全文获取类型
收费全文 | 5487篇 |
免费 | 461篇 |
国内免费 | 3篇 |
专业分类
5951篇 |
出版年
2023年 | 33篇 |
2022年 | 56篇 |
2021年 | 106篇 |
2020年 | 75篇 |
2019年 | 87篇 |
2018年 | 107篇 |
2017年 | 96篇 |
2016年 | 145篇 |
2015年 | 237篇 |
2014年 | 247篇 |
2013年 | 314篇 |
2012年 | 412篇 |
2011年 | 380篇 |
2010年 | 299篇 |
2009年 | 236篇 |
2008年 | 344篇 |
2007年 | 320篇 |
2006年 | 283篇 |
2005年 | 306篇 |
2004年 | 249篇 |
2003年 | 225篇 |
2002年 | 236篇 |
2001年 | 77篇 |
2000年 | 53篇 |
1999年 | 64篇 |
1998年 | 80篇 |
1997年 | 48篇 |
1996年 | 66篇 |
1995年 | 47篇 |
1994年 | 45篇 |
1993年 | 47篇 |
1992年 | 44篇 |
1991年 | 45篇 |
1990年 | 30篇 |
1989年 | 28篇 |
1988年 | 31篇 |
1987年 | 33篇 |
1986年 | 22篇 |
1985年 | 42篇 |
1984年 | 27篇 |
1983年 | 26篇 |
1982年 | 29篇 |
1981年 | 24篇 |
1980年 | 28篇 |
1979年 | 19篇 |
1978年 | 18篇 |
1977年 | 19篇 |
1976年 | 25篇 |
1974年 | 16篇 |
1973年 | 21篇 |
排序方式: 共有5951条查询结果,搜索用时 0 毫秒
881.
A panel of 30 putative Mycoplasma fermentans strains, isolated from various sources including human, ovine and cell lines, were tested by a previously described polymerase chain reaction (PCR) to confirm their identity by amplification of a conserved 206 bp region of the insertion sequence IS1550. In addition, the application of another PCR based on the major part of the IS1550 element showed one or two products of different length (1144 and 1341 bp) enabling M. fermentans strains to be divided into two types designated as Type A and Type B. A PCR, which amplifies the macrophage activating lipopeptide gene (malp), supported the identification of all the strains as M. fermentans. Thirteen other species of Mycoplasma from human sources gave negative results in these tests, with the exception of Mycoplasma orale, which was detected by both IS1550-PCRs based on the major part and the conserved 206 bp region of the IS1550 element. This study suggests that all M. fermentans isolates possess both the IS1550 element and the malp gene. In contrast to the IS1550, the malp gene is shown to be species-specific and the use of a malp PCR described here could prove to be a useful adjunct to IS1550 detection as confirmation of the presence of M. fermentans in clinical material. 相似文献
882.
Homologous recombination is a high fidelity, template-dependent process that is used in repair of damaged DNA, recovery of broken replication forks, and disjunction of homologous chromosomes in meiosis. Much of what is known about recombination genes and mechanisms comes from studies on baker's yeast. Ustilago maydis, a basidiomycete fungus, is distant evolutionarily from baker's yeast and so offers the possibility of gaining insight into recombination from an alternative perspective. Here we have surveyed the genome of U. maydis to determine the composition of its homologous recombination system. Compared to baker's yeast, there are fundamental differences in the function as well as in the repertoire of dedicated components. These include the use of a BRCA2 homolog and its modifier Dss1 rather than Rad52 as a mediator of Rad51, the presence of only a single Rad51 paralog, and the absence of Dmc1 and auxiliary meiotic proteins. 相似文献
883.
Estimating the population size of social bee colonies in the wild is often difficult because nests are highly cryptic. Because
of the honeybee (Apis mellifera) mating behaviour, which is characterized by multiple mating of queens at drone congregation areas (DCA), it is possible
to use genotypes of drones caught at these areas to infer the number of colonies in a given region. However, DCAs are difficult
to locate and we assess the effectiveness of an alternative sampling technique to determine colony density based on inferring
male genotypes from queen offspring. We compare these methods in the same population of wild honeybees, Apis mellifera scutellata. A set of linked microsatellite loci is used to decrease the frequency of recombination among marker loci and therefore increase
the precision of the estimates. Estimates of population size obtained through sampling of queen offspring is significantly
larger than that obtained by sampling drones at DCAs. This difference may be due to the more extensive flying range of queens
compared with drones on mating flights. We estimate that the population size sampled through queen offspring is about double
that sampled through drones. 相似文献
884.
Andersson R Bruder CE Piotrowski A Menzel U Nord H Sandgren J Hvidsten TR Diaz de Ståhl T Dumanski JP Komorowski J 《Bioinformatics (Oxford, England)》2008,24(6):751-758
MOTIVATION: Copy number profiling methods aim at assigning DNA copy numbers to chromosomal regions using measurements from microarray-based comparative genomic hybridizations. Among the proposed methods to this end, Hidden Markov Model (HMM)-based approaches seem promising since DNA copy number transitions are naturally captured in the model. Current discrete-index HMM-based approaches do not, however, take into account heterogeneous information regarding the genomic overlap between clones. Moreover, the majority of existing methods are restricted to chromosome-wise analysis. RESULTS: We introduce a novel Segmental Maximum A Posteriori approach, SMAP, for DNA copy number profiling. Our method is based on discrete-index Hidden Markov Modeling and incorporates genomic distance and overlap between clones. We exploit a priori information through user-controllable parameterization that enables the identification of copy number deviations of various lengths and amplitudes. The model parameters may be inferred at a genome-wide scale to avoid overfitting of model parameters often resulting from chromosome-wise model inference. We report superior performances of SMAP on synthetic data when compared with two recent methods. When applied on our new experimental data, SMAP readily recognizes already known genetic aberrations including both large-scale regions with aberrant DNA copy number and changes affecting only single features on the array. We highlight the differences between the prediction of SMAP and the compared methods and show that SMAP accurately determines copy number changes and benefits from overlap consideration. 相似文献
885.
