全文获取类型
收费全文 | 1236篇 |
免费 | 101篇 |
国内免费 | 1篇 |
出版年
2024年 | 2篇 |
2023年 | 5篇 |
2022年 | 6篇 |
2021年 | 21篇 |
2020年 | 8篇 |
2019年 | 16篇 |
2018年 | 17篇 |
2017年 | 23篇 |
2016年 | 40篇 |
2015年 | 73篇 |
2014年 | 85篇 |
2013年 | 90篇 |
2012年 | 109篇 |
2011年 | 88篇 |
2010年 | 69篇 |
2009年 | 54篇 |
2008年 | 74篇 |
2007年 | 75篇 |
2006年 | 78篇 |
2005年 | 50篇 |
2004年 | 58篇 |
2003年 | 57篇 |
2002年 | 55篇 |
2001年 | 17篇 |
2000年 | 18篇 |
1999年 | 15篇 |
1998年 | 15篇 |
1997年 | 16篇 |
1996年 | 9篇 |
1995年 | 13篇 |
1994年 | 13篇 |
1993年 | 11篇 |
1992年 | 17篇 |
1991年 | 9篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 8篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有1338条查询结果,搜索用时 15 毫秒
1.
Abstract. Cell aggregation in Dictyostelium discoideum is a chemotactic process mediated by cyclic adenosine monophosphate (CAMP), which is detected by cell surface receptors. The cAMP signal is degraded by cAMP phosphodiesterase. The possibility that cAMP signals are also used for cell communication in the multicellular stages was studied by determining whether the cAMP receptors, which are essential for signal transduction, continue to function in these stages. During slug migration, the number of binding sites per cell decreases to about 15% of the maximum level acquired during aggregation. At the onset of fruiting body formation, a three- to Four-Fold increase in cAMP binding activity occurs. This increase coincides with an increase in cAMP phosphodiesterase. Both phenomena suggest that cell-cell communication mediated by cAMP is used during culmination. During both slug migration and early culmination, the prestalk cells exhibit about twice as much binding activity as the prespore cells. 相似文献
2.
Hermógenes Fernández-Marín David R. Nash Sarah Higginbotham Catalina Estrada Jelle S. van Zweden Patrizia d'Ettorre William T. Wcislo Jacobus J. Boomsma 《Proceedings. Biological sciences / The Royal Society》2015,282(1807)
Fungus-farming ant colonies vary four to five orders of magnitude in size. They employ compounds from actinomycete bacteria and exocrine glands as antimicrobial agents. Atta colonies have millions of ants and are particularly relevant for understanding hygienic strategies as they have abandoned their ancestors'' prime dependence on antibiotic-based biological control in favour of using metapleural gland (MG) chemical secretions. Atta MGs are unique in synthesizing large quantities of phenylacetic acid (PAA), a known but little investigated antimicrobial agent. We show that particularly the smallest workers greatly reduce germination rates of Escovopsis and Metarhizium spores after actively applying PAA to experimental infection targets in garden fragments and transferring the spores to the ants'' infrabuccal cavities. In vitro assays further indicated that Escovopsis strains isolated from evolutionarily derived leaf-cutting ants are less sensitive to PAA than strains from phylogenetically more basal fungus-farming ants, consistent with the dynamics of an evolutionary arms race between virulence and control for Escovopsis, but not Metarhizium. Atta ants form larger colonies with more extreme caste differentiation relative to other attines, in societies characterized by an almost complete absence of reproductive conflicts. We hypothesize that these changes are associated with unique evolutionary innovations in chemical pest management that appear robust against selection pressure for resistance by specialized mycopathogens. 相似文献
3.
The histone H1°/H5 variant and terminal differentiation of cells during development of Xenopus laevis 总被引:1,自引:0,他引:1
Antoon F.M. Moorman Piet A.J. de Boer Rob Charles Wouter H. Lamers 《Differentiation; research in biological diversity》1987,35(2):100-107
The maintenance of the differentiated condition is supposed to be associated with the presence of a histone of the H1(0)/H5 subclass. If the H1(0)/H5 variant has an important role in differentiation distinct from that of H1, it should display differential expression in time and position during development. Here we report that this prediction is verified during Xenopus laevis development, in which tadpoles exhibit a very characteristic, developmentally regulated pattern of histone H1(0)/H5 expression that is different for the derivatives of each embryonic germ layer. However, the pattern of appearance of this variant during development does not reflect a simple correlation between its presence and the state of differentiation. Therefore, these results are pertinent to current ideas on differentiation and the involvement of lysine-rich histones in the repression of eukaryotic genes. 相似文献
4.
