全文获取类型
收费全文 | 19210篇 |
免费 | 1455篇 |
国内免费 | 378篇 |
出版年
2023年 | 124篇 |
2022年 | 154篇 |
2021年 | 471篇 |
2020年 | 346篇 |
2019年 | 398篇 |
2018年 | 486篇 |
2017年 | 460篇 |
2016年 | 622篇 |
2015年 | 971篇 |
2014年 | 1037篇 |
2013年 | 1268篇 |
2012年 | 1462篇 |
2011年 | 1309篇 |
2010年 | 845篇 |
2009年 | 696篇 |
2008年 | 966篇 |
2007年 | 998篇 |
2006年 | 888篇 |
2005年 | 822篇 |
2004年 | 759篇 |
2003年 | 745篇 |
2002年 | 655篇 |
2001年 | 410篇 |
2000年 | 375篇 |
1999年 | 361篇 |
1998年 | 199篇 |
1997年 | 152篇 |
1996年 | 123篇 |
1995年 | 116篇 |
1994年 | 140篇 |
1993年 | 113篇 |
1992年 | 208篇 |
1991年 | 194篇 |
1990年 | 181篇 |
1989年 | 184篇 |
1988年 | 155篇 |
1987年 | 126篇 |
1986年 | 137篇 |
1985年 | 125篇 |
1984年 | 95篇 |
1983年 | 94篇 |
1982年 | 74篇 |
1981年 | 61篇 |
1979年 | 70篇 |
1977年 | 87篇 |
1976年 | 66篇 |
1975年 | 56篇 |
1974年 | 77篇 |
1973年 | 52篇 |
1968年 | 49篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
On choice of substrate and habitat in brachionid rotifers 总被引:9,自引:9,他引:0
Information on the distribution of 28 rotifers of the family Brachionidae from diverse waters in south and central Sweden was analyzed to reveal their relationships to substrate and habitat. Some brachionids are preferably planktic, others periphytic and/or benthic. Some non-planktic habitats are utilized more than others, but there is no evidence of a chemical attraction from any substrate. Instead, some substrates seem to be avoided, possibly depending on a poorer flora of periphytic algae. Besides substrate type, the following factors are found to be important for creating separate ecological niches in the brachionid family: temperature, oxygen content, trophic degree, chemical environment, food choice and sensitivity to predation. It is possible to delineate separate ecological niches for all brachionid rotifers, implying that Hutchinson's ideas about the plankton paradox are contradicted. Some species are specialists, other are generalists, the latter being characterized by a great morphological variation. The species are adapted in different ways to their preferential habitats, as regards foot, egg-carrying, protrusions and other lorical structures etc. Longer spines, for instance, are generally found in more transparent water, being a supposed protection against visual predators. 相似文献
72.
D. Matteuzzi Patrizia Brigidi Maddalena Rossi Diana Di 《Letters in applied microbiology》1990,11(4):220-223
The small cryptic plasmid pMB1 (1.9 kb), previously isolated from Bifidobacterium longum, has been characterized by physical mapping. Two cloning vectors, pMR3 and pDG7, carrying chloramphenicol and ampicillin resistances derived from pJH101, have been electroporated in Escherichia coli. 相似文献
73.
Transformation of NIH 3T3 cells by overexpression of the normal coding sequence of the rat neu gene. 总被引:8,自引:2,他引:6 下载免费PDF全文
While the normal human erbB-2 gene is potently transforming when overexpressed in NIH 3T3 cells, its rat homolog, the neu gene, seems to acquire transforming properties only upon alteration of its coding sequence. In this study, we compared the effects of different levels of expression of normal erbB-2 and neu in NIH 3T3 cells. Our results revealed that the normal rat neu gene acts as a potent oncogene when sufficiently overexpressed in NIH 3T3 cells. 相似文献
74.
75.
Summary The biosynthesis of rhodanese was studied in human hepatoma cell lines by immunoblotting and pulselabeling experiments using polyclonal antibodies raised against the bovine liver enzyme. Rhodanese, partially purified from human liver, showed an apparent molecular weight of 33,000 daltons, coincident with that of rhodanese from Hep 3B cells. After pulse labeling of Hep 3B cells both at 37°C and 25°C, rhodanese in the cytosol fraction exhibited the same molecular weight as the enzyme isolated from the particulate fraction containing mitochondria. Moreover, newly synthesized rhodanese from total Hep 3B RNA translation products showed the same electrophoretic mobility as rhodanese from Hep 3B cells. These results suggest that rhodanese, unlike most mitochondrial proteins, is not synthesized as a higher molecular weight precursor. 相似文献
76.
Thermodynamics of local linkage effects. Contracted partition functions and the analysis of site-specific energetics 总被引:5,自引:0,他引:5
E Di Cera 《Biophysical chemistry》1990,37(1-3):147-164
77.
