全文获取类型
收费全文 | 839篇 |
免费 | 101篇 |
国内免费 | 1篇 |
专业分类
941篇 |
出版年
2021年 | 9篇 |
2019年 | 8篇 |
2017年 | 9篇 |
2016年 | 20篇 |
2015年 | 24篇 |
2014年 | 23篇 |
2013年 | 29篇 |
2012年 | 26篇 |
2011年 | 44篇 |
2010年 | 31篇 |
2009年 | 30篇 |
2008年 | 34篇 |
2007年 | 26篇 |
2006年 | 20篇 |
2005年 | 18篇 |
2004年 | 23篇 |
2003年 | 22篇 |
2002年 | 25篇 |
2001年 | 26篇 |
2000年 | 22篇 |
1999年 | 15篇 |
1998年 | 31篇 |
1997年 | 20篇 |
1996年 | 14篇 |
1995年 | 10篇 |
1993年 | 22篇 |
1992年 | 13篇 |
1991年 | 13篇 |
1990年 | 19篇 |
1989年 | 11篇 |
1988年 | 10篇 |
1987年 | 14篇 |
1986年 | 8篇 |
1985年 | 8篇 |
1984年 | 15篇 |
1983年 | 14篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1979年 | 16篇 |
1978年 | 8篇 |
1977年 | 12篇 |
1975年 | 8篇 |
1974年 | 10篇 |
1972年 | 12篇 |
1971年 | 7篇 |
1970年 | 6篇 |
1968年 | 7篇 |
1966年 | 6篇 |
1944年 | 6篇 |
排序方式: 共有941条查询结果,搜索用时 0 毫秒
1.
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
2.
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
3.
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
4.
5.
6.
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
7.
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
8.
L Kurlansik T J Williams J E Campana B N Green L W Anderson J M Strong 《Biochemical and biophysical research communications》1983,111(2):478-483
In addition to dimerization and polymerization of samples as previously suggested, it appears that during FAB-MS, reactions in the sample matrix can occur to yield new compounds that are recombinations of molecular fragments. This type of reaction may be especially critical to the integrity of peptide sequencing using FAB, since the reactions cited in this report involve the formation and rupture of amides or peptide bonds. 相似文献
9.
10.
Some Properties of Heat-Resistant and Heat-Sensitive Strains of Clostridium perfringens I. Heat Resistance and Toxigenicity 总被引:4,自引:2,他引:2 下载免费PDF全文
Heat resistance at 100 C (D-values), sporulating ratios, toxigenicity for mice, and lecithinase activity (as micrograms per milliliter of enzyme, ascertained by the lecithovitellin reaction) were determined for four strains of Clostridium perfringens. A definite inverse relationship between thermal resistance and toxigenicity was found. The D-values ranged from 17.6 for the most heat-resistant strain to 0.3 for the strain possessing the least heat resistance, with corresponding lecithinase activities from 25 to 133 mug/ml of enzyme. The sporulating ratios did not differ greatly between the strains. The heat stability of the toxin was greater at 100 C than at 75 C. There was a noticeable difference between the heat stabilities of the toxin in the culture fluids of the heat-sensitive and heat-resistant strains at pH 7.0 when the toxic filtrates were held at 100 C. At a holding temperature of 75 C, a similar but lesser difference was observed at pH 5.5. Heat resistance and lecithinase activity did not change when a substrain of the least heat-resistant parent strain was obtained through heat selection by a single transfer, or when the most heat-resistant strain was transferred serially 12 times. 相似文献