全文获取类型
收费全文 | 8439篇 |
免费 | 778篇 |
国内免费 | 2篇 |
出版年
2022年 | 54篇 |
2021年 | 101篇 |
2020年 | 69篇 |
2019年 | 97篇 |
2018年 | 99篇 |
2017年 | 120篇 |
2016年 | 174篇 |
2015年 | 272篇 |
2014年 | 349篇 |
2013年 | 406篇 |
2012年 | 515篇 |
2011年 | 497篇 |
2010年 | 367篇 |
2009年 | 295篇 |
2008年 | 408篇 |
2007年 | 443篇 |
2006年 | 405篇 |
2005年 | 371篇 |
2004年 | 377篇 |
2003年 | 354篇 |
2002年 | 356篇 |
2001年 | 163篇 |
2000年 | 149篇 |
1999年 | 160篇 |
1998年 | 135篇 |
1997年 | 84篇 |
1996年 | 106篇 |
1995年 | 80篇 |
1994年 | 84篇 |
1993年 | 82篇 |
1992年 | 88篇 |
1991年 | 110篇 |
1990年 | 104篇 |
1989年 | 72篇 |
1988年 | 106篇 |
1987年 | 61篇 |
1986年 | 59篇 |
1985年 | 89篇 |
1984年 | 72篇 |
1983年 | 58篇 |
1982年 | 67篇 |
1981年 | 59篇 |
1980年 | 54篇 |
1979年 | 51篇 |
1977年 | 53篇 |
1976年 | 53篇 |
1975年 | 42篇 |
1974年 | 54篇 |
1973年 | 40篇 |
1972年 | 46篇 |
排序方式: 共有9219条查询结果,搜索用时 15 毫秒
31.
Flavonoid synthesis in Petunia hybrida: partial characterization of dihydroflavonol-4-reductase genes 总被引:15,自引:0,他引:15
Marcel Beld Cathie Martin Henk Huits Antoine R. Stuitje Anton G. M. Gerats 《Plant molecular biology》1989,13(5):491-502
In this paper we describe the organization and expression of the genes encoding the flavonoid-biosynthetic enzyme dihydroflavonol-4-reductase (DFR) in Petunia hybrida. A nearly full-size DFR cDNA clone (1.5kb), isolated from a corolla-specific cDNA library was compared at the nucleotide level with the pallida gene from Antirrhinum majus and at the amino acid level with enzymes encoded by the pallida gene and the A1 gene from Zea mays.The P. hybrida and A. majus DFR genes transcribed in flowers contain 5 introns, at identical positions; the three introns of the A1 gene from Z. mays coincide with first three introns of the other two species. P. hybrida line V30 harbours three DFR genes (A, B, C) which were mapped by RFLP analysis on three different chromosomes (IV, II and VI respectively).Steady-state levels of DFR mRNA in the line V30 follow the same pattern during development as chalcone synthase (CHS) and chalcone flavanone isomerase (CHI) mRNA. Six mutants that accumulate dihydroflavonols in mature flowers were subjected to Northern blot analysis for the presence of DFR mRNA. Five of these mutants lack detectable levels of DFR mRNA. Four of these five also show drastically reduced levels of activity for the enzyme UDPG: flavonoid-3-O-glucosyltransferase (UFGT), which carries out the next step in flavonoid biosynthesis; these mutants might be considered as containing lesions in regulatory genes, controlling the expression of the structural genes in this part of the flavonoid biosynthetic pathway. Only the an6 mutant shows no detectable DFR mRNA but a wild-type level for UFGT activity. Since both an6 and DFR-A are located on chromosome IV and DFR-A is transcribed in floral tissues, it is postulated that the An6 locus contains the DFR structural gene. The an9 mutant shows a wild-type level of DFR mRNA and a wild-type UFGT activity. 相似文献
32.
Arnold Tiehm 《Brittonia》1989,41(2):152-155
Types are indicated for 28 new names inadvertently published in Rydberg’sFlora of Colorado. Included are a discussion as to the validity of these new names and an explanation of the causes of this oversight. A lectotype ofLupinus decumbens var.argentatus is herein designated by Rupert C. Barneby. 相似文献
33.
