全文获取类型
收费全文 | 60389篇 |
免费 | 4939篇 |
国内免费 | 30篇 |
专业分类
65358篇 |
出版年
2022年 | 413篇 |
2021年 | 855篇 |
2020年 | 528篇 |
2019年 | 638篇 |
2018年 | 834篇 |
2017年 | 771篇 |
2016年 | 1359篇 |
2015年 | 2393篇 |
2014年 | 2516篇 |
2013年 | 3339篇 |
2012年 | 4241篇 |
2011年 | 4244篇 |
2010年 | 2758篇 |
2009年 | 2442篇 |
2008年 | 3534篇 |
2007年 | 3593篇 |
2006年 | 3394篇 |
2005年 | 3424篇 |
2004年 | 3357篇 |
2003年 | 3151篇 |
2002年 | 3120篇 |
2001年 | 714篇 |
2000年 | 533篇 |
1999年 | 733篇 |
1998年 | 866篇 |
1997年 | 597篇 |
1996年 | 602篇 |
1995年 | 591篇 |
1994年 | 572篇 |
1993年 | 597篇 |
1992年 | 536篇 |
1991年 | 466篇 |
1990年 | 389篇 |
1989年 | 405篇 |
1988年 | 402篇 |
1987年 | 339篇 |
1986年 | 340篇 |
1985年 | 396篇 |
1984年 | 444篇 |
1983年 | 374篇 |
1982年 | 459篇 |
1981年 | 409篇 |
1980年 | 363篇 |
1979年 | 240篇 |
1978年 | 293篇 |
1977年 | 284篇 |
1976年 | 231篇 |
1975年 | 216篇 |
1974年 | 236篇 |
1973年 | 219篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The analysis and results presented in this paper provide conclusive evidence to distinguish between the delay effect and the
lag as two biologically distinct phenomena. It therefore dispels the incorrect notion that delay effects represented by delay
differential equations are the biological reason behind the lag phase in microorganism growth. The resulting consequence so
far is that the only other reason for the lag phase is the existence of unstable stationary states. The latter are a result
of accounting for the microbial metabolic mass transfer in the population growth process. 相似文献
992.
993.
994.
Identification, characterization, and localization of a novel kidney polycystin-1-polycystin-2 complex 总被引:9,自引:0,他引:9
Newby LJ Streets AJ Zhao Y Harris PC Ward CJ Ong AC 《The Journal of biological chemistry》2002,277(23):20763-20773
The functions of the two proteins defective in autosomal dominant polycystic kidney disease, polycystin-1 and polycystin-2, have not been fully clarified, but it has been hypothesized that they may heterodimerize to form a "polycystin complex" involved in cell adhesion. In this paper, we demonstrate for the first time the existence of a native polycystin complex in mouse kidney tubular cells transgenic for PKD1, non-transgenic kidney cells, and normal adult human kidney. Polycystin-1 is heavily N-glycosylated, and several glycosylated forms of polycystin-1 differing in their sensitivity to endoglycosidase H (Endo H) were found; in contrast, native polycystin-2 was fully Endo H-sensitive. Using highly specific antibodies to both proteins, we show that polycystin-2 associates selectively with two species of full-length polycystin-1, one Endo H-sensitive and the other Endo H-resistant; importantly, the latter could be further enriched in plasma membrane fractions and co-immunoprecipitated with polycystin-2. Finally, a subpopulation of this complex co-localized to the lateral cell borders of PKD1 transgenic kidney cells. These results demonstrate that polycystin-1 and polycystin-2 interact in vivo to form a stable heterodimeric complex and suggest that disruption of this complex is likely to be of primary relevance to the pathogenesis of cyst formation in autosomal dominant polycystic kidney disease. 相似文献
995.
