全文获取类型
收费全文 | 515504篇 |
免费 | 53001篇 |
国内免费 | 778篇 |
出版年
2018年 | 4991篇 |
2017年 | 4991篇 |
2016年 | 8188篇 |
2015年 | 13644篇 |
2014年 | 14626篇 |
2013年 | 17761篇 |
2012年 | 19445篇 |
2011年 | 17464篇 |
2010年 | 12250篇 |
2009年 | 11259篇 |
2008年 | 14253篇 |
2007年 | 14651篇 |
2006年 | 13835篇 |
2005年 | 18860篇 |
2004年 | 17694篇 |
2003年 | 15209篇 |
2002年 | 12612篇 |
2001年 | 19740篇 |
2000年 | 19083篇 |
1999年 | 16168篇 |
1998年 | 5892篇 |
1997年 | 5762篇 |
1996年 | 5433篇 |
1995年 | 5265篇 |
1994年 | 4969篇 |
1993年 | 4940篇 |
1992年 | 13947篇 |
1991年 | 13972篇 |
1990年 | 13757篇 |
1989年 | 13190篇 |
1988年 | 12353篇 |
1987年 | 11631篇 |
1986年 | 10992篇 |
1985年 | 10784篇 |
1984年 | 8717篇 |
1983年 | 7467篇 |
1982年 | 5487篇 |
1981年 | 4949篇 |
1980年 | 4595篇 |
1979年 | 8221篇 |
1978年 | 6558篇 |
1977年 | 5774篇 |
1976年 | 5456篇 |
1975年 | 6436篇 |
1974年 | 7082篇 |
1973年 | 6912篇 |
1972年 | 6206篇 |
1971年 | 5822篇 |
1970年 | 4937篇 |
1969年 | 4862篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
931.
932.
Ronald L. Calabrese 《Current biology : CB》2012,22(18):R803-R804
933.
I. A. Parfenov T. A. Revina P. P. Pashkovsky N. L. Radyukina T. A. Valueva 《Applied Biochemistry and Microbiology》2011,47(4):361-365
Product of polymerase chain reaction designated as PKPIJ-B was isolated after amplification from genomic DNA of potato (Solarium tuberosum L., Zhukov Jubilee cultivar) using the designed primers. Nucleotide sequence of PKPIJ-B was determined and amino acid sequence of protein was restored. Analysis of this sequence has allowed us to suggest that
isolated gene fragment encodes chymotrypsin and trypsin inhibitor protein (PKCI-23 potato Kunitz-type chymotrypsin inhibitor)
of potato tubers. 相似文献
934.
935.
936.
Kinga Michno David Knight Jorge M. Campussano Diana van de Hoef Gabrielle L. Boulianne 《PloS one》2009,4(9)
Much of our current understanding about neurodegenerative diseases can be attributed to the study of inherited forms of these disorders. For example, mutations in the presenilin 1 and 2 genes have been linked to early onset familial forms of Alzheimer''s disease (FAD). Using the Drosophila central nervous system as a model we have investigated the role of presenilin in one of the earliest cellular defects associated with Alzheimer''s disease, intracellular calcium deregulation. We show that expression of either wild type or FAD-mutant presenilin in Drosophila CNS neurons has no impact on resting calcium levels but does give rise to deficits in intracellular calcium stores. Furthermore, we show that a loss-of-function mutation in calmodulin, a key regulator of intracellular calcium, can suppress presenilin-induced deficits in calcium stores. Our data support a model whereby presenilin plays a role in regulating intracellular calcium stores and demonstrate that Drosophila can be used to study the link between presenilin and calcium deregulation. 相似文献
937.
938.
J W Wray W A Baase J D Lindstrom L H Weaver A R Poteete B W Matthews 《Journal of molecular biology》1999,292(5):1111-1120
The mutation Glu108-->Val (E108V) in T4 lysozyme was previously isolated as a second-site revertant that specifically compensated for the loss of function associated with the destabilizing substitution Leu99-->Gly (L99G). Surprisingly, the two sites are 11 A apart, with Leu99 in the core and Glu108 on the surface of the protein. In order to better understand this result we have carried out a detailed thermodynamic, enzymatic and structural analysis of these mutant lysozymes as well as a related variant with the substitution Leu99-->Ala. It was found that E108V does increase the stability of L99G, but it also increases the stability of both the wild-type protein and L99A by essentially equal amounts. The effects of E108V on enzymatic activity are more complicated. The mutation slightly reduces the maximal rate of cell wall hydrolysis of wild-type, L99G and L99A. At the same time, L99G is an unstable protein and rapidly loses activity during the course of the assay, especially at temperatures above 20 degrees C. Thus, even though the double mutant L99G/E108V has a slightly lower maximal rate than L99G, over a period of 20-30 minutes it hydrolyzes more substrate. This decrease in the rate of thermal inactivation appears to be the basis of the action of E108V as a second-site revertant of L99G. Mutant L99A creates a cavity of volume 149 A(3). Instead of enlarging this cavity, mutant L99G results in a 4-5 A displacement of part of helix F (residues 108-113), creating a solvent-accessible declivity. In the double mutant, L99G/E108V, this helix returns to a position akin to wild-type, resulting in a cavity of volume 203 A(3). Whether the mutation Glu108-->Val is incorporated into either wild-type lysozyme, or L99A or L99G, it results in a decrease in crystallographic thermal factors, especially in the helices that include residues 99 and 108. This increase in rigidity, which appears to be due to a combination of increased hydrophobic stabilization plus a restriction of conformational fluctuation, provides a structural basis for the increase in thermostability. 相似文献
939.
The majority of terrestrial biomass is wood, but the elemental composition of its potential consumers, xylophages, differs hugely from that of wood. This causes a severe nutritional imbalance. We studied the stoichiometric relationships of 11 elements (C, N, P, K, Ca, Mg, Fe, Zn, Mn, Cu, Na) in three species of pine-xylem-feeding insects, Stictoleptura rubra, Arhopalus rusticus (Coleoptera, Cerambycidae) and Chalcophora mariana (Coleoptera, Buprestidae), to elucidate their mechanisms of tissue growth and to match their life histories to their dietary constraints. These beetles do not differ from other Coleoptera in their absolute elemental compositions, which are approximately 1000 (N), 100 (P, Cu) and 50 (K, Na) times higher than in dead but undecayed pine wood. This discrepancy diminishes along the wood decay gradient, but the elemental concentrations remain higher by an order of magnitude in beetles than in highly decayed wood. Numerical simulation of the life history of S. rubra shows that feeding on nutrient-poor undecayed wood would extend its development time to implausible values, whereas feeding on highly decomposed wood (heavily infected with fungi) would barely balance its nutritional budget during the long development period of this species. The changes in stoichiometry indicate that the relative change in the nutrient levels in decaying wood cannot be attributed solely to carbon loss resulting from decomposer respiration: the action of fungi substantially enriches the decaying wood with nutritional elements imported from the outside of the system, making it a suitable food for wood-eating invertebrates. 相似文献
940.
The photometric assay of reducing sugars with 2,2'-bicinchoninate was improved and its conditions were optimized. Optical density is linear between 1 and 25 nmol sugar/sample, and the assay is not affected by borate, phosphate, or other buffer anions. The molar extinction coefficients produced by the standard procedure with the 12 most common monosaccharides occurring in carbohydrates and conjugated glycocompounds are listed. 相似文献