全文获取类型
收费全文 | 6497篇 |
免费 | 850篇 |
国内免费 | 57篇 |
出版年
2021年 | 83篇 |
2019年 | 59篇 |
2016年 | 90篇 |
2015年 | 132篇 |
2014年 | 173篇 |
2013年 | 240篇 |
2012年 | 301篇 |
2011年 | 294篇 |
2010年 | 183篇 |
2009年 | 189篇 |
2008年 | 256篇 |
2007年 | 222篇 |
2006年 | 237篇 |
2005年 | 219篇 |
2004年 | 225篇 |
2003年 | 168篇 |
2002年 | 199篇 |
2001年 | 190篇 |
2000年 | 194篇 |
1999年 | 176篇 |
1998年 | 118篇 |
1997年 | 67篇 |
1996年 | 78篇 |
1995年 | 95篇 |
1994年 | 85篇 |
1993年 | 78篇 |
1992年 | 150篇 |
1991年 | 153篇 |
1990年 | 127篇 |
1989年 | 147篇 |
1988年 | 142篇 |
1987年 | 123篇 |
1986年 | 109篇 |
1985年 | 103篇 |
1984年 | 91篇 |
1983年 | 89篇 |
1982年 | 63篇 |
1981年 | 69篇 |
1980年 | 59篇 |
1979年 | 106篇 |
1978年 | 71篇 |
1977年 | 82篇 |
1976年 | 72篇 |
1975年 | 78篇 |
1974年 | 96篇 |
1973年 | 75篇 |
1972年 | 79篇 |
1971年 | 68篇 |
1970年 | 66篇 |
1968年 | 54篇 |
排序方式: 共有7404条查询结果,搜索用时 31 毫秒
91.
Posttranslational modification at the N terminus of the human adenovirus type 12 E1A 235R tumor antigen. 总被引:2,自引:1,他引:1 下载免费PDF全文
The adenovirus E1A transforming region, which encodes immortalization, partial cell transformation, and gene activation functions, expresses two early mRNAs, 13S and 12S. Multiple-T antigen species with different electrophoretic mobilities are formed from each mRNA, presumably by unknown posttranslational modifications. The adenovirus type 12 (Ad12) 13S and 12S mRNAs encode E1A T antigens of 266 and 235 amino acid residues (266R and 235R), respectively. To study possible posttranslational processing at the N and C termini and to distinguish between the Ad12 266R and 235R T antigens, we prepared antibodies targeted to synthetic peptides encoded at the common C (peptide 204) and N (peptide 202) termini of the 266R and 235R T antigens and at the unique internal domain of the 266R T antigen (peptide 206). The specificity of each anti-peptide antibody was confirmed by immunoprecipitation of the 266R and 235R T antigens produced in Escherichia coli. Immunoprecipitation analysis of the E1A T antigens synthesized in Ad12-infected KB cells revealed the following. Antibody to the common C terminus recognized three T antigens with apparent Mrs of 43,000, 42,000, and 39,000 (43K, 42K, and 39K). All three forms were phosphorylated and were present in both the nucleus and the cytoplasm. The 43K and 42K T antigens were rapidly synthesized during a 10-min pulse with [35S]methionine in Ad12-infected cells. The 43K T antigen had a half-life of 20 min, the 42K T antigen had a longer half-life of about 40 min, and the 39K T antigen became the predominant E1A T antigen. Antibodies to the unique region immunoprecipitated the 43K T antigen but not the 42K and 39K T antigens. Antibody to the N terminus immunoprecipitated the 43K and 42K T antigens but not the 39K T antigen, suggesting that the 39K T antigen possessed a modified N terminus. Partial N-terminal amino acid sequence analysis showed that the 43K and 42K T antigens contain methionine at residues 1 and 5, as predicted from the DNA sequence, whereas no methionine was released from the 39K T antigen during the first six cycles of Edman degradation. We propose that the short-lived 43K T antigen is the primary product of the 13S mRNA, the 266R T antigen; the somewhat more stable 42K T antigen is the primary product of the 12S mRNA, the 235R T antigen.(ABSTRACT TRUNCATED AT 400 WORDS) 相似文献
92.
Rubella virus antigens: localization of epitopes involved in hemagglutination and neutralization by using monoclonal antibodies. 总被引:15,自引:5,他引:10 下载免费PDF全文
Monoclonal antibodies (MAbs) against the rubella virion were used to locate epitopes involved in hemagglutination and neutralization. The MAbs exhibiting high-level hemagglutination-inhibiting activity were shown by Western blot analysis to be specific for the E1 polypeptide; this is consistent with the presence of the hemagglutinin on the E1 polypeptide. Some of the E1-specific MAbs also neutralized viral infectivity. However, hemagglutination-inhibiting activity and neutralizing activity did not always correlate. Three distinct functional epitopes were identified on the E1 polypeptide by competition analyses: one which reacted with MAbs with high-level hemagglutination-inhibiting activity and with neutralizing activity, one which reacted with MAbs with low-level hemagglutination-inhibiting activity and with neutralizing activity, and one which reacted with MAbs with only hemagglutination-inhibiting activity. A MAb specific for the E2 polypeptide exhibited neutralizing activity. This E2-specific MAb and two E1-specific MAbs with neutralizing activity were capable of precipitating intact virus which indicates that at least three epitopes involved in neutralization are accessible on the surface of the virion. 相似文献
93.
