全文获取类型
收费全文 | 3065篇 |
免费 | 334篇 |
国内免费 | 4篇 |
出版年
2021年 | 25篇 |
2020年 | 26篇 |
2019年 | 28篇 |
2018年 | 32篇 |
2017年 | 36篇 |
2016年 | 47篇 |
2015年 | 99篇 |
2014年 | 88篇 |
2013年 | 106篇 |
2012年 | 132篇 |
2011年 | 111篇 |
2010年 | 94篇 |
2009年 | 64篇 |
2008年 | 117篇 |
2007年 | 127篇 |
2006年 | 110篇 |
2005年 | 106篇 |
2004年 | 94篇 |
2003年 | 100篇 |
2002年 | 127篇 |
2001年 | 108篇 |
2000年 | 124篇 |
1999年 | 112篇 |
1998年 | 67篇 |
1997年 | 37篇 |
1996年 | 49篇 |
1995年 | 33篇 |
1994年 | 31篇 |
1993年 | 47篇 |
1992年 | 76篇 |
1991年 | 78篇 |
1990年 | 76篇 |
1989年 | 57篇 |
1988年 | 61篇 |
1987年 | 58篇 |
1986年 | 54篇 |
1985年 | 39篇 |
1984年 | 52篇 |
1983年 | 40篇 |
1982年 | 43篇 |
1981年 | 36篇 |
1979年 | 38篇 |
1978年 | 37篇 |
1976年 | 32篇 |
1975年 | 47篇 |
1974年 | 29篇 |
1973年 | 26篇 |
1972年 | 36篇 |
1971年 | 25篇 |
1967年 | 23篇 |
排序方式: 共有3403条查询结果,搜索用时 15 毫秒
31.
Michael Schmid Herbert Steinbeisser Hans-Henning Epperlein Michael F. Trendelenburg Hans J. Lipps 《Development genes and evolution》1992,201(6):340-345
Summary An expression vector was constructed containing the entire bovine papilloma virus (BPV-1) genome and part of the a-actin gene of Xenopus laevis cloned in the antisense orientation into the neomycin resistance gene under the control of the herpes simplex virus (HSV) thymidine kinase (TK) promoter. When this vector is microinjected into X. laevis embryos it replicates extrachromosomally, at least up to the tadpole stage, and a fusion RNA is synthesized after the mid blastula transition (MBT). The expression of the antisense gene results in a morphological abnormality of somites demonstrating that antisense RNA generated by an episomal replicating expression vector can inhibit the expression of a selected gene during early embryogenesis of X. laevis. 相似文献
32.
Human Alu subfamilies and their methylation revealed by blot hybridization. 总被引:13,自引:4,他引:9
下载免费PDF全文
![点击此处可从《Nucleic acids research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
C W Schmid 《Nucleic acids research》1991,19(20):5613-5617
33.
H. C. Howland M. J. Rowland K. Schmid J. D. Pettigrew 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1991,168(3):299-303
Summary In an examination of the focusing abilities of 15 species of owls, the North American barn owl, Tyto alba pratincola (Bonaparte 1838), was an outstanding accommodator, having a range of accommodation exceeding 10 diopters (Murphy and Howland 1983). Using comparable methods, we examined the accommodation of 4 specimens of the Australian barn owl, Tyto alba delicatula (Gould 1837). We failed to elicit accommodation greater than two diopters, and most stimuli failed to evoke any discernable accommodation at all. Furthermore, examination of other Australian tytonid owls, the grass owl, T. longimembris, the sooty owl, T. tenebricosa, and both the mainland and Tasmanian subspecies of the masked owl, T. novaehollandiae novaehollandiae and T. novaehollandiae castanops, also failed to reveal anything but very moderate accommodative ranges. We conclude that the outstanding accommodative ability of the American barn owl is truly an exception to the modest accommodative abilities of the tytonid owls generally. 相似文献
34.
Collagen type conservation during metamorphic repatterning of the dermal fibers in salamanders 总被引:1,自引:0,他引:1
The orientation of the fibers in the dermis of the tiger salamander, Ambystoma tigrinum, undergoes a dramatic repatterning at metamorphosis. The pre-metamorphic, larval dermis is a tight layer composed of crossed fibers that wind helically around the trunk. This condition is retained by neotenic adults which do not undergo metamorphosis. In contrast, the neotenic adults which do not undergo metamorphosis. In contrast, the metamorphosed adult dermis consists of a superficial, loose network of fibers invested with large multicellular glands--the stratum spongiosum--and a deeper tight layer of fibers--the stratum densum. However, unlike the crossed fibers of the pre-metamorphic dermis, there is no preferred orientation to the fibers in either layer of the post-metamorphic dermis. In order to evaluate whether these two distinctly different fiber patterns are constructed from biochemically similar fibers, the collagen types present in the pre- and post-metamorphic dermis were determined using SDS-polyacrylamide gel electrophoresis. Type I collagen is the predominant collagen of the dermis and the same major collagen types are present for all individuals, whether pre- or post-metamorphic. Thus, the major types of collagen that compose the dermal fibers do not change during metamorphic repatterning of the dermis. 相似文献
35.