Bruno Schwarzkopf Brigitte Reuke Andreas Kiener Adelbert Bacher 《Archives of microbiology》1990,153(3):259-263
Growing cultures of Methanobacterium thermoautotrophicum were supplemented with [U-14C]adenosine or [1-14C]adenosine. 7,8-Didemethyl-8-hydroxy-5-deazariboflavin (factor F0) and 7-methylpterin were isolated from the culture medium. Hydrolysis of cellular RNA yielded purine and pyrimidine nucleotides. The ribose side chain of proffered adenosine is efficiently incorporated into cellular adenosine and guanosine nucleotide pools but not into pyrimidine nucleotides. Thus, M. thermoautotrophicum can utilize exogenous adenosine by direct phosphorylation without hydrolysis of the glycosidic bond, and AMP can be efficiently converted to GMP. Factor F0 and 7-methylpterin had approximately the same specific activities as the purine nucleotides. It follows that the ribityl side chain of factor F0 is derived from the ribose side chain of a nucleotide precursor by reduction. The pyrazine ring of methanopterin is formed by ring expansion involving the ribose side chain of the precursor, GTP.Abbreviations Factor F0
8-hydroxy-6,7-didemethyl-5-deazariboflavin
- APRT
adenine phosphoribosyltransferase
- GPRT
guanine phosphoribosyltransferase
- PRPP
phosphoribosylpyrophosphate
- HPLC
high performance liquid chromatography 相似文献
78.
C C Bianchi de Di Risio M Gronda A Malka M Barbich M T Santarelli 《Folia haematologica (Leipzig, Germany : 1928)》1990,117(1):141-150
The object of this study was to determine whether the "in vitro" parameters of medullary and blood granulopoiesis in patients with MDS, furnish information of either prognostic or diagnostic value. This study covered 94 patients with MDS. All patients were studied at the onset of disease. In order to identify the factors related to patients' survival, Cox Multiple Regression analysis was performed by the BMD P2L program. When analyzing by means of actuarial curves the survival probability of patients with benign development versus those of malignant development (those who developed ANLL), the significance between both groups was p = 0.0001. Different variables of patients included in this study were analyzed and all showed great significances. Fab: p = 0.0022, disease evolution: p = 0.0001 and presence of blastic aggregates: p = 0.0011. Cox's regression analysis revealed that the only predictable survival variable is the presence of blastic colonies and/or clusters. Accordingly, two groups were constructed: favourable and unfavourable. In the favourable group, 40% of the patients belonged to the RA group, while in the unfavourable group, 55% belonged to the RAEB group. This study shows the validity of the elaboration of prognostic groups in MDS according to the presence of blastic colonies and/or clusters in CFUGM medullary and/or peripheral cultures. The "in vitro" myeloid progenitors culture techniques may therefore be advantageously applied in these disorders for formulating a diagnosis and predicting the patient's short term evolution. 相似文献
79.
Structure and function of hemoglobin in antarctic fishes and evolutionary implications 总被引:2,自引:0,他引:2
Guido di Prisco Rossana D'Avino Laura Camardella Carla Caruso Mario Romano Bruno Rutigliano 《Polar Biology》1990,10(4):269-274
Summary The hematological features of cold-adapted, red-blooded Antarctic teleosts has prompted this study on the relationship between hemoglobin molecular structure and oxygen-binding properties. The hemolysates from 21 species of 5 families contained one component (Hb 1), often accompanied by an additional, minor one (Hb 2, 5%–10% of total). On the other hand, 3 species of Zoarcidae, a non-endemic family, had 4–5 components. All purified hemoglobins from the former group, but only 1–2 of the 4–5 hemoglobins of Zoarcidae, showed a strong Root effect (pH regulation of oxygen binding). Globins from each hemoglobin have been purified and characterised with respect to molecular structure in several species. The similarity between the complete amino acid sequence of one -chain and those of non-Antarctic -chains is lower than that among the latter sequences, suggesting independent pathways of evolution.Presented at the 5th SCAR Symposium on Antarctic Biology, Hobart, Australia (August 29th-September 3rd, 1988) 相似文献
80.
Bruno W. S. Sobral Rhonda J. Honeycutt Alan G. Atherly Michael McClelland 《Plant Molecular Biology Reporter》1990,8(4):253-275
TheOryza sativa (rice) genome is small (600 to 900 megabase pairs) when compared to that of other monocotyledonous plants. Rice was the first
of the major cereals to be successfully transformed and regenerated. An RFLP map with approximately 300 markers is readily
available, and the DNA content per map unit is only two to three times that ofArabidopsis thaliana. Rice is also the main staple food for the majority of peoples in the world. We developed techniques for the preparation
of intact genomic DNA from Indica and Japonica subspecies of rice, used statistical methods to determine which restriction
endonucleases are rare-cutting, and used pulsed-field gel electrophoresis (PFE) to separate large fragments of rice DNA. Southern
hybridization to blotted rice PFE gels was used to show that the digests were complete. The long-term goal of our work is
to generate an integrated genetic/physical map for the rice genome, as well as helping to establish rice as a model for map-based
gene cloning and genome analysis. 相似文献