全文获取类型
收费全文 | 682735篇 |
免费 | 79819篇 |
国内免费 | 901篇 |
出版年
2016年 | 7889篇 |
2015年 | 11531篇 |
2014年 | 13476篇 |
2013年 | 18585篇 |
2012年 | 21471篇 |
2011年 | 21885篇 |
2010年 | 14733篇 |
2009年 | 13513篇 |
2008年 | 19452篇 |
2007年 | 20133篇 |
2006年 | 18912篇 |
2005年 | 18413篇 |
2004年 | 18255篇 |
2003年 | 17574篇 |
2002年 | 16958篇 |
2001年 | 27180篇 |
2000年 | 27202篇 |
1999年 | 22009篇 |
1998年 | 8467篇 |
1997年 | 8688篇 |
1996年 | 8326篇 |
1995年 | 7941篇 |
1994年 | 7617篇 |
1993年 | 7626篇 |
1992年 | 18829篇 |
1991年 | 18651篇 |
1990年 | 18291篇 |
1989年 | 17724篇 |
1988年 | 16571篇 |
1987年 | 15953篇 |
1986年 | 15067篇 |
1985年 | 15021篇 |
1984年 | 12483篇 |
1983年 | 11063篇 |
1982年 | 8542篇 |
1981年 | 7899篇 |
1980年 | 7316篇 |
1979年 | 12295篇 |
1978年 | 9820篇 |
1977年 | 9052篇 |
1976年 | 8511篇 |
1975年 | 9630篇 |
1974年 | 10554篇 |
1973年 | 10271篇 |
1972年 | 9340篇 |
1971年 | 8639篇 |
1970年 | 7395篇 |
1969年 | 7364篇 |
1968年 | 6642篇 |
1967年 | 5891篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
132.
Neurospora grows vegetatively as a syncytium in which multiple nuclei exist within a connected cytoplasm. Because of the ability of separate and distinct mycelia to fuse, the possibility exists of generating heterocaryotic cultures in which the nuclei and cytoplasms of two different strains are comingled into the same syncytium. We have used such heterocaryons, in which the component parts differed with respect to their circadian clock phase, to examine whether or not clock-dominant phases exist in the circadian cycle. To this end, the phase subsequent to the formation of heterocaryons by pairs of mycelial discs that are initially at different circadian phases was examined in Neurospora crassa. The resulting phase was an average of the parent phases in many cases, but was sometimes observed to correspond more closely to just one of the original parental phases. In these cases, we did not observe any dominant phases in the circadian cycle; the phase of a particular parent disc was more dominant in the heterocaryon when the proportion of the nuclei from that parent was greater in the heterocaryon. In some instances, which occurred mostly when the difference in phase of the parental discs was large, the resultant phase could not be related in a simple way to the parental phases. An interpretation based on a limit cycle model of the circadian oscillation is possible. 相似文献
133.
134.
Romain Forestier Julia L. Blanchard Kirsty L. Nash Elizabeth A. Fulton Craig Johnson Asta Audzijonyte 《Ecology and evolution》2020,10(24):14033
- Fishing is a strong selective force and is supposed to select for earlier maturation at smaller body size. However, the extent to which fishing‐induced evolution is shaping ecosystems remains debated. This is in part because it is challenging to disentangle fishing from other selective forces (e.g., size‐structured predation and cannibalism) in complex ecosystems undergoing rapid change.
- Changes in maturation size from fishing and predation have previously been explored with multi‐species physiologically structured models but assumed separation of ecological and evolutionary timescales. To assess the eco‐evolutionary impact of fishing and predation at the same timescale, we developed a stochastic physiologically size‐structured food‐web model, where new phenotypes are introduced randomly through time enabling dynamic simulation of species'' relative maturation sizes under different types of selection pressures.
- Using the model, we carried out a fully factorial in silico experiment to assess how maturation size would change in the absence and presence of both fishing and predation (including cannibalism). We carried out ten replicate stochastic simulations exposed to all combinations of fishing and predation in a model community of nine interacting fish species ranging in their maximum sizes from 10 g to 100 kg. We visualized and statistically analyzed the results using linear models.
- The effects of fishing on maturation size depended on whether or not predation was enabled and differed substantially across species. Fishing consistently reduced the maturation sizes of two largest species whether or not predation was enabled and this decrease was seen even at low fishing intensities (F = 0.2 per year). In contrast, the maturation sizes of the three smallest species evolved to become smaller through time but this happened regardless of the levels of predation or fishing. For the four medium‐size species, the effect of fishing was highly variable with more species showing significant and larger fishing effects in the presence of predation.
