全文获取类型
收费全文 | 5222篇 |
免费 | 391篇 |
国内免费 | 255篇 |
专业分类
5868篇 |
出版年
2024年 | 8篇 |
2023年 | 71篇 |
2022年 | 125篇 |
2021年 | 231篇 |
2020年 | 178篇 |
2019年 | 215篇 |
2018年 | 195篇 |
2017年 | 175篇 |
2016年 | 279篇 |
2015年 | 317篇 |
2014年 | 355篇 |
2013年 | 414篇 |
2012年 | 477篇 |
2011年 | 422篇 |
2010年 | 251篇 |
2009年 | 200篇 |
2008年 | 282篇 |
2007年 | 223篇 |
2006年 | 193篇 |
2005年 | 172篇 |
2004年 | 182篇 |
2003年 | 132篇 |
2002年 | 141篇 |
2001年 | 82篇 |
2000年 | 48篇 |
1999年 | 73篇 |
1998年 | 48篇 |
1997年 | 54篇 |
1996年 | 38篇 |
1995年 | 29篇 |
1994年 | 32篇 |
1993年 | 21篇 |
1992年 | 33篇 |
1991年 | 12篇 |
1990年 | 12篇 |
1989年 | 15篇 |
1988年 | 15篇 |
1987年 | 13篇 |
1986年 | 10篇 |
1985年 | 11篇 |
1984年 | 12篇 |
1983年 | 19篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1975年 | 6篇 |
1974年 | 5篇 |
1973年 | 6篇 |
1970年 | 3篇 |
排序方式: 共有5868条查询结果,搜索用时 0 毫秒
121.
Lei Li Den-bang Chen Chao Lin Kang Cao Yang Wan Xin-yu Zhao Chun-lai Nie Zhu Yuan Yu-quan Wei 《Apoptosis : an international journal on programmed cell death》2013,18(4):467-479
PNAS-4, a novel pro-apoptotic gene, was activated during the early response to DNA damage. Previous studies have shown that hPNAS-4 can inhibit tumor growth when over-expressed in ovarian cancer cells. However, the underlying action mechanism remains elusive. In this work, we found that hPNAS-4 expression was significantly increased in SKOV3 cells when exposed to cisplatin, methyl methanesulfonate or mitomycin C, and that its overexpression could induce proliferation inhibition, S phase arrest and apoptosis in A2780s and SKOV3 ovarian cancer cells. The S phase arrest caused by hPNAS-4 was associated with up-regulation of p21. p21 is p53-dispensable and correlates with activation of ERK, and activation of the Cdc25A-Cdk2-Cyclin E/Cyclin A pathway, while the pro-apoptotic effects of hPNAS-4 were mediated by activation of caspase-9 and -3 other than caspase-8, and accompanied by release of AIF, Smac and cytochrome c into the cytosol. Taken together, these data suggest a new mechanism by which hPNAS-4 inhibits proliferation of ovarian cancer cells by inducing S phase arrest and apoptosis via activation of Cdc25A-Cdk2-Cyclin E/Cyclin A axis and mitochondrial dysfunction-mediated caspase-dependent and -independent apoptotic pathways. To our knowledge, we provide the first molecular evidence for the potential application of hPNAS-4 as a novel target in ovarian cancer gene therapy. 相似文献
122.
Virtualization is widely used in cloud computing environments to efficiently manage resources, but it also raises several challenges. One of them is the fairness issue of resource allocation among virtual machines. Traditional virtualized resource allocation approaches distribute physical resources equally without taking into account the actual workload of each virtual machine and thus often leads to wasting. In this paper, we propose a virtualized resource auction and allocation model (VRAA) based on incentive and penalty to correct this wasting problem. In our approach, we use Nash equilibrium of cooperative games to fairly allocate resources among multiple virtual machines to maximize revenue of the system. To illustrate the effectiveness of the proposed approach, we then apply the basic laws of auction gaming to investigate how CPU allocation and contention can affect applications’ performance (i.e., response time), and its effect on CPU utilization. We find that in our VRAA model, the fairness index is high, and the resource allocation is closely proportional to the actual workloads of the virtual machines, so the wasting of resources is reduced. Experiment results show that our model is general, and can be applied to other virtualized non-CPU resources. 相似文献
123.
