全文获取类型
收费全文 | 211篇 |
免费 | 18篇 |
出版年
2022年 | 2篇 |
2020年 | 1篇 |
2019年 | 2篇 |
2018年 | 5篇 |
2017年 | 2篇 |
2016年 | 7篇 |
2015年 | 7篇 |
2014年 | 6篇 |
2013年 | 5篇 |
2012年 | 14篇 |
2011年 | 11篇 |
2010年 | 7篇 |
2009年 | 3篇 |
2008年 | 9篇 |
2007年 | 6篇 |
2006年 | 12篇 |
2005年 | 9篇 |
2004年 | 11篇 |
2003年 | 15篇 |
2002年 | 6篇 |
2001年 | 4篇 |
2000年 | 5篇 |
1999年 | 5篇 |
1998年 | 4篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 4篇 |
1994年 | 1篇 |
1992年 | 8篇 |
1991年 | 1篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1988年 | 5篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 6篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1972年 | 1篇 |
1967年 | 1篇 |
1962年 | 1篇 |
1960年 | 1篇 |
排序方式: 共有229条查询结果,搜索用时 15 毫秒
51.
Erik Arner Martti T Tammi Anh-Nhi Tran Ellen Kindlund Bjorn Andersson 《BMC bioinformatics》2006,7(1):155-11
Background
Many genome projects are left unfinished due to complex, repeated regions. Finishing is the most time consuming step in sequencing and current finishing tools are not designed with particular attention to the repeat problem. 相似文献52.
Parvinen K 《Bulletin of mathematical biology》1999,61(3):531-550
In this paper a general deterministic discrete-time metapopulation model with a finite number of habitat patches is analysed
within the framework of adaptive dynamics. We study a general model and prove analytically that (i) if the resident populations
state is a fixed point, then the resident strategy with no migration is an evolutionarily stable strategy, (ii) a mutant population
with no migration can invade any resident population in a fixed point state, (iii) in the uniform migration case the strategy
not to migrate is attractive under small mutational steps so that selection favours low migration. Some of these results have
been previously observed in simulations, but here they are proved analytically in a general case. If the resident population
is in a two-cyclic orbit, then the situation is different. In the uniform migration case the invasion behaviour depends both
on the type of the residents attractor and the survival probability during migration. If the survival probability during migration
is low, then the system evolves towards low migration. If the survival probability is high enough, then evolutionary branching
can happen and the system evolves to a situation with several coexisting types. In the case of out-of-phase attractor, evolutionary
branching can happen with significantly lower survival probabilities than in the in-phase attractor case. Most results in
the two-cyclic case are obtained by numerical simulations. Also, when migration is not uniform we observe in numerical simulations
in the two-cyclic orbit case selection for low migration or evolutionary branching depending on the survival probability during
migration. 相似文献
53.
54.
Anni?M.?HarjuEmail author Martti?Ven?l?inen Seija?Anttonen Hannu?Viitanen Pirjo?Kainulainen Pekka?Saranp?? Elina?Vapaavuori 《Trees - Structure and Function》2003,17(3):263-268
The cell wall chemistry (amount of hemicellulose, f-cellulose, and total lignin) and the concentration of extractives (total acetone-soluble extractives, resin acids, pinosylvins and the total phenolics quantified as tannin acid equivalents) were studied in brown-rot resistant and susceptible juvenile heartwood of Scots pine (Pinus sylvestris L.). The study material consisted of a total of 18 trees from two 34-year-old progeny trials at Korpilahti and Kerimäki. The trees were selected from among 783 trees whose decay rate had previously been screened in a laboratory test using a brown-rot fungus, Coniophora puteana. Samples from neither location showed any significant difference in the concentration (mg/cm3) of hemicellulose, f-cellulose and total lignin between the decay resistant and susceptible trees. At both locations only the concentration of total phenolics was higher in the decay-resistant heartwood than in the decay-susceptible heartwood. At Korpilahti, the amount of acetone-soluble extractives and the concentration of pinosylvin and its derivatives were higher in the resistant than in the susceptible trees. 相似文献
55.
Proliferation and differentiation of bovine type A spermatogonia during long-term culture 总被引:21,自引:0,他引:21
Izadyar F Den Ouden K Creemers LB Posthuma G Parvinen M De Rooij DG 《Biology of reproduction》2003,68(1):272-281
The present study was aimed at developing a method for long-term culture of bovine type A spermatogonia. Testes from 5-mo-old calves were used, and pure populations of type A spermatogonia were isolated. Cells were cultured in minimal essential medium (MEM) or KSOM (potassium-rich medium prepared according to the simplex optimization method) and different concentrations of fetal calf serum (FCS) for 2-4 wk at 32 degrees C or 37 degrees C. Culture in MEM resulted in more viable cells and more proliferation than culture in KSOM, and better results were obtained at 37 degrees C than at 32 degrees C. After 1 wk of culture in the absence of serum, only 20% of the cells were alive. However, in the presence of 2.5% FCS, approximately 80% of cells were alive and proliferating. Higher concentrations of FCS only enhanced numbers of somatic cells. In long-term culture, spermatogonia continued to proliferate, and eventually, type A spermatogonial colonies were formed. The majority of colonies consisted mostly of groups of cells connected by intercellular bridges. Most of the cells in these colonies underwent differentiation because they were c-kit positive, and ultimately, cells with morphological and molecular characteristics of spermatocytes and spermatids were formed. Occasionally, large round colonies consisting of single, c-kit-negative, type A spermatogonia (presumably spermatogonial stem cells) were observed. For the first time to our knowledge, a method has been developed to allow proliferation and differentiation of highly purified type A spermatogonia, including spermatogonial stem cells during long-term culture. 相似文献
56.
