全文获取类型
收费全文 | 126171篇 |
免费 | 8625篇 |
国内免费 | 25篇 |
专业分类
134821篇 |
出版年
2023年 | 664篇 |
2022年 | 707篇 |
2021年 | 1541篇 |
2020年 | 1250篇 |
2019年 | 1374篇 |
2018年 | 3108篇 |
2017年 | 2727篇 |
2016年 | 3954篇 |
2015年 | 5887篇 |
2014年 | 6104篇 |
2013年 | 8033篇 |
2012年 | 9768篇 |
2011年 | 9212篇 |
2010年 | 5821篇 |
2009年 | 4435篇 |
2008年 | 7400篇 |
2007年 | 7247篇 |
2006年 | 6619篇 |
2005年 | 6177篇 |
2004年 | 5765篇 |
2003年 | 5351篇 |
2002年 | 4806篇 |
2001年 | 2251篇 |
2000年 | 2210篇 |
1999年 | 1953篇 |
1998年 | 870篇 |
1997年 | 693篇 |
1996年 | 595篇 |
1995年 | 612篇 |
1994年 | 623篇 |
1993年 | 494篇 |
1992年 | 1275篇 |
1991年 | 1196篇 |
1990年 | 1070篇 |
1989年 | 985篇 |
1988年 | 934篇 |
1987年 | 789篇 |
1986年 | 727篇 |
1985年 | 814篇 |
1984年 | 699篇 |
1983年 | 594篇 |
1982年 | 454篇 |
1981年 | 454篇 |
1979年 | 602篇 |
1978年 | 470篇 |
1977年 | 420篇 |
1976年 | 400篇 |
1975年 | 447篇 |
1974年 | 460篇 |
1973年 | 469篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
G Némethy 《Biochimie》1975,57(4):471-482
This review surveys current approaches to the problem of determining the solution conformation of polypeptides. The basic principles of energy computations are described. The utility, problems, and limitations of various theoretical methods are summarized: conformational energy mapping, energy minimization, scanning of selected local conformations, statistical predictive schemes. The need for combining the calculations with experimental studies is pointed out. The information content of various physico-chemical methods is compared for this purpose. The analysis of nmr coupling constants is discussed in more detail. In combination with energy computations, it can furnish specific information on local aspects of the conformation. Examples of such combined studies on small peptides are summarized. 相似文献
182.
183.
184.
Phosphor persistence, video bandwidth, DC restoration and high-voltage regulation affect the appearance of images presented on cathode-ray tubes (CRTs), potentially resulting in differences between nominal and actual stimuli. We illustrate these effects by measuring physical parameters of horizontal and vertical static and counter-phase flickering gratings, and we illustrate problems for vision research by measuring contrast sensitivity to these gratings. We also measured the extent to which calibration protocols actually result in the monitor being calibrated over its entire area regardless of image size. The results of our physical measurements indicate substantial differences between gratings that nominally differ only as to orientation. Consistent with these differences, our psychophysical measurements indicate different sensitivities when the bars of the gratings are parallel or orthogonal to raster lines, regardless of the retinal orientation of the gratings. The results of our calibration check show that only a small region around the target area of calibration can be regarded as effectively linearized, and only if the size of the test image used during the check is similar to the size of the calibration patch. Overall, our results indicate potentially severe problems with the use of CRTs in vision research, and we discuss some published results that are likely to have been affected by these problems. 相似文献
185.
186.
The emergence pattern of Schistosoma curassoni cercariae from Bulinus umbilicatus, whose adult worms parasitize bovine, caprine, and ovine ungulates in Niger, is of a circadian type with a mean emission time at 0855 hr +/- 1 hr 6 min, characteristic of the schistosome species parasitizing domestic or wild cattle. The comparison of this cercarial emergence pattern with those of the other 3 sympatric species of schistosomes (Schistosoma haematobium, Schistosoma bovis, and Schistosoma mansoni) shows a significant difference between the chronobiology of the cercariae infective for human and those infective for bovine hosts. This difference may improve epidemiological surveys based on snail prevalences by allowing the distinction between bulinids infected with human and bovine parasites. 相似文献
187.
P Navarro-Rosinés C Roux A Bellemère 《Comptes rendus de l'Académie des sciences. Série III, Sciences de la vie》2001,324(5):443-451
An ultrastructural study of Weddellomyces epicallopisma (ascomata wall, asci, ascospores and vegetative hyphae), the first done on the family Dacampiaceae, confirms most of the observations made in light microscopy. Moreover it shows that ascospores are provided with an endospore (not visible in light microscope) and that the structure of the ascospore septum is more complex. The similarity of the wall structure between the ascospore and the hyphoid appendages, developed on the upper part of the ascoma, is emphasized. 相似文献
188.
The yeast exosome is a complex of at least 10 essential 3'-5' riboexonucleases which is involved in 3'-processing of many RNA species. An exosome-like complex has been found or predicted to exist in other eukaryotes but not in Escherichia coli. The unicellular parasite Trypanosoma brucei diverged very early in eukaryotic evolution. We show here that T.brucei contains at least eight exosome subunit homologs, but only a subset of these associate in a complex. Accordingly, the T.brucei exosome is smaller than that of yeast. Both free and complex-associated homologs are essential for cell viability and are involved in 5.8S rRNA maturation. We suggest that the exosome was present in primitive eukaryotes, and became increasingly complex during subsequent evolution. 相似文献
189.
A heterozygous mutant for the two- and six-rowed character was isolated in the barley cultivar Igri through application of sodium azide to isolated microspore cultures and posterior regeneration. Six-rowed and two-rowed homozygotic plants were subsequently identified in the self-pollinated M2 progenies of the original heterozygous M1. Detailed molecular markers confirmed the isogenic nature of this recovered mutant and the original cultivar Igri. A comparative study of the anther culture response of this six-rowed induced mutant vs. diploid 'Igri' was performed to assess whether the two- or six-rowed gene influences anther culture response in barley through a pleiotropic effect or via linkage disequilibrium. No significant differences for any of the recorded variables throughout the in vitro regeneration process were detected between the 'Igri' six-rowed mutant and any of their two-rowed isogenic lines. This suggests that row-type association with anther culture response in barley cultivars is due to the effect of a tight linkage with other genes directly responsible for androgenic response. 相似文献