全文获取类型
收费全文 | 72064篇 |
免费 | 6398篇 |
国内免费 | 43篇 |
出版年
2022年 | 443篇 |
2021年 | 943篇 |
2020年 | 641篇 |
2019年 | 784篇 |
2018年 | 1041篇 |
2017年 | 927篇 |
2016年 | 1551篇 |
2015年 | 2594篇 |
2014年 | 2902篇 |
2013年 | 3764篇 |
2012年 | 4833篇 |
2011年 | 4744篇 |
2010年 | 3133篇 |
2009年 | 2881篇 |
2008年 | 4024篇 |
2007年 | 4206篇 |
2006年 | 4053篇 |
2005年 | 3765篇 |
2004年 | 3744篇 |
2003年 | 3579篇 |
2002年 | 3336篇 |
2001年 | 729篇 |
2000年 | 505篇 |
1999年 | 801篇 |
1998年 | 1000篇 |
1997年 | 728篇 |
1996年 | 651篇 |
1995年 | 630篇 |
1994年 | 608篇 |
1993年 | 655篇 |
1992年 | 584篇 |
1991年 | 499篇 |
1990年 | 493篇 |
1989年 | 481篇 |
1988年 | 473篇 |
1987年 | 461篇 |
1986年 | 461篇 |
1985年 | 527篇 |
1984年 | 650篇 |
1983年 | 577篇 |
1982年 | 696篇 |
1981年 | 749篇 |
1980年 | 690篇 |
1979年 | 426篇 |
1978年 | 473篇 |
1977年 | 457篇 |
1976年 | 403篇 |
1975年 | 361篇 |
1974年 | 416篇 |
1973年 | 363篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
971.
972.
Partial sequencing and mapping of clones from two maize cDNA libraries 总被引:11,自引:0,他引:11
973.
Robert G. Gregerson Susan S. Miller Mary Petrowski J. Stephen Gantt Carroll P. Vance 《Plant molecular biology》1994,25(3):387-399
Genomic clones encoding two isozymes of aspartate aminotransferase (AAT) were isolated from an alfalfa genomic library and their DNA sequences were determined. The AAT1 gene contains 12 exons that encode a cytosolic protein expressed at similar levels in roots, stems and nodules. In nodules, the amount of AAT1 mRNA was similar at all stages of development, and was slightly reduced in nodules incapable of fixing nitrogen. The AAT1 mRNA is polyadenylated at multiple sites differing by more than 250 bp. The AAT2 gene contains 11 exons, with 5 introns located in positions identical to those found in animal AAT genes, and encodes a plastid-localized isozyme. The AAT2 mRNA is polyadenylated at a very limited range of sites. The transit peptide of AAT2 is encoded by the first two and part of the third exon. AAT2 mRNA is much more abundant in nodules than in other organs, and increases dramatically during the course of nodule development. Unlike AAT1, expression of AAT2 is significantly reduced in nodules incapable of fixing nitrogen. Phylogenetic analysis of deduced AAT proteins revealed 4 separate but related groups of AAT proteins; the animal cytosolic AATs, the plant cytosolic AATs, the plant plastid AATs, and the mitochondrial AATs. 相似文献
974.
Nitrogen fixation by periphyton and plankton on the Amazon floodplain at Lake Calado 总被引:2,自引:1,他引:1
Nitrogen fixation by periphyton and plankton was measured on the Amazon flood-plain using the acetylene reduction method calibrated with15N-N2. The average ratio (± SD) of moles C2H4 reduced per mole N2-N fixed was 3.4 ± 0.7, similar to other studies. Periphyton and plankton had high rates of light-dependent nitrogen fixation, with dark nitrogen fixation averaging 26% of the average rates in the light. The average daily (24 h) rates for periphyton nitrogen fixation in 1989 and 1990 were 1.79 and 0.51 mmol N2-N·m–2·d–1 respectively, which are comparable to summer rates in many temperate cyanobacterial assemblages. Nitrogen fixation was depressed at N03
– concentrations as low as 0.5 M, and was below detection limits at concentrations of 4 M, which occurred during periods of river flooding. Planktonic nitrogen fixation rates were high (0.5–0.8 mmol N2-N·m–2·d–1) during the high-water and drainage phases of the annual hydrograph when the floodplain waters were draining towards the river (low NO3
–), but rates were undetectable (< 0.05 mmol N2-N·m–2·d–1) when there was river flooding (high NO3
–). Nitrogen fixation by periphyton and plankton in 1989–1990 accounted for approximately 8% of previously reported total annual nitrogen inputs to the floodplain at Lake Calado. 相似文献
975.
