全文获取类型
收费全文 | 20754篇 |
免费 | 2198篇 |
国内免费 | 11篇 |
专业分类
22963篇 |
出版年
2022年 | 161篇 |
2021年 | 351篇 |
2020年 | 232篇 |
2019年 | 257篇 |
2018年 | 321篇 |
2017年 | 337篇 |
2016年 | 470篇 |
2015年 | 825篇 |
2014年 | 904篇 |
2013年 | 1011篇 |
2012年 | 1425篇 |
2011年 | 1357篇 |
2010年 | 927篇 |
2009年 | 847篇 |
2008年 | 1151篇 |
2007年 | 1261篇 |
2006年 | 1022篇 |
2005年 | 1049篇 |
2004年 | 1062篇 |
2003年 | 999篇 |
2002年 | 956篇 |
2001年 | 356篇 |
2000年 | 355篇 |
1999年 | 306篇 |
1998年 | 273篇 |
1997年 | 186篇 |
1996年 | 146篇 |
1995年 | 138篇 |
1994年 | 159篇 |
1993年 | 136篇 |
1992年 | 224篇 |
1991年 | 218篇 |
1990年 | 185篇 |
1989年 | 188篇 |
1988年 | 189篇 |
1987年 | 197篇 |
1986年 | 174篇 |
1985年 | 192篇 |
1984年 | 188篇 |
1983年 | 139篇 |
1982年 | 114篇 |
1981年 | 122篇 |
1980年 | 98篇 |
1979年 | 113篇 |
1978年 | 122篇 |
1976年 | 101篇 |
1974年 | 108篇 |
1973年 | 90篇 |
1972年 | 85篇 |
1971年 | 90篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
A simple and efficient method for the oligodeoxyribonucleotide-directed mutagenesis of double-stranded plasmid DNA. 总被引:4,自引:0,他引:4
A method for the oligodeoxyribonucleotide-directed mutagenesis of double-stranded DNA without the necessity for phenotypic selection is described. Plasmids denatured with alkali and purified by adsorption to and elution from nitrocellulose have single-stranded regions where primers can hybridize and serve as templates for a T7 DNA polymerase-catalyzed synthesis of complementary mutant DNA strands. When this procedure was carried out such that the original nonmutant strand contained uracil [method of Kunkel, Proc. Natl. Acad. Sci. USA 82(1985)488-492], mutation frequencies of between 30% and 40% were obtained. The technique has been used to generate mutant genes in plasmids of a wide variety of sizes. The largest plasmid manipulated and successfully mutagenized was 22 kb. The method is rapid and efficient and is not dependent upon either f1 phage vectors or the presence of restriction sites in the vicinity of the sequence targeted for mutation. 相似文献
72.
Expression kinetics of the lactose (lac) operon in Escherichia coli are reviewed for both wild-type and recombinant cell cultures under chemostatic conditions. A unified model which involves regulation of active inducer (lactose) transport, promoter-operator regulated expression of the lac operon, glucose-mediated inducer exclusion, and catabolite repression is summarized and supporting data is shown to verify its accuracy. The synthesis of alpha-amylase with a recombinant form of Bacillus subtilis is also reviewed to point out generic features in transport regulation, the lac operon model providing a point of departure. While there are many similarities in the influence of transport on both regulating models, there are also important differences. In a chemostat system, the synthesis of alpha-amylase is nongrowth associated, while beta-galactosidase is a growth-associated enzyme. Nevertheless, transport regulation is an important feature in both instances. 相似文献
73.
In humans, a deficiency of the lysosomal hydrolase α-
-iduronidase (IDUA; EC 3.2.1.76) results in the lysosomal storage of the glycosaminoglycans heparan sulfate and dermatan sulfate, thereby causing the lysosomal storage disorder mucopolysaccharidosis type I. The gene for IDUA is split into 14 exons spanning approximately 19 kb. We report the sequence of two noncontiguous segments of the IDUA gene, one 1.8-kb segment containing exons 1 and 2 and surrounding sequences and a second segment of 4.5 kb containing the last 12 exons. The potential promoter for IDUA has only GC box type consensus sequences consistent with a housekeeping promoter and is bounded by an Alu repeat sequence. The first two exons of IDUA are separated by an intron of 566 bp, then there is a large intron of approximately 13 kb, and the last 12 exons are clustered within 4.5 kb. No consensus polyadenylation signal was found in the 3′ untranslated region, although two variant polyadenylation signals are proposed. 相似文献
74.
75.
