全文获取类型
收费全文 | 953篇 |
免费 | 119篇 |
国内免费 | 57篇 |
出版年
2024年 | 1篇 |
2023年 | 20篇 |
2022年 | 19篇 |
2021年 | 42篇 |
2020年 | 28篇 |
2019年 | 40篇 |
2018年 | 40篇 |
2017年 | 37篇 |
2016年 | 41篇 |
2015年 | 31篇 |
2014年 | 65篇 |
2013年 | 78篇 |
2012年 | 36篇 |
2011年 | 48篇 |
2010年 | 32篇 |
2009年 | 41篇 |
2008年 | 46篇 |
2007年 | 56篇 |
2006年 | 43篇 |
2005年 | 44篇 |
2004年 | 31篇 |
2003年 | 30篇 |
2002年 | 24篇 |
2001年 | 19篇 |
2000年 | 18篇 |
1999年 | 18篇 |
1998年 | 17篇 |
1997年 | 22篇 |
1996年 | 19篇 |
1995年 | 15篇 |
1994年 | 12篇 |
1993年 | 19篇 |
1992年 | 16篇 |
1991年 | 11篇 |
1990年 | 7篇 |
1989年 | 9篇 |
1988年 | 11篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1985年 | 12篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1979年 | 2篇 |
1978年 | 2篇 |
排序方式: 共有1129条查询结果,搜索用时 15 毫秒
991.
The CH(2)Cl(2) extract of dried leaves of Helianthus annuus L. cv. Peredovick(R) has yielded, in addition to the known sesquiterpene lactones annuolide E and leptocarpin, and the sesquiterpenes heliannuols A, C, D, F, G, H, I, the new bisnorsesquiterpene, annuionone E, and the new sesquiterpenes heliannuol L, helibisabonol A and helibisabonol B. Structural elucidation was based on extensive spectral (one and two-dimensional NMR experiments) and theoretical studies. The sesquiterpenes heliannuol A and helibisabonol A and the sesquiterpene lactone leptocarpin inhibited the growth of etiolated wheat coleoptiles. 相似文献
992.
Cholesterol-fed ovariectomized monkeys are good animal models for human atherosclerosis of postmenopausal women 总被引:1,自引:0,他引:1
Torii R Shiomi M Ito T Yamada S Eguchi Y Ikeda N 《Primates; journal of primatology》2003,44(3):247-252
Although it is well known that the incidence of atherosclerosis is markedly increased in postmenopausal women, antiatherosclerotic
effects of estrogen replacement therapies are not clear. One of the reasons for this is due to the lack of appropriate animal
models for atherosclerosis of postmenopausal women. Therefore, we attempted to develop an animal model for atherosclerosis
of postmenopausal women and examined the antiatherosclerotic effects of estrogen replacement therapy. Adult ovariectomized
Japanese monkeys were fed 2% cholesterol diet alone (C-group) or in combination with conjugated estrogen (CE-group) for 30 months.
The serum estradiol-17β levels of the CE-group were varied between 10 and 204.5 ng/dl during treatment. In the C-group, the
serum total cholesterol levels were increased from 110 to 270 mg/dl, and atheroma was first observed after 3-months treatment
with angioscopy. In the CE-group, the levels of the serum total cholesterol during treatment were 30% lower than those of
the C-group, and the aortic lesions were first observed after 12-months treatment with angioscopy. The aortic intimal thickness
of the CE-group was 58% of the C-group. This finding showed good agreement with the angioscopic observation. The aortic lesions
were of a fibromuscular type in both groups. In conclusion, a cholesterol-fed ovariectomized monkey is an appropriate animal
model for atherosclerosis of postmenopausal women. Furthermore, angiofiberscopic and histopathological observations suggested
that estrogen replacement therapy was valid for atherosclerosis of postmenopausal women. 相似文献
993.
