全文获取类型
收费全文 | 5641篇 |
免费 | 720篇 |
国内免费 | 3篇 |
出版年
2022年 | 37篇 |
2021年 | 84篇 |
2020年 | 40篇 |
2019年 | 50篇 |
2018年 | 61篇 |
2017年 | 50篇 |
2016年 | 97篇 |
2015年 | 168篇 |
2014年 | 184篇 |
2013年 | 227篇 |
2012年 | 273篇 |
2011年 | 292篇 |
2010年 | 150篇 |
2009年 | 135篇 |
2008年 | 266篇 |
2007年 | 254篇 |
2006年 | 243篇 |
2005年 | 242篇 |
2004年 | 210篇 |
2003年 | 196篇 |
2002年 | 187篇 |
2001年 | 203篇 |
2000年 | 212篇 |
1999年 | 156篇 |
1998年 | 75篇 |
1997年 | 73篇 |
1996年 | 67篇 |
1995年 | 59篇 |
1994年 | 74篇 |
1993年 | 78篇 |
1992年 | 147篇 |
1991年 | 134篇 |
1990年 | 129篇 |
1989年 | 113篇 |
1988年 | 105篇 |
1987年 | 121篇 |
1986年 | 98篇 |
1985年 | 101篇 |
1984年 | 96篇 |
1983年 | 71篇 |
1982年 | 63篇 |
1981年 | 66篇 |
1980年 | 48篇 |
1979年 | 67篇 |
1978年 | 51篇 |
1977年 | 43篇 |
1976年 | 46篇 |
1975年 | 39篇 |
1974年 | 42篇 |
1973年 | 58篇 |
排序方式: 共有6364条查询结果,搜索用时 463 毫秒
81.
Mixtures of pre-formed microtubules, polymerized from chicken erythrocyte and brain tubulin, rapidly anneal end-to-end in vitro in standard microtubule assembly buffer. The erythrocyte tubulin segments in annealed heteropolymers can be distinguished by an immunoelectron microscopic assay that uses an antibody specific for chicken erythrocyte beta-tubulin. An annealing process is consistent with the following observations: (a) Microtubule number decreases while the polymer mass remains constant. (b) As the total number of microtubules declines, the number of heteropolymers, and the number of segments contained in each heteropolymer, increases. (c) The size of the segments determined after annealing and antibody labeling is the same as the original microtubule polymers. (d) Points of discontinuity in the annealing heteropolymers can be observed directly by electron microscopy, and correspond to type-specific polymer domains. The junctions probably represent initial contact points during the annealing process. Microtubule annealing occurs rapidly in vitro and may be significant for determining properties of microtubule dynamics in vivo. 相似文献
82.
Growth inhibition of 3T3 fibroblasts by lysosomotropic amines: correlation with effects on intravesicular pH but not vacuolation 总被引:5,自引:0,他引:5
The effects of five lysosomotropic amines on the growth of Swiss 3T3 fibroblasts were measured and compared with effects on intravesicular pH. Tributylamine and benzylamine, amines that affect intravesicular pH without causing vacuolation, were found to inhibit cell growth to a similar extent as vacuologenic amines previously tested. Excellent correlation between the half-maximal concentrations for the growth and pH effects were found for tributylamine, benzylamine, chloroquine, and ammonium chloride. The results suggest that growth inhibition by these amines is a direct result of their effects on pH and not due to other effects (such as vacuolation). In contrast, a 100-fold difference in the half-maximal concentrations was found for methylamine, suggesting that methylamine inhibits growth by a mechanism unrelated to pH. 相似文献
83.
Independence of exercise hyperpnea and acidosis during high-intensity exercise in ponies 总被引:1,自引:0,他引:1
Pan L. G.; Forster H. V.; Bisgard G. E.; Murphy C. L.; Lowry T. F. 《Journal of applied physiology》1986,60(3):1016-1024
We investigated arterial PCO2 (PaCO2) and pH (pHa) responses in ponies during 6-min periods of high-intensity treadmill exercise. Seven normal, seven carotid body-denervated (2 wk-4 yr) (CBD), and five chronic (1-2 yr) lung (hilar nerve)-denervated (HND) ponies were studied during three levels of constant load exercise (7 mph-11%, 7 mph-16%, and 7 mph-22% grade). Mean pHa for each group of ponies became alkaline in the first 60 s (between 7.45 and 7.52) (P less than 0.05) at all work loads. At 6 min pHa was at or above rest at 7 mph-11%, moderately acidic at 7 mph-16% (7.32-7.35), and markedly acidic at 7 mph-22% (7.20-7.27) for all groups of ponies. Yet with no arterial acidosis at 7 mph 11%, normal ponies decreased PaCO2 below rest (delta PaCO2) by 5.9 Torr at 90 s and 7.8 Torr by 6 min of exercise (P less than 0.05). With a progressively more acid pHa at the two higher work loads in normal ponies, delta PaCO2 was 7.3 and 7.8 Torr by 90 s and 9.9 and 11.4 Torr by 6 min, respectively (P less than 0.05). CBD ponies became more hypocapnic than the normal group at 90 s (P less than 0.01) and tended to have greater delta PaCO2 at 6 min. The delta PaCO2 responses in normal and HND ponies were not significantly different (P greater than 0.1).(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
84.