Bactericidal effect of lactoferrin on Legionella pneumophila: effect of the physiological state of the organism 总被引:6,自引:0,他引:6
Lactoferrin has been previously shown to be bactericidal for Legionella pneumophila. The current study showed that CaCl2, Mg(NO3)2, and MgCl2, but not NaCl, blocked killing. Activity was pH dependent with the greatest activity at 5.0. Sensitivity of the organism was dramatically affected by the growth conditions. Log phase 12 h, broth-grown cells were most sensitive, with older cultures becoming more resistant. Plate-grown cells were completely resistant. Lactoferrin binding, as detected by immunofluorescence microscopy, was temperature dependent (no binding at 4 degrees C), but was independent of killing. 相似文献
34.
Lactococci are fastidious bacteria which require an external source of amino acids and many other nutrients. These compounds have to pass the membrane. However, detailed analysis of transport processes in membrane vesicles has been hampered by the lack of a suitable protonmotive force (pmf)-generating system in these model systems. A membrane-fusion procedure has been developed by which pmf-generating systems can be functionally incorporated into the bacterial membrane. This improved model system has been used to analyze the properties of amino acid transport systems in lactococci. Detailed studies have been made of the specificity and kinetics of amino acid transport and also of the interaction of the transport systems with their lipid environment. The properties of a pmf-independent, arginine-catabolism specific transport system in lactococci will be discussed.Abbreviations pmf
protonmotive force
-
transmembrane electrical potential
- pH
transmembrane pH gradient
- PE
phosphatidylethanolamine
- PC
phosphatidylcholine
Paper adapted from a treatise Secondary Transport of Amino Acids by Membrane Vesicles Derived from Lactic Acid Bacteria and awarded the Kluyver Prize 1988 by the Netherlands Society of Microbiology. 相似文献
35.
Viviane Hechler Marcel Mersel Henri Dreyfus Michel Maitre 《Molecular and cellular biochemistry》1990,93(1):87-94
Summary -Hydroxybutyric acid (GHB) is a natural compound of mammalian brain synthesized from GABA. The characteristics of its synthesis, transport, release, distribution and turnover, in addition to the presence of a high affinity binding site for this substance in brain are in favor of a modulator role for GHB. The effects of hydrolytic enzymes on the specific binding capacity of GHB have been studied in the present work. Phospholipases A2 and C, neuraminidase and Pronase markedly decrease GHB binding to crude synaptosomal membranes from rat brain. This effect is time and enzyme concentration dependent. Trypsin, under the conditions employed, is less active. The inhibitory effects of phospholipases is correlated with phospholipid hydrolysis. Lysophospholipids, in the absence of bovine fatty acid free serum albumin partially inhibit GHB binding. The action of neuraminidase has been followed by sialic acid release and modifications of the ganglioside profile. The effects of phospholipase C and of neuraminidase are completely different to those on GABA binding sites. These results represent further data concerning the molecular existence of specific GHB binding sites on rat brain membranes.Abbreviations GHB
-hydroxybutyrate
- LPC
L--lysophosphatidylcholine
- LPE
Lysophosphatidylethanolamine
- PC
Phosphatidylcholine
- PE
Phosphatidylethanolamine
- BSA
Bovine Serum Albumin 相似文献
36.
Recombinant plasmid pCED3 was structurally unstable inBacillus subtilis cultures grown in the presence of kanamycin to eliminate the effects of segregational instability. Analysis of 96 modified plasmids indicated that deletions in the plasmid occur at many different sites. The presence of plasmid pCED3 slowed the growth rate of theB. subtilis host. Cells that contained modified plasmids grew faster than the parental cells and took over the population. Two different methodologies were developed to reduce the cultural instability of the plasmid-directed LacZ+ phenotype. By growing the cells in a medium that supports a low growth rate, the growth rate ratio between modified and parental cells was reduced, resulting in a partial stabilization (40 generations) of the LacZ+ phenotype in the population [35]. Removal of a 4.77 kbEcoRI fragment (which consists primarily of the pBR322 replicon) from plasmid pCED3 produced a more stable plasmid derivative, designated pYS1. Cells harboring plasmid pYS1 grew faster than pCED3-bearing cells, although the level of activity of -galactosidase was similar in both strains. By combining the two approaches (i.e., growth of pYS1-bearing cells in a medium that supports low growth rate), the LacZ+ phenotype was stably maintained in the cell population for over 170 generations. Under these conditions, there was no detectable difference between the growth rates of cells bearing the pYS1 plasmid and further modified plasmids. 相似文献
37.
