全文获取类型
收费全文 | 86819篇 |
免费 | 7978篇 |
国内免费 | 169篇 |
专业分类
94966篇 |
出版年
2021年 | 635篇 |
2019年 | 595篇 |
2018年 | 1086篇 |
2017年 | 1060篇 |
2016年 | 1599篇 |
2015年 | 2372篇 |
2014年 | 2662篇 |
2013年 | 3513篇 |
2012年 | 4157篇 |
2011年 | 4038篇 |
2010年 | 2569篇 |
2009年 | 2044篇 |
2008年 | 3345篇 |
2007年 | 3411篇 |
2006年 | 3286篇 |
2005年 | 3152篇 |
2004年 | 3083篇 |
2003年 | 2925篇 |
2002年 | 2945篇 |
2001年 | 3462篇 |
2000年 | 3484篇 |
1999年 | 2775篇 |
1998年 | 1068篇 |
1997年 | 959篇 |
1996年 | 938篇 |
1995年 | 860篇 |
1994年 | 868篇 |
1993年 | 852篇 |
1992年 | 1910篇 |
1991年 | 1927篇 |
1990年 | 1859篇 |
1989年 | 1751篇 |
1988年 | 1661篇 |
1987年 | 1517篇 |
1986年 | 1380篇 |
1985年 | 1325篇 |
1984年 | 1096篇 |
1983年 | 979篇 |
1982年 | 790篇 |
1981年 | 752篇 |
1980年 | 643篇 |
1979年 | 1075篇 |
1978年 | 834篇 |
1977年 | 734篇 |
1976年 | 617篇 |
1975年 | 831篇 |
1974年 | 843篇 |
1973年 | 773篇 |
1972年 | 762篇 |
1971年 | 652篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
72.
73.
U H Lewinski G M Mavligit P M Epstein E M Hersh R A Ortez W J Thompson 《Biomedicine / [publiée pour l'A.A.I.C.I.G.]》1980,33(7):217-219
Endogenous cyclic adenosine and guanosine monophosphate (cAMP, cGMP) levels were studied in human peripheral blood lymphocytes during mixed leukocyte reactions (MLR). cAMP level was consistently elevated in one-way MLR, with good correlation to 3H-thymidine uptake in these reactions. In contrast, cGMP level was practically unchanged. Irradiation of reacting cell populations resulted in inhibition of cyclic nucleotide phosphodiesterase (PDE) activity. These results suggest that metabolic alterations in cAMP may be associated with immune reactions of cellular recognition. 相似文献
74.
75.
Neurons in the visual cortex are responsive to the presentation of oriented and curved line segments, which are thought to act as primitives for the visual processing of shapes and objects. Prolonged adaptation to such stimuli gives rise to two related perceptual effects: a slow change in the appearance of the adapting stimulus (perceptual drift), and the distortion of subsequently presented test stimuli (adaptational aftereffects). Here we used a psychophysical nulling technique to dissociate and quantify these two classical observations in order to examine their underlying mechanisms and their relationship to one another. In agreement with previous work, we found that during adaptation horizontal and vertical straight lines serve as attractors for perceived orientation and curvature. However, the rate of perceptual drift for different stimuli was not predictive of the corresponding aftereffect magnitudes, indicating that the two perceptual effects are governed by distinct neural processes. Finally, the rate of perceptual drift for curved line segments did not depend on the spatial scale of the stimulus, suggesting that its mechanisms lie outside strictly retinotopic processing stages. These findings provide new evidence that the visual system relies on statistically salient intrinsic reference stimuli for the processing of visual patterns, and point to perceptual drift as an experimental window for studying the mechanisms of visual perception. 相似文献
76.
