全文获取类型
收费全文 | 7395篇 |
免费 | 727篇 |
国内免费 | 2篇 |
专业分类
8124篇 |
出版年
2021年 | 92篇 |
2018年 | 90篇 |
2017年 | 66篇 |
2016年 | 93篇 |
2015年 | 205篇 |
2014年 | 233篇 |
2013年 | 308篇 |
2012年 | 350篇 |
2011年 | 312篇 |
2010年 | 206篇 |
2009年 | 194篇 |
2008年 | 295篇 |
2007年 | 276篇 |
2006年 | 279篇 |
2005年 | 266篇 |
2004年 | 223篇 |
2003年 | 257篇 |
2002年 | 218篇 |
2001年 | 210篇 |
2000年 | 190篇 |
1999年 | 173篇 |
1998年 | 91篇 |
1997年 | 77篇 |
1996年 | 70篇 |
1995年 | 76篇 |
1994年 | 77篇 |
1993年 | 79篇 |
1992年 | 153篇 |
1991年 | 155篇 |
1990年 | 137篇 |
1989年 | 133篇 |
1988年 | 135篇 |
1987年 | 141篇 |
1986年 | 109篇 |
1985年 | 94篇 |
1984年 | 107篇 |
1983年 | 97篇 |
1981年 | 75篇 |
1980年 | 74篇 |
1979年 | 76篇 |
1978年 | 85篇 |
1977年 | 86篇 |
1975年 | 85篇 |
1974年 | 88篇 |
1973年 | 93篇 |
1972年 | 88篇 |
1971年 | 92篇 |
1970年 | 68篇 |
1969年 | 73篇 |
1968年 | 67篇 |
排序方式: 共有8124条查询结果,搜索用时 0 毫秒
991.
Fuller BT Molleson TI Harris DA Gilmour LT Hedges RE 《American journal of physical anthropology》2006,129(1):45-54
Historical documents indicate that breastfeeding and weaning practices have fluctuated in England through history. In order to obtain evidence for general breastfeeding patterns in Late/Sub-Roman Britain, stable carbon and nitrogen isotope values were measured in juvenile and adult skeletons (n = 87) from the cemetery of Queenford Farm, Dorchester-on-Thames, Oxfordshire. As the site contained few individuals between 0-1.5 years of age, it was not possible to determine the initial timing for the introduction of weaning foods. Between ages 2-4 years, the mean +/- SD delta(13)C results (-20.2 +/- 0.3 per thousand) are significantly more negative (t = -4.03, P < 0.001) compared to adult females (-19.7 +/- 0.3 per thousand). This is interpreted as evidence of a different diet being fed to children during weaning. After age 2, the delta(15)N values gradually decline, indicating complete cessation of breastfeeding by 3-4 years. Among adults, stature (males = 1.68 +/- 0.06 m; females = 1.58 +/- 0.07 m) and sexual dimorphism (106) were low, suggesting that the population was possibly under environmental stress. The delta(13)C results for adults are similar, but females show a small but statistically significantly (t = -2.86, P < 0.01) lower mean delta(15)N value (9.9 +/- 0.9 per thousand) compared to males (10.6 +/- 0.5 per thousand). These lower female delta(15)N values possibly reflect the different physiology of the sexes (pregnancy and/or lactation) or the reduced consumption of animal/fish protein by women, and this may have been influenced by individual preference, family needs, or societal values of the era. 相似文献
992.
Attempts to understand the current distributions of plants and animals require both historical and ecological information. Phylogeography has proved highly effective in elucidating historical events such as postglacial colonisations in north temperate zones. However, interesting questions still await resolution. Lusitanian distributions of fauna and flora in western Europe, for example, have puzzled biogeographers for more than 150 years. Lusitanian species have highly disjunct distributions in Ireland and in Iberia, often with few or no other populations inbetween. Despite much debate, no agreed explanation for Lusitanian distributions has yet emerged. We investigated the phylogeographic structure of one Lusitanian species, the natterjack toad Bufo calamita, using mitochondrial DNA control region sequences and allelic variation at eight microsatellite loci. Our results show that this amphibian must have survived in north European refugia, as well as in Iberia, during and since the last (Weichselian) glacial maximum around 20,000 years before present (BP). Subsequent local recolonisation after the Younger Dryas cooling around 11,000 years BP best explains the Lusitanian aspect of natterjack toad distribution. 相似文献
993.
994.
Vanden Wymelenberg A Minges P Sabat G Martinez D Aerts A Salamov A Grigoriev I Shapiro H Putnam N Belinky P Dosoretz C Gaskell J Kersten P Cullen D 《Fungal genetics and biology : FG & B》2006,43(5):343-356
The white-rot basidiomycete Phanerochaete chrysosporium employs extracellular enzymes to completely degrade the major polymers of wood: cellulose, hemicellulose, and lignin. Analysis of a total of 10,048 v2.1 gene models predicts 769 secreted proteins, a substantial increase over the 268 models identified in the earlier database (v1.0). Within the v2.1 'computational secretome,' 43% showed no significant similarity to known proteins, but were structurally related to other hypothetical protein sequences. In contrast, 53% showed significant similarity to known protein sequences including 87 models assigned to 33 glycoside hydrolase families and 52 sequences distributed among 13 peptidase families. When grown under standard ligninolytic conditions, peptides corresponding to 11 peptidase genes were identified in culture filtrates by mass spectrometry (LS-MS/MS). Five peptidases were members of a large family of aspartyl proteases, many of which were localized to gene clusters. Consistent with a role in dephosphorylation of lignin peroxidase, a mannose-6-phosphatase (M6Pase) was also identified in carbon-starved cultures. Beyond proteases and M6Pase, 28 specific gene products were identified including several representatives of gene families. These included 4 lignin peroxidases, 3 lipases, 2 carboxylesterases, and 8 glycosyl hydrolases. The results underscore the rich genetic diversity and complexity of P. chrysosporium's extracellular enzyme systems. 相似文献
995.
