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   2960篇
  免费   337篇
  国内免费   2篇
  2023年   16篇
  2022年   31篇
  2021年   76篇
  2020年   51篇
  2019年   75篇
  2018年   63篇
  2017年   76篇
  2016年   100篇
  2015年   149篇
  2014年   178篇
  2013年   200篇
  2012年   223篇
  2011年   223篇
  2010年   153篇
  2009年   133篇
  2008年   163篇
  2007年   146篇
  2006年   134篇
  2005年   126篇
  2004年   96篇
  2003年   111篇
  2002年   88篇
  2001年   62篇
  2000年   67篇
  1999年   63篇
  1998年   44篇
  1997年   22篇
  1996年   31篇
  1995年   15篇
  1994年   22篇
  1993年   22篇
  1992年   30篇
  1991年   27篇
  1990年   27篇
  1989年   32篇
  1988年   19篇
  1987年   29篇
  1986年   26篇
  1985年   18篇
  1983年   15篇
  1982年   8篇
  1981年   6篇
  1980年   9篇
  1979年   9篇
  1978年   10篇
  1977年   8篇
  1976年   6篇
  1975年   7篇
  1974年   7篇
  1973年   9篇
排序方式: 共有3299条查询结果,搜索用时 140 毫秒
1.
2.
Fiddler crabs (Uca spp., Decapoda: Ocypodidae) are commonly found forming large aggregations in intertidal zones, where they perform rhythmic waving displays with their greatly enlarged claws. While performing these displays, fiddler crabs often synchronize their behavior with neighboring males, forming the only known synchronized visual courtship displays involving reflected light and moving body parts. Despite being one of the most conspicuous aspects of fiddler crab behavior, little is known about the mechanisms underlying synchronization of male displays. In this study we develop a spatially explicit model of fiddler crab waving displays using coupled logistic map equations. We explored two alternative models in which males either direct their attention at random angles or preferentially toward neighbors. Our results indicate that synchronization is possible over a fairly large region of parameter space. Moreover, our model was capable of generating local synchronization neighborhoods, as commonly observed in fiddler crabs under natural conditions.  相似文献   
3.
Bleach treatment of plants was studied as a simple alternative to axenic tissue cultures for demonstrating phytodegradation of aqueous and gas-phase environmental contaminants. Parrotfeather (Myriophyllum aquaticum), spinach (Spinacia oleracea), and wheat (Triticum aestivum) were exposed to 0.525% NaC10 solutions for 15 s, then rinsed in deionized water. Plate counts indicated that 97 to 100% of viable bacteria were removed from parrotfeather and spinach. Transformation rates for 2,4,6-trinitrotoluene (TNT) by bleached and untreated parrotfeather were virtually identical. Similarly, treated and untreated spinach, wheat heads, and wheat leaves removed methyl bromide (MeBr) from air at the same rates. However, wheat root with attendant adhering soil was rendered inactive by bleach treatment. Parrotfeather roots examined by dissecting microscope and by electron microscope showed no significant damage caused by bleach treatment.  相似文献   
4.
Abstract. The selection of polarity in cells of the cambium in higher plants is a regulated process of development which results in horizontally or vertically oriented cells. A set of mathematical equations suggestive of this developmental dichotomy is given a new biologic interpretation. As a result, a molecular scheme capable of acting as a biochemical switch is suggested. The model features two structural protein monomers whose synthesis is controlled autogenously by feedback repression. The implications of this and similar mechanisms to other differentiating systems is discussed.  相似文献   
5.
6.
Five polymorphic microsatellite loci were characterized for Penaeus (Litopenaeus) vannamei. Loci were isolated using a partial Sau3A1 genomic library by the sequencing of randomly selected clones and by a biotinylated (CT)10 and (GT)10 probes screening procedure. The last strategy resulted in the most useful data. About 40% of the clones showed a previously reported satellite/microsatellite (PVS1), reducing the chance of finding new microsatellite regions. Whereas two of the microsatellite loci with more than 10 alleles will be useful for mating analysis in a breeding program, the others might prove useful for population genetic studies.  相似文献   
7.
8.
We describe a simple, sensitive, and quantitative procedure for measurement of triglycerides and protein contents in formalin-fixed liver sections. The method can detect as little as 0.27 microgram of triglycerides per mg of protein. It is based on selective binding of Sudan IV and Fast Green FCF to fat and total proteins, respectively, and their sequential elution with solvents. Sudan IV is eluted readily with acetone and Fast Green with NaOH-methanol, and the absorbances obtained at 500 and 610 nm can be used to determine the amount of triglycerides and total protein. The color equivalence for Fast Green was obtained after destaining the sections and measuring the protein contents by micro-Kjeldahl analysis. The color equivalence of Sudan IV was estimated by determining the triglyceride content in liver homogenates by an enzymatic procedure and then measuring the amount of dye bound to multiple fixed sections. There was a strong linear correlation between the triglyceride content as determined chemically and that obtained using the equivalence colors (r2 = 0.98). This method is useful to measure fat content in tissue samples and could be applied to evaluate the progression of liver disease.  相似文献   
9.
Understanding people’s knowledge, attitudes, and concerns about genetic biocontrol can help researchers understand the challenges and opportunities that may be encountered during development of these technologies. This study conducted eight focus groups in the United States Great Lakes and Lake Champlain region to assess different stakeholders’ views about genetic biocontrol technology, factors affecting whether or not they support its use, and recommendations on how to proceed with its development. Stakeholders were excited about having a new invasive species control tool, but they were deeply concerned about potential unintended consequences. The primary concerns relate to ecological impacts, along with the cost of development and the possibility that such efforts will distract from other, ongoing control work. Participants made a number of recommendations to genetic biocontrol developers, including setting up regulatory systems, conducting independent cost benefit analyses and risk assessments, and engaging stakeholders throughout the development process.  相似文献   
10.
The XcpR protein of Pseudomonas aeruginosa dimerizes via its N-terminus   总被引:2,自引:1,他引:1  
Extracellular protein secretion by the main terminal branch of the general secretory pathway in Pseudomonas aeruginosa requires a secretion machinery comprising the products of at least 12 genes. One of the components of this machinery, the XcpR protein, belongs to a large family of related proteins distinguished by the presence of a highly conserved nucleotide binding domain (Walker box A). The XcpR protein is essential for the process of extracellular secretion and amino acid substitutions within the Walker A sequence result in inactive XcpR. The same mutations exert a dominant negative effect on protein secretion when expressed in wild-type bacteria. Transdominance of XcpR mutants suggests that this protein is involved in interactions with other components of the secretion machinery or that it functions as a multimer. In this study, the amino-terminal portion of the cI repressor protein of phage λ was used as a reporter of dimerization in Escherichia coli following fusion to full-length as well as a truncated form of XcpR. The cI–XcpR hybrid proteins were able to dimerize, as demonstrated by the immunity of bacteria expressing them to killing by λ phage. The full-length XcpR as well as several deletion mutants of XcpR were able to disrupt the dimerization of the chimeric cI–XcpR protein. The disruption of cI–XcpR dimers using the deletion mutants of XcpR, combined with the analysis of their dominant negative effects on protein secretion, was used to map the minimal dimerization domain of XcpR, which is located within an 85 amino acid region in its N-terminal domain. Taken together, the data presented in this paper suggest that the XcpR protein dimerizes via its N-terminus and that this dimerization is essential for extracellular protein secretion.  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