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   859198篇
  免费   92614篇
  国内免费   500篇
  952312篇
  2018年   7732篇
  2016年   10231篇
  2015年   13379篇
  2014年   15912篇
  2013年   23491篇
  2012年   25746篇
  2011年   26438篇
  2010年   17854篇
  2009年   16517篇
  2008年   23290篇
  2007年   24342篇
  2006年   22899篇
  2005年   21885篇
  2004年   21806篇
  2003年   20992篇
  2002年   20455篇
  2001年   39743篇
  2000年   39767篇
  1999年   31419篇
  1998年   10542篇
  1997年   11265篇
  1996年   10495篇
  1995年   9773篇
  1994年   9519篇
  1993年   9606篇
  1992年   25970篇
  1991年   25127篇
  1990年   24625篇
  1989年   24050篇
  1988年   22343篇
  1987年   21023篇
  1986年   19511篇
  1985年   19768篇
  1984年   16282篇
  1983年   13949篇
  1982年   10549篇
  1981年   9488篇
  1980年   8942篇
  1979年   15507篇
  1978年   11996篇
  1977年   11196篇
  1976年   10393篇
  1975年   11401篇
  1974年   12333篇
  1973年   12080篇
  1972年   11150篇
  1971年   10170篇
  1970年   8749篇
  1969年   8559篇
  1968年   7968篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
The serum amyloid A (SAA) superfamily comprises a number of differentially expressed genes with a high degree of homology in mammalian species. SAA4, an apolipoprotein constitutively expressed only in humans and mice, is associated almost entirely with lipoproteins of the high-density range. The presence of SAA4 mRNA and protein in macrophage-derived foam cells of coronary and carotid arteries suggested a specific role of human SAA4 during inflammation including atherosclerosis. Here we underline the importance of ribosome binding site (rbs)-like sequences (also known as Shine-Dalgarno sequences) in the SAA4 cDNA for expression of recombinant SAA4 protein in Escherichia coli. In contrast to rbs sequences coded by the expression vectors, rbs-like sequences in the cDNA of target protein(s) are known to interfere with protein translation via binding to the small 16S ribosome subunit, yielding low or even no expression. Here we show that PCR mutations of two rbs-like sequences in the human SAA4 cDNA promote expression of considerable amounts of recombinant SAA4 in E.coli.  相似文献   
52.
53.
54.
Gymnocharacinus bergi (Pisces, Characidae), an endemic fish in the headwaters of the Valcheta stream in northern Patagonia, displays a fright reaction. As this species inhabits a rapid stream, we hypothesized that the alarm reaction might be socially transmitted through visual information, because the detection of alarm pheromone would be restricted to those fish located downstream from the releaser. Alarm substances elicited a decrease in the swimming activity and a tendency to remain at the bottom of the aquarium. Individuals that observed the fright reaction, but were not exposed to the pheromone, spent significantly more time at the bottom of the aquarium imitating the alarm response, and tended to approach the others. Gregarious behaviour was observed; however, interactions (number of approaches) were stronger between those fish held prior to testing in the same aquarium; smaller fish in particular tended to get closer to bigger ones. Greater group cohesion was found between individuals that were exposed to the alarm pheromone.  相似文献   
55.
To cope with changes in the environment, organisms not only show behavioural but also phenotypic adjustments. This is well established for the digestive tract. Here we present a first case of birds adjusting their flight machinery in response to predation risk. In an indoor experiment, ruddy turnstones Arenaria interpres were subjected to an unpredictable daily appearance of either a raptor or a small gull (as a control). Ruddy turnstones experiencing threat induced by a flying raptor model, longer than after similar passage by the gull model, refrained from feeding after this disturbance. Pectoral muscle mass, but not lean mass, responded in a course of a few days to changes in the perceived threat of predation. Pectoral muscle mass increased after raptor scares. Taking the small increases in body mass into account, pectoral muscle mass was 3.6% higher than aerodynamically predicted for constant flight performance. This demonstrates that perceived risk factors may directly affect organ size.  相似文献   
56.
57.
The aim of this paper is to test with genomic in situ hybridization the genomic affinities between maize and its putative progenitor Zea mays subsp. parviglumis. Blocking procedures were applied for the purpose of improving discrimination among chromosome regions. Unlabeled genomic DNA from Z. mays subsp. parviglumis as a blocking agent and labeled genomic DNA from maize were hybridized on maize chromosomes. On the other hand, mitotic metaphases from Z. mays subsp. parviglumis were blocked with unlabeled genomic DNA of maize and hybridized with labeled genomic DNA from Z. mays subsp. parviglumis. Both experiments showed that either maize or Z. mays subsp. parviglumis chromosomes have their own unique sequences. This means an unexpected degree of divergence if Z. mays subsp. parviglumis is the only progenitor of maize, a result that is discussed in relation to our previous genomic in situ hybridization observations and to the different scenarios proposed about the origin of maize.  相似文献   
58.
The members of the family Syngnathidae stand out among the bony fish for their specific parental care: the male bear embryos in a particular brood chamber or pouch. The physiological role of the latter is discussed in the literature in detail. The pouch is rather multifunctional; its osmoregulatory function is the most well studied. The unusual parental care supposes that some specific features should exist in the reproduction biology and development in these fish. This idea is supported by detailed studies of the ovaries and oogenesis in pipefish. On the other hand, the structure of the testes and specific features of spermatogenesis in the representatives of the family Syngnathidae remain poorly studied. The interest in the reproduction and development of pipefish is also determined, to a certain degree, by the fact that in the countries of Southeast Asia seahorses are objects of fishing and are used as a source of pharmacological materials comprising active compounds. Attempts to cultivate these fish have also been undertaken in these countries [13].  相似文献   
59.
60.
Microelement preparations obtained in the course of processing of flint powder stimulate the biological activity of Bifidobacterium adolescentis 94 BIM, grown on complex and synthetic nutritive media. The composition of the microelement preparations differed in the content of cations and anions. Introduction of the preparations into the cultures of physiologically active or anabiotic forms of bifidobacteria changed the parameters of exponential growth: compared to controls, the cultures were characterized by increased specific growth rate and decreased generation time. In the presence of microelements, the development of populations of bifidobacteria was associated with more pronounced accumulation of metabolic products (acetate, lactate, and ethanol). Introduction of microelement preparations increased the rate of synthesis of the extracellular proteinase (maximum content of the enzyme was observed after 3 h, whereas control cultures attained this level only after 6 h).  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