全文获取类型
收费全文 | 2640篇 |
免费 | 208篇 |
国内免费 | 1篇 |
专业分类
2849篇 |
出版年
2023年 | 6篇 |
2022年 | 27篇 |
2021年 | 46篇 |
2020年 | 24篇 |
2019年 | 33篇 |
2018年 | 45篇 |
2017年 | 34篇 |
2016年 | 58篇 |
2015年 | 152篇 |
2014年 | 120篇 |
2013年 | 217篇 |
2012年 | 216篇 |
2011年 | 219篇 |
2010年 | 121篇 |
2009年 | 117篇 |
2008年 | 149篇 |
2007年 | 201篇 |
2006年 | 159篇 |
2005年 | 140篇 |
2004年 | 127篇 |
2003年 | 123篇 |
2002年 | 115篇 |
2001年 | 23篇 |
2000年 | 14篇 |
1999年 | 30篇 |
1998年 | 39篇 |
1997年 | 26篇 |
1996年 | 23篇 |
1995年 | 16篇 |
1994年 | 25篇 |
1993年 | 20篇 |
1992年 | 6篇 |
1991年 | 13篇 |
1990年 | 10篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 7篇 |
1985年 | 4篇 |
1984年 | 8篇 |
1982年 | 7篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1976年 | 4篇 |
1975年 | 6篇 |
1974年 | 4篇 |
1973年 | 7篇 |
1968年 | 4篇 |
1937年 | 3篇 |
排序方式: 共有2849条查询结果,搜索用时 15 毫秒
71.
72.
Natalia Rusnac Florence Spitzenstetter Patricia Tassi 《Chronobiology international》2016,33(7):937-941
Inclination toward eveningness is often associated with risky behavior. But the existing studies are scarce, inconsistent and usually limited to self-reported measures. We sought to investigate in young adults whether morningness-eveningness is associated with risky behavior in dangerous driving situations, with self-reported drunk driving and with alcohol consumption. Results show that, indeed, inclination toward eveningness is associated with these risky behaviors. We also demonstrate a link between morningness-eveningness and sensation seeking. Therefore, young adults with a tendency toward eveningness might be more at risk to face negative consequences of alcohol abuse or to be involved in a road accident. 相似文献
73.
Background
Except for testicular cancer and Hodgkin’s disease, baseline data on semen quality in case of cancers as well as systemic pathologies of the young adult are scarce or based on low sample size.Methods
Semen quality in patients having testicular cancer (TGCT, n?=?2315), Hodgkin’s disease (HD, n?=?1175), non-Hodgkin’s lymphoma (NHL, n?=?439), leukemia (L, n?=?360), sarcoma (S, n?=?208), brain tumour (BT, n?=?40), Behcet’s disease (Behcet’s, n?=?68) or multiple sclerosis (MS, n?=?73) was studied and compared to that of 1448 fertile men candidates for sperm donation (CSD) and 208 partners of pregnant women (PPW). All samples were studied following the same methodology in a single laboratory. Post freezing and thawing semen characteristics were also studied.Results
The percentage of normozoospermic men was only 37 % for L patients and lower than 60 % for TGCT, NHL, S and BT. The level of sperm production was differently decreased according to pathologies, the median total sperm count in TC and L patients being four times lower (p?<?0.01 when compared to CSD and PPW). The lowest percentage of progressively motile spermatozoa was found for L and BT patients (both, p?<?0.01 compared to CSD and PPW). The percentage of morphologically normal spermatozoa was also reduced in cancer patients, especially in BT patients. Progressive motility after thawing in patients was about half that observed among candidates for sperm donation. In almost half of the semen of patients with testicular cancer or leukemia, the total number of motile spermatozoa per straw was less than 0.5?×?106 compared to 4.3?×?106 in CSD.Conclusions
The present data confirm on large series the deleterious impact of various cancers of the young adult on semen quality, establishing thus baseline data for future studies. Owing to the post-thaw quality of the frozen straws, future fertility projects for the majority of the patients studied (in case there is no post-treatment recovery of spermatogenesis) should necessitate an ICSI to provide the best chance of paternity whatever the fertility check-up in the female partner.74.
75.
Since their first formulations about half a century ago, the soft and hard selection models have become classical frameworks to study selection in subdivided populations. These models differ in the timing of density regulation and represent two extreme types of selection: density- and frequency-dependent selection (soft) and density- and frequency-independent selection (hard). Yet only few attempts have been made so far to model intermediate scenarios. Here, we design a model where migration may happen twice during the life cycle: before density regulation with probability d(J) (juvenile migration) and after density regulation with probability d(A) (adult migration). In the first step, we analyze the conditions for the coexistence of two specialists. We find that coexistence is possible under a large range of selection types, even when environmental heterogeneity is low. Then, we investigate the different possible outcomes obtained through gradual evolution. We show that polymorphism is more likely to evolve when the trade-off is weak, environmental heterogeneity is high, migration is low, and in particular when juvenile migration is low relative to adult migration, because the timing of migration affects the magnitude of frequency-dependent selection relative to gene flow. This model may provide a more general theoretical framework to experimentally study evolution in heterogeneous environments. 相似文献
76.