Jian Wan Jianliang Zhang Li Zhou Yicheng Wang Congfeng Jiang Yongjian Ren Jue Wang 《Cluster computing》2013,16(4):625-638
Taking advantage of distributed storage technology and virtualization technology, cloud storage systems provide virtual machine clients customizable storage service. They can be divided into two types: distributed file system and block level storage system. There are two disadvantages in existing block level storage system: Firstly, Some of them are tightly coupled with their cloud computing environments. As a result, it’s hard to extend them to support other cloud computing platforms; Secondly, The bottleneck of volume server seriously affects the performance and reliability of the whole system. In this paper we present a lightweighted block-level storage system for clouds—ORTHRUS, based on virtualization technology. We first design the architecture with multiple volume servers and its workflows, which can improve system performance and avoid the problem. Secondly, we propose a Listen-Detect-Switch mechanism for ORTHRUS to deal with contingent volume servers’ failure. At last we design a strategy that dynamically balances load between multiple volume servers. We characterize machine capability and load quantity with black box model, and implement the dynamic load balance strategy which is based on genetic algorithm. Extensive experimental results show that the aggregated I/O throughputs of ORTHRUS are significantly improved (approximately two times of that in Orthrus), and both I/O throughputs and IOPS are also remarkably improved (about 1.8 and 1.2 times, respectively) by our dynamic load balance strategy. 相似文献
124.
Effect of the primary host for production of both sexes on the mating interaction in an autoparasitoid species 总被引:2,自引:0,他引:2
Encarsia sophia (Girault and Dodd) is an autoparasitoid in the hymenopteran family Aphelinidae. The females develop as primary parasitoids on whitefly nymphs (primary hosts), whereas the males develop as hyperparasitoids on their own species or on other primary parasitoid species (secondary hosts). The autoparasitoids not only parasitise whiteflies but also kill them with strong host-feeding capacity. In this study, female and male E. sophia were reared on the primary hosts Trialeurodes vaporariorum and Bemisia tabaci ‘Q’, and the host-feeding and parasitism of wasps on both whitefly species were determined for the four possible different mating combinations: (i) E. sophia females reared on B. tabaci (ESF-BT) mated with E. sophia males from B. tabaci (ESM-BT), (ii) E. sophia females reared on T. vaporariorum (ESF-TV) mated with E. sophia males from T. vaporariorum (ESM-TV), (iii) ESF-BT mated with ESM-TV, and (iv) ESF-TV mated with ESM-BT. ESF-TV mated with ESM-TV killed the largest percentage of whitefly nymphs through host feeding. The ESF-TV with larger body size mating with larger ESM-TV killed more whitefly nymphs through host feeding than those mating with smaller ESM-BT. Whether B. tabaci or T. vaporariorum were used as hosts, ESF-TV mated with ESM-TV and ESM-BT and ESF-BT mated with ESM-BT significantly parasitised more whitefly nymphs than ESF-BT mated with ESM-TV. In general, ESF-BT mated with ESM-TV killed significantly fewer whitefly nymphs through parasitism and host feeding than the other three mating combinations on both whitefly species. These results indicated that the performance of autoparasitoids on insect pests was not only dependent on females but was also affected by mating with males from different primary host species. 相似文献
125.
126.
Jun Seo Goo Yo Na Kim Kyung Mi Choi In Sik Hwang Ji Eun Kim Young Ju Lee Moon Hwa Kwak Sun Bo Shim Seung Wan Jee Chul Joo Lim Je Kyung Seong Dae Youn Hwang 《Clinical proteomics》2013,10(1):10
Background
To characterize changes in global protein expression in kidneys of transgenic rats overexpressing human selenoprotein M (SelM) in response to increased bioabivility of selenium (Sel), total proteins extracted from kidneys of 10-week-old CMV/hSelM Tg and wild-type rats were separated by 2-dimensional gel electrophoresis and measured for changes in expression.Results
Ten and three proteins showing high antioxidant enzymatic activity were up- and down-regulated, respectively, in SelM-overexpressing CMV/hSelM Tg rats compared to controls based on an arbitrary 2-fold difference. Up-regulated proteins included LAP3, BAIAP2L1, CRP2, CD73 antigen, PDGF D, KIAA143 homolog, PRPPS-AP2, ZFP313, HSP-60, and N-WASP, whereas down-regulated proteins included ALKDH3, rMCP-3, and STC-1. After Sel treatment, five of the up-regulated proteins were significantly increased in expression in wild-type rats, whereas there were no changes in CMV/hSelM Tg rats. Only two of the down-regulated proteins showed reduced expression in wild-type and Tg rats after Sel treatment.Conclusions
These results show the primary novel biological evidences that new functional protein groups and individual proteins in kidneys of Tg rats relate to Sel biology including the response to Sel treatment and SelM expression. 相似文献127.