Switching species tropism: an effective way to manipulate the feline coronavirus genome 总被引:11,自引:0,他引:11 下载免费PDF全文
Feline infectious peritonitis virus (FIPV), a coronavirus, is the causative agent of an invariably lethal infection in cats. Like other coronaviruses, FIPV contains an extremely large positive-strand RNA genome of ca. 30 kb. We describe here the development and use of a reverse genetics strategy for FIPV based on targeted RNA recombination that is analogous to what has been described for the mouse hepatitis virus (MHV) (L. Kuo et al., J. Virol. 74:1393-1406, 2000). In this two-step process, we first constructed by targeted recombination a mutant of FIPV, designated mFIPV, in which the ectodomain of the spike glycoprotein was replaced by that of MHV. This switch allowed for the selection of the recombinant virus in murine cells: mFIPV grows to high titers in these cells but has lost the ability to grow in feline cells. In a second, reverse process, mFIPV was used as the recipient, and the reintroduction of the FIPV spike now allowed for selection of candidate recombinants by their regained ability to grow in feline cells. In this fashion, we reconstructed a wild-type recombinant virus (r-wtFIPV) and generated a directed mutant FIPV in which the initiation codon of the nonstructural gene 7b had been disrupted (FIPV Delta 7b). The r-wtFIPV was indistinguishable from its parental virus FIPV 79-1146 not only for its growth characteristics in tissue culture but also in cats, exhibiting a highly lethal phenotype. FIPV Delta 7b had lost the expression of its 7b gene but grew unimpaired in cell culture, confirming that the 7b glycoprotein is not required in vitro. We establish the second targeted RNA recombination system for coronaviruses and provide a powerful tool for the genetic engineering of the FIPV genome. 相似文献
996.
Finger JN Lich JD Dare LC Cook MN Brown KK Duraiswami C Bertin J Bertin JJ Gough PJ 《The Journal of biological chemistry》2012,287(30):25030-25037
Nucleotide-binding domain leucine-rich repeat proteins (NLRs) play a key role in immunity and disease through their ability to modulate inflammation in response to pathogen-derived and endogenous danger signals. Here, we identify the requirements for activation of NLRP1, an NLR protein associated with a number of human pathologies, including vitiligo, rheumatoid arthritis, and Crohn disease. We demonstrate that NLRP1 activity is dependent upon ASC, which associates with the C-terminal CARD domain of NLRP1. In addition, we show that NLRP1 activity is dependent upon autolytic cleavage at Ser(1213) within the FIIND. Importantly, this post translational event is dependent upon the highly conserved distal residue His(1186). A disease-associated single nucleotide polymorphism near His(1186) and a naturally occurring mRNA splice variant lacking exon 14 differentially affect this autolytic processing and subsequent NLRP1 activity. These results describe key molecular pathways that regulate NLRP1 activity and offer insight on how small sequence variations in NLR genes may influence human disease pathogenesis. 相似文献
997.
Peter J. Gebicke-Haerter Andr s Seregi Angelika Schobert Georg Hertting 《Neurochemistry international》1988,13(4):475-480
The role of protein kinase C (PKC) and calcium in the stimulation of prostaglandin D2 (PGD2) synthesis was investigated in primary rat astroglial cultures using the phorbol esters phorbol 12-myristate, 13-acetate (PMA), phorbol 12,13-dibutyrate (PDB) and the calcium ionophore A23187. Both phorbol esters and the ionophore were able to stimulate PGD2 synthesis in a concentration dependent manner. The inactive stereoisomers of PMA and PDB had no significant effect. Combinations of subthreshold concentrations of phorbol esters (10 nM PMA or 10 nM PBD) potentiated PG formation induced by 100 nM A23187. An even more pronounced effect was observed when phorbol ester concentrations were increased to 100nM. The contribution of extra- and intracellular calcium in phorbol ester or A23187 stimulated PGD2 synthesis was evaluated by carrying out experiments with calcium-free media plus EGTA or with the intracellular calcium-chelating agent TMB-8. Ionophore stimulated PGD2 release was shut down to basal values upon removal of extracellular calcium, whereas phorbol ester stimulated PGD2 formation persisted at a reduced level. It was unabated also upon further addition of EGTA. In the presence of TMB-8, however, phorbol ester stimulated PGD2 synthesis was completely suppressed. These data strongly suggest that PKC has an additional effect on the activation of phospholipase A2 and subsequent prostanoid synthesis, which is independent from extracellular calcium and, thus, support the concept of more than one metabolic pathway in astrocytes that synergistically regulate phospholipase A2 activity. 相似文献
998.
