全文获取类型
收费全文 | 1195篇 |
免费 | 135篇 |
出版年
2021年 | 11篇 |
2020年 | 9篇 |
2019年 | 11篇 |
2018年 | 18篇 |
2017年 | 20篇 |
2016年 | 18篇 |
2015年 | 39篇 |
2014年 | 33篇 |
2013年 | 53篇 |
2012年 | 63篇 |
2011年 | 45篇 |
2010年 | 35篇 |
2009年 | 30篇 |
2008年 | 54篇 |
2007年 | 62篇 |
2006年 | 36篇 |
2005年 | 42篇 |
2004年 | 39篇 |
2003年 | 37篇 |
2002年 | 35篇 |
2001年 | 38篇 |
2000年 | 38篇 |
1999年 | 24篇 |
1998年 | 20篇 |
1997年 | 11篇 |
1996年 | 14篇 |
1995年 | 11篇 |
1993年 | 17篇 |
1992年 | 29篇 |
1991年 | 15篇 |
1990年 | 19篇 |
1989年 | 29篇 |
1988年 | 14篇 |
1987年 | 16篇 |
1986年 | 12篇 |
1985年 | 19篇 |
1984年 | 14篇 |
1982年 | 16篇 |
1981年 | 12篇 |
1979年 | 11篇 |
1977年 | 17篇 |
1976年 | 15篇 |
1975年 | 16篇 |
1974年 | 16篇 |
1973年 | 20篇 |
1972年 | 19篇 |
1971年 | 18篇 |
1970年 | 8篇 |
1967年 | 8篇 |
1966年 | 9篇 |
排序方式: 共有1330条查询结果,搜索用时 31 毫秒
11.
We have examined the possible role of two signal transducing mechanisms, tyrosine phosphorylation and activation of protein kinase C (PKC), during fibroblast growth factor (FGF)-induced mesoderm induction in Xenopus. Tyrosine phosphorylation was examined through the use of a monoclonal anti-phosphotyrosine antibody. This antibody was shown to recognize the FGF receptor crosslinked to radioiodinated FGF. We also studied the response of Xenopus ectodermal explants to sodium orthovanadate, a compound that has been shown to elevate intracellular phosphotyrosine levels. Thirty percent of explants cultured in 100 microM vanadate were induced. In addition, vanadate synergized with FGF to give inductions that were more dorsal in nature than either vanadate or FGF alone. The role of PKC was evaluated by measuring PKC activity during mesoderm induction by FGF and by examining the effect of the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) on explants. TPA did not induce mesoderm, however, activation of PKC was detected in FGF-treated explants. Therefore, activation of the PKC pathway alone is not sufficient for mesoderm induction. Simultaneous treatment with TPA and FGF resulted in a significant inhibition of mesoderm induction by FGF, suggesting that activation of PKC could be part of a negative feedback mechanism. In contrast, TPA had no effect on induction by activin A. 相似文献
12.
G H Thorpe L J Kricka E Gillespie S Moseley R Amess N Baggett T P Whitehead 《Analytical biochemistry》1985,145(1):96-100
6-Hydroxybenzothiazole, 2-cyano-6-hydroxybenzothiazole, and 2-(6-hydroxy-2-benzothiazolyl)thiazole-4-carboxylic acid (dehydroluciferin) dramatically enhance light emission from the horseradish peroxidase conjugate catalyzed oxidation of luminol, isoluminol, N-(6-aminobutyl)-N-ethyl isoluminol, and 7-dimethylaminonaphthalene-1,2-dicarboxylic acid hydrazide by either peroxide or perborate. Light emission is enhanced by up to 1000-fold, which is an improvement over the enhancement previously observed using firefly luciferin (4,5-dihydro-2-(6-hydroxy-2-benzothiazolyl)thiazole-4-carboxylic acid). Enhancement is influenced by enhancer concentration and pH. Spectral scans of light emitted in enhanced and unenhanced reactions are similar, suggesting that aminophthalate products, and not the enhancers, are the emitters. 相似文献
13.
A comparative description of mitochondrial DNA differentiation in selected avian and other vertebrate genera 总被引:14,自引:1,他引:13
Levels of mitochondrial DNA (mtDNA) sequence divergence between species
within each of several avian (Anas, Aythya, Dendroica, Melospiza, and
Zonotrichia) and nonavian (Lepomis and Hyla) vertebrate genera were
compared. An analysis of digestion profiles generated by 13-18 restriction
endonucleases indicates little overlap in magnitude of mtDNA divergence for
the avian versus nonavian taxa examined. In 55 interspecific comparisons
among the avian congeners, the fraction of identical fragment lengths (F)
ranged from 0.26 to 0.96 (F = 0.46), and, given certain assumptions, these
translate into estimates of nucleotide sequence divergence (p) ranging from
0.007 to 0.088; in 46 comparisons among the fish and amphibian congeners, F
values ranged from 0.00 to 0.36 (F = 0.09), yielding estimates of P greater
than 0.070. The small mtDNA distances among avian congeners are associated
with protein-electrophoretic distances (D values) less than approximately
0.2, while the mtDNA distances among assayed fish and amphibian congeners
are associated with D values usually greater than 0.4. Since the
conservative pattern of protein differentiation previously reported for
many avian versus nonavian taxa now appears to be paralleled by a
conservative pattern of mtDNA divergence, it seems increasingly likely that
many avian species have shared more recent common ancestors than have their
nonavian taxonomic counterparts. However, estimates of avian divergence
times derived from mtDNA- and protein-calibrated clocks cannot readily be
reconciled with some published dates based on limited fossil remains. If
the earlier paleontological interpretations are valid, then protein and
mtDNA evolution must be somewhat decelerated in birds. The empirical and
conceptual issues raised by these findings are highly analogous to those in
the long-standing debate about rates of molecular evolution and times of
separation of ancestral hominids from African apes.
