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   29606篇
  免费   2398篇
  国内免费   2019篇
  2024年   13篇
  2023年   381篇
  2022年   427篇
  2021年   1647篇
  2020年   1034篇
  2019年   1340篇
  2018年   1279篇
  2017年   928篇
  2016年   1271篇
  2015年   1856篇
  2014年   2208篇
  2013年   2473篇
  2012年   2766篇
  2011年   2458篇
  2010年   1480篇
  2009年   1276篇
  2008年   1504篇
  2007年   1312篇
  2006年   1152篇
  2005年   941篇
  2004年   791篇
  2003年   664篇
  2002年   594篇
  2001年   535篇
  2000年   470篇
  1999年   483篇
  1998年   269篇
  1997年   290篇
  1996年   284篇
  1995年   281篇
  1994年   253篇
  1993年   179篇
  1992年   276篇
  1991年   185篇
  1990年   151篇
  1989年   151篇
  1988年   92篇
  1987年   85篇
  1986年   60篇
  1985年   69篇
  1984年   29篇
  1983年   32篇
  1982年   18篇
  1981年   15篇
  1980年   12篇
  1979年   9篇
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
991.
In the present investigation, we used directed evolution approach to engineer a lipase from metagenomic origin. A variant S311C, was generated, characterized in detail and compared with wild type. Wild type and variant lipases were overexpressed and purified to homogeneity. The temperature optima of the purified lipases (Variant and wild type) were almost same, and found to be 45 and 50 °C, respectively. The variant protein was highly thermostable (54 times) as compared with the wild type at 60 °C. The variant displayed very high kinetic efficiency over the wild type protein. Analysis of the homology models of wild type and variant lipase showed that the substitution is on the surface of the protein. This substitution, along with hydrophobic residues in near vicinity may be involved in formation of strong hydrophobic channel leading to active site. This study identifies the role of hydrophobic interactions in protein stability along with enhancement of enzyme activity.  相似文献   
992.
The wheat aleurone is formed from surface endosperm cells, and its developmental status reflects its biogenesis, structural characteristics, and physiological functions. In this report, wheat caryopses at different development stages were embedded in Spurr’s low-viscosity embedding medium for observation of the development of aleurone cells (ACs) by light microscopy, scanning electron microscopy, and fluorescence microscopy, respectively. According to their structures and physiological characterization, the ACs development process was divided into five stages: endosperm cellulization, spherosome formation, aleurone grain formation, filling material proliferation, and maturation. Furthermore, ACs in different parts of the caryopsis formed differently. ACs near the vascular bundle developed earlier and formed transfer cells, but other ACs formed slowly and did not form transfer cells. ACs on the caryopsis backside were a regular square shape; however, ACs in the caryopsis abdomen were mainly irregular. There were also differences in development between wheat varieties. ACs were rectangular in hard wheat but square in soft wheat. ACs were larger and showed a greater degree of filling in hard compared to soft wheat. The storage materials in ACs were different compared to inner endosperm cells (IECs). The concentrations of minerals such as sodium, magnesium, silicon, phosphorus and potassium were higher in ACs than in IECs. ACs contained many aleurone grains and spherosomes, which store lipids and mineral nutrients, respectively. The cell nucleus did not disappear and the cells were still alive during aleurone maturation. However, IECs were dead and mainly contained amyloplast and protein bodies, which store starch and protein, respectively. Overall, the above results characterized major structural features of aleurone and revealed that the wheat aleurone has mainly four functions.  相似文献   
993.
One 1.2 kbp long sequence was cloned by using PCR with primers that were designed from cDNA sequence of CsH1 gene (Genbank: EU716314) from tea plant (Camellia sinensis). According to the 1.2 kbp sequence, a 0.6 kbp sequence was isolated from tea plant genomic DNA using DNA Walking Method. Sequence analysis revealed that the 1.2 kbp sequence is a CsH1 gene consisting of 1 exon and 2 introns, the border of exton and intron sequences conforming to the GT–AG rule, and the 0.6 kbp sequence was found to be the promoter of CsH1 gene which contains basic promoter elements, TATA-box and CAAT-box. Abscisic acid responsiveness cis-acting element, elictor-responsive element, GA response element, light response cis-acting element and TC-rich repeats were also represented. To further study the activity of this promoter, the sequence was used to drive a GUS fusion gene in Agrobacterium-mediated transformation of tea plant somatic embryos, leaf discs and calli of tobacco (Nicotiana tabacum L.) where a high level of GUS expression was both observed in the tobacco calli and tea plant somatic embryos. These results suggest that the CsH1 gene promoter isolated is capable of conferring nuclear gene expression.  相似文献   
994.