Purification and characterization of a bone metalloproteinase that degrades gelatin and types IV and V collagen 总被引:12,自引:0,他引:12
A third metalloendopeptidase activity, gelatinase, has been completely separated from the collagenase and proteoglycanase activities of rabbit bone culture medium. Although the proteinase could not be purified to homogeneity in large amounts, it was possible to obtain accurate molecular weight values and activity after electrophoresis on non-reduced SDS/polyacrylamide gels. The latent form had an Mr of 65 000 which could be activated with 4-aminophenylmercuric acetate, APMA, to a form of Mr 61 000; under reducing conditions the latent and active forms had Mr of 72 000 and 65 000, respectively. Trypsin was a very poor activator of the latent enzyme. Gelatinase degraded gelatins derived from the interstitial collagens and it also had low activity on native types IV and V collagen and on insoluble elastin. Gelatinase acted synergistically with collagenase in degrading insoluble interstitial collagen. The specific mammalian tissue inhibitor of metalloproteinases inhibited gelatinase by forming a stable inactive complex. Comparison of the properties of gelatinase with those of collagenase and proteoglycanase suggest that the three proteinases form a family which together are capable of degrading all the major macromolecules of connective tissue matrices. 相似文献
85.
Octapeptide analogs of somatostatin exhibiting greatly enhanced in vivo and in vitro inhibition of growth hormone secretion in the rat 总被引:1,自引:0,他引:1
W A Murphy M L Heiman V A Lance I Mezo D H Coy 《Biochemical and biophysical research communications》1985,132(3):922-928
Analogs of a superactive somatostatin (SRIF) octapeptide (code named SMS 201-995 (1)) were synthesized using solid-phase synthetic methodology and assayed for their ability to inhibit growth hormone release from cultured rat anterior pituitary cells and in sodium pentobarbital-anesthetized rats. One analog: (Formula: see text) exhibited greatly enhanced in vitro inhibitory activity (greater than 1,000x) relative to both the parent octapeptide molecule and to the 14 amino acid SRIF molecule. This analog which was also very potent in vivo contains a tyrosine residue and, given its high in vitro activity, may be of investigative importance as a radioiodinated ligand in receptor assays. An octapeptide retro-inverso analog also exhibited significant SRIF-like activity. Several very low activity octapeptide analogs were synthesized and were found to be devoid of SRIF-antagonist activity. A dodecapeptide analog previously shown to be superactive in vivo also demonstrated high in vitro activity. 相似文献
86.
The effects of 5 alpha-dihydroprogesterone (5 alpha-DHP) and 3 alpha, 5 alpha-tetrahydroprogesterone (3 alpha, 5 alpha-THP) on follicle-stimulating hormone (FSH) and luteinizing hormone (LH) release were examined in the pregnant mare's serum gonadotropin (PMSG)-primed immature female rat (8 IU PMSG at 28 days of age) maintained in constant light. Control rats kept in 14L:10D conditions exhibited proestrous-like surges of LH and FSH release with peak levels attained at 1800 h on the second day after PMSG treatment. In rats exposed to constant light, the PMSG-induced surges of LH and FSH were not only delayed until 1000 h on the third day after PMSG, resulting in a delay in ovulation, but were also significantly attenuated when compared to the gonadotropin surges that occurred on Day 2 in rats kept under normal light-dark conditions. The administration of 5 alpha-DHP significantly enhanced the release of FSH at 1000 h on Day 3 when compared to constant light-exposed controls, but had no effect on LH. Treatment with 3 alpha, 5 alpha-THP selectively potentiated the release of LH at 1000 h on Day 3 and had an attenuating effect on FSH release on Days 2 and 3. These observations confirm earlier findings in the immature ovariectomized estrogen-primed rat and suggest that 5 alpha-DHP and 3 alpha, 5 alpha-THP may have significant roles in the regulation of FSH and LH secretion. 相似文献
87.