miR-124-1 is a tumour suppressor microRNA (miR). Epigenetic deregulation of miRs is implicated in carcinogenesis. Promoter DNA methylation and histone modification of miR-124-1 was studied in 5 normal marrow controls, 4 lymphoma, 8 multiple myeloma (MM) cell lines, 230 diagnostic primary samples of acute myeloid leukaemia (AML), acute lymphoblastic leukaemia (ALL), chronic myeloid leukaemia (CML), chronic lymphocytic leukaemia (CLL), MM, and non-Hodgkin's lymphoma (NHL), and 53 MM samples at stable disease or relapse. Promoter of miR-124-1 was unmethylated in normal controls but homozygously methylated in 4 of 4 lymphoma and 4 of 8 myeloma cell lines. Treatment of 5-Aza-2'-deoxycytidine led to miR-124-1 demethylation and re-expression of mature miR-124, which also associated with emergence of euchromatic trimethyl H3K4 and consequent downregulation of CDK6 in myeloma cells harboring homozygous miR-124-1 methylation. In primary samples at diagnosis, miR-124-1 methylation was absent in CML but detected in 2% each of MM at diagnosis and relapse/progression, 5% ALL, 15% AML, 14% CLL and 58.1% of NHL (p<0.001). Amongst lymphoid malignancies, miR-124-1 was preferentially methylated in NHL than MM, CLL or ALL. In primary lymphoma samples, miR-124-1 was preferentially hypermethylated in B- or NK/T-cell lymphomas and associated with reduced miR-124 expression. In conclusion, miR-124-1 was hypermethylated in a tumour-specific manner, with a heterochromatic histone configuration. Hypomethylation led to partial restoration of euchromatic histone code and miR re-expression. Infrequent miR-124-1 methylation detected in diagnostic and relapse MM samples showed an unimportant role in MM pathogenesis, despite frequent methylation found in cell lines. Amongst haematological cancers, miR-124-1 was more frequently hypermethylated in NHL, and hence warrants further study. 相似文献
77.
Alterations in the mitochondrial proteome of neuroblastoma cells in response to complex 1 inhibition
Increasing evidence points to mitochondrial dysfunction in Parkinson's disease (PD) associated with complex I dysfunction, but the exact pathways which lead to cell death have not been resolved. 2D-gel electrophoresis profiles of isolated mitochondria from neuroblastoma cells treated with subcytotoxic concentrations of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a well-characterized complex I inhibitor, were assessed to identify associated targets. Up to 27 differentially expressed proteins were observed, of which 16 were identified using peptide mass fingerprinting. Changes in protein levels were validated by immunoprobing 1D blots, confirming increases in heat shock cognate 71 kDa (Hsc70), 60 kDa heat shock protein (Hsp60), fumarase, glutamate oxaloacetate transaminase 2, ATP synthase subunit d, and voltage-dependent anion-channel 1 (VDAC1). Immunoprobing of 2D blots revealed isoform changes in Hsc70, Hsp60, and VDAC1. Subcytotoxic concentrations of MPTP modulated a host of mitochondrial proteins including chaperones, metabolic enzymes, oxidative phosphorylation-related proteins, an inner mitochondrial protein (mitofilin), and an outer mitochondrial membrane protein (VDAC1). Early changes in chaperones suggest a regulated link between complex 1 inhibition and protein folding. VDAC1, a multifunctional protein, may have a key role in signaling between mitochondria and the rest of the cell prior to cell death. Our work provides new important information of relevance to PD. 相似文献
78.
Medicago truncatula mtpt4 mutants reveal a role for nitrogen in the regulation of arbuscule degeneration in arbuscular mycorrhizal symbiosis 总被引:1,自引:0,他引:1
Javot H Penmetsa RV Breuillin F Bhattarai KK Noar RD Gomez SK Zhang Q Cook DR Harrison MJ 《The Plant journal : for cell and molecular biology》2011,68(6):954-965
Plants acquire essential mineral nutrients such as phosphorus (P) and nitrogen (N) directly from the soil, but the majority of the vascular plants also gain access to these mineral nutrients through endosymbiotic associations with arbuscular mycorrhizal (AM) fungi. In AM symbiosis, the fungi deliver P and N to the root through branched hyphae called arbuscules. Previously we identified MtPT4, a Medicago truncatula phosphate transporter located in the periarbuscular membrane that is essential for symbiotic phosphate transport and for maintenance of the symbiosis. In mtpt4 mutants arbuscule degeneration occurs prematurely and symbiosis fails. Here, we show that premature arbuscule degeneration occurs in mtpt4 mutants even when the fungus has access to carbon from a nurse plant. Thus, carbon limitation is unlikely to be the primary cause of fungal death. Surprisingly, premature arbuscule degeneration is suppressed if mtpt4 mutants are deprived of nitrogen. In mtpt4 mutants with a low N status, arbuscule lifespan does not differ from that of the wild type, colonization of the mtpt4 root system occurs as in the wild type and the fungus completes its life cycle. Sulphur is another essential macronutrient delivered to the plant by the AM fungus; however, suppression of premature arbuscule degeneration does not occur in sulphur-deprived mtpt4 plants. The mtpt4 arbuscule phenotype is strongly correlated with shoot N levels. Analyses of an mtpt4-2 sunn-1 double mutant indicates that SUNN, required for N-mediated autoregulation of nodulation, is not involved. Together, the data reveal an unexpected role for N in the regulation of arbuscule lifespan in AM symbiosis. 相似文献
79.
80.
Jenny Hill Jenna Hoyt Florence Achieng Peter Ouma Anne L’lanziva Simon Kariuki Meghna Desai Jayne Webster 《PloS one》2016,11(3)