全文获取类型
收费全文 | 413篇 |
免费 | 50篇 |
出版年
2021年 | 4篇 |
2020年 | 3篇 |
2019年 | 7篇 |
2018年 | 7篇 |
2017年 | 6篇 |
2016年 | 7篇 |
2015年 | 13篇 |
2014年 | 14篇 |
2013年 | 13篇 |
2012年 | 22篇 |
2011年 | 19篇 |
2010年 | 5篇 |
2009年 | 14篇 |
2008年 | 16篇 |
2007年 | 17篇 |
2006年 | 17篇 |
2005年 | 22篇 |
2004年 | 14篇 |
2003年 | 14篇 |
2002年 | 16篇 |
2001年 | 21篇 |
2000年 | 21篇 |
1999年 | 20篇 |
1998年 | 8篇 |
1997年 | 9篇 |
1996年 | 3篇 |
1995年 | 6篇 |
1994年 | 6篇 |
1993年 | 5篇 |
1992年 | 10篇 |
1991年 | 9篇 |
1990年 | 11篇 |
1989年 | 10篇 |
1988年 | 11篇 |
1987年 | 10篇 |
1986年 | 7篇 |
1985年 | 2篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1975年 | 3篇 |
1972年 | 2篇 |
1971年 | 3篇 |
1970年 | 2篇 |
1969年 | 1篇 |
1968年 | 2篇 |
排序方式: 共有463条查询结果,搜索用时 31 毫秒
21.
22.
K. E. Mock J. C. Brim Box J. P. Chong J. Furnish J. K. Howard 《Molecular ecology》2013,22(24):6060-6073
We investigate population genetic structuring in Margaritifera falcata, a freshwater mussel native to western North America, across the majority of its geographical range. We find shallow rangewide genetic structure, strong population‐level structuring and very low population diversity in this species, using both mitochondrial sequence and nuclear microsatellite data. We contrast these patterns with previous findings in another freshwater mussel species group (Anodonta californiensis/A. nuttalliana) occupying the same continental region and many of the same watersheds. We conclude that differences are likely caused by contrasting life history attributes between genera, particularly host fish requirements and hermaphroditism. Further, we demonstrate the occurrence of a ‘hotspot’ for genetic diversity in both groups of mussels, occurring in the vicinity of the lower Columbia River drainage. We suggest that stream hierarchy may be responsible for this pattern and may produce similar patterns in other widespread freshwater species. 相似文献
23.
Martha M. Muñoz Nicholas G. Crawford Thomas J. McGreevy Jr Nicholas J. Messana Rebecca D. Tarvin Liam J. Revell Rosanne M. Zandvliet Juanita M. Hopwood Elbert Mock André L. Schneider Christopher J. Schneider 《Molecular ecology》2013,22(10):2668-2682
Adaptive divergence in coloration is expected to produce reproductive isolation in species that use colourful signals in mate choice and species recognition. Indeed, many adaptive radiations are characterized by differentiation in colourful signals, suggesting that divergent selection acting on coloration may be an important component of speciation. Populations in the Anolis marmoratus species complex from the Caribbean island of Guadeloupe display striking divergence in the colour and pattern of adult males that occurs over small geographic distances, suggesting strong divergent selection. Here we test the hypothesis that divergence in coloration results in reduced gene flow among populations. We quantify variation in adult male coloration across a habitat gradient between mesic and xeric habitats, use a multilocus coalescent approach to infer historical demographic parameters of divergence, and examine gene flow and population structure using microsatellite variation. We find that colour variation evolved without geographic isolation and in the face of gene flow, consistent with strong divergent selection and that both ecological and sexual selection are implicated. However, we find no significant differentiation at microsatellite loci across populations, suggesting little reproductive isolation and high levels of contemporary gene exchange. Strong divergent selection on loci affecting coloration probably maintains clinal phenotypic variation despite high gene flow at neutral loci, supporting the notion of a porous genome in which adaptive portions of the genome remain fixed whereas neutral portions are homogenized by gene flow and recombination. We discuss the impact of these findings for studies of colour evolution and ecological speciation. 相似文献
24.