Stabilizing selection for an intermediate optimum is generally considered to deplete genetic variation in quantitative traits. However, conflicting results from various types of models have been obtained. While classical analyses assuming a large number of independent additive loci with individually small effects indicated that no genetic variation is preserved under stabilizing selection, several analyses of two-locus models showed the contrary. We perform a complete analysis of a generalization of Wright's two-locus quadratic-optimum model and investigate numerically the ability of quadratic stabilizing selection to maintain genetic variation in additive quantitative traits controlled by up to five loci. A statistical approach is employed by choosing randomly 4000 parameter sets (allelic effects, recombination rates, and strength of selection) for a given number of loci. For each parameter set we iterate the recursion equations that describe the dynamics of gamete frequencies starting from 20 randomly chosen initial conditions until an equilibrium is reached, record the quantities of interest, and calculate their corresponding mean values. As the number of loci increases from two to five, the fraction of the genome expected to be polymorphic declines surprisingly rapidly, and the loci that are polymorphic increasingly are those with small effects on the trait. As a result, the genetic variance expected to be maintained under stabilizing selection decreases very rapidly with increased number of loci. The equilibrium structure expected under stabilizing selection on an additive trait differs markedly from that expected under selection with no constraints on genotypic fitness values. The expected genetic variance, the expected polymorphic fraction of the genome, as well as other quantities of interest, are only weakly dependent on the selection intensity and the level of recombination. 相似文献
77.
New class I and II HLA alleles strongly associated with opposite patterns of progression to AIDS. 总被引:10,自引:0,他引:10
H Hendel S Caillat-Zucman H Lebuanec M Carrington S O'Brien J M Andrieu F Sch?chter D Zagury J Rappaport C Winkler G W Nelson J F Zagury 《Journal of immunology (Baltimore, Md. : 1950)》1999,162(11):6942-6946
The genetics of resistance to infection by HIV-1 cohort consists of 200 slow and 75 rapid progressors to AIDS corresponding to the extremes of HIV disease outcome of 20,000 Caucasians of European descent. A comprehensive analysis of HLA class I and class II genes in this highly informative cohort has identified HLA alleles associated with fast or slow progression, including several not described previously. A quantitative analysis shows an overall HLA influence independent of and equal in magnitude (for the protective effect) to the effect of the CCR5-Delta32 mutation. Among HLA class I genes, A29 (p = 0.001) and B22 (p < 0.0001) are significantly associated with rapid progression, whereas B14 (p = 0.001) and C8 (p = 0.004) are significantly associated with nonprogression. The class I alleles B27, B57, C14 (protective), and C16, as well as B35 (susceptible), are also influential, but their effects are less robust. Influence of class II alleles was only observed for DR11. These results confirm the influence of the immune system on disease progression and may have implications on peptide-based vaccine development. 相似文献
78.
Mononuclear phagocytes release interleukin-1 (IL-1), a 17-kDa polypeptide with diverse biological activities. IL-1 is synthesized as a precursor (31 kDa) which lacks a signal sequence or hydrophobic domains that could facilitate transmembrane translocation. Possible postsynthetic modifications of IL-1 that might account for its cellular transport were examined. We found that lipopolysaccharide stimulated, but not unstimulated, murine macrophages incorporated 32PO4 into the IL-1 alpha precursor (31 kDa) predominantly at residue serine 90. Released IL-1 alpha (17 kDa) is not phosphorylated in agreement with peptide sequence data that the site of 32P incorporation is in the amino-terminal one-third of the precursor. Approximately 10% of the phosphorylated IL-1 alpha precursor is membrane bound and associated with a fraction enriched in lysosomal vesicles. Together these data suggest mechanisms by which the postsynthetic proteolysis of the IL-1 alpha precursor may be modified and cellular transport of IL-1 alpha is accomplished. 相似文献
79.
80.
This paper presents the results of karyological analysis of seven Alchemilla species collected from north-east Anatolia, Turkey, belonging to Alchemilla sect. Alchemilla subsect. Heliodrosium ser. Vulgares and subsect. Calycanthum ser. Elatae and ser. Calycinae. The following chromosome numbers were determined: A. haraldi 2n = 85–105, A. heterophylla 2n = 85–97, A. hirtipedicellata 2n = 86–100, A. oriturcica 2n = 86–102, A. persica 2n = 78–99, A. procerrima 2n = 69–78 and A. trabzonica 2n = 78–88. The chromosome numbers of three of these seven species are presented for the first time. 相似文献