全文获取类型
收费全文 | 11313篇 |
免费 | 1126篇 |
国内免费 | 3篇 |
出版年
2023年 | 48篇 |
2022年 | 112篇 |
2021年 | 228篇 |
2020年 | 99篇 |
2019年 | 152篇 |
2018年 | 185篇 |
2017年 | 171篇 |
2016年 | 310篇 |
2015年 | 475篇 |
2014年 | 520篇 |
2013年 | 615篇 |
2012年 | 884篇 |
2011年 | 881篇 |
2010年 | 563篇 |
2009年 | 516篇 |
2008年 | 725篇 |
2007年 | 702篇 |
2006年 | 622篇 |
2005年 | 652篇 |
2004年 | 659篇 |
2003年 | 588篇 |
2002年 | 597篇 |
2001年 | 135篇 |
2000年 | 109篇 |
1999年 | 126篇 |
1998年 | 144篇 |
1997年 | 98篇 |
1996年 | 99篇 |
1995年 | 91篇 |
1994年 | 82篇 |
1993年 | 90篇 |
1992年 | 63篇 |
1991年 | 69篇 |
1990年 | 78篇 |
1989年 | 84篇 |
1988年 | 68篇 |
1987年 | 51篇 |
1986年 | 49篇 |
1985年 | 47篇 |
1984年 | 51篇 |
1983年 | 61篇 |
1982年 | 57篇 |
1981年 | 50篇 |
1980年 | 50篇 |
1979年 | 30篇 |
1978年 | 37篇 |
1977年 | 36篇 |
1975年 | 24篇 |
1973年 | 28篇 |
1971年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
Photocopying was found to be a rapid method of making a permanent record of a root sample. The method used produced a copy
with white roots against a black background.
Manual estimates of root length were made from photocopies using a light box. The number of intersections visible when laid
over a copy of a white on black regular square grid was counted. Automated estimates of root length were made by scanning
a photocopy with a bar code reader in place of a pen in a computer-driven graph plotter. Roots >0.2 mm diameter were resolved
with precision and speed. 相似文献
43.
The potential for radioprotection of growing cartilage by the thiophosphate WR2721 was evaluated in weanling rats using single fractions of irradiation. Protection of acute skin toxicity was monitored simultaneously. Single doses of 600, 1200, 1800, or 2400 cGy were administered to the left tibia of CrL:CD(SD)BR female rats in groups of 12. Identically treated groups were injected with 310 mg/kg WR2721 (2/3 the determined LD50/30) in a concentration of 26 mg/ml intraperitoneally 15 min prior to irradiation. Rats untreated or given WR2721 without radiation served as control groups. Radiographs of the irradiated and unirradiated tibiae for each animal were obtained weekly to the date of sacrifice at 80 days following the initial treatment. Skin toxicity was assessed weekly starting on the second week using Moulder's scale (J.E. Moulder, J.J. Fischer, and A. Casey, Radiology 115, 465-470 (1975]. No significant difference in bone growth as measured by tibial lengths for the WR2721-treated or untreated animals was observed. Skin toxicity including moist desquamation occurred in irradiated limbs and was substantially less in rats treated with WR2721. As opposed to previous work with cysteamine, WR2721 as administered had no significant radioprotective effect on tibial growth in weanling rats but substantially reduced the accompanying skin toxicity. 相似文献
44.
Post-translational processing of the porcine gastrin precursor by phosphorylation of the COOH-terminal fragment 总被引:5,自引:0,他引:5
G J Dockray A Varro H Desmond J Young H Gregory R A Gregory 《The Journal of biological chemistry》1987,262(18):8643-8647
The gene sequence encoding porcine preprogastrin is known; in order to clarify pathways of post-translational processing of the predicted precursor peptide we have characterized material reacting with antibodies to a synthetic peptide corresponding to the expected extreme COOH-terminal portion of the precursor. Radioimmunoassay was used to identify and monitor the purification of peptides in porcine antral mucosa. Two peptides (I and II) were isolated to homogeneity by steps involving gel filtration, ion exchange, and reversed-phase high performance liquid chromatography. The two co-eluted on gel filtration but were separated on anion-exchange chromatography. The more acidic peptide (II) was less hydrophobic on high performance liquid chromatography. Automated gas-phase microsequencing revealed the less acidic peptide (I) to have the sequence of porcine preprogastrin 96-104 (SAEEGDQRP); it would be produced by tryptic-like cleavage of Arg95-Ser96. The second peptide did not yield a phenylthiohydantoin-derivative on the first cycle but thereafter it sequenced as the first peptide (i.e. -AEEGDQRP). Incubation in alkali liberated almost equimolar amounts of phosphate from peptide II but not from I. In addition, alkaline phosphatase liberated phosphate and converted the acidic peptide to the less acidic one. The results suggest that serine in the first position is phosphorylated in peptide II but not I. The tripeptide -Ser(P)-Ala-Glu- also occurs in adrenocorticotropic hormone; this tripeptide is a substrate for physiological casein kinase. Potential phosphorylation sites occur at comparable positions in the precursors of a number of regulatory peptides. 相似文献
45.