Boomsma JJ 《Trends in ecology & evolution》1991,6(3):92-95
Colony-level predictions about sex-ratio optimization in eusocial Hymenoptera are different from the ones that apply to the population level. A recent empirical study on the sweat bee Halictus rubicundus has revealed a distinct pattern in the colony sex ratio of the summer brood: eusocial colonies produced more female-biased sex ratios and non-eusocial colonies produce more male-biased sex ratios. These data are consistent with theoretical hypotheses as put forward by Trivers and Hare and several later authors. When interpreted in the light of these theoretical contentions, the sex-ratio variation in Halictus rubicundus appears to be adaptive for workers, replacement queens and - under reasonable additional assumptions - also for foundresses. 相似文献
5.
Wouter J. Middelhoven Heleen de Kievit André L. Biesbroek 《Antonie van Leeuwenhoek》1985,51(3):289-301
Yeast strains utilizing uric acid, adenine, monoamines or diamines as sole source of carbon and energy were isolated from
several soil samples by the enrichment culture method. The most common species wasTrichosporon cutaneum. Strains ofCandida catenulata, C. famata, C. parapsilosis, C. rugosa, Cryptococcus laurentii, Stephanoascus ciferrii andTr. adeninovorans were also isolated. All strains utilizing uric acid as sole carbon source utilized some primaryn-alkyl-l-amines hydroxyamines or diamines as well. The ascomycetous yeast strains showing these characteristics all belonged
to species known to assimilate hydrocarbons. Type strains of hydrocarbon-positive yeast species which were not found in the
enrichment cultures generally assimilated putrescine, some type strains also butylamine or pentylamine, but none assimilated
uric acid. Methanol-positive species were not isolated. Type strains of methanol-positive and of hydrocarbon-negative species
did not assimilate uric acid, butylamine or putrescine. Assimilation of putrescine as sole source of carbon and energy may
be a valuable diagnostic criterion in yeast taxonomy. 相似文献
6.
Wouter J. Middelhoven Alex Coenen Bart Kraakman Maarten D. Sollewijn Gelpke 《Antonie van Leeuwenhoek》1992,62(3):181-187
The imperfect ascomycetous yeastsCandida parapsilosis andArxula adeninivorans degraded 3-hydroxybenzoic acid via gentisate which was the cleavage substrate. 4-Hydroxybenzoic acid was metabolized via protocatechuate. No cleavage enzyme for the latter was detected. In stead of this NADH- and NADPH-dependent monooxygenases were present. In cells grown at the expense of hydroquinone and 4-hydroxygenzoic acid, enzymes of the hydroxyhydroquinone variant of the 3-oxoadipate pathway were demonstrated, which also took part in the degradation of 2,4-dihydroxybenzoic acid byC. parapsilosis.Abbreviations HHQ
Hydroxyhydroquinone (1,2,4-trihydroxybenzene)
- GSH
reduced Glutathione 相似文献
7.
Instar-specific mortalities of Daphnia hyalina and D.cucullatawere studied from May 19 to September 29, 1988 in combinationwith invertebrate predator and phytoplankton abundance. Simultaneouslife-table experiments were conducted under semi-natural conditionsin the laboratory to estimate juvenile mortality in a predator-freeenvironment. Juvenile mortality by predation was calculatedas the difference between juvenile mortality in the field andin the experiments and was the most important factor for thedifferences in abundance of the two species. For D.hyalina juvenilemortality was higher in early summer and probably caused byselective predation by Chaoborus flavicans. Predation by Leptodorakindtii was probably more important during the rest of the summer.Estimated mortality by predation adequately explained juvenilemortality, except for a 3-week period in August. Decreasingflagellate densities in July were accompanied by increased juvenilemortalities of D.hyalina and D.cucullata in the life-table experimentsin August and coincided with a Daphnia population decrease. 相似文献
8.