Intercellular organelle traffic through cytoplasmic bridges in early spermatids of the rat: mechanisms of haploid gene product sharing 下载免费PDF全文
Stable cytoplasmic bridges (or ring canals) connecting the clone of spermatids are assumed to facilitate the sharing of haploid gene products and synchronous development of the cells. We have visualized these cytoplasmic bridges under phase-contrast optics and recorded the sharing of cytoplasmic material between the spermatids by a digital time-lapse imaging system ex vivo. A multitude of small (ca. 0.5 microm) granules were seen to move continuously over the bridges, but only 28% of those entering the bridge were actually transported into other cell. The average speed of the granules decreased significantly during the passage. Immunocytochemistry revealed that some of the shared granules contained haploid cell-specific gene product TRA54. We also demonstrate the novel function for the Golgi complex in acrosome system formation by showing that TRA54 is processed in Golgi complex and is transported into acrosome system of neighboring spermatid. In addition, we propose an intercellular transport function for the male germ cell-specific organelle chromatoid body. This mRNA containing organelle, ca. 1.8 microm in diameter, was demonstrated to go over the cytoplasmic bridge from one spermatid to another. Microtubule inhibitors prevented all organelle movements through the bridges and caused a disintegration of the chromatoid body. This is the first direct demonstration of an organelle traffic through cytoplasmic bridges in mammalian spermatogenesis. Golgi-derived haploid gene products are shared between spermatids, and an active involvement of the chromatoid body in intercellular material transport between round spermatids is proposed. 相似文献
57.
Parvinen K 《Journal of mathematical biology》2002,45(2):106-124
Dispersal polymorphism and evolutionary branching of dispersal strategies has been found in several metapopulation models.
The mechanism behind those findings has been temporal variation caused by cyclic or chaotic local dynamics, or temporally
and spatially varying carrying capacities. We present a new mechanism: spatial heterogeneity in the sense of different patch
types with sufficient proportions, and temporal variation caused by catastrophes. The model where this occurs is a generalization
of the model by Gyllenberg and Metz (2001). Their model is a size-structured metapopulation model with infinitely many identical
patches. We present a generalized version of their metapopulation model allowing for different types of patches. In structured
population models, defining and computing fitness in polymorphic situations is, in general, difficult. We present an efficient
method, which can be applied also to other structured population or metapopulation models.
Received: 6 March 2001 / Revised version: 12 February 2002 / Published online: 17 July 2002 相似文献
58.
We study the evolution of dispersal in a structured metapopulation model. The metapopulation consists of a large (infinite)
number of local populations living in patches of habitable environment. Dispersal between patches is modelled by a disperser
pool and individuals in transit between patches are exposed to a risk of mortality. Occasionally, local catastrophes eradicate
a local population: all individuals in the affected patch die, yet the patch remains habitable. We prove that, in the absence
of catastrophes, the strategy not to migrate is evolutionarily stable. Under a given set of environmental conditions, a metapopulation
may be viable and yet selection may favor dispersal rates that drive the metapopulation to extinction. This phenomenon is
known as evolutionary suicide. We show that in our model evolutionary suicide can occur for catastrophe rates that increase
with decreasing local population size. Evolutionary suicide can also happen for constant catastrophe rates, if local growth
within patches shows an Allee effect. We study the evolutionary bifurcation towards evolutionary suicide and show that a discontinuous
transition to extinction is a necessary condition for evolutionary suicide to occur. In other words, if population size smoothly
approaches zero at a boundary of viability in parameter space, this boundary is evolutionarily repelling and no suicide can
occur.
Received: 10 November 2000 / Revised version: 13 February 2002 / Published online: 17 July 2002 相似文献
59.
De-O-acylated lipopolysaccharides (LPS) of three polymyxin-resistant Salmonella typhimurium pmrA mutants and their parent strains were analysed by 31P-NMR (nuclear magnetic resonance) in order to assess, in relation to polymyxin resistance, the types and degree of substitution of phosphates of the LPS and lipid A. in the pmrA mutant LPS phosphate diesters predominated over phosphate monoesters, whereas the latter were more abundant in the parent wild-type LPS. The increase in the proportion of phosphate diesters was traced to both the core oligosaccharide and the lipld A part. In the latter, the ester-linked phosphate at position 4’was to a large extent (79–88%) substituted with 4-amino-4-deoxy-l -arabinose, whereas in the wild-type LPS the 4′-phosphate was mainly present as monoester. In each LPS, regardless of the pmrA mutation, the glycosidically linked phosphate of lipid A was largely unsubstituted. 相似文献
60.