Bahamontes-Rosa N Robin A Ambrosio AR Messias-Reason I Beitz E Flitsch SL Kun JF 《Parasitology international》2008,57(2):132-137
The phospholipid metabolism of Plasmodium falciparum-infected erythrocytes has been shown to be an effective pharmacological target for novel chemotherapy. Thirty-seven monoquaternary ammonium derivatives analogous to choline were screened for their potential antiprotozoal activity against P. falciparum and Leishmania braziliensis. Twenty-three compounds inhibited chloroquine resistant and sensitive P. falciparum strains with inhibitory concentrations ranging from 0.001 μM to 47 μM. Among the inhibitors were six compounds with nanomolar activity containing at least one ethyl group in the polar head and a hydrophobic alkyl chain with 10 to 14 methylene groups. Four compounds also exhibited in vitro antileishmanial properties in the micromolar range. 相似文献
886.
Biological invasions offer excellent systems to study the evolutionary processes involved in introductions of species to new
ranges. Molecular markers can reveal invasion histories and the effects of introductions on amounts and structuring of genetic
variation. We used five polymorphic microsatellite loci to elucidate genetic diversity and population structure between native
range and introduced range populations of a prominent North American rangeland weed, Centaurea diffusa (Asteraceae). We found that the total number of alleles and the number of private alleles was slightly higher in the native
Eurasian range, and that allelic richness did not differ between the ranges, indicating overall levels of diversity were similar
in Eurasia and North America. It therefore seems unlikely that this invasion has been affected by genetic bottlenecks or founder
effects. Indeed, results of assignment tests suggest that multiple introductions have contributed to North America’s C. diffusa invasion. Additionally, assignment tests show that both Eurasian and North American sites had a strong pattern of mixed genetic
ancestry. This mixed assignment corresponded to a lack of geographic population structure among Eurasian samples. The lack
of population structure in the native range conflicts with general expectations and findings to date for invasion genetics,
and cautions that even species’ native ranges may show signs of recent ecological upheaval. Despite the mixed assignments,
North American samples showed strong population structure, suggesting that the invasion has been characterized by long-range
dispersal of genetically distinct propagules across the introduced range. 相似文献
887.
Jones HE Strid J Osman M Uronen-Hansson H Dixon G Klein N Wong SY Callard RE 《Cellular microbiology》2008,10(8):1634-1645
Phagocytosis of microbial pathogens is essential for the host immune response to infection. Our previous work has shown that lipooligosaccharide (LOS) expression on the surface of Neisseria meningitidis (Nm) is essential for phagocytosis, but the receptor involved remained unclear. In this study, we show that human CR3 (CD11b/CD18) and CR4 (CD11c/CD18) are phagocytic receptors for Nm as illustrated by the capacity of CR3- and CR4-transfected Chinese hamster ovary (CHO) cells to facilitate Nm uptake. A CR3-signalling mutant failed to internalize Nm, showing that the ability of CR3 to signal is essential for phagocytosis. Internalization of Nm by CR3-transfected CHO cells could be inhibited by the presence of CR3-specific antibodies. Furthermore, dendritic cells from leukocyte adhesion deficiency-1 patients, who have diminished expression of β2 integrins, showed markedly reduced phagocytosis of Nm. The CR3-mediated phagocytosis required the presence of lipopolysaccharide-binding protein (LBP). Furthermore, the expression of LOS by Nm was essential for LBP binding and phagocytosis via CR3. These results reveal a critical role of CR3 and LBP in the phagocytosis of Nm and provide important insights into the initial interaction meningococci have with the immune system. 相似文献
888.
Robin Doley Susanta Pahari Stephen P Mackessy R Manjunatha Kini 《BMC evolutionary biology》2008,8(1):196
Background
Snake venoms consist primarily of proteins and peptides showing a myriad of potent biological activities which have been shaped by both adaptive and neutral selective forces. Venom proteins are encoded by multigene families that have evolved through a process of gene duplication followed by accelerated evolution in the protein coding region. 相似文献889.
890.
Andrews RM Díaz-Paniagua C Marco A Portheault A 《Physiological and biochemical zoology : PBZ》2008,81(3):336-344
Embryonic development of the common chameleon, Chamaeleo chamaeleon, was monitored from oviposition to hatching at a field site in southwestern Spain and in the laboratory under five experimental temperature regimes. Embryos were diapausing gastrulae at the time of oviposition; developmental arrest in the field continued as cold torpor during winter. Postarrest development in the field commenced in April, and hatching occurred in August, for a total incubation period of 10.5 mo. In the laboratory, one group of eggs was incubated at a constant warm (26 degrees C) temperature. The remaining treatments simulated field conditions and consisted of initial periods of warm temperature of 0, 27, 46, and 71 d, a subsequent 4-mo period of cold winter (16 degrees C) temperature, and a final period of warm (26 degrees C) temperature. Embryos in the constant warm temperature treatment were in diapause an average of 3 mo, with clutch means ranging from 2 to 4 mo. Hatching among clutches occurred over 2 mo. In contrast, for field and experimental eggs that experienced cold winter conditions, hatching within treatments occurred over 2-14 d; "winter" conditions synchronized development. The length of time between the end of cold conditions and hatching did not differ among treatments; development thus resumed as soon as temperature was suitable regardless of the initial period of warm temperature. Diapause in nature thus insures that embryos remain gastrulae after oviposition despite nest temperatures that may be warm enough to support development. 相似文献