25.
26.
27.
Martins BM Grimm B Mock HP Huber R Messerschmidt A 《The Journal of biological chemistry》2001,276(47):44108-44116
The enzymatic catalysis of many biological processes of life is supported by the presence of cofactors and prosthetic groups originating from the common tetrapyrrole precursor uroporphyrinogen-III. Uroporphyrinogen-III decarboxylase catalyzes its conversion into coproporphyrinogen-III, leading in plants to chlorophyll and heme biosynthesis. Here we report the first crystal structure of a plant (Nicotiana tabacum) uroporphyrinogen-III decarboxylase, together with the molecular modeling of substrate binding in tobacco and human enzymes. Its structural comparison with the homologous human protein reveals a similar catalytic cleft with six invariant polar residues, Arg(32), Arg(36), Asp(82), Ser(214) (Thr in Escherichia coli), Tyr(159), and His(329) (tobacco numbering). The functional relationships obtained from the structural and modeling analyses of both enzymes allowed the proposal for a refined catalytic mechanism. Asp(82) and Tyr(159) seem to be the catalytic functional groups, whereas the other residues may serve in substrate recognition and binding, with Arg(32) steering its insertion. The crystallographic dimer appears to represent the protein dimer under physiological conditions. The dimeric arrangement offers a plausible mechanism at least for the first two (out of four) decarboxylation steps. 相似文献
28.
A transporter present in intestinal cells and in choriocarcinoma cells has been shown to transport both pantothenic acid and biotin at similar transporter affinities. However, the concentration of pantothenic acid in most foods and biological fluids is approximately 200 times the concentration of biotin; theoretically, pantothenic acid might substantially reduce biotin transport via competition. In the present study, we sought to determine whether pantothenic acid reduces biotin transport by the biotin transporter in peripheral blood mononuclear cells (PBMC). PBMC were isolated from human blood by gradient centrifugation. Incubations with [(3)H]biotin and pantothenic acid were conducted at physiologic concentrations. Intracellular [(3)H]biotin was quantified after washing by liquid scintillation counting. Pantothenic acid at 10 to 1,000 nmol/L reduced biotin (475 pmol/L) uptake by less than 12% (P < 0.05). Based on Lineweaver-Burk plots, the competition was reversible. Several structural analogs of pantothenic acid at 1,000 nmol/L reduced biotin transport by only 7 to 15% (P = 0.13). No pattern of molecular structure required for recognition by the transporter was apparent. Extracellular pantothenic acid did not affect biotin efflux from [(3)H]biotin-loaded PBMC (P > 0.05), suggesting that countertransport of extracellular pantothenic acid and intracellular biotin does not increase biotin efflux from PBMC. We conclude that the physiologic effects of pantothenic acid on the transport of biotin in PBMC are likely to be quantitatively minor. 相似文献
29.
Zinc ion in solution yields a 560-fold enhancement in the kinetic inhibition of carboxypeptidase A by the simple heterocycle imidazole, behavior attributed to formation of a ternary complex of the three species. However, the effect is partially negated by formation of less-inhibitory Zn2+(C3H4N2)2-4 coordination complexes, providing for the enzyme an anomalous profile of catalytic rate versus imidazole concentration. 相似文献
30.
The gene encoding ClpC in Bacillus anthracis was amplified from the chromosome by polymerase chain reaction using degenerate oligonucleotide primers. These primers also amplified a second DNA fragment identified as a clpB homolog. Both genes were suggested to be functional. Contrary to Bacillus subtilis which possesses clpC but not clpB, many Bacillus species were found to harbor both clpC and clpB. We also found that Clostridium strains could possess clpB, clpC, or both. All the Gram-negative strains tested had clpB only. 相似文献