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   3502篇
  免费   196篇
  国内免费   7篇
  3705篇
  2022年   21篇
  2021年   35篇
  2020年   25篇
  2019年   35篇
  2018年   55篇
  2017年   35篇
  2016年   61篇
  2015年   107篇
  2014年   115篇
  2013年   199篇
  2012年   173篇
  2011年   234篇
  2010年   137篇
  2009年   154篇
  2008年   212篇
  2007年   241篇
  2006年   226篇
  2005年   239篇
  2004年   234篇
  2003年   222篇
  2002年   222篇
  2001年   35篇
  2000年   36篇
  1999年   41篇
  1998年   57篇
  1997年   30篇
  1996年   40篇
  1995年   41篇
  1994年   34篇
  1993年   47篇
  1992年   34篇
  1991年   25篇
  1990年   36篇
  1989年   25篇
  1988年   25篇
  1987年   19篇
  1986年   25篇
  1985年   15篇
  1984年   16篇
  1983年   20篇
  1982年   14篇
  1981年   16篇
  1980年   12篇
  1979年   7篇
  1978年   9篇
  1977年   7篇
  1975年   6篇
  1974年   5篇
  1968年   7篇
  1965年   5篇
排序方式: 共有3705条查询结果,搜索用时 9 毫秒
81.
Larvae of bigmouth manefish Caristius macropus are described and illustrated on the basis of seven specimens (4.2–10.5 mm in body length) from the Kuroshio waters (0–60 m depth) and the transition waters (surface) between the Kuroshio and Oyashio fronts of the western North Pacific. The present larvae of C. macropus are distinguished from those of Paracaristius maderensis that inhabit the North Pacific by having 39–40 myomeres, 34 dorsal-fin rays, and 22 anal-fin rays. The present study, along with previous studies of the early life stages of caristiids, shows that larvae of the family may be defined by the following characters: body elongate in preflexion stage but becoming deep bodied and hatchet shaped after notochord flexion; anus located near vertical through base of pectoral fin; head large, without spination or serration; a distinct vertical band on the posterior tail throughout the larval stages, and two bands gradually appearing on the tail and trunk during the flexion and postflexion stages; and melanophores present around the notochord tip by the flexion stage. Adult C. macropus are found in the subarctic and temperate waters of the North Pacific; however, the present study and other occurrences of early life stages of the species probably indicate that C. macropus may spawn over a wide area in the North Pacific.  相似文献   
82.
Vascular endothelial growth factor (VEGF) and its receptors are highly expressed in Kaposi's sarcoma (KS) lesion and play a key role in angiogenesis. Latency-associated nuclear antigen (LANA) of Kaposi's sarcoma-associated herpesvirus (KSHV/HHV8) has multiple functions related to viral latency and KSHV-induced oncogenesis. In this report, we have identified Daxx as a LANA-binding protein by co-immunoprecipitation analysis of HeLa cells stably expressing LANA. LANA associated with Daxx in a PEL cell line infected with KSHV. LANA and Daxx also bound in vitro, suggesting direct interaction. From the results of binding assays, a region containing the Glu/Asp-rich domain within LANA, and a central region including the second paired amphipathic helix within Daxx contributed to the interaction. To address the physiological significance of this interaction, we focused on a Daxx-mediated VEGF receptor gene regulation. We found that Daxx repressed Ets-1-dependent Flt-1/VEGF receptor-1 gene expression, and that LANA inhibited the repression by Daxx in a reporter assay. Analyses of flow cytometry and real-time PCR revealed that expression of VEGF receptor-1 and -2 in LANA-expressing human umbilical vein endothelial cells (HUVECs) significantly increased. Co-immunoprecipitation and immunoblotting experiments suggested that LANA-bound Daxx to inhibit the interaction between Daxx and Ets-1. Chromatin immunoprecipitation assays showed that Daxx associated with VEGF receptor-1 promoter in HUVECs, and that LANA expression reduced this association. These results suggested that LANA contributes to a high expression of VEGF receptors in KS lesion by interfering with the interaction between Daxx and Ets-1.  相似文献   
83.
Calcium plays a fundamental role as second messenger in intracellular signaling and bone serves as the body's calcium reserve to tightly maintain blood calcium levels. Calcium in ingested meal is the main supply and inadequate calcium intake causes osteoporosis and bone fracture. Here, we describe a novel mechanism of how ingested calcium is deposited on bone. Meal ingestion elicits secretion of the gut hormone gastric inhibitory polypeptide (GIP) from endocrine K cells in the duodenum. Bone histomorphometrical analyses revealed that bone formation parameters in the mice lacking GIP receptor (GIPR(-/-)) were significantly lower than those of wild-type (GIPR(+/+)) mice, and that the number of osteoclasts, especially multinuclear osteoclasts, was significantly increased in GIPR(-/-) mice, indicating that GIPR(-/-) mice have high-turnover osteoporosis. In vitro examination showed the percentage of osteoblastic cells undergoing apoptosis to be significantly decreased in the presence of GIP. Because GIPR(-/-) mice exhibited an increased plasma calcium concentration after meal ingestion, GIP directly links calcium contained in meal to calcium deposition on bone.  相似文献   
84.
