全文获取类型
收费全文 | 4420篇 |
免费 | 442篇 |
专业分类
4862篇 |
出版年
2024年 | 10篇 |
2023年 | 43篇 |
2022年 | 93篇 |
2021年 | 176篇 |
2020年 | 71篇 |
2019年 | 117篇 |
2018年 | 120篇 |
2017年 | 100篇 |
2016年 | 202篇 |
2015年 | 268篇 |
2014年 | 261篇 |
2013年 | 288篇 |
2012年 | 390篇 |
2011年 | 347篇 |
2010年 | 202篇 |
2009年 | 181篇 |
2008年 | 240篇 |
2007年 | 228篇 |
2006年 | 194篇 |
2005年 | 178篇 |
2004年 | 192篇 |
2003年 | 134篇 |
2002年 | 158篇 |
2001年 | 35篇 |
2000年 | 39篇 |
1999年 | 30篇 |
1998年 | 49篇 |
1997年 | 30篇 |
1996年 | 24篇 |
1995年 | 17篇 |
1994年 | 20篇 |
1993年 | 23篇 |
1992年 | 20篇 |
1991年 | 19篇 |
1990年 | 19篇 |
1989年 | 14篇 |
1988年 | 12篇 |
1987年 | 13篇 |
1986年 | 11篇 |
1985年 | 15篇 |
1984年 | 19篇 |
1983年 | 15篇 |
1982年 | 10篇 |
1981年 | 11篇 |
1980年 | 9篇 |
1979年 | 17篇 |
1977年 | 11篇 |
1974年 | 14篇 |
1972年 | 12篇 |
1970年 | 9篇 |
排序方式: 共有4862条查询结果,搜索用时 15 毫秒
1.
Emma Bricchi Marco Fornaciari Claudia Giannoni Francesca Greco Daniela Fascini Giuseppe Frenguelli Gianfranco Mincigrucci Bruno Romano 《Aerobiologia》1992,8(3):401-406
Summary Gramineae pollination from a pollen monitoring station located in the eastern suburb of Perugia and meteorological correlations are reported. The data refers to the year 1989. Grass pollen peak pollination was from May to July; in this period the influence of relative humidity and of temperature on pollen concentration was very high. Phenological observations, to identify the time of maximum stamen extension in the most common genera in the area, are also reported. During the period of investigation the counts of pollen grains over four-hour periods showed a regular diurnal rhythm with peaks of concentration in the four-hour period 16.00–20.00. Aerosporological data and meteorological data related to four-hour periods were correlated following different criteria. 相似文献
2.
3.
Recent data have demonstrated that differences in sIg density on B lymphocytes distinguish functionally distinct subpopulations of these cells. Other reports suggest that cyropreservation may change the frequency of sIg-bearing lymphocytes. To determine if cryopreservation alters either the frequency of sIg cells or the distribution of sIg density, PBM from normals and patients with CLL and LCL were analyzed using the FACS. Aliquots of Ficoll-Hypaque-separated PBM were controlled-rate frozen (1 °C/min) in 7.5% Me2SO in RPMI 1640 and thawed in a 37 °C water bath on the same day. Fresh and frozen-thawed PBM aliquots were labeled with fluorescein conjugates of F(ab′) fragments of affinity chromatography-purified anti-Fab or class-specific anti-μ, anti-δ, anti-γ, or anti-α. Histograms of relative cell fluorescence, reflecting sIg density, were prepared for each aliquot with the FACS. The frequency of sIg-bearing PBM labeled with each reagent was not significantly altered by freezing. Likewise, FACS profiles demonstrated that the distribution of sIg density on normal and CLL PBM was unchanged after freezing. However, the fluorescence peak produced by frozen-thawed unlabeled cells was occasionally slightly broader than that of fresh cells, suggesting increased autofluorescence induced by freezing. These data indicate that frozen cell preparations may be utilized for the study of B-lymphocyte subsets as determined by sIg density. 相似文献
4.