Nucleotide sequence of a spectinomycin adenyltransferase AAD(9) determinant from Staphylococcus aureus and its relationship to AAD(3") (9) 总被引:16,自引:0,他引:16
Ellen Murphy 《Molecular & general genetics : MGG》1985,200(1):33-39
Summary The nucleotide sequence of the spc determinant of the Staphylococcus aureus transposon Tn554 has been determined. This gene encodes a spectinomycin adenyltransferase, AAD(9), that mediates resistance to spectinomycin but not to streptomycin. The sequence predicts a 260 amino acid protein of molecular weight 28,943. A spectinomycin-sensitive mutant (spc-1) contains a GA transition resulting in substitution of threonine (ACA) for alanine (GCA) at residue 165. The predicted amino acid sequence is 36% homologous to that of a widely distributed, gramnegative streptomycin/spectinomycin adenyltransferase, AAD(3) (9), specified by the aadA determinant (Holingshead and Vapnek 1985). 相似文献
88.
We tested a number of inhibitory monovalent anions for their primary site of action on photosystem II(PSII) in chloroplasts. We find that the inhibitory effects of F−, HCO2−, NO2−, NO3−, and CH3CO2− are all reversed by addition of a high concentration of HCO3−. This class of anions competitively inhibits H14CO3− binding to PSII. All of those anions tested reduced H14CO3− binding more in the light than in the dark. We conclude that the primary inhibitory site of action of a number of monovalent anions is at the HCO3− binding site(s) on the PSII complex. The carbonic anhydrase inhibitor gold cyanide, and also azide, inhibit PSII but at a site other than the HCO3− binding site. We suggest that the unique ability of HCO3− to reverse the effects of inhibitory anions reflects its singular ability to act as a proton donor/acceptor at the anion binding site. A similar role has been proposed for non-substrate-bound HCO3− on carbonic anhydrase by Yeagle et al. (1975 Proc Natl Acad Sci USA 72: 454-458). 相似文献
89.
Biosynthesis of the glycoprotein tissue inhibitor of metalloproteinases (TIMP) by human fibroblasts in culture has been characterized by functional assays, immunoprecipitation, and immunocytochemistry with a monospecific antiserum. As determined by radiolabeling with [35S]methionine, immunoprecipitation, and analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the secreted form of TIMP had an Mr of 29,000, whereas the form associated with the cell layer had an Mr of 24,000. Unstimulated human lung fibroblasts (HFL-1) secreted TIMP at the rate of approximately 2 micrograms/10(6) cells/24 h, and normal foreskin fibroblasts (HS 27) and skin fibroblasts from a patient with Hurler's disease (GM 1391) secreted TIMP at 0.3 and 0.2 micrograms/10(6) cells/24 h, respectively. Secretion of TIMP was stimulated up to 10-fold by treating the cells with 20-100 ng/ml of 12-O-tetradecanoylphorbol 13-acetate or 10 units/ml of human interleukin 1. In the stimulated HFL-1 cells, TIMP accounted for 0.03-0.09% of the total [35S]methionine incorporated into protein, and 0.3-0.8% of the [35S]methionine in secreted protein. Although TIMP accounted for a relatively small proportion of total protein synthesis of the fibroblasts, greater than 80% of untreated and greater than 95% of stimulated fibroblasts synthesized TIMP, as determined by indirect immunofluorescence. The treatments of the human fibroblasts that increased TIMP secretion also induced synthesis and secretion of proenzyme forms of collagenase, indicating that degradative enzymes and their controlling inhibitors may be synthesized in parallel under certain conditions. 相似文献
90.
Detection and physical map of a omega tox+-related defective prophage in Corynebacterium diphtheriae Belfanti 1030(-)tox-. 总被引:1,自引:0,他引:1
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A library of chromosomal DNA from Corynebacterium diphtheriae Belfanti 1030(-)tox- was cloned in the lambda phage vector EMBL4 and screened for sequences homologous to corynephage omega tox+ and the attB1-attB2 region of the C7(-)tox- chromosome. Two portions of the 1030(-)tox- chromosome, 35 and 30.5 kilobases long which contain, respectively, the entire region homologous to corynephage omega tox+ and the attB1-attB2 sites, were mapped with the restriction endonucleases BamHI and EcoRI. Chromosomal DNA from 1030(-)tox- was shown to contain a 15.5-kilobase region that was homologous to ca. 42% of the corynephage omega tox+ genome. These sequences were found to hybridize to three regions of the phage genome and do not contain either the diphtheria tox operon or the attP site. These sequences are distant from the chromosomal region that contains the attB1-attB2 sites. Moreover, unlike other known defective prophages, the physical map of this prophage starts at the cos site and is colinear with the vegetative phage map. The 30.5-kilobase region of the 1030(-)tox- chromosome, which contains the attB1-attB2 sites, has a central core region that is almost identical to the corresponding region of the C7(-)tox- chromosome; however, the flanking sequences in these two strains of C. diphtheriae are different. 相似文献