全文获取类型
收费全文 | 4966篇 |
免费 | 334篇 |
国内免费 | 4篇 |
专业分类
5304篇 |
出版年
2023年 | 22篇 |
2022年 | 44篇 |
2021年 | 98篇 |
2020年 | 79篇 |
2019年 | 81篇 |
2018年 | 93篇 |
2017年 | 78篇 |
2016年 | 164篇 |
2015年 | 200篇 |
2014年 | 207篇 |
2013年 | 286篇 |
2012年 | 324篇 |
2011年 | 301篇 |
2010年 | 205篇 |
2009年 | 192篇 |
2008年 | 221篇 |
2007年 | 250篇 |
2006年 | 216篇 |
2005年 | 201篇 |
2004年 | 177篇 |
2003年 | 166篇 |
2002年 | 163篇 |
2001年 | 127篇 |
2000年 | 106篇 |
1999年 | 135篇 |
1998年 | 62篇 |
1997年 | 47篇 |
1996年 | 48篇 |
1995年 | 43篇 |
1994年 | 35篇 |
1993年 | 30篇 |
1992年 | 64篇 |
1991年 | 81篇 |
1990年 | 71篇 |
1989年 | 66篇 |
1988年 | 69篇 |
1987年 | 53篇 |
1986年 | 56篇 |
1985年 | 51篇 |
1984年 | 36篇 |
1983年 | 28篇 |
1982年 | 19篇 |
1981年 | 19篇 |
1980年 | 24篇 |
1979年 | 34篇 |
1977年 | 21篇 |
1975年 | 21篇 |
1974年 | 19篇 |
1973年 | 17篇 |
1971年 | 19篇 |
排序方式: 共有5304条查询结果,搜索用时 31 毫秒
41.
Summary We have studied members of three generations of the same family affected by brachydactyly, which is accompanied by intraventricular conduction defects in three cases (proband's father and two of his sons) and sick sinus syndrome in the proband.The brachydactyly described affects mainly the middle phalanges of both hands; the index and fifth fingers are more severely affected than the other fingers. It also includes a rare variant with an ossicle on the proximal phalanx of both index fingers, which reduces them in length and causes them to deviate towards the ulnar border of the hand. The feet also tend to be affected, but to a lesser degree. No other pathological findings were recorded.It is therefore suggested that the anomalies detected in this family are transmitted by an autosomal dominant mode of inheritance, thus forming a syndrome. 相似文献
42.
Anel L de Paz P Alvarez M Chamorro CA Boixo JC Manso A González M Kaabi M Anel E 《Theriogenology》2003,60(7):1293-1308
Glycerol has been the most widely used cryopreservation agent for spermatozoa and a wide range of factors affect its action on sperm viability and fertilizing capacity. We tested three methods for freezing ram semen packed in 0.25 ml straws (final cellular concentration: 100 x 10(6) spz/ml). Method M1: Two-thirds of the final volume of diluent was added as solution A (without glycerol) to the pure semen at 35 degrees C. The sample was cooled to 5 degrees C (-0.30 degrees C/min), one-third of final diluent volume was added as solution B (final concentration of glycerol 4%) and the sample was maintained at 5 degrees C for 2h. It was then frozen in a programmable biofreezer (-20 degrees C/min down to -100 degrees C). Method M2: The sample was diluted with a specific solution at 35 degrees C (final concentration of glycerol 3%), cooled to 5 degrees C (-0.20 degrees C/min) and left for 2h. After that, it was frozen in nitrogen vapours. Method M3: Semen was diluted 1:1 in a specific solution (concentration of glycerol 2%) and cooled to 5 degrees C (-0.25 degrees C/min). The sample was then diluted again in the same solution to the final cellular concentration (final concentration of glycerol 4%). It was left for 1h at 5 degrees C and then frozen in a programmable biofreezer (-20 degrees C/min down to -100 degrees C). Best total motility (TM) and progressive motility (PM) (75.8 and 55.18%) were obtained using Method M3. Methods M1 and M3 gave significantly higher values (P<0.05) for kinetic parameters: average path velocity (VAP) (81.3 and 85.2 microm/s), straight-line velocity (VSL) (72.8 and 77.3 microm/s) and linearity (LIN) (66.6 and 68.8%). Method M2 showed the lowest kinetic parameters of motility (VAP 74.4, VSL 67.3 and LIN 62.5) and the highest percentage of cells with damaged plasma membrane (53.8%). Method M1 gave the worst results in viability and acrosome status assessed using fluorescence probes (31.3%-dead cells with damaged acrosomes-versus 25.4% in M2 and 23.3% in M3). A field trial carried out on fertility showed a significantly higher percentage of pregnant or lambing ewes (P<0.05) with Method M3 (67.3% versus 51.1% for M1 and 58.8% for M2). We concluded that the use of a simple dilution medium (test-fructose-glycerol-egg yolk) with the addition of glycerol (to 2% at 35 degrees C and to 4% at 5 degrees C) in two steps together with a programmable biofreezer was a productive method for freezing ram semen. 相似文献
43.