Susceptibility of cartilage collagens type II, IX, X, and XI to human synovial collagenase and neutrophil elastase 总被引:1,自引:0,他引:1
S J Gadher D R Eyre V C Duance S F Wotton L W Heck T M Schmid D E Woolley 《European journal of biochemistry》1988,175(1):1-7
The action of purified rheumatoid synovial collagenase and human neutrophil elastase on the cartilage collagen types II, IX, X and XI was examined. At 25 degrees C, collagenase attacked type II and type X (45-kDa pepsin-solubilized) collagens to produce specific products reflecting one and at least two cleavages respectively. At 35 degrees C, collagenase completely degraded the type II collagen molecule to small peptides whereas a large fragment of the type X molecule was resistant to further degradation. In contrast, collagen type IX (native, intact and pepsin-solubilized type M) and collagen type XI were resistant to collagenase attack at both 25 degrees C and 35 degrees C even in the presence of excess enzyme. Mixtures of type II collagen with equimolar amounts of either type IX or XI did not affect the rate at which the former was degraded by collagenase at 25 degrees C. Purified neutrophil elastase, shown to be functionally active against soluble type III collagen, had no effect on collagen type II at 25 degrees C or 35 degrees C. At 25 degrees C collagen types IX (pepsin-solubilized type M) and XI were also resistant to elastase, but at 35 degrees C both were susceptible to degradation with type IX being reduced to very small peptides. Collagen type X (45-kDa pepsin-solubilized) was susceptible to elastase attack at 25 degrees C and 35 degrees C as judged by the production of specific products that corresponded closely with those produced by collagenase. Although synovial collagenase failed to degrade collagen types IX and XI, all the cartilage collagen species examined were degraded at 35 degrees C by conditioned culture medium from IL1-activated human articular chondrocytes. Thus chondrocytes have the potential to catabolise each cartilage collagen species, but the specificity and number of the chondrocyte-derived collagenase(s) has yet to be resolved. 相似文献
36.
Mitotic chromosomes, interphase cell nuclei, and male meiosis of 41 species representing all vertebrate classes were analyzed with distamycin A/mithramycin counterstaining. The purpose of the study was to recognize differences and common characteristics in the reverse (R) fluorescent banding patterns in the chromosomes of vertebrate species at various stages of evolution. In contrast to the warm-blooded mammals and birds, the euchromatic segments in the chromosomes of most reptiles, amphibians, and fishes contain no multiple fluorescent R-bands. This is thought to be due to the absence of the long homogeneous regions (isochores) in the DNA of the cold-blooded vertebrates. Distamycin A/mithramycin banding specifically reveals the GC-rich constitutive heterochromatin in all vertebrates. In most of the vertebrate chromosomes examined, the heterochromatic regions have opposite staining properties with mithramycin and quinacrine. Mithramycin labels the nucleolus organizer regions very brightly in the karyotypes of fishes, amphibians, reptiles and birds, but not of mammals. The lack of mithramycin fluorescence at the nucleolus organizer regions of mammals is attributed to the relatively low level of redundancy of the GC-rich ribosomal DNA in their genomes. Studies on the various meiotic stages of the cold-blooded vertebrates show that the mithramycin labeling of the nucleolus organizers is independent of their state of activity. This can be confirmed by mithramycin fluorescence at the nucleoli of actinomycintreated cells.Dedicated to the memory of Professor Dr. Hans Bauer 相似文献
37.
N-Glycosylation occurs cotranslationally as soon as the growing polypeptide chain enters the endoplasmic reticulum, before the final native-like folded state is reached. We examined the role of the carbohydrate chains in the mechanism of protein folding. The in vitro folding and association of yeast invertase are used as an experimental system. External invertase contains approximately 50% carbohydrate, whereas cytoplasmic invertase is not glycosylated. The functional native state of both proteins is a homodimer. At pH greater than or equal to 6.5 and at protein concentrations below 3 micrograms/ml, the kinetics of reactivation and the final yields are similar for the two invertases. For both proteins, reactivation is a sequential reaction with a lag phase at the beginning. The nonglycosylated protein tends to aggregate during reactivation at low pH and at protein concentrations above 3 micrograms/ml. After separation of inactive material, the renatured protein is indistinguishable from the original native state by a number of physicochemical and functional criteria. The results suggest that the carbohydrate moiety does not affect the mechanism of folding and association of invertase. However, glycosylation improves the solubility of unfolded or partially folded invertase molecules and hence leads to a suppression of irreversible aggregation. Such a protective effect may also be important for the in vivo maturation of nascent glycosylated protein chains. 相似文献
38.