Biosynthesis of the Macrocyclic Diterpene Casbene in Castor Bean (Ricinus communis L.) Seedlings : The Purification and Properties of Farnesyl Transferase from Elicited Seedlings 总被引:1,自引:0,他引:1 下载免费PDF全文
Farnesyl transferase (farnesyl pyrophosphate: isopentenyl pyrophosphate farnesyl transferase; geranylgeranyl pyrophosphate synthetase) was purified at least 400-fold from extracts of castor bean (Ricinus communis L.) seedlings that were elicited by exposure for 10 h to Rhizopus stolonifer spores. The purified enzyme was free of isopentenyl pyrophosphate isomerase and phosphatase activities which interfere with prenyl transferase assays. The purified enzyme showed a broad optimum for farnesyl transfer between pH 8 and 9. The molecular weight of the enzyme was estimated to be 72,000 ± 3,000 from its behavior on a calibrated G-100 Sephadex molecular sieving column. Mg2+ ion at 4 millimolar gave the greatest stimulation of activity; Mn2+ ion gave a small stimulation at 0.5 millimolar, but was inhibitory at higher concentrations. Farnesyl pyrophosphate (Km = 0.5 micromolar) in combination with isopentenyl pyrophosphate (Km = 3.5 micromolar) was the most effective substrate for the production of geranylgeranyl pyrophosphate. Geranyl pyrophosphate (Km = 24 micromolar) could replace farnesyl pyrophosphate as the allylic pyrophosphate substrate, but dimethylallyl pyrophosphate was not utilized by the enzyme. One peak of farnesyl transferase activity (geranylgeranyl pyrophosphate synthetase) and two peaks of geranyl transferase activity (farnesyl pyrophosphate synthetases) from extracts of whole elicited seedlings were resolved by DEAE A-25 Sephadex sievorptive ion exchange chromatography. These results suggest that the pathway for geranylgeranyl pyrophosphate synthesis in elicited castor bean seedlings involves the successive actions of two enzymes—a geranyl transferase which utilizes dimethylallypyrophosphate and isopentenyl pyrophosphate as substrates and a farnesyl transferase which utilizes the farnesyl pyrophosphate produced in the first step and isopentenyl pyrophosphate as substrates. 相似文献
94.
95.
The NADPH:O2 oxidoreductase of human neutrophils. Stoichiometry of univalent and divalent reduction of O2 总被引:1,自引:0,他引:1
The ratio of superoxide production to oxidation of NADPH affected by the NADPH:O2 oxidoreductase of human neutrophils is strongly influenced by pH, NADPH substrate concentration, aging of the enzyme, or its exposure to excess deoxycholate. Freshly prepared enzyme exhibited a Km for NADPH of 52 microM as determined by assaying NADPH oxidase activity, or approximately 33 microM by measurement of superoxide formation. In the range of 100-150 microM NADPH at pH 7.6 and in the presence of 0.06% deoxycholate, the univalent flux of electron equivalents given up by NADPH to O2 was 99%. Following storage of the oxidoreductase overnight on ice, its Km for NADPH rose to 125 microM as determined by monitoring oxidation of NADPH but was unaltered when measured in terms of superoxide production. Concomitantly, its capacity to affect univalent reduction of O2 fell approximately 20-30% under the same assay conditions. Univalent flux rates of less than 40% were observed with exposure of the enzyme to concentrations of deoxycholate in excess of 0.1% or to pH values below 6.0 or above 8.0. The capacity of the enzyme to carry out univalent reduction fell with increasing NADPH concentrations in a manner resembling that previously reported with increasing concentrations of xanthine in the case of xanthine oxidase (Fridovich, I. (1970) J. Biol. Chem. 245, 4053-4057). The reduced form of the neutrophil oxidoreductase, like xanthine oxidase, thus appears to be capable of conducting both 1- and 2-electron transfer steps in reducing O2. Its capacity to affect univalent reduction of O2 depends upon the concentration of electron donor (NADPH) supplied as well as conditions of storage and assay of the native enzyme. 相似文献
96.