Eric Glasgow †Robert K. Druger †Chana Fuchs †Edward M. Levine †Suzanne Giordano † Nisson Schechter 《Journal of neurochemistry》1994,63(2):470-481
Abstract: In efforts to determine the primary structure of intermediate filament proteins in the goldfish visual pathway, we isolated clones from a retinal λgt11 cDNA expression library that represent goldfish vimentin. We show that there are at least two forms of goldfish vimentin, designated as vimentin α and vimentin β. RNase protection assays indicate that vimentin α mRNA is expressed in low amounts in retina, optic nerve, and brain and in higher amounts in spinal cord. In contrast, vimentin β mRNA is expressed in low amounts in retina, optic nerve, brain, and spinal cord and in very high amounts in eye lens. Immunohistochemical studies show that in the optic nerve, vimentin α is mainly restricted to blood vessels, meninges, and septa. Light staining is observed with this antibody in an astrocytic glial pattern throughout the optic nerve. Two-dimensional gel analysis shows that all of these goldfish vimentins are low abundant components of optic nerve cytoskeletal preparations. 相似文献
976.
Raquel Fernández-Durango José A. de Juan Horatio Zimman Francisco J. Moya Mario Garcia de la Coba Arturo Fernández-Cruz 《Journal of neurochemistry》1994,62(4):1482-1488
Abstract: Specific endothelin (ET) binding sites were characterized in membranes prepared from human cerebral cortices using binding assay and cross-linking analysis. The presence of immunoreactive (IR) ET-1 was studied by radioimmunoassay. Saturation binding experiments revealed that the K D and B max for 125 I-ET-1 and 125 l-ET-3 to membranes from gray matter were 25 ± 6 pM and 115 ± 15 fmol/mg of protein and 24 ± 5 p M and 108 ± 13 fmol/mg of protein, respectively. Similar results were obtained for white matter. In the presence of 10 n M sarafotoxin-6c, which is selective for ETB receptors, 125 I-ET-1 and 125 l-ET-3 binding was totally abolished. However, in the presence of 1 μ M BQ123, which is selective for ETA receptors, both bindings were not affected. These results suggest that the human cerebral cortex contains only ETB receptors. Cross-linking of 125 I-ET-1 and 125 l-ET-3 to membranes with disuccinimidyl suberate resulted in the labeling of two bands of 48 and 31 kDa. Concentrations of IR-ET-1 in the gray and white matter were 7.0 ± 3.2 and 2.5 ± 1.7 fmol/g wet weight, respectively. The demonstration of high-affinity ETB receptors and the presence of IRET-1 suggest that the peptide may act as a neurotransmitter or neuromodulator in the human cerebral cortex. 相似文献
977.
Ann-ping Tsou Alan Kosaka Chinh Bach Patti Zuppan Calvin Yee Leonard Tom Robert Alvarez Scott Ramsey Douglas W. Bonhaus Eric Stefanich Lyn Jakeman Richard M. Eglen Hardy W. Chan 《Journal of neurochemistry》1994,63(2):456-464
Abstract: In a number of different cell types, phosphorylation of a 63-kDa protein has been shown to increase rapidly in response to stimuli that lead to an increase in intracellular calcium. Here, a stimulus-sensitive protein at this molecular weight is identified in PC12 cells and rat cortical synaptosomes as phosphoglucomutase. In addition, the added phosphate is shown to be in an oligosaccharide terminating in phosphodiester-linked glucose. In synaptosomes, incorporated radioactivity, following incubation with [14 C]glucose or the [β-35 S]phosphorothioate analogue of UDP-glucose, was found to increase within 5 s of stimulation and return to baseline within 25 s. Despite the many pathways utilizing glucose, this was the only detectable protein glycosylation observed in synaptosomes. These results indicate that cytoplasmic glycosylation is reversible and rapidly regulated, and suggest that phosphoglucomutase undergoes an alteration in function and/or topography in response to increases in intracellular calcium. 相似文献
978.
