全文获取类型
收费全文 | 16534篇 |
免费 | 1184篇 |
国内免费 | 2篇 |
专业分类
17720篇 |
出版年
2023年 | 122篇 |
2022年 | 232篇 |
2021年 | 466篇 |
2020年 | 315篇 |
2019年 | 375篇 |
2018年 | 478篇 |
2017年 | 421篇 |
2016年 | 672篇 |
2015年 | 996篇 |
2014年 | 1088篇 |
2013年 | 1339篇 |
2012年 | 1605篇 |
2011年 | 1429篇 |
2010年 | 880篇 |
2009年 | 778篇 |
2008年 | 938篇 |
2007年 | 910篇 |
2006年 | 845篇 |
2005年 | 700篇 |
2004年 | 672篇 |
2003年 | 594篇 |
2002年 | 480篇 |
2001年 | 145篇 |
2000年 | 104篇 |
1999年 | 132篇 |
1998年 | 108篇 |
1997年 | 106篇 |
1996年 | 82篇 |
1995年 | 62篇 |
1994年 | 69篇 |
1993年 | 52篇 |
1992年 | 74篇 |
1991年 | 50篇 |
1990年 | 46篇 |
1989年 | 36篇 |
1988年 | 34篇 |
1987年 | 35篇 |
1986年 | 28篇 |
1985年 | 25篇 |
1984年 | 26篇 |
1983年 | 26篇 |
1982年 | 9篇 |
1981年 | 13篇 |
1980年 | 14篇 |
1979年 | 11篇 |
1978年 | 14篇 |
1977年 | 16篇 |
1976年 | 9篇 |
1975年 | 9篇 |
1972年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
991.
Juan?A.?FargalloEmail author José?A.?Dávila Jaime?Potti Ana?de?León Vicente?Polo 《Polar Biology》2004,27(6):339-343
Variation in the sex ratio at hatching in the chinstrap penguin Pygoscelis antarctica was investigated, using molecular sexing to test predictions of sex allocation theory. The sex ratio was slightly male-biased (0.54) but did not differ significantly from parity. The proportion of males increased with nest size, an estimator of parental quality in chinstrap penguins. High-quality parents were able to produce and rear a higher proportion of male offspring, the more costly sex in this slightly sexually dimorphic species. Our results may be in agreement with Trivers and Willards (1973) argument on biases in the offspring sex ratio being contingent on parental condition or quality. 相似文献
992.
Eirín-López JM González-Tizón AM Martínez A Méndez J 《Molecular biology and evolution》2004,21(10):1992-2003
Histones are small basic nuclear proteins with critical structural and functional roles in eukaryotic genomes. The H1 multigene family constitutes a very interesting histone class gathering the greatest number of isoforms, with many different arrangements in the genome, including clustered and solitary genes, and showing replication-dependent (RD) or replication-independent (RI) expression patterns. The evolution of H1 histones has been classically explained by concerted evolution through a rapid process of interlocus recombination or gene conversion. Given such intriguing features, we have analyzed the long-term evolutionary pattern of the H1 multigene family through the evaluation of the relative importance of gene conversion, point mutation, and selection in generating and maintaining the different H1 subtypes. We have found the presence of an extensive silent nucleotide divergence, both within and between species, which is always significantly greater than the nonsilent variation, indicating that purifying selection is the major factor maintaining H1 protein homogeneity. The results obtained from phylogenetic analysis reveal that different H1 subtypes are no more closely related within than between species, as they cluster by type in the topologies, and that both RD and RI H1 variants follow the same evolutionary pattern. These findings suggest that H1 histones have not been subject to any significant effect of interlocus recombination or concerted evolution. However, the diversification of the H1 isoforms seems to be enhanced primarily by mutation and selection, where genes are subject to birth-and-death evolution with strong purifying selection at the protein level. This model is able to explain not only the generation and diversification of RD H1 isoforms but also the origin and long-term persistence of orphon RI H1 subtypes in the genome, something that is still unclear, assuming concerted evolution. 相似文献
993.