Louise Balfour Johanna N. Spaans Dean Fergusson Harold Huff Edward J. Mills Charles J. la Porte Sharon Walmsley Neera Singhal Ron Rosenes Nancy Tremblay M. John Gill Hugues Loemba Brian Conway Anita Rachlis Edward Ralph Mona Loutfy Ranjeeta Mallick Rika Moorhouse D. William Cameron 《PloS one》2014,9(1)
Introduction
The MAINTAIN study is an on-going RCT comparing high-dose micronutrient and anti-oxidant supplementation versus recommended daily allowance (RDA) vitamins in slowing HIV immune deficiency progression in ART-naïve people with HIV infection.Objective
We planned analysis of the first 127 participants to determine the baseline prevalence of serum micronutrient deficiencies and correlates, as well as tolerance and adherence to study interventions.Methods
Participants receive eight capsules twice daily of 1) high-dose or 2) RDA supplements for two years and are followed-up quarterly for measures of immune deficiency progression, safety and tolerability. Regression analysis was used to identify correlates of micronutrient levels at baseline. Adherence was measured by residual pill count, self-report using the General Treatment Scale (GTS) and short-term recall HIV Adherence Treatment Scale (HATS).Results
Prior micronutrient supplementation (within 30 days) was 27% at screening and 10% of study population, and was not correlated with baseline micronutrient levels. Low levels were frequent for carotene (24%<1 nmol/L), vitamin D (24%<40 nmol/L) and serum folate (20%<15 nmol/L). The proportion with B12 deficiency (<133 pmol/L) was 2.4%. Lower baseline levels of B12 correlated lower baseline CD4 count (r = 0.21, p = 0.02) with a 21 pmol/L reduction in B12 per 100 cells/µL CD4. Vitamin D levels were higher in men (p<0.001). After a median follow-up of 1.63 years, there were 19 (15%) early withdrawals from the study treatment. Mean treatment adherence using pill count was 88%. Subjective adherence by the GTS was 81% and was moderately but significantly correlated with pill count (r = 0.29, p<0.001). Adherence based on short-term recall (HATS) was >80% in 75% of participants.Conclusion
Micronutrient levels in asymptomatic HIV+ persons are in keeping with population norms, but micronutrient deficiencies are frequent. Adherence levels are high, and will permit a valid evaluation of treatment effects.Trial Registration
ClinicalTrials.gov NCT00798772相似文献44.
Christian Gortazar Leslie A. Reperant Thijs Kuiken José de la Fuente Mariana Boadella Beatriz Martínez-Lopez Francisco Ruiz-Fons Agustin Estrada-Pe?a Christian Drosten Graham Medley Richard Ostfeld Townsend Peterson Kurt C. VerCauteren Christian Menge Marc Artois Constance Schultsz Richard Delahay Jordi Serra-Cobo Robert Poulin Frederic Keck Alonso A. Aguirre Heikki Henttonen Andrew P. Dobson Susan Kutz Juan Lubroth Atle Mysterud 《PLoS pathogens》2014,10(6)
45.