Background
Somatic cell nuclear transfer (SCNT) is a promising technique to produce transgenic cloned mammalian, including transgenic goats which may produce Human Lactoferrin (hLF). However, success percentage of SCNT is low, because of gestational and neonatal failure of transgenic embryos. According to the studies on cattle and mice, DNA methylation of some imprinted genes, which plays a vital role in the reprogramming of embryo in NT maybe an underlying mechanism.Methodology/Principal Findings
Fibroblast cells were derived from the ear of a two-month-old goat. The vector expressing hLF was constructed and transfected into fibroblasts. G418 selection, EGFP expression, PCR, and cell cycle distribution were applied sequentially to select transgenic cells clones. After NT and embryo transfer, five transgenic cloned goats were obtained from 240 cloned transgenic embryos. These transgenic goats were identified by 8 microsatellites genotyping and southern blot. Of the five transgenic goats, 3 were lived after birth, while 2 were dead during gestation. We compared differential methylation regions (DMR) pattern of two paternally imprinted genes (H19 and IGF2R) of the ear tissues from the lived transgenic goats, dead transgenic goats, and control goats from natural reproduction. Hyper-methylation pattern appeared in cloned aborted goats, while methylation status was relatively normal in cloned lived goats compared with normal goats.Conclusions/Significance
In this study, we generated five hLF transgenic cloned goats by SCNT. This is the first time the DNA methylation of lived and dead transgenic cloned goats was compared. The results demonstrated that the methylation status of DMRs of H19 and IGF2R were different in lived and dead transgenic goats and therefore this may be potentially used to assess the reprogramming status of transgenic cloned goats. Understanding the pattern of gene imprinting may be useful to improve cloning techniques in future. 相似文献128.
Shengfu Chen Rongpei Wu Yanping Huang Fufu Zheng Yangbin Ou Xiangan Tu Yadong Zhang Yong Gao Xin Chen Tao Zheng Qiyun Yang Zi Wan Yuanyuan Zhang Xiangzhou Sun Guihua Liu Chunhua Deng 《PloS one》2013,8(12)
Background
Insulin resistance (IR) triggers endothelial dysfunction, which contributes to erectile dysfunction (ED) and cardiovascular disease.Aim
To evaluate whether IR was related to ED in young adult patients.Methods
A total of 283 consecutive men complaining of ED at least six months were enrolled, with a full medical history, physical examination, and laboratory tests collected. Quantitative Insulin Sensitivity Check Index (QUICKI) was used to determine IR. The severity of ED was assessed by IIEF-5 questionnaire. Endothelial function was assessed by ultrasonographic examination of brachial artery flow mediated dilation (FMD).Results
IR was detected in 52% patients. Subjects with IR had significant higher total cholesterol, triglycerides, low density lipoprotein-cholesterol (LDL-c), glycated haemoglobin (HBA1c), high sensitivity C-reactive protein (hs-CRP) and body mass index (BMI), but showed significant lower IIEF-5 score, FMD%, high density lipoprotein -cholesterol (HDL-c), testosterone, sex hormone binding globulin (SHBG) levels than patients without IR. Multiple regression analysis showed QUICKI and testosterone were independent predictors of IIEF-5 score. Furthermore, the incidence of IR was correlated with the severity of ED.Conclusions
Compared with other CVFs, IR was found as the most prevalent in our subjects. Besides, IR was independently associated with ED and its severity, suggesting an adverse effect of insulin resistance on erectile function. 相似文献129.
Objective
The aim of this study was to assess the results of troponin I (cTnI) in non- acute Coronary Syndrome (ACS) patients with chronic kidney disease (CKD). We also examined the risk factors for elevated cTnI in non-ACS patients with CKD and whether stage 5 CKD modifies the associations of elevated cTnI and the risk factors in non-ACS patients with CKD.Methods
A retrospective study was performed. Logistic regression models were used.Results
293 non-ACS patients with CKD were included in the current study. 43.34% non-ACS patients with CKD have an elevated cTnI level and 5.12% have an elevated cTnT level in MI range. In CKD patients without ACS and heart failure, only 26.03% (38/146) patients have an elevated cTnT level. In adjusted analyses, age, diastolic blood pressure and congestive heart failure is associated with an elevated cTnI level in non-ACS patients with CKD. Congestive heart failure is associated with an elevated cTnI level in non-ACS patients with CKD (OR 2.30, 95% CI 1.08,4.88, P=0.03). Stage 5 CKD does not modify the association of congestive heart failure and an elevated cTnI level.Conclusion
43.34% non-ACS patients with CKD and 26.03% CKD patients without ACS and congestive heart failure have an elevated cTnI level. Congestive heart failure is associated with an elevated cTnI level in non-ACS patients with CKD. Stage 5 CKD does not modify the association of congestive heart failure and an elevated cTnI level. 相似文献130.
Weiwei Jiao Zhiguang Liu Rui Han Xiuqin Zhao Fang Dong Haiyan Dong Hairong Huang Jianling Tian Qinjing Li Lulu Lian Qingqin Yin Wenqi Song Kanglin Wan A-dong Shen 《PloS one》2013,8(12)