Chacon F Cano P Jimenez V Cardinali DP Marcos A Esquifino AI 《Chronobiology international》2004,21(3):393-404
This work analyzes the effect of calorie restriction on the 24 h variation of pituitary-testicular function in young male Wistar rats by measuring the circulating levels of prolactin, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and testosterone. Control animals were provided an equilibrium calorie diet and the experimental animals a calorie-restriction diet equivalent to 66% of food restriction for four weeks starting on day 35 of life. Different groups of control and experimental rats were killed at 6 h intervals around the clock, beginning 1 h after light on (HALO). Compared to the control animals, the mean secretion of prolactin was augmented and that of LH and testosterone decreased in calorie-restricted rats, whereas FSH release remained unchanged. Significant changes in the 24 h secretory pattern of circulating prolactin, LH, and testosterone occurred in the calorie-restricted rats. These include the appearance of a second maximum of plasma prolactin at 21 HALO, blunting of the LH peak seen at 13 HALO, and phase-shift of the testosterone peak from 13 HALO in controls to 17 HALO in calorie-restricted rats. The significant positive correlation between individual LH and testosterone levels found in controls was no longer observed in calorie-restricted rats. Availability of nutrients presumably affects the mechanisms that modulate the circadian variation of the pituitary-gonadal axis in growing male rats. 相似文献
994.
In vivo ozone exposure induces antioxidant/stress-related responses in murine lung and skin 总被引:2,自引:0,他引:2
Valacchi G Pagnin E Corbacho AM Olano E Davis PA Packer L Cross CE 《Free radical biology & medicine》2004,36(5):673-681
Lung and skin are the organs directly exposed to environmental pollution. Ozone (O(3)) is a toxic, oxidant air pollutant, and exposure has been shown to induce antioxidant depletion as well as oxidation of lipids and proteins within the outermost skin layer (stratum corneum) and the lung respiratory tract lining fluids (RTLFs). To further define skin and lung responses to O(3) exposure, SKH-1 hairless mice were exposed to either 0.8 ppm of O(3) (a level occasionally reached in very polluted areas) or ambient air 6 h/day for 6 consecutive days. O(3) exposure resulted in the depletion of alpha-tocopherol in lung and plasma and induction in both skin and lung of heme oxygenase 1, cyclooxygenase 2, and proliferating cell nuclear antigen. O(3)-exposed animals showed a similar extent of upregulation of COX-2 and PCNA in lung and skin, whereas HO-1 was more responsive in skin than in lung (7-fold induction vs. 2-fold induction). In addition to these measures of response to oxidative stress, O(3) exposure led to the activation of nuclear factor kappaB measured as IkappaBalpha phosphorylation in both tissues. We conclude that in this model, O(3) at high pollutant levels is able to affect both lung and skin biology, inducing depletion of alpha-tocopherol and inducing stress-related responses in both skin epidermis and respiratory tract epithelium. 相似文献
995.
Filadelfi AM Ramanzini GC Visconti MA Castrucci AM 《Pigment cell research / sponsored by the European Society for Pigment Cell Research and the International Pigment Cell Society》2004,17(5):480-487
Endothelins (ETs) and sarafotoxins (SRTXs) have been reported to exert ET(B)-mediated effects on vertebrate pigment cells. GEM-81 cell line, a red pigment cell-derived cutaneous tumor of the teleost Carassius auratus, expresses ET(B) receptors and can be differentiated with 1.5% DMSO treatment, thus constituting an useful model to investigate ET and SRTX effects on cultured fish pigment cells. Our aim was to characterize the pharmacology and biological effects mediated by ET receptors in DMSO-differentiated and undifferentiated cells. ET subtype receptors and their respective Ki values in both cell types were determined by competitive binding assays using (125)I ET-1 and BQ-485 (an ET(A) antagonist) or BQ-788 (an ET(B) antagonist). BQ-788, but not BQ-485, significantly reduced (125)I-ET-1 binding in both cell types, with similar low (Ki > nM) affinities. To determine the proliferation effects of ETs/SRTXs, cells were treated for 72 h with the hormones, and counted in a hemocytometer. The proliferation assays were repeated for SRTX S6c in the presence or absence of BQ-788. The results demonstrated that, with the exception of ET-1 (biphasic effect) and ET-3 (no significant effect) in undifferentiated GEM-81 cells, all the tested hormones induced increases in the proliferation of both types of cells. The hormones were equipotent in DMSO-differentiated cells, which exhibited increased sensitivity to ETs, but not to SRTXs, as compared with undifferentiated cells. The BQ-788 antagonistic effect was also exerted on the proliferation responses to SRTX S6c. These results corroborate the long and important evolutionary history of the ET/SRTX receptor system in vertebrate pigment cells. 相似文献
996.
