全文获取类型
收费全文 | 4325篇 |
免费 | 438篇 |
国内免费 | 3篇 |
专业分类
4766篇 |
出版年
2022年 | 36篇 |
2021年 | 68篇 |
2020年 | 41篇 |
2019年 | 49篇 |
2018年 | 43篇 |
2017年 | 65篇 |
2016年 | 88篇 |
2015年 | 143篇 |
2014年 | 148篇 |
2013年 | 194篇 |
2012年 | 237篇 |
2011年 | 236篇 |
2010年 | 157篇 |
2009年 | 143篇 |
2008年 | 181篇 |
2007年 | 192篇 |
2006年 | 178篇 |
2005年 | 178篇 |
2004年 | 176篇 |
2003年 | 134篇 |
2002年 | 137篇 |
2001年 | 148篇 |
2000年 | 150篇 |
1999年 | 102篇 |
1998年 | 71篇 |
1997年 | 63篇 |
1996年 | 42篇 |
1995年 | 50篇 |
1994年 | 36篇 |
1993年 | 38篇 |
1992年 | 77篇 |
1991年 | 45篇 |
1990年 | 81篇 |
1989年 | 75篇 |
1988年 | 62篇 |
1987年 | 47篇 |
1986年 | 50篇 |
1985年 | 64篇 |
1984年 | 38篇 |
1983年 | 32篇 |
1980年 | 31篇 |
1979年 | 54篇 |
1978年 | 44篇 |
1977年 | 48篇 |
1976年 | 32篇 |
1975年 | 30篇 |
1974年 | 47篇 |
1973年 | 35篇 |
1972年 | 40篇 |
1969年 | 29篇 |
排序方式: 共有4766条查询结果,搜索用时 0 毫秒
101.
K. -H. Becker 《Helgoland Marine Research》1975,27(1):96-100
Several stages of the copepodBasipodella harpacticola n. gen., n. sp., which lives as a parasite on harpacticoid copepods, are described on the basis of four individuals obtained from the Peruvian deep-sea. The taxonomic position of the new species is discussed. 相似文献
102.
The assembly of lipoprotein(a) (Lp(a)) particles occurs via a two-step mechanism in which noncovalent interactions between apolipoprotein(a) (apo(a)) and the apolipoproteinB-100 component of low density lipoprotein precede the formation of a single disulfide bond. Although we have previously demonstrated that the rate constant for the covalent step of Lp(a) assembly can be enhanced by altering the conformational status of apo(a), the resultant rates of covalent Lp(a) particle formation measured in vitro are relatively slow. The large excess of Lp(a) (over apo(a)) observed in vivo can be accounted for by a preferential clearance of apo(a) over Lp(a) and/or a sufficiently high rate of covalent Lp(a) assembly. In the present study, we report that cultured human hepatoma cells secrete an oxidase activity that dramatically enhances the rate of covalent Lp(a) assembly. This activity is likely possessed by a protein because it is heat-sensitive and is retained in the concentrate following ultrafiltration through a 5 kDa cutoff filter. However, a small molecule cofactor for the activity is suggested by the observation that the activity is lost upon dialysis. Plots of Lp(a) assembly rate versus input apo(a) concentration gave rectangular hyperbolae; the reaction displayed an unusual dependence on the concentration of apoB-100, with increasing concentrations of apoB-100 resulting in slower rates of Lp(a) assembly at low concentrations of apo(a), an effect that was alleviated by higher apo(a) concentrations. Interestingly, V(max(app))/K(m(app)) ratios were insensitive to apoB-100 concentration, which is diagnostic of a ping-pong reaction mechanism. In this way, the putative Lp(a) oxidase may be functionally analogous to protein disulfide isomerase, which exhibits a similar mechanism during the catalysis of disulfide bond formation during protein folding, although we have ruled out a role for this enzyme in Lp(a) assembly. 相似文献
103.
Kraiczy P Hellwage J Skerka C Becker H Kirschfink M Simon MM Brade V Zipfel PF Wallich R 《The Journal of biological chemistry》2004,279(4):2421-2429
The etiologic agent of Lyme disease, Borrelia burgdorferi, is capable of circumventing the immune defense of a variety of potential vertebrate hosts. Previous work has shown that interaction of host-derived complement regulators, factor H and factor H-like protein 1 (FHL-1), with up to five complement regulator-acquiring surface proteins (CRASPs) expressed by resistant B. burgdorferi sensu lato isolates conferred complement resistance. In addition expression of CRASP-1 is directly correlated with complement resistance of Borrelia species. This work describes the functional characterization of BbCRASP-1 as the dominant factor H and FHL-1-binding protein of B. burgdorferi. The corresponding gene, zs7.a68, is located on the linear plasmid lp54 and is different from factor H-binding Erp proteins that are encoded by genes localized on circular plasmids (cp32). Deletion mutants of BbCRASP-1 were generated, and a high affinity binding site for factor H and FHL-1 was mapped to the C terminus of BbCRASP-1. Similarly, the predominant binding site of factor H and FHL-1 was localized to the short consensus repeat 7. Factor H and FHL-1 maintain their cofactor activity for factor I-mediated C3b inactivation when bound to BbCRASP-1, and factor H is up to 6-fold more efficient in mediating C3b conversion than FHL-1. In conclusion, BbCRASP-1 (i). binds the host complement regulators factor H and FHL-1 with high affinity, (ii). is the key molecule of the complement resistance of spirochetes, and (iii). is distinct from the Erp protein family. Thus, BbCRASP-1 most likely contributes to persistence of B. burgdorferi and to pathogenesis of Lyme disease. 相似文献
104.