Scott Pownall Christine A. Kozak Keith Schappert Mohan Sarkar Eric Hull Harry Schachter Jamey D. Marth 《Genomics》1992,12(4)
The biosynthesis of protein-bound complex N-glycans in mammals requires a series of covalent modifications governed by a large number of specific glycosyltransferases and glycosidases. The addition of oligosaccharide to an asparagine residue on a nascent polypeptide chain begins in the endoplasmic reticulum. Oligosaccharide processing continues in the Golgi apparatus to produce a diversity of glycan structures. UDP-N-acetylglucosamine:α-3-
-mannoside β-1,2-N-acetylglucosaminyltransferase I (EC 2.4.1.101; GlcNAc-TI) is a key enzyme in the process because it is essential for the conversion of high-mannose N-glycans to complex and hybrid N-glycans. We have isolated the mouse gene encoding GlcNAc-TI (Mgat-1) from a genomic DNA library. The mouse sequence is highly conserved with respect to the human and rabbit homologs and exists as a single protein-encoding exon. Mgat-1 was mapped to mouse Chromosome 11, closely linked to the gene encoding interleukin-3 by the analysis of multilocus interspecies backcrosses. RNA analyses of Mgat-1 expression levels revealed significant variation among normal tissues and cells. 相似文献
76.
Neurons require a large amount of intracellular transport. Cytoplasmic polypeptides and membrane-bounded organelles move from
the perikaryon, down the length of the axon, and to the synaptic terminals. This movement occurs at distinct rates and is
termed axonal transport. Axonal transport is divided into the slow transport of cytoplasmic proteins including glycolytic
enzymes and cytoskeletal structures and the fast transport of membrane-bounded organelles along linear arrays of microtubules.
The polypeptide compositions of the rate classes of axonal transport have been well characterized, but the underlying molecular
mechanisms of this movement are less clear. Progress has been particularly slow toward understanding force-generation in slow
transport, but recent developments have provided insight into the molecular motors involved in fast axonal transport. Recent
advances in the cellular and molecular biology of one fast axonal transport motor, kinesin, have provided a clearer understanding
of organelle movement along microtubules. The availability of cellular and molecular probes for kinesin and other putative
axonal transport motors have led to a reevaluation of our understanding of intracellular motility. 相似文献
77.
D Scott S M Galloway R R Marshall M Ishidate D Brusick J Ashby B C Myhr 《Mutation research》1991,257(2):147-205
78.
Photophosphorylation in Attached Leaves of Helianthus annuus at Low Water Potentials 总被引:5,自引:2,他引:3
The in situ response of photophosphorylation and coupling factor activity to low leaf water potential (ψL) was investigated using kinetic spectroscopy to measure the flash-induced electrochromic absorption change in attached sunflower (Helianthus annuus L. cv IS894) leaves. The electrochromic change is caused by the formation of an electric potential across the thylakoid membrane associated with proton uptake. Since depolarization of the thylakoid membrane following flash excitation is normally dominated by proton efflux through the coupling factor during ATP formation, this measurement can provide direct information about the catalytic activity of the coupling factor. Under low ψL conditions in which a clear nonstomatal limitation of net photosynthesis could be demonstrated, we found a strong inhibition of coupling factor activity in dark-adapted leaves which was probably caused by an increase in the energetic threshold for the activation of the enzyme at low ψL. While this result supported earlier in vitro findings, we further discovered that the light-dependent reduction of coupling factor reversed any observable effect of low ψL on the energetics of activation or on photophosphorylation competence. Furthermore, coupling factor was reduced, even in severely droughted sunflower, almost immediately upon illumination. Based on these measurements, we conclude that the nonstomatal limitation of photosynthesis observed by us and others in droughted plants cannot be explained by impaired coupling factor activity. 相似文献
79.
A micropropagation system was developed to test concepts for automation and microenvironment alteration. Amelanchier x grandiflora Rehd. Princess Diana (serviceberry) shoot cultures were grown in seven-liter polycarbonate containers. Through computer control, the apparatus intermittently applied culture medium to the plant material according to a selected schedule. Shoot growth in the programmable system was compared with four micropropagation treatments: gelled and liquid medium in baby food jars and gelled and non-cycling liquid medium in a seven-liter vessel. Plants cultured in continuous contact with liquid medium became increasingly vitrified. Significantly greater shoot number, shoot length, shoot weight, and culture weight occurred in the programmable system than in jars with gelled medium. The combination of liquid medium, 7-liter vessel, and intermittent contact with medium caused a significantly higher proliferation rate than any combination of jar/vessel and gelled/liquid medium tested. 相似文献
80.
Traditionally, the major biotic determinants of animal community structure were assumed to be competition and predation. Recent theoretical, experimental and field studies indicate that parasitism is also important. The spectrum of effects that parasites have on host individuals is as broad as the range of parasite-host interactions. This review considers evidence for direct effects of parasites on fecundity and survival that influence host demography, and for indirect effects on host physiology and behavior that interface with competition and predation. 相似文献