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   9464篇
  免费   999篇
  国内免费   6篇
  2021年   114篇
  2020年   86篇
  2019年   89篇
  2018年   117篇
  2017年   98篇
  2016年   183篇
  2015年   301篇
  2014年   329篇
  2013年   419篇
  2012年   563篇
  2011年   509篇
  2010年   318篇
  2009年   310篇
  2008年   429篇
  2007年   480篇
  2006年   387篇
  2005年   397篇
  2004年   403篇
  2003年   373篇
  2002年   388篇
  2001年   204篇
  2000年   237篇
  1999年   202篇
  1998年   142篇
  1997年   97篇
  1996年   92篇
  1995年   104篇
  1994年   86篇
  1993年   87篇
  1992年   167篇
  1991年   147篇
  1990年   188篇
  1989年   137篇
  1988年   122篇
  1987年   120篇
  1986年   109篇
  1985年   125篇
  1984年   103篇
  1983年   84篇
  1982年   107篇
  1981年   92篇
  1980年   72篇
  1979年   102篇
  1978年   91篇
  1977年   80篇
  1976年   60篇
  1975年   75篇
  1974年   60篇
  1973年   68篇
  1971年   60篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
131.
An attempt has been made to illustrate the quite complicated process of ethnogenesis in South Asia from the viewpoint of physical anthropology. The numerous invading waves which reached the Indian subcontinent from the northwest played an important role in this process. Most important for the ethnogenesis of South Asia was the invasion of Indo-Aryan groups in the middle of the 2nd millenium B.C. known from historical sources. In large parts of the Indo-Pakistan region they assimilated the aboriginal population in ethnic, cultural and linguistic respects in the course of time. Furthermore, the ethnogenesis of the Indian region is determined by the caste system of Hinduism which, however, is not as rigid as generally assumed. There are numerous evidences that since more than 2000 years a slow but steady process of assimilation and integration of tribal groups, living in the forest areas of Central India, into the Hindu caste system took place, a process which is still going on. It is intended to demonstrate to what degree the ethnogenetic processes in South Asia, known from prehistoric and historical sources, can be traced in human skeletal findings of different time periods as well as in the anthropological structure of the living population. Finally, hypotheses and theories, especially those of Risley and von Eickstedt are discussed, who attempted to interpret the great variability of anthropological and morphological traits in the Indian subcontinent by taking into consideration the existence of different old population substrata and their mixing and assimilation.  相似文献   
132.
Molecular cloning of cDNA for rat L-type pyruvate kinase and aldolase B   总被引:13,自引:0,他引:13  
Two double-stranded cDNA recombinant pBR322 plasmid libraries were constructed starting from high carbohydrate diet rat liver poly(A)+ mRNA, either fractionated by denaturing sucrose gradient centrifugation for the cloning of L-type pyruvate kinase cDNA, or nonfractionated for aldolase B. Both libraries were screened with single-stranded cDNA probes reverse transcribed from fasted or high carbohydrate diet rat liver mRNAs. mRNAs from fasted animals were also fractionated by sucrose gradient centrifugation and mRNAs from the fed animals were, in addition, further purified by high performance liquid gel filtration chromatography. Those clones hybridizing with the "positive" probe (from animals fed the high carbohydrate diet) and not with the "negative" one (from fasted animals) were preselected and their plasmid DNA was purified and analyzed by positive hybridization-selection. Thirty of 4500 bacteria colonies transformed by recombinant plasmids were preselected by differential screening for pyruvate kinase, and 8 of 864 colonies for aldolase B. Twenty-two recombinant plasmids for pyruvate kinase and two for aldolase B were shown to contain specific cDNA inserts by positive hybridization-selection. Plasmids DNAs of some pyruvate kinase and aldolase B clones (whose inserts ranged from 700 to 1050 bases in length) were labeled by nick translation and used as probes for Northern blot hybridization. The pyruvate kinase cDNA probes recognized mainly a 3400-base RNA species which was detected in high carbohydrate diet rat liver, but not in fasted rat liver and in tissues which do not synthesize L-type pyruvate kinase. In addition, some pyruvate kinase probes hybridized with minor RNA species of about 2000 bases in length, only observed after carbohydrate diet. For aldolase B, the recombinant plasmid DNA hybridized with a single RNA species of 1750 bases. This RNA, detected in kidney, small intestine and liver, was induced by a high carbohydrate diet and increased with liver development. The rat probe cross-hybridized with human aldolase B messenger RNA.  相似文献   
133.
Abstract: Membranes of the secretory vesicles from bovine adrenal medulla were investigated for the presence of the endogenous protein phosphorylation activity. Seven phosphoprotein bands in the molecular weight range of 250,000 to 30,000 were observed by means of the sodium dodecyl sulphate electrophoresis and autoradiography. On the basis of the criteria of molecular weight, selective stimulation of the phosphorylation by cyclic AMP (as compared with cyclic GMP) and immunoprecipitation by specific antibodies, band 5 (molecular weight 60,300) was found to represent the phosphorylated form of the secretory vesicle-bound tyrosine hydroxylase. The electrophoretic mobility, the stimulatory and inhibitory effects of cyclic AMP in presence of Mg2+ and Zn,2+ respectively, and immunoreactivity toward antibodies showed band 6 to contain two forms of the regulatory subunits of the type II cyclic AMP-dependent protein kinase, distinguishable by their molecular weights (56,000 and 52,000, respectively). Phosphorylation of band 7 (molecular weight 29,800) was stimulated about 2 to 3 times by Ca2+ and calmodulin in the concentration range of both agents believed to occur in the secretory tissues under physiological conditions.  相似文献   
134.