Elizabeth Ortiz-Gutiérrez Karla García-Cruz Eugenio Azpeitia Aaron Castillo María de la Paz Sánchez Elena R. álvarez-Buylla 《PLoS computational biology》2015,11(9)
Cell cycle control is fundamental in eukaryotic development. Several modeling efforts have been used to integrate the complex network of interacting molecular components involved in cell cycle dynamics. In this paper, we aimed at recovering the regulatory logic upstream of previously known components of cell cycle control, with the aim of understanding the mechanisms underlying the emergence of the cyclic behavior of such components. We focus on Arabidopsis thaliana, but given that many components of cell cycle regulation are conserved among eukaryotes, when experimental data for this system was not available, we considered experimental results from yeast and animal systems. We are proposing a Boolean gene regulatory network (GRN) that converges into only one robust limit cycle attractor that closely resembles the cyclic behavior of the key cell-cycle molecular components and other regulators considered here. We validate the model by comparing our in silico configurations with data from loss- and gain-of-function mutants, where the endocyclic behavior also was recovered. Additionally, we approximate a continuous model and recovered the temporal periodic expression profiles of the cell-cycle molecular components involved, thus suggesting that the single limit cycle attractor recovered with the Boolean model is not an artifact of its discrete and synchronous nature, but rather an emergent consequence of the inherent characteristics of the regulatory logic proposed here. This dynamical model, hence provides a novel theoretical framework to address cell cycle regulation in plants, and it can also be used to propose novel predictions regarding cell cycle regulation in other eukaryotes. 相似文献
46.
The colonial urochordate Botryllus schlosseri is a sedentary species of Mediterranean origin that became cosmopolitan, probably because of postglacial-period dispersal
and human-mediated invasions of colonies attached to ship hulls. Here we studied microsatellite allele diversity of Atlantic
coast populations from an area ranging from European regions south of the last glacial front to regions that had been permanently
ice-covered. Gene diversity levels varied dramatically among populations residing in areas subject to different glacial conditions.
Five populations from the Iberian Peninsula, in an area south of the last glacial front, as well as two populations from presumed
refugia in Brittany, expressed high gene diversity values (expected heterozygosity [He]: 0.76–0.80; average number of alleles
per locus [A]: 7.25–8.75). Two populations inhabiting areas that experienced permafrost conditions (Helgoland Island, Germany,
and Plymouth, England) had intermediate values (He: 0.40–0.42; A: 3.0–4.0), whereas the Auchenmalg, Scotland, population,
from an area previously covered by ice, showed a remarkably low value (He: 0.17; A: 1.75). Therefore, most European populations
of B. schlosseri mirrored the movement of the ice front in the last ice age. A second population from the area that was covered by permanent
ice (Lossiemouth, Scotland), however, had a high He of 0.61 and an intermediate A of 3.67. Results were compared with recent
invasions (populations less than 200 years old) in the United States and New Zealand that had a higher degree of genetic variation
than the European native populations established thousands of years ago. Given the overall dearth of studies on this subject,
we suggest that in contemporary established Botryllus populations, gene diversity is affected by ecological factors, some of which can be traced directly to the last ice age.
Other parameters of gene diversity are influenced by selection pressure, which might be more intense in northern regions. 相似文献
47.
J. Garcia de la Torre 《European biophysics journal : EBJ》1994,23(5):307-322
Segmentally flexible macromolecules are composed of a few rigid subunits linked by joints which are more or less flexible. The dynamics in solution of this type of macromolecule present special aspects that are reviewed here. Three alternative approaches are described. One is the rigid-body treatment, which is shown to be valid for overall dynamic properties such as translational diffusion and intrinsic viscosity. Another approach is the Harvey-Wegener treatment, which is particularly suited for rotational diffusion. The simplest version of this treatment, which ignores hydrodynamic interaction (HI) effects, is found to be quite accurate when compared to a more rigorous version including HI. A third approach is the Brownian dynamics simulation that, albeit at some computational cost, might describe rigorously cases of arbitrary complexity. This technique has been used to test the approximations in the rigid-body and Harvey-Wegener treatments, thus allowing a better understanding of their validity. Brownian trajectories of simplified models such as the trumbbell and the broken rod have been simulated. The comparison of the decay rates of some correlation functions with the predictions of the two treatments leads to a general conclusion: the Harvey-Wegener treatment determines the initial rate, while the long-time behavior is dominated by the rigid-body relaxation time. As an example of application to a specific biological macromolecule, we present a simulation of an immunoglobulin molecule, showing how Brownian Dynamics can be used to predict rotational and internal dynamics. Another typical example is myosin. Literature data of hydrodynamic properties of whole myosin and the myosin rod are compared with predictions from the Harvey-Wegener and rigid-body treatments. The present situation of the problem on myosin flexibility is analyzed, and some indications are given for future experimental and simulation work. 相似文献
48.