- Ultimately our results suggest that the interactive effects of predation and fishing can have marked effects on species'' maturation sizes, but that, at least for the largest species, predation does not counterbalance the evolutionary effect of fishing. Our model also produced relative maturation sizes that are broadly consistent with empirical estimates for many fish species.
135.
Prediction of sequential antigenic regions in proteins 总被引:30,自引:0,他引:30
Prediction of antigenic regions in a protein will be helpful for a rational approach to the synthesis of peptides which may elicit antibodies reactive with the intact protein. Earlier methods are based on the assumption that antigenic regions are primarily hydrophilic regions at the surface of the protein molecule. The method presented here is based on the amino acid composition of known antigenic regions in 20 proteins which is compared with that of 314 proteins [(1978) Atlas of Protein Sequence and Structure, vol. 5, suppl. 3, 363-373]. Antigenicity values were derived from the differences between the two data sets. The method was applied to bovine ribonuclease, the B-subunit of cholera toxin and herpes simplex virus type 1 glycoprotein D. There was a good correlation between the predicted regions and previously determined antigenic regions. 相似文献
136.
J R Vandenheede S D Yang W Merlevede 《Biochemical and biophysical research communications》1983,115(3):871-877
The major active protein phosphatase present in a rabbit skeletal muscle extract is associated with the glycogen particle and migrates in sucrose density gradient centrifugation as a Mr = 70,000 protein and contains modulator activity. Addition of extra modulator protein causes a time- and concentration-dependent conversion of the enzyme to an inactive FA-ATP, Mg-dependent form. The intrinsic modulator in the active phosphatase is destroyed by limited proteolysis without an appreciable change in the phosphatase activity. The proteolyzed active enzyme has a lower molecular weight (Mr = 40,000) and it reassociates with the modulator producing a FA-ATP, Mg-dependent enzyme form (Mr = 60,000). The modulator protein is used stoichiometrically in the activation of the ATP, Mg-dependent phosphatase. This is in agreement with the presence of one unit of modulator activity per unit of native spontaneously active phosphatase. 相似文献
137.
L B Andersen P Henckel B Saltin 《European journal of applied physiology and occupational physiology》1987,56(1):74-82
A random sample of schoolchildren, 119 boys and 153 girls, was tested in the fall of 1983. The data presented here are anthropometric data (height, weight, fat % and vital capacity) and oxygen uptake directly measured on a bicycle ergometer. The mean height and weight for boys were 179.1 cm and 67.7 kg, and those for girls were 168.0 cm and 59.6 kg. The mean fat content was 9.1% for boys and 19.1% for girls, and their mean vital capacities were 4.91 and 3.61 respectively. The boys had a high maximal oxygen uptake (51.7 ml X kg-1 X min-1) showing no reduction over the age span studied. The girls' maximal oxygen uptake was lower (overall mean 40.0 ml X kg-1 X min-1) with a small reduction from 16 to 19 years of age. When comparing maximal oxygen uptake per kg lean body mass in the two sexes, the boys had 18.4% higher values than the girls, indicating that girls of this age have the lower fitness level. The results of maximal aerobic power measurement in the boys compare well with findings from other investigations using direct measurements, indicating that the fitness of teenage boys is kept at a high level. Comparable data from various countries for girls show different pictures, but it appears that in general they have a low fitness level. 相似文献
138.
139.
140.
Gregory L. Dilworth 《Archives of biochemistry and biophysics》1982,219(1):30-38
The nicotinic acid hydroxylase from Clostridium barkeri is a selenoenzyme, as evidenced by the copurification of selenium with enzyme activity. This conclusion is supported by data showing a 23-fold increase in nicotinic acid hydroxylase activity when C. barkeri was cultured in media supplemented with selenium. A labile, selenium-containing compound was released from the native protein by treatment with either chaotropic agents and heat or by heating alone. A stable selenium compound was formed when the enzyme was alkylated prior to denaturation. This compound had the same chromatographic properties as dialykyl selenide in a number of systems. The formation of dialkyl selenide upon alkylation is not consistent with the selenium moiety being selenocysteine. Thus, nicotinic acid hydroxylase represents a new type of selenoenzyme. 相似文献