Smyth DR Mrozkiewicz MK McGrath WJ Listwan P Kobe B 《Protein science : a publication of the Protein Society》2003,12(7):1313-1322
The fusion of a protein of interest to a large-affinity tag, such as the maltose-binding protein (MBP), thioredoxin (TRX), or glutathione-S-transferase (GST), can be advantageous in terms of increased expression, enhanced solubility, protection from proteolysis, improved folding, and protein purification via affinity chromatography. Unfortunately, crystal growth is hindered by the conformational heterogeneity induced by the fusion tag, requiring that the tag is removed by a potentially problematic cleavage step. The first three crystal structures of fusion proteins with large-affinity tags have been reported recently. All three structures used a novel strategy to rigidly fuse the protein of interest to MBP via a short three- to five-amino acid spacer. This strategy has the potential to aid structure determination of proteins that present particular experimental challenges and are not conducive to more conventional crystallization strategies (e.g., membrane proteins). Structural genomics initiatives may also benefit from this approach as a way to crystallize problematic proteins of significant interest. 相似文献
994.
Significantly different patterns of amino acid replacement after gene duplication as compared to after speciation 总被引:8,自引:0,他引:8
We have performed a large-scale analysis of amino acid sequence evolution after gene duplication by comparing evolution after gene duplication with evolution after speciation in over 1,800 phylogenetic trees constructed from manually curated alignments of protein domains downloaded from the PFAM database. The site-specific rate of evolution is significantly altered by gene duplication. A significant increase in the proportion of amino acid substitutions at constrained (slowly evolving) sites after duplication was observed. An increase in the proportion of replacements at normally constrained amino acid sites could result from relaxation of purifying selective pressure. However, the proportion of amino acid replacements involving radical changes in amino acid properties after duplication does not appear to be significantly increased by relaxed selective pressure. The increased proportion of replacements at constrained sites was observed over a relatively large range of protein change (up to 25% amino acid replacements per site). These findings have implications for our understanding of the nature of evolution after duplication and may help to shed light on the evolution of novel protein functions through gene duplication. 相似文献
995.
Gene therapy and the production of mutated cell lines or model animals both require the development of efficient, controlled gene-targeting strategies. Classical approaches are based on the ability of cells to use homologous recombination to integrate exogenous DNA into their own genome. The low frequency of homologous recombination in mammalian cells leads to inefficient targeting. Here, we review the limiting steps of classical approaches and the new strategies developed to improve the efficiency of homologous recombination in gene-targeting experiments. 相似文献
996.
Barrón-González MP Villarreal-Treviño L Verduzco-Martínez JA Mata-Cárdenas BD Morales-Vallarta MR 《Experimental parasitology》2005,110(3):318-321
The current media for axenic cultivation of Entamoeba histolytica and Entamoeba invadens are supplemented with bovine or equine serum, which provides several essential nutrients to amoebas. Serum has also been considered an essential component in encystation media for E. invadens. A substitute of serum, PACSR has been described as an alternative for growth of E. histolytica and also maintains growth of E. invadens. When PACSR was used instead of serum for encystation of E. invadens the efficiency was the same as for serum. Our present data show that PACSR can support the growth and induction of encystation of E. invadens strain IP-1. 相似文献
997.
