全文获取类型
收费全文 | 732197篇 |
免费 | 72216篇 |
国内免费 | 410篇 |
专业分类
804823篇 |
出版年
2018年 | 12188篇 |
2017年 | 11733篇 |
2016年 | 13158篇 |
2015年 | 13922篇 |
2014年 | 15417篇 |
2013年 | 21626篇 |
2012年 | 27049篇 |
2011年 | 31014篇 |
2010年 | 21584篇 |
2009年 | 19398篇 |
2008年 | 26751篇 |
2007年 | 28609篇 |
2006年 | 20938篇 |
2005年 | 20995篇 |
2004年 | 20359篇 |
2003年 | 19690篇 |
2002年 | 18841篇 |
2001年 | 29524篇 |
2000年 | 29880篇 |
1999年 | 23485篇 |
1998年 | 8396篇 |
1997年 | 8525篇 |
1996年 | 8111篇 |
1995年 | 7422篇 |
1994年 | 7398篇 |
1993年 | 7251篇 |
1992年 | 18740篇 |
1991年 | 18238篇 |
1990年 | 17708篇 |
1989年 | 17498篇 |
1988年 | 15874篇 |
1987年 | 15106篇 |
1986年 | 13843篇 |
1985年 | 13889篇 |
1984年 | 11627篇 |
1983年 | 10034篇 |
1982年 | 7543篇 |
1981年 | 6799篇 |
1980年 | 6437篇 |
1979年 | 10943篇 |
1978年 | 8533篇 |
1977年 | 7744篇 |
1976年 | 7223篇 |
1975年 | 7981篇 |
1974年 | 8776篇 |
1973年 | 8548篇 |
1972年 | 8252篇 |
1971年 | 7623篇 |
1970年 | 6089篇 |
1969年 | 5933篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
12.
In a recent study, we demonstrated that the conversion of carboxyl residues in the C-termini of tubulin to neutral amides with glycine ethyl ester enhanced the ability of the protein to assemble into microtubules and decreased its interaction with microtubule-associated proteins (MAPs). In this work, we investigated the effects of carboxyl modification on the dynamic behavior of microtubules at polymer mass steady state. After steady state, microtubules assembled from unmodified tubulin were sheared, and the mean polymer lengths decreased to 5 microns and then increased to 29 microns within 130 min. In contrast, lengths of sheared microtubules polymerized from tubulin containing 23 modified carboxyl groups increased by only 2-fold. Stabilization of polymer lengths was also observed directly by video-enhanced light microscopy of microtubules grown off of axonemes. Rapid shortening was seen in microtubules composed of unmodified but not modified tubulin. Further evidence for the less dynamic behavior of microtubules as a result of carboxyl modification was obtained from kinetic studies of the elongation phase during assembly which showed a 3-fold lower off-rate constant, k-, for modified microtubules. Another effect of the modification was a 12-fold reduction in the steady-state rate constant for GTP hydrolysis (165 s-1 for unmodified and 14 s-1 for modified). These results suggest that reduction of the negative charges in the C-termini by modification of the acidic residues stabilizes microtubules against depolymerization. MAPs may stabilize microtubules in an analogous manner. 相似文献
13.
The effect of carbohydrate depletion on procoagulant activity and in vivo survival of highly purified human factor VIII 总被引:1,自引:0,他引:1
P J Fay S I Chavin J E Malone D Schroeder F E Young V J Marder 《Biochimica et biophysica acta》1984,800(2):152-158
Human factor VIII procoagulant protein (factor VIII) was purified using a modification of our previously described method, in which Sephacryl S-400 elution, rather than QAE-cellulose chromatography, served as the final purification step. The protein had a specific activity of more than 2500 U/mg and consisted of a single polypeptide (Mr 100 000) when analyzed by SDS-polyacrylamide gel electrophoresis. Factor VIII was shown to be a glycoprotein by staining with periodic acid-Schiff's reagent following electrophoresis. Treatment of factor VIII with a mixture of exo- and endoglycosidases caused a reduction by about 50% in the intensity of periodic acid-Schiff staining, as determined by scanning densitometry, and an increase in electrophoretic mobility (equivalent to a new Mr 95 000). Removal of this portion of the total carbohydrate had no significant effect on factor VIII clotting activity or on thrombin potentiation of clotting activity. The in vivo survival curves of a native and sugar-depleted 125I-labeled factor VIII both showed similar patterns of initial rapid decay to 60 and 40% activity, respectively, followed by a one-half decay time of 4 h for both. These results suggest that the carbohydrate portion of human factor VIII does not contribute significantly to either clotting function in vitro or to biological turnover in vivo. 相似文献
14.
Electrophysiological and behavioral correlates of sleep in the desert iguana, Dipsosaurus dorsalis Hallowell 总被引:1,自引:0,他引:1
The circadian behavior of the desert iguana, Dipsosaurus dorsalis, was investigated on the basis of behavioral observation and electrophysiological recording. D. dorsalis adequately complies with accepted criteria for both behavioral sleep and paradoxical sleep. At 20 degrees C, 12% of the photophase is spent in sleep, 95% of the scotophase is spent in sleep. Paradoxical sleep occurs at all times of the year, but only at temperatures of 20 and 30 degrees C. Amounts of behavioral sleep are affected by both temperature and time of year. Total sleep increases with decreased day length and decreased temperature. Daytime sleep increases with decreased temperature. 相似文献
15.
16.
L. N. GILLMAN D. J. KEELING R. C. GARDNER S. D. WRIGHT 《Journal of evolutionary biology》2010,23(6):1327-1330
A faster rate of nuclear DNA evolution has recently been found for plants occupying warmer low latitudes relative to those in cooler high latitudes. That earlier study by our research group compared substitution rates within the variable internal transcribed spacer (ITS) region of the ribosomal gene complex amongst 45 congeneric species pairs, each member of which differed in their latitudinal distributions. To determine whether this rate differential might also occur within highly conserved DNA, we sequenced the 18S ribosomal gene in the same 45 pairs of plants. We found that the rate of evolution in 18S was 51% faster in the tropical plant species relative to their temperate sisters and that the substitution rate in 18S correlated positively with that in the more variable ITS. This result, with a gene coding for ribosomal structure, suggests that climatic influences on evolution extend to functionally important regions of the genome. 相似文献
17.
Y Tani I Ohkubo S Higashiyama M Kunimatsu M Sasaki 《Comparative biochemistry and physiology. B, Comparative biochemistry》1987,88(2):429-441
1. High mol. wt kininogen (HMW kininogen) was purified to a homogeneous state from porcine plasma. 2. The protein exhibited a strong inhibitory activity for thiol proteinases such as ficin, papain and calpain I, whereas it did not inhibit serine proteinases, trypsin and chymotrypsin. 3. The mol. wt, isoelectric point, amino acid and carbohydrate compositions, stabilities to temperature and pH, kinetic constants, and immunological properties of the porcine HMW kininogen were determined and compared with those of human HMW kininogen. 相似文献
18.
M C Bateson 《BMJ (Clinical research ed.)》1984,289(6453):1163-1164
19.