Using an interspecies microinsemination assay with bovine oocytes, it was examined whether centrosomes of Antarctic minke whale spermatozoa function as the microtubule-organizing centre (MTOC). Bull and rat spermatozoa were used as positive and negative controls, respectively. Vitrified-warmed bovine mature oocytes were subjected to immunostaining against alpha-tubulin 4-6 h after intracytoplasmic injection (ICSI) of 5 mM dithiothreitol-treated spermatozoa. Aster formation occurred from whale spermatozoa (33%) and bull spermatozoa (33%), but very little from rat spermatozoa (3%). Activation treatment for the microinseminated oocytes with 7% ethanol + 2 mM 6-dimethylaminopurine resulted in a similar proportion of oocytes forming a whale sperm aster (35% vs 27% in the non-treated group; 4 h after ICSI) but a significantly larger aster (ratio of aster diameter to oocyte diameter, 0.57 vs 0.30 in the non-treated group). These results indicate that the centrosome introduced into bovine oocytes by whale spermatozoa contributes to the MTOC and that assembly of the microtubule network is promoted by oocyte activation.  相似文献   
85.
Summary Effect of pH of culture media on intracellular accumulation of poly-(hydroxybutyrate) (PHB) by a non-sulfur photosynthetic bacterium, Rhodobacter sphaeroides strain RV was studied in pH-stat cultures. Sub-optimal pH for growth, 8.0 or 8.5 gave the higher content of PHB rather than optimal pH 7.5 for growth. These results show that growth and PHB accumulation of the bacteria can be controlled by pH of culture media.  相似文献   
86.
Lactobacillus delbrueckii was grown on sugarcane molasses, sugarcane juice and sugar beet juice in batch fermentation at pH 6 and at 40°C. After 72 h, the lactic acid from 13% (w/v) sugarcane molasses (119 g total sugar l−1) and sugarcane juice (133 g total sugar l−1) was 107 g l−1 and 120 g l−1, respectively. With 10% (w/v) sugar beet juice (105 g total sugar l−1), 84 g lactic acid l−1 was produced. The optical purities of d-lactic acid from the feedstocks ranged from 97.2 to 98.3%.  相似文献   
87.
The effect of a high-pressure carbonation treatment on the change in quality of sake during storage was investigated. Measurements of the amino acidity and isovaleraldehyde content of carbonated sake (20 MPa pressure at 40, 45 and 50 degrees C for 7, 21 and 33 min, respectively) as well as of heat-treated sake (reaching temperature of 65 degrees C and immediately cooled) were almost unchanged during storage at 3 and 20 degrees C. Glucose in the sake subjected to these treatments was retained at an almost constant under the same storage conditions, except for the sake carbonated at 40 degrees C and stored at 20 degrees C. In contrast, the amino acidity, and glucose and isovaleraldehyde contents of non-pasteurized (fresh) sake increased during storage at both temperatures. The sake samples subjected to the carbonation treatment and heat treatment both gave better sensory scores than the fresh sake sample after 6 month of storage at 3 and 20 degrees C, especially at 3 degrees C for the flavor. These results suggest that the high-pressure carbonation treatment is an effective new technique for preserving the quality of sake.  相似文献   
88.
Streptomyces sp. No. B-1625, which was identified as a strain of Streptomyces antibioticus, is a typical producer of actinomycin, but also produces minor acidic antibiotic components (FA), besides actinomycins X2, D and X. The FA-components, which were obtained with a high-producing mutant, 11M-21, showed antibacterial and antitumor activities, and also similar visible and UV absorption spectra to those characteristic of actinomycin. The FA-components were separated into five components, FA1 FA, FA, FA and FA, on TLC. Among them, one component, FA, isolated in a purified state as an orange powder, has a composition of C, 52.97: H, 6.34: N, 10.48%, and is active against B. subtilis at a MIC of 5mcg/ml. The FA component showed pKa′ of 5.4 and 12.0 and λmax at 443, 427 and 233 nm. From these properties, FA is considered to be an acidic actinomycin congener.  相似文献   
89.
The processes and mechanisms underlying the diversification of host–microbe endosymbiotic associations are of evolutionary interest. Here we investigated the bacteriocyte-associated primary symbionts of weevils wherein the ancient symbiont Nardonella has experienced two independent replacement events: once by Curculioniphilus symbiont in the lineage of Curculio and allied weevils of the tribe Curculionini, and once by Sodalis-allied symbiont in the lineage of grain weevils of the genus Sitophilus. The Curculioniphilus symbiont was detected from 27 of 36 Curculionini species examined, the symbiont phylogeny was congruent with the host weevil phylogeny, and the symbiont gene sequences exhibited AT-biased nucleotide compositions and accelerated molecular evolution. These results suggest that the Curculioniphilus symbiont was acquired by an ancestor of the tribe Curculionini, replaced the original symbiont Nardonella, and has co-speciated with the host weevils over evolutionary time, but has been occasionally lost in several host lineages. By contrast, the Sodalis-allied symbiont of Sitophilus weevils exhibited no host–symbiont co-speciation, no AT-biased nucleotide compositions and only moderately accelerated molecular evolution. These results suggest that the Sodalis-allied symbiont was certainly acquired by an ancestor of the Sitophilus weevils and replaced the original Nardonella symbiont, but the symbiotic association must have experienced occasional re-associations such as new acquisitions, horizontal transfers, replacements and/or losses. We detected Sodalis-allied facultative symbionts in populations of the Curculionini weevils, which might represent potential evolutionary sources of the Sodalis-allied primary symbionts. Comparison of these newcomer bacteriocyte-associated symbiont lineages highlights potential evolutionary trajectories and consequences of novel symbionts after independent replacements of the same ancient symbiont.  相似文献   
90.
The effects of solute and hydrochloric acid concentrations on optical rotation were studied using 20 naturally occuring amino acids.

There appeared to be no common factor among the amino acids as far as the inclination of optical rotation was concerned. Lutz-Jirgenson’s rule could be applied to few amino acids in the cationic form. Therefore, in the determination of the optical rotation, the concentration of the solute, nature of solvent and temperature must be rigorously controlled. The optical conditions of measurement and the specific rotation of 20 amino acids were recommended based on this work.  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