Timothy J. Strabala Sebastian Y. Bednarek Gregory Bertoni Richard M. Amasino 《Molecular & general genetics : MGG》1989,216(2-3):388-394
Summary A 1.9 kb clone of the T-DNA region of the Agrobacterium tumefaciens Ti plasmid Bo542 which exhibited homology to the isopentenyl transferase (ipt) locus of pTiA6 was identified by low stringency DNA hybridization. Introduction of this segment of pTiBo542 DNA into cells of Nicotiana tabacum or N. glauca caused tumor formation in vivo, and allowed hormone independent growth in vitro. Furthermore, this DNA segment complemented ipt mutant strains of A. tumefaciens, restoring their ability to cause tumors on Kalanchöe leaves and tomato stems. The complete DNA sequence of this segment has been determined, revealing an open reading frame homologous to other known Agrobacterium ipt genes. 相似文献
46.
Biotransformation of mercury by bacteria isolated from a river collecting cinnabar mine waters 总被引:1,自引:0,他引:1
One hundred six strains of aerobic bacteria were isolated from the Fiora River which drains an area of cinnabar deposits in southern Tuscany, Italy. Thirty-seven of the strains grew on an agar medium containing 10g/ml Hg (as HgCl2) with all of these strains producing elemental mercury. Seven of the 37 strains also degraded methylmercury. None of 106 sensitive and resistant strains produced detectable monomethylmercury although 15 strains produced a benzene-soluble mercury species. Two strains of alkylmercury (methyl-, ethyl- and phenylmercury) degrading bacteria were tested for the ability to degrade several other analogous organometals and organic compounds, but no activity was detected toward these compounds. Mercury methylation is not a mechanism of Hg resistance in aerobic bacteria from this environment. Growth of bacteria on the agar medium containing 10g/ml HgCl2 was diagnostic for Hg detoxification based on reduction. 相似文献
47.
The paratypes of Trebius akajeii Shiino, 1954, originally collected from Dasyatis akajei (Müller & Henle) at Owase, Japan, and the paratypes and newly collected specimens of T. latifurcatus Wilson, 1921, previously collected from Urolophus and Myliobatis at Venice, California, are redescribed. New host records for T. latifurcatus are: Torpedo californica Ayres, Squatina californica Ayres, Rhinobatos productus (Ayres), Platyrhinoides triseriata (Jordan & Gilbert), Raja inornata (Jordan & Gilbert) and Gymnura marmorata (Cooper). A new species, Trebius heterodonti, is described from the horn shark Heterodontus francisci (Girard) collected from southern California waters. These three species of Trebius are apparently closely related to each other. Diagnostic characters of the species are provided along with a discussion on the taxonomic value of selected features. 相似文献
48.
The constitution and properties of phosphorylated and unphosphorylated C-terminal fragments of progastrin from dog and ferret antrum 总被引:1,自引:0,他引:1
Antibodies to the extreme C-terminal tryptic (nona-) peptide fragment of porcine progastrin have been used in radioimmunoassay to identify progastrin fragments in dog, ferret and pig antral mucosa extracts and to monitor their purification. In addition to previously characterised phosphorylated and unphosphorylated C-terminal tryptic peptides of porcine progastrin a minor form corresponding to the C-terminal octapeptide (i.e. des-Ser C-terminal nonapeptide) was isolated and characterised. The latter form together with phosphorylated and unphosphorylated forms of the nonapeptides were also isolated and chemically characterised from dog antrum, and the unphosphorylated nonapeptide was characterised from ferret antrum. The primary amino acid sequences of the dog, ferret and pig nonapeptides were identical. In ferret the unphosphorylated nonapeptide predominated, and in dog the phosphorylated form predominated; in pig both forms of the nonapeptide were well represented. Intact progastrin was identified in gel filtration eluates of extracts of all 3 species, but occurred only in relatively low concentrations. The nonapeptides did not stimulate acid secretion in the conscious gastric fistula rat and they did not modify the acid response to G17. Phosphorylation of progastrin-derived peptides is evidently well conserved across a range of species even though there appear to be differences in the relative proportions of phosphorylated and unphosphorylated forms. 相似文献
49.
Class II MHC molecules and the HIV gp 120 envelope protein interact with functionally distinct regions of the CD4 molecule. 总被引:9,自引:1,他引:8 下载免费PDF全文
A murine T cell hybridoma with a receptor specific for the class I molecule H-2 Dd was transfected with an expressible cDNA for human CD4. Expression of the human class II MHC molecule HLA-DP on Dd-positive murine fibroblasts resulted in a greatly enhanced response of the CD4-positive T cell hybridoma, measured either by lymphokine production or by rosette formation. Inhibition of these functional assays with anti-CD4 monoclonal antibodies implicated the two amino-terminal domains of CD4 in an interaction with the HLA-DP molecule. This interaction was blocked by incubation with recombinant gp120 envelope protein of HIV. In contrast, recombinant soluble CD4 did not inhibit and was able to prevent the inhibition by gp120. Anti-CD4 antibody blocking experiments clearly indicated that distinct regions of CD4 interact respectively with gp120 and with class II MHC molecules. 相似文献
50.
Effect of orally-administered epidermal growth factor on intestinal iron absorption and mucosal permeability 总被引:1,自引:0,他引:1
A progressive increase in intestinal 59Fe3+ absorption was observed on oral feeding of mice with physiological doses of EGF/UGO. Maximal changes were apparent after 3d and appeared to be dose-dependent. In addition to a small increase in intestinal cell proliferation, as reflected by increased ornithine decarboxylase activity, EGF/UGO-feeding increased mucosal permeability (evaluated with [51Cr]-EDTA): the latter could account for the increase in iron absorption. Sialoadenectomy, to remove the major source of endogenous EGF/UGO, had no appreciable effect on the intestinal absorption of iron. 相似文献