Association between angiotensinogen (AGT) M235T gene polymorphism and end-stage renal disease (ESRD) risk is still controversial. This meta-analysis was performed to evaluate the association of AGT M235T gene polymorphism with ESRD susceptibility. A predefined literature search and selection of eligible relevant studies were performed to collect data from electronic databases of PubMed, Embase and Cochrane Library. Sixteen literatures were identified for the analysis of association of AGT M235T gene polymorphism with ESRD risk. T allele and TT genotype were associated with ESRD susceptibility in Caucasians (T: OR = 1.13, 95 % CI: 1.02–1.25, P = 0.02; TT: OR = 1.22, 95 % CI: 1.03–1.45, P = 0.02). However, MM genotype might not play a protective role against ESRD risk in Caucasians. Furthermore, there was no a markedly positive association between AGT M235T gene polymorphism and ESRD susceptibility in overall populations, Asians and Africans. In conclusion, T allele or TT homozygote is associated with the onset of ESRD in Caucasians. However, more studies should be performed in the future.  相似文献   
995.
Cutaneous mucormycosis is a rare opportunistic infection caused by zygomycetes that can be rapidly fatal if unrecognized. We describe the clinical, histopathological, fungal and molecular features of a case of gangrenous cutaneous mucormycosis. The patient presented with great necrosis on his right forearm at the site of detained intravenous cannula needle. He had type II diabetes and chronic renal insufficiency. KOH mount of black eschar showed many broad, aseptate fungal hyphae with right-angle branching. PAS staining of the tissue sample revealed similar broad hyphae in the dermis and cutis. Fungal culture and ITS sequence analysis identified this fungus as Rhizopus oryzae. As no organ involvement was detected, the patient was diagnosed with primary cutaneous mucormycosis. Considering the poor state of the patient, complete excision of the infectious tissue was performed without skin graft instead of amputation. At the same time, intravenous liposomal amphotericin B was given, starting from a small dosage and increased to a total dosage amount of 5.45 g. The wound recovered well with granulation. We emphasize that early recognition and prompt therapy including the control of the primary diseases were important. In this article, we also reviewed the features of primary cutaneous mucormycosis reported in China over the last 20 years.  相似文献   
996.
997.
Monocyte differentiation antigen CD14 is considered an important cell-activating mediator of inflammatory responses that may result in atherosclerosis, coronary heart disease (CHD), thrombus formation, and myocardial infarction (MI). A common C-260T polymorphism in the promoter of the CD14 gene, the trans-membrane receptor of lipopolysaccharides, has been inconsistently associated with CHD. To investigate this inconsistency, we performed a meta-analysis of 28 studies involving a total of 13,335 CHD cases and 7,979 controls for C-260T of the CD14 gene to evaluate the effect of CD14 on genetic susceptibility for CHD. An overall random effects odds ratio of 1.24 (95 % CI: 1.12–1.36, P < 10?5) was found for T allele. Significant results were also observed using dominant (OR = 1.34, 95 % CI: 1.17–1.54, P < 10?4) or recessive genetic model (OR = 1.25, 95 % CI: 1.10–1.41, P = 0.0004). There was strong evidence of heterogeneity (P < 10?5), which largely disappeared after stratification by ethnicity. After stratified by ethnicity, significant results were found in East Asians; whereas no significant associations were found among Caucasians and other ethnic populations in all genetic models. In the stratified analysis according to sample size, CHD endpoints, and HWE status, significantly increased risks for the polymorphism were found in all genetic models. In conclusion, our results indicate that the CD14 C-260T polymorphism is a risk factor of CHD, especially in East Asians. However, additional very large-scale studies are warranted to confirm our results.  相似文献   
998.
Protein phosphatases type 2C (PP2Cs) from group A, which includes the ABI1/HAB1 and PP2CA branches, are key negative regulators of ABA signaling. HAI-1 gene had been shown to affect both seed and vegetative responses to ABA, which is one of PP2Cs clade A in Arabidopsis thaliana. Transgenic plants containing pHAI-1::GUS (β-glucuronidase) displayed GUS activity existing in the vascular system of leave veins, stems and petioles. Green fluorescent protein fused HAI-1 (HAI-1-GFP) was found in the nucleus through transient transformation assays with onion epidermal cells. The water-loss assays indicated the loss-of-function mutants did not show symptoms of wilting and they had still turgid green rosette leaves. The assays of seed germination by exogenous ABA and NaCl manifested that the loss-of-function mutants displayed higher insensitivity than wild-type plants. Taken together, the final results suggest that the HAI-1 (AT5G59220) encoded a nuclear protein and it can be highly induced by ABA and wound in Arabidposis, the stress-tolerance phenotype showed a slightly improvement when HAI-1 gene was disrupted.  相似文献   
999.
A 62-year-old immunocompetent rural woman who represents an isolated cryptococcal skull infection without systematic involvement is described. Diagnosis was based on positive India ink staining, positive histopathologic examination, and positive culture. Species identification was performed by growth on Sabouraud dextrose agar and CHROMagar medium and by sequencing of the intergenic and internal transcribed spacer regions of the rRNA genes. This case describes a rare presentation of Cryptococcus neoformans infection in a human immunodeficiency virus-negative patient. The lesions were significantly improved with treatment of daily oral itraconazole 400 mg. A maintenance therapy with a low-dose itraconazole was prescribed to warrant a clinical and mycological eradication. A two-year follow-up did not show any recurrence of infection.  相似文献   
1000.
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