全文获取类型
收费全文 | 181篇 |
免费 | 13篇 |
专业分类
194篇 |
出版年
2021年 | 1篇 |
2019年 | 1篇 |
2018年 | 5篇 |
2017年 | 3篇 |
2016年 | 4篇 |
2015年 | 9篇 |
2014年 | 8篇 |
2013年 | 3篇 |
2012年 | 4篇 |
2011年 | 4篇 |
2010年 | 4篇 |
2009年 | 5篇 |
2008年 | 2篇 |
2007年 | 8篇 |
2006年 | 7篇 |
2005年 | 3篇 |
2004年 | 6篇 |
2003年 | 3篇 |
2002年 | 6篇 |
2001年 | 3篇 |
1999年 | 5篇 |
1998年 | 11篇 |
1997年 | 8篇 |
1996年 | 8篇 |
1995年 | 5篇 |
1994年 | 1篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1990年 | 5篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 7篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1972年 | 2篇 |
1971年 | 1篇 |
1964年 | 1篇 |
排序方式: 共有194条查询结果,搜索用时 15 毫秒
1.
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
2.
3.
4.
Identification of the signalling molecules involved in mesoderm formation in amphibian embryos still presents problems. None of the original candidates, such as activin, have been definitively ruled out, and the new factors, such as the nodal-related genes, have come on to the scene. Of the original candidates, activin has been definitively shown to act as a morphogen, whereas bone morphogenetic protein (BMP)-4 has emerged as a ventral inducer and an inhibitor of neural differentiation. The effects of BMP-4 are antagonized by chordin, a molecule related to the product of the Drosophila gene short gastrulation. 相似文献
5.
Theoretical detection of a dark contrast line in twinned apatite bicrystals and its possible correlation with the chemical properties of human dentin and enamel crystals. 下载免费PDF全文
Electron microscope images of twinned apatite bicrystals oriented along the [1120] crystallographic direction have been simulated for various experimental conditions, and the validity of the calculation has been checked. These images show a dark contrast line similar to the one observed experimentally in enamel and dentin crystals and therefore strongly suggest the presence of a twin plane parallel to the (1100) crystallographic planes, in these crystals. The presence of a twin boundary in teeth and bone crystals is of prime importance for the adsorption and the dissolution properties of the calcified tissues as a whole. 相似文献
6.
Ants are powerful model systems for the study of cooperation and sociality. In this review, we discuss how recent advances in ant genomics have contributed to our understanding of the evolution and organization of insect societies at the molecular level. 相似文献
7.
8.
9.
Richert L Schneider A Vautier D Vodouhe C Jessel N Payan E Schaaf P Voegel JC Picart C 《Cell biochemistry and biophysics》2006,44(2):273-285
The adhesion of primary chondrocytes to polyelectrolyte multilayer films, made of poly(l-lysine) (PLL) and hyaluronan (HA), was investigated for native and crosslinked films, either ending by PLL or HA. Crosslinking
the film was achieved by means of a water-soluble carbodiimide in combination with N-hydroxysulfosuccinimide. The adhesion of macrophages and primary chondrocytes was investigated by microscopical techniques
(optical, confocal, and atomic), providing useful information on the cell/film interface. Native films were found to be nonadhesive
for the, primary chondrocytes, but could be degraded by macrophages, as could be visualized by confocal laser scanning microscopy
after film labeling. Confocal microscopy images show that these films can be deformed by the condrocytes and that PLL diffuses
at the chondrocyte membrane. In contrast, the cells adhered and proliferated well on the crosslinked films, which were not
degraded by the macrophages. These results were confirmed by a MTT test over a 6-d period and by atomic force microscopy observations.
We thus prove that chemical crosslinking can dramatically change cell adhesion properties, the cells being more stably anchored
on the crosslinked films.
Both authors kcontributed equally. 相似文献
10.
Multi-bead-and-spring model to interpret protein detachment studied by AFM force spectroscopy 下载免费PDF全文
This article deals with the detachment of molecules (fibrinogen) from a surface studied experimentally with an atomic force microscope. The detachment (or rupture) forces are measured as a function of the retraction velocity and exhibit a clear dependence on this parameter, even though the interaction between the molecules and the surface are nonspecific. To interpret these data, a mechanical multi-bead-and-spring model is developed. It consists of one to several parallel, "molecular" springs connected to an extra spring representing the cantilever that is moved at constant velocity. The free end of each molecular spring terminates with a particle that interacts with the surface through a Lennard-Jones potential. This Brownian dynamics model is used to analyze the experimental findings. In the framework of this model, it appears that the fibrinogen molecule must be ascribed a stiffness much smaller than that of the cantilever. In addition, several bonds between the molecule and the surface must be taken into account for the range of the molecule-surface interaction not to be unrealistically small. In future work, this model will be extended to more complex mechanisms such as the detachment of cells from a surface. 相似文献