Bunney TD Harris R Gandarillas NL Josephs MB Roe SM Sorli SC Paterson HF Rodrigues-Lima F Esposito D Ponting CP Gierschik P Pearl LH Driscoll PC Katan M 《Molecular cell》2006,21(4):495-507
Ras proteins signal to a number of distinct pathways by interacting with diverse effectors. Studies of ras/effector interactions have focused on three classes, Raf kinases, ral guanylnucleotide-exchange factors, and phosphatidylinositol-3-kinases. Here we describe ras interactions with another effector, the recently identified phospholipase C epsilon (PLCepsilon). We solved structures of PLCepsilon RA domains (RA1 and RA2) by NMR and the structure of the RA2/ras complex by X-ray crystallography. Although the similarity between ubiquitin-like folds of RA1 and RA2 proves that they are homologs, only RA2 can bind ras. Some of the features of the RA2/ras interface are unique to PLCepsilon, while the ability to make contacts with both switch I and II regions of ras is shared only with phosphatidylinositol-3-kinase. Studies of PLCepsilon regulation suggest that, in a cellular context, the RA2 domain, in a mode specific to PLCepsilon, has a role in membrane targeting with further regulatory impact on PLC activity. 相似文献
996.
Recursive SVM feature selection and sample classification for mass-spectrometry and microarray data 总被引:3,自引:0,他引:3
Xuegong Zhang Xin Lu Qian Shi Xiu-qin Xu Hon-chiu E Leung Lyndsay N Harris James D Iglehart Alexander Miron Jun S Liu Wing H Wong 《BMC bioinformatics》2006,7(1):197-13
Background
Like microarray-based investigations, high-throughput proteomics techniques require machine learning algorithms to identify biomarkers that are informative for biological classification problems. Feature selection and classification algorithms need to be robust to noise and outliers in the data. 相似文献997.
998.
Bello CG Harris JM Manthey MK Coggins JR Abell C 《Bioorganic & medicinal chemistry letters》2000,10(5):407-409
Mechanistic differences between type I and type II dehydroquinases have been exploited in the design of type specific inhibitors. (2R)-2-Bromo-3-dehydroquinic acid (3), (2R)-2-fluoro-3-dehydroquinic acid (5) and 2-bromo-3-dehydroshikimic acid (4), all excellent substrates for type II dehydroquinase, are shown to be irreversible inhibitors of type I dehydroquinase. 相似文献
999.
Anderson KM Alrefai WA Dudeja PK Jadko S Bonomi P Hu Y Ou D Harris JE 《Prostaglandins, leukotrienes, and essential fatty acids》2002,66(4):443-452
MK 886, an arachidonic acid-related analog which inhibits the enzyme, 5-lipoxygenase by an indirect mechanism involving the 5-lipoxygenase activating protein, rapidly increased U937 cytosol Ca(2+), much of which localized around the cell nuclei. Five-lipoxygenase activity was not directly involved since the direct redox-dependent 5-LPOx inhibitor, SC-41661A did not increase Ca(2+). U937 cells subsequently undergo classic type 1 programmed cell death. At least initially the ionized calcium originates from internal stores. Coincident with the rise in U937 ionized calcium, MK 886 rapidly increased reactive oxygen species and reduced mitochondrial membrane potential, as judged by several fluorescent probes. The Ca(2+) response of myeloid leukemia-derived HL-60 cells to MK 886 was similar and both cell lines express Bcl-2 protein. Bcl-2-negative Panc-1 and PC-3 cells did not respond to MK 886 with a Ca(2+) signal but did develop oxidative stress and a decline in mitochondrial membrane potential; these events are thought to contribute to the inhibition of cell proliferation and induction of a type 2 PCD. In addition to its marked inhibition of Bcl-2 mRNA synthesis, an interesting hypothesis is that MK 886, serving as a low molecular weight ligand, either by direct or indirect inhibition of U937 Bcl-2 protein function, possibly related to an ion channel activity, alters the distribution of intracellular, possibly nuclear Ca(2+), thereby promoting the development of type 1 programmed cell death. 相似文献
1000.
Miller CP Jirkovsky I Tran BD Harris HA Moran RA Komm BS 《Bioorganic & medicinal chemistry letters》2000,10(2):147-151
A diastereomerically pure series of 7alpha-thioestratrienes was prepared and evaluated for its affinity for both the human estrogen receptor alpha and the more recently discovered estrogen receptor beta. The functional estrogenic activities of the compounds were measured in a MCF-7 ERE-tk-luciferase assay. The activities and selectivities of the compounds were sensitive to the nature of the thioether side chain. 相似文献