The incubation of sciatic nerve slices in Krebs Ringer bicarbonate (KRB) buffer (pH 7.4) at 37°C, or the incubation of freshly isolated myelin in ammonium bicarbonate buffer (pH 8), resulted in the generation of a 24kDa protein with a concomitant decrease of PO protein. The conversion of PO into 24kDa protein was blocked by heating isolated myelin at 100°C for 5 min suggesting that the reaction is enzyme mediated. Inclusion of the protease inhibitors and chelating agent to isolated myelin did not prevent the formation of 24kDa protein. Similarly, addition of CaCl2 to isolated myelin did not accentuate the formation of 24kDa protein suggesting that the conversion of PO into 24kDa protein may not be due to Ca2+ activated protease. It is postulated that the formation of 24kDa protein may be due to neutral protease and/or metalloproteinase associated with the PNS myelin. 24kDa protein was purified and characterized. The N-terminal sequence of 1–17 amino acid residues of 24kDa protein was identical to PO. 24kDa protein was immunostained and immunoprecipitated with anti-PO antiserum indicating the immunological similarities between PO and 24kDa protein. Labeling of 24kDa protein with [35S]methionine provided evidence that PO may be in all probability cleaved between Met-168 and Met-193. Further studies were carried out to demonstrate that 24kDa protein was phosphorylated, glycosylated and acylated like PO. Phosphorylation of 24kDa protein in the nerve slices was increased five-fold by phorbol esters and phosphoserine was the only phosphoamino acid identified after partial acid hydrolysis of 24kDa protein. These results suggested that serine residue phosphorylated by protein kinase C may be located in amino acid residues 1-168. 24kDa protein was stained with periodic Schiff reagent. In addition, 24kDa protein was fucosylated and the fucosylation of 24kDa protein was inhibited (70%) by tunicamycin, providing evidence that it is N-glycosylated. Recently, it was demonstrated that both PO and 24kDa protein were fatty acylated with [3H]palmitic acid in the nerve slices and fatty acids are covalently linked to these proteins (Agrawal, H.C. and Agrawal, D. 1989, Biochem. J. 263:173–177). The time course of inhibition of acylation by cycloheximide of 24kDa protein was identical to PO. Cycloheximide inhibited acylation of PO and 24kDa protein by 61% and 58% respectively, whereas, monensin had little affect on the fatty acylation of these proteins. Less [3H]palmitic acid and14C-amino acids were incorporated into 24kDa protein when compared to PO between 5–30 min after incubation of the nerve slices. However, more radioactivity was incorporated into 24kDa protein after 60 min when compared to PO under identical conditions. These results provided evidence of a precursor-product relationship between PO and 24kDa protein. Therefore, PO may be cleaved into 24kDa protein in the myelin membrane following its acylation and glycosylation in the Schwann cells. 相似文献
38.
39.
The aim of the study was to see whether adults who had been sexually abused in childhood were vulnerable to physical symptoms and therefore investigation and intervention. The case histories of seven patients who were aged 22-39, were under the care of three consultant psychiatrists, had experienced childhood sexual abuse, and had a history of medical or surgical intervention were surveyed. The patients had had a mean of 18 contacts with non-psychiatric consultant teams and a mean of eight operations, with a high rate (66-70%) of normal findings. They had experienced many somatic symptoms, which led to investigations and interventions in the specialties of gynaecology, obstetrics, gastroenterology, urology, rheumatology, haematology, orthopaedics, neurology, and neuropsychiatry. The history of childhood sexual abuse was recognised only in the later stages of this medical and surgical intervention. The possibility of childhood sexual abuse should be considered earlier in such cases to prevent further unnecessary intervention. 相似文献
40.
A water-soluble fraction from adult bone stimulates the differentiation of cartilage in explants of embryonic muscle 总被引:2,自引:0,他引:2
Paul A. Lucas Glenn T. Syftestad Arnold I. Caplan 《Differentiation; research in biological diversity》1988,37(1):47-52
A water-soluble fraction of a 4 M guanidine HCl extract of demineralized adult bovine bone stimulated the differentiation of cartilage in explants of minced skeletal muscle from embryonic chick legs; cartilage was also induced by a semipurified protein preparation. Cartilage could be identified in treated cultures at 1 week with muscle from day-9 embryos, not before 2 weeks with muscle from day-12 embryos, and not before 3 weeks with muscle from day-19 embryos. The ability to respond to this water-soluble fraction by exhibiting cartilage differentiation was dose-dependent, but not confined to any particular muscle region of the day-12 embryonic leg. These observations indicate that bone-derived soluble chondroinductive agents act on cells in minced embryonic muscle preparations. The induction of cartilage is dependent upon the accessibility of the responding cells to the agents, on the concentration of inductive agents, and on the developmental age of the responsive tissue. 相似文献