Yiwen Sun Zexuan Zhu Siping Chen Jega Balakrishnan Derek Abbott Anil T. Ahuja Emma Pickwell-MacPherson 《PloS one》2012,7(11)
The GP2 peptide is derived from the Human Epidermal growth factor Receptor 2 (HER2/nue), a marker protein for breast cancer present in saliva. In this paper we study the temperature dependent behavior of hydrated GP2 at terahertz frequencies and find that the peptide undergoes a dynamic transition between 200 and 220 K. By fitting suitable molecular models to the frequency response we determine the molecular processes involved above and below the transition temperature (T
D). In particular, we show that below T
D the dynamic transition is dominated by a simple harmonic vibration with a slow and temperature dependent relaxation time constant and that above T
D, the dynamic behavior is governed by two oscillators, one of which has a fast and temperature independent relaxation time constant and the other of which is a heavily damped oscillator with a slow and temperature dependent time constant. Furthermore a red shifting of the characteristic frequency of the damped oscillator was observed, confirming the presence of a non-harmonic vibration potential. Our measurements and modeling of GP2 highlight the unique capabilities of THz spectroscopy for protein characterization. 相似文献
5.
Genetic mechanisms of tumor-specific loss of 11p DNA sequences in Wilms tumor. 总被引:12,自引:7,他引:5 下载免费PDF全文
D D Dao W T Schroeder L Y Chao H Kikuchi L C Strong V M Riccardi S Pathak W W Nichols W H Lewis G F Saunders 《American journal of human genetics》1987,41(2):202-217
Wilms tumor, a common childhood renal tumor, occurs in both a heritable and a nonheritable form. The heritable form may occasionally be attributed to a chromosome deletion at 11p13, and tumors from patients with normal constitutional chromosomes often show deletion or rearrangement of 11p13. It has been suggested that a germinal or somatic mutation may occur on one chromosome 11 and predispose to Wilms tumor and that a subsequent somatic genetic event on the normal homologue at 11p13 may permit tumor development. To study the frequency and mechanism of such tumor-specific genetic events, we have examined the karyotype and chromosome 11 genotype of normal and tumor tissues from 13 childhood renal tumor patients with different histologic tumor types and associated clinical conditions. Tumors of eight of the 12 Wilms tumor patients, including all viable tumors examined directly, show molecular evidence of loss of 11p DNA sequences by somatic recombination (four cases), chromosome loss (two cases), and recombination (two cases) or chromosome loss and duplication. One malignant rhabdoid tumor in a patient heterozygous for multiple 11p markers did not show any tumor-specific 11p alteration. These findings confirm the critical role of 11p sequences in Wilms tumor development and reveal that mitotic recombination may be the most frequent mechanism by which tumors develop. 相似文献
6.
The details of Fröhlich's theory and some recent experiments on the rouleau formation of human erythrocytes which exhibit a strong interaction that appears to satisfy the prerequisites of the Fröhlich theory, are summarized. To verify whether the Fröhlich theory of long-range coherence in biological systems is applicable to the phenomenon of rouleau formation in human erythrocytes, the interactions between erythrocytes are modelled as those between two large, coupled oscillating dipoles. Relevant expressions for the resonant long-range and the van der Waals interaction are then derived. Using the available numerical data, the eigenfrequencies and the interaction energies corresponding to the experimental conditions are then derived. In the range of postulated frequencies (1011–1012 Hz) the effective interaction coefficient due to the resonant long-range forces is, indeed, found to agree with its experimental value of 3.0. However, the same value of can also be achieved through the ordinary van der Waals interactions between dipoles oscillating at lower frequencies. It is concluded that the resonant long-range interaction between erythrocytes may be responsible for the onset of rouleau formation. However, other mechanisms cannot be ruled out at this stage, especially since the Fröhlich mechanism requires a number of unconfirmed preconditions. 相似文献
7.
8.
9.
S Ghanshani M Pak J D McPherson M Strong B Dethlefs J J Wasmuth L Salkoff G A Gutman K G Chandy 《Genomics》1992,12(2):190-196
10.
Previous experiments have suggested that hemicholinium-3 might directly antagonize certain actions of beta-bungarotoxin at the neuromuscular junction. Data presented here show that, on the contrary, hemicholinium-3 neither inhibits the phospholipase activity of beta-bungarotoxin nor does it affect the characteristic pattern of transmitter release observed at end plates exposed to the toxin. Lanthanum ions were found to promote the release of acetylcholine from sartorius nerve-muscle preparations that had been paralyzed by botulinum toxin. However, the acceleration of transmitter release by lanthanum was not nearly as great as in control preparations as monitored either electrophysiologically or by chemical measurement of ACh. 相似文献