The adaptation to extrauterine nutrition involves complex physiological changes at birth which may be regulated by genetic endowment; enteral nutrients, secretions, and bacteria; and endogenous hormones and exogenous hormones in breast milk. The hypothesis is explored that enteral feeding after birth may trigger key adaptations in the gut and in metabolism partly through the mediation of gastrointestinal hormone secretion. Gut peptides are found in the early human fetal gut and by the second trimester some are found in high concentrations in the fetal circulation and amniotic fluid. Major plasma hormonal surges occur during the neonatal period in term and preterm infants: notably in enteroglucagon, gastrin, motilin, neurotensin, gastrointestinal peptide, and pancreatic polypeptide. These events do not occur in neonates deprived of enteral feeding. A progressive development of dynamic gut hormonal responses to feeding is observed. The pattern of gut endocrine changes after birth is influenced by the type and route of feeding. Potential pathophysiological effects of depriving high risk neonates of enteral feeding are considered. It is speculated that infants committed to prolonged total parenteral nutrition from birth may benefit from the biological effects of intraluminal nutrients used in subnutritional quantities. 相似文献
97.
Prevost, I. and Le PageDegivry, M. Th. 1985. Changesin absicisic acid content in axis and cotyledons of developingPhaseolus vulgaris embryos and their physiological consequences.J.exp. Bot. 36: 19001905.Changes in abscisic acid (ABA)content with time were measured in embryonic axes and in cotyledonsof Phaseolus vulgaris embryos using a radioimmunoassay.During embryogenesis, a similar pattern was observed in bothtissues: ABA increased to a maximum 29 d after an thesis, followedby a decrease as the seed matured. The level of ABA in the cotyledonswas always much higher than that in the axes. In in vitro cultures,the duration of the lag phase before germination of isolatedembryonic axes increased with ABA content. The presence of cotyledonsalways lengthened the lag phase; longer lag phases were associatedwith greater concentrations of ABA in the cotyledons. Moreoverthe presence of cotyledons stimulated the growth of seedlings. Key words: ABA distribution, embryo maturation, axis and embryo germinability 相似文献
98.
R S Green 《Journal of general microbiology》1985,131(11):2871-2876
Three proteinase isoenzymes from one benign strain of Bacteroides nodosus and five proteinase isoenzymes from each of two virulent strains of B. nodosus were purified by horizontal slab polyacrylamide gel electrophoresis. The purified isoenzymes hydrolysed casein, collagen I, collagen III, elastin, alpha-elastin, fibrinogen, gelatin, haemoglobin and alpha-keratin. The pH optima of all the isoenzymes lay between 7.25 and 9.5, the range of 8.75-9.25 being common to all. The isoenzymes were inhibited by phenylmethylsulphonyl fluoride, diphenylcarbamyl chloride, L-(1-tosylamide-2-phenyl)ethyl chloromethyl ketone, EGTA and EDTA, indicating that they were chymotrypsin-like serine proteinases that require a metal ion for stability or activity. EDTA inhibition was not reversed by addition of Ca2+ or Mg2+. Some isoenzymes were activated by Mg2+, Ca2+, Cr3+ and Se4+ and all were inhibited by Fe2+, Co2+, Cu2+, Zn2+, Cd2+ and Hg2+. Isoenzymes from benign strains had a lower temperature stability, losing all activity at 55 degrees C, whereas those from virulent strains lost all activity at 60 degrees C. 相似文献
99.
Suppression and contrasuppression in the induction of contact sensitivity by the administration of cellbound antigen-antibody complexes 总被引:1,自引:0,他引:1
W Ptak M Bereta M Ptak G M Iverson D R Green 《Journal of immunology (Baltimore, Md. : 1950)》1985,135(4):2312-2318
The tolerogenic signal produced by the i.v. injection of haptenated peritoneal exudate cells can be converted to an immunogenic signal by treating the cells with antibody to the hapten before administration. We examined this phenomenon and found that immunity induced by antigen-antibody complexes, as opposed to skin sensitization, is resistant to suppressor T cell influences. This resistance to suppression is due to the activation of an I-J+, Ly-1 T cell population which adheres to the Vicia villosa lectin, all characteristics of contrasuppressor T cells. Because haptenated cells can induce immunity if injected subcutaneously or into cyclophosphamide-pretreated recipients (thereby avoiding the induction of suppressor cells), we suggest that the activation of contrasuppressor cells by antigen-antibody complexes overrides suppressive influences in the host, allowing immunity to become dominant. The possible roles of suppression and contrasuppression in channeling the effector arm of the immune response (e.g., contact sensitivity vs humoral immunity) are discussed. 相似文献
100.
D. G. Green 《Plant and Soil》1985,86(2):291-294
Summary The effect of applying (2-chloroethyl)trimethylammonium chloride (CCC) or gibberellic acid (GA) as foliar sprays on internodal development of barley was studied. CCC applied to the whole plant at main tiller leaf stages 1, 2 or 3 decreased shoot elongation, and prevented elongation of internode 6 (internode 5 subtended the head). CCC at all stages delayed senescence of the lower leaves. CCC sprayed at all 6 leaf stages and GA sprayed at main tiller leaf stages 1, 2, 3 or 4 reduced plant height at maturity. GA treatment at leaf stages 2, 3 or 4 initially stimulated internodal elongation; elongation of later developed internodes was inhibited resulting in shorter plants at maturity. Only the treatment with GA at leaf stages 5 and 6 resulted in increased plant height. 相似文献