全文获取类型
收费全文 | 23248篇 |
免费 | 1613篇 |
国内免费 | 5篇 |
专业分类
24866篇 |
出版年
2023年 | 206篇 |
2022年 | 404篇 |
2021年 | 721篇 |
2020年 | 533篇 |
2019年 | 638篇 |
2018年 | 852篇 |
2017年 | 718篇 |
2016年 | 985篇 |
2015年 | 1335篇 |
2014年 | 1448篇 |
2013年 | 1746篇 |
2012年 | 2000篇 |
2011年 | 1826篇 |
2010年 | 1106篇 |
2009年 | 987篇 |
2008年 | 1241篇 |
2007年 | 1136篇 |
2006年 | 1068篇 |
2005年 | 907篇 |
2004年 | 791篇 |
2003年 | 751篇 |
2002年 | 706篇 |
2001年 | 327篇 |
2000年 | 299篇 |
1999年 | 268篇 |
1998年 | 157篇 |
1997年 | 143篇 |
1996年 | 103篇 |
1995年 | 85篇 |
1994年 | 87篇 |
1993年 | 75篇 |
1992年 | 133篇 |
1991年 | 95篇 |
1990年 | 80篇 |
1989年 | 90篇 |
1988年 | 74篇 |
1987年 | 76篇 |
1986年 | 57篇 |
1985年 | 60篇 |
1984年 | 65篇 |
1983年 | 36篇 |
1982年 | 39篇 |
1981年 | 36篇 |
1980年 | 29篇 |
1979年 | 46篇 |
1978年 | 25篇 |
1976年 | 25篇 |
1975年 | 31篇 |
1974年 | 22篇 |
1973年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
Ho WW Fernandes CC Alves-Gomes JA Smith GT 《Ethology : formerly Zeitschrift fur Tierpsychologie》2010,116(11):1050-1064
The South American weakly-electric knifefish (Apteronotidae) produce highly diverse and readily quantifiable electrocommunication signals. The electric organ discharge frequency (EODf), and EOD modulations (chirps and gradual frequency rises (GFRs)), vary dramatically across sexes and species, presenting an ideal opportunity to examine the proximate and ultimate bases of sexually dimorphic behavior. We complemented previous studies on the sexual dimorphism of apteronotid communication signals by investigating electric signal features and their hormonal correlates in Apteronotus bonapartii, a species which exhibits strong sexual dimorphism in snout morphology. Electrocommunication signals were evoked and recorded using a playback paradigm, and were analyzed for signal features including EOD frequency and the structure of EOD modulations. To investigate the androgenic correlates of sexually dimorphic EOD signals, we measured plasma concentrations of testosterone and 11-ketotestosterone. A. bonapartii responded robustly to stimulus playbacks. EODf was sexually monomorphic, and males and females produced chirps with similar durations and amounts of frequency modulation. However, males were more likely than females to produce chirps with multiple frequency peaks. Sexual dimorphism in apteronotid electrocommunication signals appears to be highly evolutionarily labile. Extensive interspecific variation in the magnitude and direction of sex differences in EODf and in different aspects of chirp structure suggest that chirp signals may be an important locus of evolutionary change within the clade. The weakly-electric fish represent a rich source of data for understanding the selective pressures that shape, and the neuroendocrine mechanisms that underlie, diversity in the sexual dimorphism of behavior. 相似文献
12.
13.
14.
The TREX enzymes process DNA as the major 3′→5′ exonuclease activity in mammalian cells. TREX2 and TREX1 are members of the DnaQ family of exonucleases and utilize a two metal ion catalytic mechanism of hydrolysis. The structure of the dimeric TREX2 enzyme in complex with single-stranded DNA has revealed binding properties that are distinct from the TREX1 protein. The TREX2 protein undergoes a conformational change in the active site upon DNA binding including ordering of active site residues and a shift of an active site helix. Surprisingly, even when a single monomer binds DNA, both monomers in the dimer undergo the structural rearrangement. From this we have proposed a model for DNA binding and 3′ hydrolysis for the TREX2 dimer. The structure also shows how TREX proteins potentially interact with double-stranded DNA and suggest features that might be involved in strand denaturation to provide a single-stranded substrate for the active site. 相似文献
15.
By freezing blocks of paraffin-embedded tissues to a convenient temperature it is possible to obtain routinely 1 micron sections that can be further processed as normal thicker sections. Normal and disposable steel knives can be used and the staining time should be increased in most procedures. Gradual freezing of blocks to the temperature of dry ice is the simplest and safest way to obtain an adequate temperature. The best results were obtained using as fixative 4% paraformaldehyde in phosphate buffered saline solution. 相似文献
16.
17.
The synthesis and characterization of rare-earth (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) thiocyanate adducts with tripiperidinophosphine oxide (tpppO) with general formula (RE)(SCN)3(tpppO)3 are reported. Conductance measurements in acetonitrile indicate the non-electrolytic nature of the complexes. Infrared absorption spectra evidence that the SCN− ion coordinates through the nitrogen atom (isothiocyanate form) and that tpppO coordinates through the phosphoryl oxygen. X-ray powder patterns suggest the existence of three different crystal forms: (1) La; (2) an isomorphous series including Ce, Nd and Pr; and (3) another isomorphous series, including Sm, Gd, Eu, Ho, Er, Tb, Lu and Y. The visible spectra of the Nd adduct and the calculated parameters β = 0.98, b1/2 = 0.072 and δ = 1.06 indicate that the metal-ligand bonds are essentially electrostactic. The emission spectra of the Eu compound showed 5D0 → 7FJ bands (J = 0, 1, 2), suggesting a C3v symmetry for the coordination polyhedron. The lifetime of the 5D0 state is 1.28 ms. The emission spectra of the Tb complex presented 5D4 → 7FJ bands (J = 4, 5, 6) and the Dy complex showed the 4F9/2 → 6H13/2 band. The structure of the Pr complex showed that the coordination polyhedron is a trigonal antiprism, with the isothiocyanate anions in one base and three tpppO ligands in the other. Thermal analyses (TG-DTG) were carried out for the Ce, Nd and Gd adducts. Mass losses start between 250 and 334 °C. The final residues at 1300 °C are the corresponding phosphates. 相似文献
18.
Adriana Calderaro Giovanna Piccolo Chiara Gorrini Sara Montecchini Sabina Rossi Maria Cristina Medici Carlo Chezzi Georges Snounou 《PloS one》2012,7(10)
It has been proposed that ovale malaria in humans is caused by two closely related but distinct species of malaria parasites: P. ovale curtisi and P. ovale wallikeri. We have extended and optimized a Real-time PCR assay targeting the parasite’s small subunit ribosomal RNA (ssrRNA) gene to detect both these species. When the assay was applied to 31 archival blood samples from patients diagnosed with P. ovale, it was found that the infection in 20 was due to P. ovale curtisi and in the remaining 11 to P. ovale wallikeri. Thus, this assay provides a useful tool that can be applied to epidemiological investigations of the two newly recognized distinct P. ovale species, that might reveal if these species also differ in their clinical manifestation, drugs susceptibility and relapse periodicity. The results presented confirm that P. ovale wallikeri is not confined to Southeast Asia, since the majority of the patients analyzed in this study had acquired their P. ovale infection in African countries, mostly situated in West Africa. 相似文献
19.
20.