相似文献
14.
Methods for computing the standard errors of branching points in an evolutionary tree and their application to molecular data from humans and apes 总被引:23,自引:2,他引:21
Statistical methods for computing the standard errors of the branching
points of an evolutionary tree are developed. These methods are for the
unweighted pair-group method-determined (UPGMA) trees reconstructed from
molecular data such as amino acid sequences, nucleotide sequences,
restriction-sites data, and electrophoretic distances. They were applied to
data for the human, chimpanzee, gorilla, orangutan, and gibbon species.
Among the four different sets of data used, DNA sequences for an
895-nucleotide segment of mitochondrial DNA (Brown et al. 1982) gave the
most reliable tree, whereas electrophoretic data (Bruce and Ayala 1979)
gave the least reliable one. The DNA sequence data suggested that the
chimpanzee is the closest and that the gorilla is the next closest to the
human species. The orangutan and gibbon are more distantly related to man
than is the gorilla. This topology of the tree is in agreement with that
for the tree obtained from chromosomal studies and DNA-hybridization
experiments. However, the difference between the branching point for the
human and the chimpanzee species and that for the gorilla species and the
human-chimpanzee group is not statistically significant. In addition to
this analysis, various factors that affect the accuracy of an estimated
tree are discussed.
相似文献
15.
Atkinson J. E.; Olson J. W.; Altiere R. J.; Gillespie M. N. 《Journal of applied physiology》1987,62(4):1562-1568
This study tested the hypothesis that the polyamines, a family of low-molecular-weight organic cations with documented regulatory roles in cell growth and differentiation, are mediators of chronic hypoxia-induced pulmonary vascular remodeling. Relative to room air controls, chronically hypoxic animals (inspired O2 fraction = 0.1; 21 days) exhibited higher pulmonary arterial pressures (measured in room air), thicker medial layers in pulmonary arteries of 50-100 microns diam, increased hematocrits, and right ventricular hypertrophy. In addition, lung contents of the polyamines, putrescine, spermidine, and spermine were greater in hypoxic animals than in controls. alpha-Difluoromethylornithine (DFMO), an inhibitor of polyamine synthesis, attenuated the hypoxia-induced elevations in lung putrescine and spermidine content and blunted the increases in pulmonary arterial pressure and medial thickness. Neither the increased hematocrit nor right ventricular hypertrophy associated with chronic hypoxia were abrogated by DFMO. In addition, DFMO failed to influence vasoconstrictor responses provoked by acute hypoxic ventilation in isolated, buffer-perfused rat lungs. These observations suggest that depression of polyamine biosynthesis with DFMO blunts the sustained increase in pulmonary arterial pressure by attenuating hypoxia-induced medial thickening. 相似文献
16.
17.
18.
A mutant of E. coli, T
s68b, selected as unable to grow at 43 C, is unable to synthesize proteins at 43 C, though it can carry out this function at 30 C. This defect is shown to be in the protein-synthetic rather than the RNA-synthetic machinery by an analysis of the response of the strain to infection with the RNA bacteriophage f2. An analysis of the capacity for RNA synthesis and the polyribosome content of these cells at 44 C indicates that the defect resides in the elongation step of protein synthesis. No defects could be detected in vitro. The results are discussed in light of similar data on other mutants and in relation to the general approach of analyzing complex mutants selected with ill-defined phenotypes.This work was supported by Grant GM14368 from the National Institute of Health. 相似文献
19.
A further study on the dietary-regulated biosynthesis of high-sulphur wool proteins 总被引:2,自引:2,他引:0
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
When the diet of sheep is supplemented by the infusion of sulphur-containing amino acids or casein into the abomasum, the newly synthesized wool shows characteristic changes in its amino acid composition, with significant increases in cystine, proline and serine and decreases in aspartic acid and phenylalanine. This modification seems to be due entirely to an alteration in the overall composition of the high-sulphur proteins and to an increase in their proportion in the fibre. These variations are not the result of a change in the composition of individual proteins, but are due to alterations in their relative proportions and to the initiation of the synthesis of `new' proteins, many of which are extremely rich in cystine. It is suggested that the heterogeneity of the high-sulphur proteins may be due, in part, to similar changes in composition caused by natural variations in the nutrition of sheep. 相似文献
20.