全文获取类型
收费全文 | 1236篇 |
免费 | 96篇 |
出版年
2023年 | 5篇 |
2022年 | 4篇 |
2021年 | 29篇 |
2020年 | 22篇 |
2019年 | 34篇 |
2018年 | 35篇 |
2017年 | 29篇 |
2016年 | 43篇 |
2015年 | 83篇 |
2014年 | 82篇 |
2013年 | 108篇 |
2012年 | 112篇 |
2011年 | 96篇 |
2010年 | 70篇 |
2009年 | 55篇 |
2008年 | 84篇 |
2007年 | 71篇 |
2006年 | 47篇 |
2005年 | 50篇 |
2004年 | 50篇 |
2003年 | 43篇 |
2002年 | 34篇 |
2001年 | 7篇 |
2000年 | 11篇 |
1999年 | 6篇 |
1998年 | 9篇 |
1997年 | 5篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1993年 | 7篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1988年 | 4篇 |
1987年 | 7篇 |
1986年 | 7篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 8篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1978年 | 4篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1972年 | 2篇 |
1970年 | 2篇 |
1944年 | 3篇 |
1936年 | 2篇 |
1925年 | 4篇 |
1924年 | 2篇 |
排序方式: 共有1332条查询结果,搜索用时 15 毫秒
1.
2.
Laura Cernat Cristina Blaj Rene Jackstadt Lydia Brandl Jutta Engel Heiko Hermeking Andreas Jung Thomas Kirchner David Horst 《PloS one》2014,9(8)
Colonic crypts are stereotypical structures with distinct stem cell, proliferating, and differentiating compartments. Colorectal cancers derive from colonic crypt epithelia but, in contrast, form morphologically disarrayed glands. In this study, we investigated to which extent colorectal cancers phenocopy colonic crypt architecture and thus preserve structural organization of the normal intestinal epithelium. A subset of colon cancers showed crypt-like compartments with high WNT activity and nuclear β-Catenin at the leading tumor edge, adjacent proliferation, and enhanced Cytokeratin 20 expression in most differentiated tumor epithelia of the tumor center. This architecture strongly depended on growth conditions, and was fully reproducible in mouse xenografts of cultured and primary colon cancer cells. Full crypt-like organization was associated with low tumor grade and was an independent prognostic marker of better survival in a collection of 221 colorectal cancers. Our findings suggest that full activation of preserved intestinal morphogenetic programs in colon cancer requires in vivo growth environments. Furthermore, crypt-like architecture was linked with less aggressive tumor biology, and may be useful to improve current colon cancer grading schemes. 相似文献
3.
Chang‐Yong Ma Warren Y. Brockelman Lydia E. O. Light Thad Q. Bartlett Peng‐Fei Fan 《American journal of primatology》2019,81(8)
According to the sexual selection hypothesis, infanticide during resident male replacement is an adaptive strategy that has evolved because the killing of unweaned offspring sired by previous males shortens the inter‐birth intervals of the mothers whose infants are targeted and thereby increases the reproductive fitness of the perpetrator. To test this hypothesis, we describe previously unreported cases of primary male replacement for two gibbon species (Hylobates lar and Nomascus nasutus), and review all other reported cases of primary male replacement in gibbons. Overall, infants were present in nearly half of all cases (16/33, 48%) and of the 18 infants present during replacement, 50% (N = 9) disappeared within 2 months of the event. In four of the five cases where there was sufficient demographic information to identify the likely sire of the subsequent offspring of females that lost infants, the new male was believed to be the sire. Infants were also less likely to die or disappear if the new male and original resident male were possible kin. However, there was no significant difference in the age of infants between those that died or disappeared following replacement and those that survived to weaning (p = .630). Our review of takeover‐related infant loss in gibbons confirms that periods of male instability are risky for unweaned infants and that replacing males benefit from infant loss. Nevertheless, variability in the context of infant loss and difficulties related to data collection in the field make it difficult to test competing hypotheses concerning the mechanisms and functions of infanticide in the small apes. 相似文献
4.