The free-living cyanobacterium Anabaena variabilis showed a biphasic pattern of 14CH3NH
3
+
uptake. Initial accumulation (up to 60 s) was independent of CH3NH
3
+
metabolism, but long-term uptake was dependent on its metabolism via glutamine synthetase (GS). The CH3NH
3
+
was converted into methylglutamine which was not further metabolised. The addition of l-methionine-dl-sulphoximine (MSX), to inhibit GS, inhibited CH3NH
3
+
metabolism, but did not affect the CH3NH
3
+
transport system.NH
4
+
, when added after the addition of 14CH3NH
3
+
, caused the efflux of free CH3NH
3
+
; when added before 14CH3NH
3
+
, NH
4
+
inhibited its uptake indicating that both NH
4
+
and CH3NH
3
+
share a common transport system. Carbonylcyanide m-chlorophenylhydrazone and triphenyl-methylphosphonium both inhibited CH3NH
3
+
accumulation indicating that the transport system was -dependent. At pH 7 and at an external CH3NH
3
+
concentration of 30 mol dm-3, A. variabilis showed a 40-fold intracellular accumulation of CH3NH
3
+
(internal concentration 1.4 mmol dm-3). Packets of the symbiotic cyanobacterium Anabaena azollae, directly isolated from the water fern Azolla caroliniana, also showed a -dependent NH
4
+
transport system suggesting that the reduced inhibitory effect of NH
4
+
on nitrogenase cannot be attributed to the absence of an NH
4
+
transport system but is probably related to the reduced GS activity of the cyanobiont.Abbreviations CCCP
carbonylcyanide m-chlorophenylhydrazone
- GS
glutamine synthetase
- HEPES
4-(2-hydroxyethyl)-1-piperazine ethanesulphonic acid
- MSX
l-methionine-dl-sulphoximine
-
membrane potential
- pH
transmembrane pH difference
- TPMP+
triphenylmethylphosphonium 相似文献
999.
Species traits and climate velocity explain geographic range shifts in an ocean‐warming hotspot 下载免费PDF全文
Jennifer M. Sunday Gretta T. Pecl Stewart Frusher Alistair J. Hobday Nicole Hill Neil J. Holbrook Graham J. Edgar Rick Stuart‐Smith Neville Barrett Thomas Wernberg Reg A. Watson Dan A. Smale Elizabeth A. Fulton Dirk Slawinski Ming Feng Ben T. Radford Peter A. Thompson Amanda E. Bates 《Ecology letters》2015,18(9):944-953
Species' ranges are shifting globally in response to climate warming, with substantial variability among taxa, even within regions. Relationships between range dynamics and intrinsic species traits may be particularly apparent in the ocean, where temperature more directly shapes species' distributions. Here, we test for a role of species traits and climate velocity in driving range extensions in the ocean‐warming hotspot of southeast Australia. Climate velocity explained some variation in range shifts, however, including species traits more than doubled the variation explained. Swimming ability, omnivory and latitudinal range size all had positive relationships with range extension rate, supporting hypotheses that increased dispersal capacity and ecological generalism promote extensions. We find independent support for the hypothesis that species with narrow latitudinal ranges are limited by factors other than climate. Our findings suggest that small‐ranging species are in double jeopardy, with limited ability to escape warming and greater intrinsic vulnerability to stochastic disturbances. 相似文献
1000.
Enhancement of 7-methoxyresorufin O-demethylation activity of human cytochrome P450 1A2 by molecular breeding 总被引:2,自引:0,他引:2
Alkylresorufins are model substrates for cytochrome P450 (P450) 1A2. The ability of human P450 1A2 to catalyze 7-methoxyresorufin O-demethylation was improved by screening of random mutant libraries (expressed in Escherichia coli) on the basis of 7-methoxyresorufin O-demethylation. After three rounds of mutagenesis and screening, the triple mutant E163K/V193M/K170Q yielded a kcat > five times faster than wild type P450 1A2 in steady-state kinetic analysis using either isolated membrane fractions or purified, reconstituted enzymes. The enhanced catalytic activity was not attributed to changes in substrate affinity. The kinetic hydrogen isotope effect of the triple mutant did not change from wild type enzyme and suggests that C-H bond cleavage is rate-limiting in both enzymes. Homology modeling, based on an X-ray structure of rabbit P450 2C5, suggests that the locations of mutated residues are not close to the substrate binding site and therefore that structural elements outside of this site play roles in changing the catalytic activity. This approach has potential value in understanding P450 1A2 and generating engineered enzymes with enhanced catalytic activity. 相似文献