Preferential somatic pairing between homologous heterochromatic regions of human chromosomes. 总被引:4,自引:0,他引:4
下载免费PDF全文
![点击此处可从《American journal of human genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The cytidine analog 5-azacytidine (5-azaC) induces an undercondensation of the heterochromatin in human chromosomes 1, 9, 15, 16, and Y when it is added in low concentrations to the late S-phase of growing lymphocyte cultures. In interphase nuclei, these heterochromatic regions are frequently somatically paired. The somatic pairing configurations are preserved up to metaphase stage in the 5-azaC-treated cultures and are thus susceptible to a direct microscopical examination. The statistical analysis of 1,000 somatic pairing configurations from 5-azaC-treated cells showed that the somatic pairing between the heterochromatic regions of homologous chromosomes is preferred over that between nonhomologous chromosomes. 相似文献
39.
Two distinct and interdependent binding sites for inhibitors of voltage-dependent Ca2+ channels have been identified. They include one site for molecules of the 1,4-dihydropyridine serie such as nitrendipine, nifedipine or PN200-110 and one site for a chemically heterogenous group of compounds comprising verapamil, D600 and desmethoxyverapamil, bepridil and diltiazem. Ca2+ binds to its own coordination site which is distinct from the receptor site for organic Ca2+ channel inhibitors. The molecular size of the native [3H] nitrendipine receptor of transverse tubule membrane, brain and heart, have been determined using the radiation inactivation technique. The [3H] nitrendipine receptor is found to have a Mr of 210,000 +/- 20,000. CHAPS solubilization and purification indicate that the dihydropyridine receptor contains polypeptides of apparent molecular weights of 142,000, 32,000 and 33,000 which copurifie with (+) [3H] PN200-110 binding activity. Two stages in which there is an increased binding of [3H]nitrendipine have been observed during chick myogenesis. The first one occurs during embryonic life and has the same properties as in the in vitro development. The second stage occurs near hatching and corresponds to a large increase in the number of nitrendipine receptors. This increase is accompanied by a decrease in the affinity of nitrendipine for its receptor by a factor of 4 to 10. The second stage of development is partly under innervation control and its expression is modulated by the intracellular cyclic AMP content. The two dihydropyridines Bay K8644 and CGP 28932 work preferentially on polarized membranes. 45Ca2+ flux experiments yielded results which are in good agreement with electrophysiological, contraction and binding data obtained with rat cardiac cells and skeletal muscle cells. 相似文献
40.
The complete amino acid sequence of the A-chain of human plasma alpha 2HS-glycoprotein 总被引:2,自引:0,他引:2
Y Yoshioka F Gejyo T Marti E E Rickli W Bürgi G D Offner R F Troxler K Schmid 《The Journal of biological chemistry》1986,261(4):1665-1676
Normal human plasma alpha 2HS-glycoprotein has earlier been shown to be comprised of two polypeptide chains. Recently, the amino acid and carbohydrate sequences of the short chain were elucidated (Gejyo, F., Chang, J.-L., Bürgi, W., Schmid, K., Offner, G. D., Troxler, R.F., van Halbeck, H., Dorland, L., Gerwig, G. J., and Vliegenthart, J.F.G. (1983) J. Biol. Chem. 258, 4966-4971). In the present study, the amino acid sequence of the long chain of this protein, designated A-chain, was determined and found to consist of 282 amino acid residues. Twenty-four amino acid doublets were found; the most abundant of these are Pro-Pro and Ala-Ala which each occur five times. Of particular interest is the presence of three Gly-X-Pro and one Gly-Pro-X sequences that are characteristic of the repeating sequences of collagens. Chou-Fasman evaluation of the secondary structure suggested that the A-chain contains 29% alpha-helix, 24% beta-pleated sheet, and 26% reverse turns and, thus, approximately 80% of the polypeptide chain may display ordered structure. Four glycosylation sites were identified. The two N-glycosidic oligosaccharides were found in the center region (residues 138 and 158), whereas the two O-glycosidic heterosaccharides, both linked to threonine (residues 238 and 252), occur within the carboxyl-terminal region. The N-glycans are linked to Asn residues in beta-turns, while the O-glycans are located in short random segments. Comparison of the sequence of the amino- and carboxyl-terminal 30 residues with protein sequences in a data bank demonstrated that the A-chain is not significantly related to any known proteins. However, the proline-rich carboxyl-terminal region of the A-chain displays some sequence similarity to collagens and the collagen-like domains of complement subcomponent C1q. 相似文献