Lydia K. Greene Sally L. Bornbusch Erin A. McKenney Rachel L. Harris Sarah R. Gorvetzian Anne D. Yoder Christine M. Drea 《American journal of primatology》2019,81(10-11)
Research on animal microbiomes is increasingly aimed at determining the evolutionary and ecological factors that govern host–microbiome dynamics, which are invariably intertwined and potentially synergistic. We present three empirical studies related to this topic, each of which relies on the diversity of Malagasy lemurs (representing a total of 19 species) and the comparative approach applied across scales of analysis. In Study 1, we compare gut microbial membership across 14 species in the wild to test the relative importance of host phylogeny and feeding strategy in mediating microbiome structure. Whereas host phylogeny strongly predicted community composition, the same feeding strategies shared by distant relatives did not produce convergent microbial consortia, but rather shaped microbiomes in host lineage‐specific ways, particularly in folivores. In Study 2, we compare 14 species of wild and captive folivores, frugivores, and omnivores, to highlight the importance of captive populations for advancing gut microbiome research. We show that the perturbational effect of captivity is mediated by host feeding strategy and can be mitigated, in part, by modified animal management. In Study 3, we examine various scent‐gland microbiomes across three species in the wild or captivity and show them to vary by host species, sex, body site, and a proxy of social status. These rare data provide support for the bacterial fermentation hypothesis in olfactory signal production and implicate steroid hormones as mediators of microbial community structure. We conclude by discussing the role of scale in comparative microbial studies, the links between feeding strategy and host–microbiome coadaptation, the underappreciated benefits of captive populations for advancing conservation research, and the need to consider the entirety of an animal's microbiota. Ultimately, these studies will help move the field from exploratory to hypothesis‐driven research. 相似文献
5.
Summary The degree of satellite association was found to be significantly higher in phytohemagglutinin (PHA)-stimulated lymphocytes from cystic fibrosis (CF) patients than from those of control individuals. PHA-stimulated lymphocytes from obligatory heterozygotes for the CF mutant allele showed an intermediate degree of satellite association. The degree of satellite association was estimated by the frequency of cells exhibiting associations, by the number of associations per cell, and by the number of chromosomes in an association. The differences in the degree of satellite association were dependent on the concentration of colchicine used for cell arrest. These findings may assist in developing a diagnostic method for the early identification of heterozygotes for the CF allele and for prenatal detection of CF homozygous fetuses.This paper is based on a portion of a dissertation to be submitted by Y. Ravia in partial fulfilment of the Ph. D. requirements in the Graduate School of Tel Aviv University 相似文献
6.
Lydia P. Gavrilova Irina N. Perminova Alexander S. Spirin 《Journal of molecular biology》1981,149(1):69-78
Rates of incorporation of [3H]phenylalanine and [14C]leucine from the aminoacylated transfer-RNA into polypeptides synthesized on poly(U) programmed Escherichia coli ribosomes have been determined in cell-free translation systems containing either elongation factors Tu and G with GTP, or just elongation factor Tu or G with GTP, or none of the elongation factors. The presence of elongation factor Tu with GTP has been shown to reduce the leucine to phenylalanine ratio in the product at relatively low concentrations of Mg2+. This error-reducing effect of elongation factor Tu has not been observed at high concentrations of Mg2+, although the factor still contributed to the speed of elongation. The results are discussed in terms of the kinetic proof-reading mechanism proposed by Hopfield (1974). 相似文献
7.
8.
9.
Kushol Gupta Lydia M. Contreras Dorie Smith Guosheng Qu Tao Huang Lynn A. Spruce Steven H. Seeholzer Marlene Belfort Gregory D. Van Duyne 《Nucleic acids research》2014,42(8):5347-5360
The stable ribonucleoprotein (RNP) complex formed between the Lactococcus lactis group II intron and its self-encoded LtrA protein is essential for the intron’s genetic mobility. In this study, we report the biochemical, compositional, hydrodynamic and structural properties of active group II intron RNP particles (+A) isolated from its native host using a novel purification scheme. We employed small-angle X-ray scattering to determine the structural properties of these particles as they exist in solution. Using sucrose as a contrasting agent, we derived a two-phase quaternary model of the protein–RNA complex. This approach revealed that the spatial properties of the complex are largely defined by the RNA component, with the protein dimer located near the center of mass. A transfer RNA fusion engineered into domain II of the intron provided a distinct landmark consistent with this interpretation. Comparison of the derived +A RNP shape with that of the previously reported precursor intron (ΔA) particle extends previous findings that the loosely packed precursor RNP undergoes a dramatic conformational change as it compacts into its active form. Our results provide insights into the quaternary arrangement of these RNP complexes in solution, an important step to understanding the transition of the group II intron from the precursor to a species fully active for DNA invasion. 相似文献
10.
Summary We have observed a large Mexican American family segregating for a low activity allele at the phosphoglucomutase-1 locus. The deficient allele is detectable by starch gel electrophoresis and by direct activity determination. The presence of the deficient allele in either the homozygous or heterozygous condition is not associated with any other phenotypic finding. 相似文献