B Levinson A E Lehesjoki A de la Chapelle J Gitschier 《American journal of human genetics》1990,46(1):53-62
We have examined the Finnish hemophilia A population for factor VIII gene mutations. This study included 83 unrelated patients and revealed 10 mutations associated with hemophilia. Using cloned cDNA, genomic, and oligonucleotide probes, we have identified three classes of mutations: five mutations causing the loss of TaqI restriction sites, a point mutation resulting in a new TaqI site, and four partial gene deletions. Although exons 5 and 6 were involved in three of the four partial gene deletions, the extent of the DNA lost differs in each case. The fourth deletion was located entirely within intron 1 and segregated with the disease in a large hemophilia pedigree. There was no history of hemophilia in eight of the 10 families. The origin of the mutation was determined in six of these pedigrees, two of which showed evidence for maternal mosaicism. 相似文献
49.
Cano F Poderoso C Cornejo Maciel F Castilla R Maloberti P Castillo F Neuman I Paz C Podestá EJ 《The Journal of steroid biochemistry and molecular biology》2006,99(4-5):197-202
The activation of the rate-limiting step in steroid biosynthesis, that is the transport of cholesterol into the mitochondria, is dependent on PKA-mediated events triggered by hormones like ACTH and LH. Two of such events are the protein tyrosine dephosphorylation mediated by protein tyrosine phosphatases (PTPs) and the release of arachidonic acid (AA) mediated by two enzymes, ACS4 (acyl-CoA synthetase 4) and Acot2 (mitochondrial thioesterase). ACTH and LH regulate the activity of PTPs and Acot2 and promote the induction of ACS4. Here we analyzed the involvement of PTPs on the expression of ACS4. We found that two PTP inhibitors, acting through different mechanisms, are both able to abrogate the hormonal effect on ACS4 induction. PTP inhibitors also reduce the effect of cAMP on steroidogenesis and on the level of StAR protein, which facilitates the access of cholesterol into the mitochondria. Moreover, our results indicate that exogenous AA is able to overcome the inhibition produced by PTP inhibitors on StAR protein level and steroidogenesis. Then, here we describe a link between PTP activity and AA release, since ACS4 induction is under the control of PTP activity, being a key event for AA release, StAR induction and steroidogenesis. 相似文献
50.
Extensively managed pastures are of crucial importance in sustaining biodiversity both in local- and landscape-level. Thus, re-introduction of traditional grazing management is a crucial issue in grassland conservation actions worldwide. Traditional grazing with robust cattle breeds in low stocking rates is considered to be especially useful to mimic natural grazing regimes, but well documented case-studies are surprisingly rare on this topic. Our goal was to evaluate the effectiveness of traditional Hungarian Grey cattle grazing as a conservation action in a mosaic alkali landscape. We asked the following questions: (i) How does cattle grazing affect species composition and diversity of the grasslands? (ii) What are the effects of grazing on short-lived and perennial noxious species? (iii) Are there distinct effects of grazing in dry-, mesophilous- and wet grassland types? Vegetation of fenced and grazed plots in a 200-ha sized habitat complex (secondary dry grasslands and pristine mesophilous- and wet alkali grasslands) was sampled from 2006–2009 in East-Hungary. We found higher diversity scores in grazed plots compared to fenced ones in mesophilous- and wet grasslands. Higher cover of noxious species was typical in fenced plots compared to their grazed counterparts in the last year in every studied grassland type. We found an increasing effect of grazing from the dry- towards the wet grassland types. The year-to-year differences also followed similar pattern: the site-dependent effects were the lowest in the dry grassland and an increasing effect was detected along the moisture gradient. We found that extensive Hungarian Grey cattle grazing is an effective tool to suppress noxious species and to create a mosaic vegetation structure, which enables to maintain high species richness in the landscape. Hungarian Grey cattle can feed in open habitats along long moisture gradient, thus in highly mosaic landscapes this breed can be the most suitable livestock type. 相似文献