Genetic determination of telomere size in humans: a twin study of three age groups. 总被引:29,自引:2,他引:27
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)
Reduction of telomere length has been postulated to be a causal factor in cellular aging. Human telomeres terminate in tandemly arranged repeat arrays consisting of the (TTAGGG) motif. The length of these arrays in cells from human mitotic tissues is inversely related to the age of the donor, indicating telomere reduction with age. In addition to telomere length differences between different age cohorts, considerable variation is present among individuals of the same age. To investigate whether this variation can be ascribed to genetic influences, we have measured the size of terminal restriction fragments (TRFs) in HaeIII-digested genomic DNA from 123 human MZ and DZ twin pairs 2-95 years of age. The average rate of telomere shortening was 31 bp/year, which is similar to that observed by others. Statistical analysis in 115 pairs 2-63 years of age indicates a 78% heritability for mean TRF length in this age cohort. The individual differences in mean TRF length in blood, therefore, seem to a large extent to be genetically determined. 相似文献
9.
Genetics of the human electroencephalogram (EEG) and event-related brain potentials (ERPs): a review
Twin and family studies of normal variation in the human electroencephalogram (EEG) and event related potentials (ERPs) are reviewed. Most of these studies are characterized by small sample sizes. However, by summarizing these studies in one paper, we may be able to gain some insight into the genetic influences on individual differences in central nervous system functioning that may mediate genetically determined differences in behavior. It is clear that most EEG parameters are to a large extent genetically determined. The results for ERPs are based on a much smaller number of studies and suggest medium to large heritability. 相似文献
10.
Jacques J. H. Hens Marina De Wit Lodewijk V. Dekker Frans Boomsma A. Beate Oestreicher Frank Margolis† Willem Hendrik Gispen Pierre N. E. De Graan 《Journal of neurochemistry》1993,60(4):1264-1273
Abstract: The involvement of B-50, protein kinase C (PKC), and PKC-mediated B-50 phosphorylation in the mechanism of Ca2+-induced noradrenaline (NA) release was studied in highly purified rat cerebrocortical synaptosomes permeated with streptolysin-O. Under optimal permeation conditions, 12% of the total NA content (8.9 pmol of NA/mg of synaptosomal protein) was released in a largely (>60%) ATP-dependent manner as a result of an elevation of the free Ca2+ concentration from 10?8 to 10?5M Ca2+ The Ca2+ sensitivity in the micromolar range is identical for [3H]NA and endogenous NA release, indicating that Ca2+-induced [3H]NA release originates from vesicular pools in noradrenergic synaptosomes. Ca2+-induced NA release was inhibited by either N- or C-terminal-directed anti-B-50 antibodies, confirming a role of B-50 in the process of exocytosis. In addition, both anti-B-50 antibodies inhibited PKC-mediated B-50 phosphorylation with a similar difference in inhibitory potency as observed for NA release. However, in a number of experiments, evidence was obtained challenging a direct role of PKC and PKC-mediated B-50 phosphorylation in Ca2+-induced NA release. PKC pseudosubstrate PKC19-36, which inhibited B-50 phosphorylation (IC50 value, 10?5M), failed to inhibit Ca2+-induced NA release, even when added before the Ca2+ trigger. Similar results were obtained with PKC inhibitor H-7, whereas polymyxin B inhibited B-50 phosphorylation as well as Ca2+-induced NA release. Concerning the Ca2+ sensitivity, we demonstrate that PKC-mediated B-50 phosphorylation is initiated at a slightly higher Ca2+ concentration than NA release. Moreover, phorbol ester-induced PKC down-regulation was not paralleled by a decrease in Ca2+-induced NA release from streptolysin-O-permeated synaptosomes. Finally, the Ca2+- and phorbol ester-induced NA release was found to be additive, suggesting that they stimulate release through different mechanisms. In summary, we show that B-50 is involved in Ca2+-induced NA release from streptolysin-O-permeated synaptosomes. Evidence is presented challenging a role of PKC-mediated B-50 phosphorylation in the mechanism of NA exocytosis after Ca2+ influx. An involvement of PKC or PKC-mediated B-50 phosphorylation before the Ca2+ trigger is not ruled out. We suggest that the degree of B-50 phosphorylation, rather than its phosphorylation after PKC activation itself, is important in the molecular cascade after the Ca2+ influx resulting in exocytosis of NA. 相似文献