Microbial activity in spent nuclear fuel pools which contain ultrapure and radioactive water has been previously observed. The aim of the present research was to isolate and identify the microorganisms attached to the nuclear pool wall of a Spanish nuclear power plant. Amplification of 16S rDNA fragments from the culturable microorganisms by PCR using universal primers for the domain 'Bacteria', followed by Denaturing Gradient Gel Electrophoresis analysis revealed the presence of six different bacteria. The complete gene for 16S rDNA of each one was sequenced and identified as belonging to three different phylogenetic groups, viz. beta-Proteobacteria, Actinomycetales and the Bacillus/Staphylococcus group. A fungus was also found and identified as Aspergillus fumigatus by sequencing the D2 region of the large subunit rDNA gene. The isolation of these microorganisms in oligotrophic and radioactive conditions is of great interest due to the possibility of their use in bioremediation processes of radionuclide-contaminated environments. 相似文献
997.
Balagué CE de Ruiz CS Rey R de Duffard AM Nader-Macías ME 《FEMS immunology and medical microbiology》2004,42(3):313-319
Interfering Escherichia coli attachment to the urinary tract, using P-fimbriation inhibitors, can prevent pyelonephritis. Clofibric and ethacrynic acids are organic compounds structurally related, but with different pharmacological uses. These agents are potentially active in the urinary tract due to its elimination in an unaltered form by the renal route. This study described a pyelonephritogenic E. coli strain, grown in the presence of sub-inhibitory concentrations of clofibric or ethacrynic acids (0.1 and 1 mM, respectively), which exhibits inhibition of P1 erythrocytes agglutination and a drastic decrease in fimbriation, using electron microscopy and quantitative analyses of superficial proteins (decrease to a 17-25% in comparison with the control). In vivo assays were performed using ascending urinary tract infection in mice. The treatment with therapeutic doses of the drugs, administered 2 days before the bacterial challenge and daily until the end of the experiment (22 days), abolished renal infection after 7-10 days of drug exposure. Within this period clofibric acid did not produce adverse effects on the renal parenchyma. However, ethacrynic acid caused pyelitis and tubular cellular desquamation. These results suggested that clofibric acid might be useful in the short-term prophylaxis of urinary tract infection. 相似文献
998.
Ramirez Apan AA Pérez-Castorena AL de Vivar AR 《Zeitschrift für Naturforschung. C, Journal of biosciences》2004,59(3-4):237-243
A new phytochemical study of Mortonia greggii (Celastraceae) afforded four friedelan derivatives (1-4), three lupanes (5-7), retusine (8), two esterified polyhydroxyagarofurans (9-10), mortonin C (11) and photomortonin C (12). The anti-inflammatory activity on carrageenan and 12-O-tetradecanoylphorbol-13-acetate induced models of inflammation, as well as the ability to inhibit the nitric oxide (NO) produced by lipopolysaccharide-stimulated mouse peritoneal macrophages were evaluated for the main metabolites. Our results showed that the friedelan dehydrocanophyllic acid methyl ester (1) exhibits an anti-inflammatory effect which could be related to an inhibition of prostaglandin and NO production. The activity of lupeol (5), 29-hydroxylupeol (6) and 29-hydroxylupenone (7) might be involved with the prostanoid synthesis. The presence of the hydroxy groups in 6 appears to be important for activity. The edema inhibition capacity of retusine (8) could be related to a reduction of the prostaglandin production. The agarofuran derivative 10 is an NO inhibitor whose activity is probably not involved in the synthesis of prostaglandins. 相似文献
999.
Silva J Carvalho AS Vitorino R Domingues P Teixeira P Gibbs P 《Protein and peptide letters》2004,11(2):189-194
Protein extraction and the proteome of Lactobacillus delbrueckii subsp. bulgaricus were studied using different stains. The reversible silver staining technique was shown to be more sensitive than the irreversible silver stain. Coomassie colloidal was demonstrated to be as sensitive as reversible silver stain; however, the Coomassie colloidal blue solution developed a higher background and for sample preparation was more time-consuming. 相似文献
1000.