Forsblad d'Elia H Christgau S Mattsson LA Saxne T Ohlsson C Nordborg E Carlsten H 《Arthritis research & therapy》2004,6(5):R457-R468
This study aimed to evaluate the effects of hormone replacement therapy (HRT), known to prevent osteoporosis and fractures, on markers of bone and cartilage metabolism. Furthermore, we assessed whether changes in these markers corresponded to alterations in bone mineral density and radiographic joint destructions in postmenopausal women with rheumatoid arthritis. Eighty-eight women were randomized to receive HRT, calcium, and vitamin D3, or calcium and vitamin D3 alone, for 2 years. Bone turnover was studied by analyzing serum levels of C-terminal telopeptide fragments of type I collagen (CTX-I), C-terminal telopeptide of type I collagen (ICTP), bone sialoprotein, and C-terminal propeptide of type I procollagen (PICP) and cartilage turnover by urinary levels of collagen type II C-telopeptide degradation fragments (CTX-II) and cartilage oligomeric matrix protein (COMP) in serum. Treatment with HRT resulted in decrease in CTX-I (P < 0.001), ICTP (P < 0.001), PICP (P < 0.05), COMP (P < 0.01), and CTX-II (P < 0.05) at 2 years. Reductions in CTX-I, ICTP, and PICP were associated with improved bone mineral density. Of the markers tested, CTX-I reflected bone turnover most sensitively; it was reduced by 53 +/- 6% in the patients receiving HRT. Baseline ICTP (P < 0.001), CTX-II (P < 0.01), and COMP (P < 0.05) correlated with the Larsen score. We suggest that biochemical markers of bone and cartilage turnover may provide a useful tool for assessing novel treatment modalities in arthritis, concerning both joint protection and prevention of osteoporosis. 相似文献
998.
Sulfation of tibolone and tibolone metabolites by expressed human cytosolic sulfotransferases 总被引:2,自引:0,他引:2
Falany JL Macrina N Falany CN 《The Journal of steroid biochemistry and molecular biology》2004,88(4-5):383-391
Tibolone is an important therapeutic agent used in the treatment of menopausal symptoms in many countries and has beneficial effects on menopausal and postmenopausal vasomotor, bone, vaginal and mood symptoms without affecting the endometrial, breast or cardiovascular systems. The rapid metabolism of tibolone to active metabolites including 3-OH-tibolone, 3β-OH-tibolone and Δ4-tibolone may be important in its tissue-specific effects. Sulfation also has a major role in the metabolism and regulation of the tissue-specific activity of tibolone and its metabolites. The ability of seven major expressed human sulfotransferase (SULT) isoforms to sulfate tibolone and its three metabolites was examined. Expressed human SULT2A1 was capable of sulfating tibolone and all three metabolites with the highest affinity for 3-OH-tibolone. SULT1E1 conjugated both 3-OH-tibolone metabolites and tibolone itself slightly. SULT2B1b sulfated both 3-OH metabolites but not tibolone or Δ4-tibolone. SULT isoforms 1A1, 1A3, 1B1 and 1C1 did not demonstrate detectable activity. Sulfation of tibolone and its metabolites by human tissue cytosols was analyzed to determine whether the pattern of tibolone sulfation corresponded to the known expression of SULT isoforms in each tissue. The tissue-specific effects of tibolone may be regulated in part by the inactivation of tibolone and its metabolites by specific human SULT isoforms. 相似文献
999.
In a riverine forest along the Menanggul River, which is a tributary of the Kinabatangan River, Sabah, Malaysia, I observed an all-male group of proboscis monkey (Nasalis larvatus) consisting of 27–30 (mean: 28.8) individuals. This large size of the all-male group seems to be attributed to habitat fragmentation because of the expansion of oil palm plantations. A few females joined this all-male group. Sub-adult females copulated with subadult or large juvenile males. Since the mean male tenure period of this monkey was estimated to be longer than female maturity, and prematured females might leave their natal one-male groups to avoid inbreeding and temporarily participate in the all-male group where males were permissive to them. Even when females joined this group, no conflicts occurred among males. 相似文献
1000.
Ant colonies are commonly thought to have a stable and simple family structure, with one or a few egg-laying queens and their worker daughters. However, recent genetic studies reveal that the identity of breeding queens can vary over time within colonies. In several species, some queens are apparently specialized to enter established colonies instead of initiating a new colony on their own. The previously overlooked occurrence of queen turnover within colonies has important consequences not only on the genetic structure and nature of kin conflict within colonies, but also on the evolution of social parasitism. 相似文献