Maria Luz Perez José Ramón Valverde Beatriz Batuecas Francisco Amat Roberto Marco Rafael Garesse 《Journal of molecular evolution》1994,38(2):156-168
From the cloned mitochondrial DNAs (mtDNAs) isolated from two bisexual species, one Mediterranean, Artemia salina, and one American, Artemia franciscana, and two parthenogenetic (diploid and tetraploid) strains of Artemia parthenogenetica collected in Spain, physical maps have been constructed and compared. They are extremely different among themselves, much more than the differences between Drosophila melanogaster and D. yakuba and in the same range of different mammalian species such as mouse/rat or man/cow. The nucleotide sequences of two regions of mtDNA encoding parts of the cytochrome c oxidase subunit I (COI) and cytochrome b (Cytb) genes have been determined in the two bisexual species and the two parthenogenetic strains. Comparisons of these sequences have revealed a high degree of divergence at the nucleotide level, averaging more than 15%, in agreement with the differences found in the physical maps. The majority of the nucleotide changes are silent and there is a strong bias toward transitions, with the CT substitutions being highly predominant. The evolutionary distance between the two Artemia parthenogenetica is high and there is no clear relationship with any of the bisexual species, including the one present nowadays in Spain. Using a combination of molecular (mtDNA) and morphological markers it is possible to conclude that all of these Artemia isolates should be actually considered as belonging to different species, even the two Artemia parthenogenetica diploidica and tetraploidica.On sabbatical leave from Departamento de Bioquímica, Facultad de Veterinaria, Universidad Complutense de Madridearly Italian artemiologists to designate the Medi-Beatriz Batuecas died in an accident during the Christmas holy days of 1988 after she had initiated this workCorrespondence to: R. Garesse 相似文献
979.
Victoriano Garre Francisco J. Murillo Santiago Torres-Martínez 《Molecular & general genetics : MGG》1994,244(3):278-286
A 5.6 kb DNA fragment from the fungus Phycomyces blakesleeanus has been cloned and sequenced. The fragment contains a gene that probably codes for the enzyme acetyl-coenzyme A synthetase (facA). The amino acid sequence deduced for the P. blakesleeanns protein is highly homologous to those of acetyl-coA-synthetases from other organisms. When placed under the control of a constitutive promoter from Aspergillus nidulans, the cloned gene complemented a facA mutation of this organism. In P. blakesleeanns, the expression of facA is induced by acetate. 相似文献
980.
A colony-level phenotype was used to map the major sex determination locus (designatedX) in the honey bee (Apis mellifera). Individual queen bees (reproductive females) were mated to single drones (fertile males) by instrumental insemination. Haploid drone progeny of an F1 queen were each backcrossed to daughter queens from one of the parental lines. Ninety-eight of the resulting colonies containing backcross progeny were evaluated for the trait low brood-viability resulting from the production of diploid drones that were homozygous atX. DNA samples from the haploid drone fathers of these colonies were used individually in polymerase chain reactions (PCR) with 10-base primers. These reactions generated random amplified polymorphic DNA (RAPD) markers that were analyzed for cosegregation with the colony-level phenotype. One RAPD marker allele was shared by 22 of 25 drones that fathered low brood-viability colonies. The RAPD marker fragment was cloned and partially sequenced. Two primers were designed that define a sequence-tagged site (STS) for this locus. The primers amplified DNA marker fragments that cosegregated with the original RAPD marker. In order to more precisely estimate the linkage betweenX and the STS locus, another group of bees consisting of progeny from one of the low-brood viability colonies was used in segregation analysis. Four diploid drones and 181 of their diploid sisters (workers, nonfertile females) were tested for segregation of the RAPD and STS markers. The cosegregating RAPD and STS markers were codominant due to the occurrence of fragment-length alleles. The four diploid drones were homozygous for these markers but only three of the 181 workers were homozygotes (recombinants). Therefore the distance betweenX and the STS locus was estimated at 1.6 cM. An additional linked marker was found that was 6.6 cM from the STS locus. 相似文献