demonstrated that the conversion of carboxyl residues in the C-termini of tubulin to neutral amides with glycine ethyl ester enhanced the ability of the protein to assemble into microtubules and decreased its interaction with microtubule-associated proteins (MAPs). In this work, we investigated the effects of carboxyl modification on the dynamic behavior of microtubules at polymer mass steady state. After steady state, microtubules assembled from unmodified tubulin were sheared, and the mean polymer lengths decreased to 5 microns and then increased to 29 microns within 130 min. In contrast, lengths of sheared microtubules polymerized from tubulin containing 23 modified carboxyl groups increased by only 2-fold. Stabilization of polymer lengths was also observed directly by video-enhanced light microscopy of microtubules grown off of axonemes. Rapid shortening was seen in microtubules composed of unmodified but not modified tubulin. Further evidence for the less dynamic behavior of microtubules as a result of carboxyl modification was obtained from kinetic studies of the elongation phase during assembly which showed a 3-fold lower off-rate constant, k-, for modified microtubules. Another effect of the modification was a 12-fold reduction in the steady-state rate constant for GTP hydrolysis (165 s-1 for unmodified and 14 s-1 for modified). These results suggest that reduction of the negative charges in the C-termini by modification of the acidic residues stabilizes microtubules against depolymerization. MAPs may stabilize microtubules in an analogous manner.  相似文献   
2.
3.
4.
The response of cytosolic calcium [Ca2+]i to angiotensin II (AII) and potassium (K+) in individual rat glomerulosa cells was determined using the calcium-sensitive fluorescent dye, fura-2 and digital imaging. Control (4 mM K+) cytosolic calcium levels were generally in the 80-120 nM range and increased monotonically as [K+] was increased from 4 to 12 mM. There was no delay in the onset of the response. In most cells the [Ca2+]i decreased from its peak after 3-4 min, even in the presence of superfusate containing elevated K+. The time course of the change in [Ca2+]i in response to AII stimulation, on the other hand, was more variable. It was most often characterized by an early decrease followed by a large delayed increase. The response also was observed to decline during sustained AII stimulation. The majority of the cells showed some response to one or the other secretagogue with a sizeable minority (25%) having an increase in [Ca2+]i in excess of 200%. While the majority showed a response, the cell to cell variation was substantial. Finally, the pattern of cytosolic calcium increase sometimes showed a marked dependence on the secretagogue used, with different regions of the same cell being more strongly affected by one agent or the other. A few cells (10%) responded to AII only at one pole, establishing a large concentration gradient of calcium across the cell. Because of differences in time course, pattern, and degree of responsiveness, it is likely that the mechanisms underlying the Ca2+ elevation with K+ and AII are different.  相似文献   
5.
6.
7.
8.
The gene of Cytotoxic T Lymphocyte-associated Antigen 4 (CTLA4), a negative regulator of T lymphocytes, contains a single-nucleotide polymorphism (SNP) at position +6230A->G (ct60A->G), which has been found associated with several autoimmune diseases and appears to reduce T-cell inhibitory activity. In Ghana, West Africa, we compared the frequencies of CTLA4 +6230 A/G and 6 haplotype-tagging SNPs in 2010 smear-positive, HIV-negative patients with pulmonary tuberculosis (TB) and 2346 controls matched for age, gender and ethnicity. We found no difference in allele frequencies between cases and controls. However, +6230A and a distinct CTLA4 haplotype and a diplotype comprising the +6230A allele were significantly less frequent among cases with large opacities in chest radiographs compared to those with small ones (Pcorrected [cor] = 0.002, Pcor = 0.00045, P = 0.0005, respectively). This finding suggests that an increased T-cell activity associated with the CTLA4 +6230G allele contributes to pathology rather than to protection in pulmonary TB.  相似文献   
9.
S ummary . Treating seedling roots of several plant species with cultures of Azotobacter paspali changed plant growth and development and significantly increased weight of leaves and roots; effects were probably caused by plant growth regulators. Culture supernatant fluids contained indolyl-3-acetic acid, at least 3 gibberellins and 2 cytokinins. The added inoculum of A. paspali survived in plant rhizospheres for only a few weeks and no nitrogen was fixed in the root zone of young Paspalum notatum , the grass with which A. paspali is associated.  相似文献   
10.
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