Spontaneous mutants of Saccharomycopsis lipolytica were selected and partially characterized. Several antibiotics and antimetabolites were used for selection of spontaneous resistant mutants from Saccharomycopsis lipolytica. The frequencies of such mutants were mainly arranged between 1 X 10(-7) and 5 X 10(-6) mutants per cell. But one class of glucosamine resistant mutants (GAMRA) occurred more frequently. Among the resistant mutants different types of dominant and recessive resistant mutants could be observed. UV light was used for inactivation of cells and induction of mutants from S. lipolytica. Comparing four haploid strains only small differences were detected in sensitivity to UV light. UV light at a dosage of 135 J/m2 was applied to increase the mutant frequencies in three haploid strains. Besides auxotrophic, temperature sensitive and colony morphology mutants, some new mutant types like small colony forming mutants, red-brown coloured mutants, some new mutant types like small colony forming mutants, red-brown coloured mutants, allylalcohol, glucosamine, 2-deoxyglucose or nystatin resistant mutants, hitherto not described for S. lipolytica, were isolated and partially characterized.  相似文献   
135.
136.
Two methods for the detection of neuraminidase antibodies were compared. The lectin neuraminidase inhibition test (LNI-test) gave results comparable with those provided by the conventional neuraminidase inhibition test (NI-test). Reproducibility and repeatability were better with the LNI-test which used smaller amounts of materials, was less time consuming than the NI-test and was more sensitive.  相似文献   
137.
Genetic identification of an endoproteinase encoded by the adenovirus genome   总被引:13,自引:0,他引:13  
The temperature-sensitive adenovirus type 2 mutant H2ts1 is defective for polypeptide processing at the non-permissive temperature. We have in the present study mapped the mutation by marker rescue and DNA sequencing techniques: the mutation is a C/T transition located at map co-ordinate 61.1. Previous sequencing studies have identified an uninterrupted translational reading frame in this part of the adenovirus genome, encoding a hypothetical 23 X 10(3) Mr polypeptide. The mutation leads to a proline/leucine substitution in the 23 X 10(3) Mr polypeptide.  相似文献   
138.
This report presents the results of an investigation designed to establish whether exposure of mice to ultraviolet radiation (UVR) is capable of influencing the factors that control the distribution of lymphoid cells in vivo. We found that such exposure resulted in a dramatic and long-lasting increase in the tropism of peripheral lymph nodes for circulating lymphoid cells. Termination of UVR exposure did not result in a reversal of this phenomenon. Since an increase in lymphocyte migration into the lymph nodes of UVR-exposed mice was apparent within 2 hr of infusion of the radiolabeled cells, we conclude that the homing assay data reflect a relatively increased binding of circulating lymphocytes to high endothelial venules (HEV) within the lymph nodes of irradiated animals. A histologic analysis of skin from UVR-exposed mice established that the dermal microvasculature had expanded in terms of size and number of vessels, a condition that also does not completely reverse after the termination of treatments. In spite of the increase in dermal microvasculature, very few inflammatory cells were detected in the irradiated skin site. These observations support our conclusion that the enhanced traffic of lymphocytes into peripheral lymph nodes of UVR-exposed mice occurs primarily via lymphocyte-HEV interactions rather than afferent drainage of the irradiated skin.  相似文献   
139.
N Geisler  K Weber 《The EMBO journal》1983,2(11):2059-2063
Determination of 50% of the sequence of the astrocyte-specific intermediate filament (IF) protein documents the hypervariable regions as well as parts of the coiled-coil array of glial fibrillary acidic protein (GFA). The results show that the four non-epithelial IF proteins (myogenic desmin, mesenchymal vimentin, GFA and neurofilament 68 K protein) known to form homopolymers are much more closely related than the epithelial keratins, which seem to form heteropolymers only. Of the four non-epithelial proteins, desmin and vimentin are the most closely related, since GFA has a shorter non-alpha-helical array at the amino terminus. We discuss the possibility that the non-alpha-helical terminal arrays, because of their sequence and length variability, are responsible for differences of distinct IF with respect to physical-chemical properties such as the low ionic strength-induced depolymerization into protofilaments.  相似文献   
140.
We found that stimulation of density-inhibited chicken embryo fibroblasts with serum, epidermal growth factor (EGF), platelet-derived growth factor, (PDGF), or multiplication-stimulating activity (MSA) leads to an increase in tyrosine phosphorylation of proteins in the region of Mr 40,000 (40K) to 42K. The increase in tyrosine phosphorylation after serum or EGF stimulation was transient, reaching a maximum at about 5 min and then declining. By fine-resolution analysis of proteins separated on sodium dodecyl sulfate-polyacrylamide gels, we found that after EGF stimulation, the major increase in phosphotyrosine content was in a 42K Mr protein, with a smaller increase in a 40K Mr protein. The increased phosphorylation in the 40K to 42K Mr region accounted for almost all of the increase in phosphotyrosine observed in these cells. These phosphotyrosine-containing proteins were different from the major phosphotyrosine-containing protein of Rous sarcoma virus-transformed chicken embryo fibroblasts, which migrates at an approximate Mr of 36K. Increased tyrosine phosphorylation of proteins of similar Mr was found in 3T3 cells treated with EGF, but not in NR-6 cells, which lack detectable EGF receptors. It is possible that the 40K to 42K Mr phosphotyrosine-containing proteins are involved in the integration of the biological response to a number of different growth factors.  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