Golden bananas in the field: elevated fruit pro‐vitamin A from the expression of a single banana transgene 下载免费PDF全文
Jean‐Yves Paul Harjeet Khanna Jennifer Kleidon Phuong Hoang Jason Geijskes Jeff Daniells Ella Zaplin Yvonne Rosenberg Anthony James Bulukani Mlalazi Pradeep Deo Geofrey Arinaitwe Priver Namanya Douglas Becker James Tindamanyire Wilberforce Tushemereirwe Robert Harding James Dale 《Plant biotechnology journal》2017,15(4):520-532
Vitamin A deficiency remains one of the world's major public health problems despite food fortification and supplements strategies. Biofortification of staple crops with enhanced levels of pro‐vitamin A (PVA) offers a sustainable alternative strategy to both food fortification and supplementation. As a proof of concept, PVA‐biofortified transgenic Cavendish bananas were generated and field trialed in Australia with the aim of achieving a target level of 20 μg/g of dry weight (dw) β‐carotene equivalent (β‐CE) in the fruit. Expression of a Fe'i banana‐derived phytoene synthase 2a (MtPsy2a) gene resulted in the generation of lines with PVA levels exceeding the target level with one line reaching 55 μg/g dw β‐CE . Expression of the maize phytoene synthase 1 (ZmPsy1) gene, used to develop ‘Golden Rice 2’, also resulted in increased fruit PVA levels although many lines displayed undesirable phenotypes. Constitutive expression of either transgene with the maize polyubiquitin promoter increased PVA accumulation from the earliest stage of fruit development. In contrast, PVA accumulation was restricted to the late stages of fruit development when either the banana 1‐aminocyclopropane‐1‐carboxylate oxidase or the expansin 1 promoters were used to drive the same transgenes. Wild‐type plants with the longest fruit development time had also the highest fruit PVA concentrations. The results from this study suggest that early activation of the rate‐limiting enzyme in the carotenoid biosynthetic pathway and extended fruit maturation time are essential factors to achieve optimal PVA concentrations in banana fruit. 相似文献
105.
KLOTHO allele status and the risk of early-onset occult coronary artery disease 总被引:10,自引:0,他引:10 下载免费PDF全文
Arking DE Becker DM Yanek LR Fallin D Judge DP Moy TF Becker LC Dietz HC 《American journal of human genetics》2003,72(5):1154-1161
We previously identified a functional variant of KLOTHO (termed "KL-VS"), which harbors two amino acid substitutions in complete linkage disequilibrium and is associated with reduced human longevity when in homozygosity. Klotho-deficient mice display extensive arteriosclerosis when fed a normal diet, suggesting a potent genetic predisposition. To determine whether klotho influences atherosclerotic risk in humans, we performed cross-sectional studies to assess the association between the KL-VS allele and occult coronary artery disease (CAD) in two independent samples of apparently healthy siblings of individuals with early-onset (age <60 years) CAD (SIBS-I [N=520] and SIBS-II [N=436]). Occult CAD was defined as the occurrence of a reversible perfusion defect during exercise thallium scintigraphy and/or as an abnormal result of an exercise electrocardiogram (SIBS-I, n=97; SIBS-II, n=56). In SIBS-I, the KL-VS allele conferred a relative odds of 1.90 (95% confidence interval 1.21-2.98) for occult CAD, after adjusting for familial intraclass correlations (P<.005). Logistic regression modeling, incorporating known CAD risk factors, demonstrated that the KL-VS allele is an independent risk factor (P<.019) and that the imposed risk of KL-VS allele status is influenced by modifiable risk factors. Hypertension (P<.022) and increasing high-density lipoprotein cholesterol (HDL-C) levels (P<.022) mask or reduce the risk conferred by the KL-VS allele, respectively, whereas current smoking (P<.004) increases the risk. Remarkably concordant effects of the KL-VS allele and modifying factors on the risk of occult CAD were seen in SIBS-II. These results demonstrate that the KL-VS allele is an independent risk factor for occult CAD in two independent high-risk samples. Modifiable risk factors, including hypertension, smoking status, and HDL-C level, appear to influence the risk imposed by this allele. 相似文献
106.
Hortega's ammoniated silver carbonate method was used to demonstrate lysosomes in the central nervous system and kidney of adult rats. Formol-CaCl2, (10%:1%) fixed, frozen sections were impregnated for 10 min in Hortega's solution: 30 ml of 10% AgNO2 and 90 ml of 5% Na2CO3, with concentrated NH4OH added until the precipitate dissolved, then distilled water to make 400 ml. This procedure revealed silver-positive cytoplasmic structures whose form, shape and distribution were similar to that seen by staining adjacent sections for acid phosphatase. A short fixation of 18-24 hr appears to be essential. A useful, nonenzymatic method for the demonstration of lysosomes is thereby available. 相似文献
107.
108.
109.
W. A. Becker 《Protoplasma》1938,29(1):355-360
Ohne Zusammenfassung 相似文献
110.
Zusammenfassung Mit Hilfe von Chlorellakulturen, deren Zellmaterial durch autotrophe Anzucht in C13-reichem CO2 markiert war, wurde nachgewiesen, daß die Veratmung von endogenem Substrat bei Zufuhr von Glucose nicht unterdrückt wird. Die endogene Atmung ist vielmehr in Gegenwart von Glucose merklich gesteigert. 相似文献