首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1036篇
  免费   99篇
  国内免费   1篇
  2023年   4篇
  2022年   14篇
  2021年   28篇
  2020年   17篇
  2019年   15篇
  2018年   17篇
  2017年   18篇
  2016年   35篇
  2015年   56篇
  2014年   56篇
  2013年   67篇
  2012年   87篇
  2011年   78篇
  2010年   50篇
  2009年   38篇
  2008年   69篇
  2007年   67篇
  2006年   42篇
  2005年   45篇
  2004年   61篇
  2003年   55篇
  2002年   33篇
  2001年   5篇
  2000年   7篇
  1999年   14篇
  1998年   5篇
  1997年   11篇
  1993年   4篇
  1992年   5篇
  1991年   4篇
  1990年   6篇
  1989年   4篇
  1988年   5篇
  1986年   4篇
  1984年   3篇
  1983年   4篇
  1982年   7篇
  1981年   6篇
  1980年   6篇
  1979年   5篇
  1978年   7篇
  1977年   3篇
  1974年   6篇
  1973年   3篇
  1969年   3篇
  1968年   6篇
  1967年   6篇
  1962年   6篇
  1960年   4篇
  1958年   3篇
排序方式: 共有1136条查询结果,搜索用时 31 毫秒
41.
The One Health initiative is a global effort fostering interdisciplinary collaborations to address challenges in human, animal, and environmental health. While One Health has received considerable press, its benefits remain unclear because its effects have not been quantitatively described. We systematically surveyed the published literature and used social network analysis to measure interdisciplinarity in One Health studies constructing dynamic pathogen transmission models. The number of publications fulfilling our search criteria increased by 14.6% per year, which is faster than growth rates for life sciences as a whole and for most biology subdisciplines. Surveyed publications clustered into three communities: one used by ecologists, one used by veterinarians, and a third diverse-authorship community used by population biologists, mathematicians, epidemiologists, and experts in human health. Overlap between these communities increased through time in terms of author number, diversity of co-author affiliations, and diversity of citations. However, communities continue to differ in the systems studied, questions asked, and methods employed. While the infectious disease research community has made significant progress toward integrating its participating disciplines, some segregation—especially along the veterinary/ecological research interface—remains.  相似文献   
42.
Processing speed is a psychological construct that refers to the speed with which an individual can perform any cognitive operation. Processing speed correlates strongly with general cognitive ability, declines sharply with age and is impaired across a number of neurological and psychiatric disorders. Thus, identifying genes that influence processing speed will likely improve understanding of the genetics of intelligence, biological aging and the etiologies of numerous disorders. Previous genetics studies of processing speed have relied on simple phenotypes (eg, mean reaction time) derived from single tasks. This strategy assumes, erroneously, that processing speed is a unitary construct. In the present study, we aimed to characterize the genetic architecture of processing speed by using a multidimensional model applied to a battery of cognitive tasks. Linkage and QTL‐specific association analyses were performed on the factors from this model. The randomly ascertained sample comprised 1291 Mexican‐American individuals from extended pedigrees. We found that performance on all three distinct processing‐speed factors (Psychomotor Speed; Sequencing and Shifting and Verbal Fluency) were moderately and significantly heritable. We identified a genome‐wide significant quantitative trait locus (QTL) on chromosome 3q23 for Psychomotor Speed (LOD = 4.83). Within this locus, we identified a plausible and interesting candidate gene for Psychomotor Speed (Z = 2.90, P = 1.86 × 10?03).  相似文献   
43.
Estimating the evolutionary potential of quantitative traits and reliably predicting responses to selection in wild populations are important challenges in evolutionary biology. The genomic revolution has opened up opportunities for measuring relatedness among individuals with precision, enabling pedigree‐free estimation of trait heritabilities in wild populations. However, until now, most quantitative genetic studies based on a genomic relatedness matrix (GRM) have focused on long‐term monitored populations for which traditional pedigrees were also available, and have often had access to knowledge of genome sequence and variability. Here, we investigated the potential of RAD‐sequencing for estimating heritability in a free‐ranging roe deer (Capreolous capreolus) population for which no prior genomic resources were available. We propose a step‐by‐step analytical framework to optimize the quality and quantity of the genomic data and explore the impact of the single nucleotide polymorphism (SNP) calling and filtering processes on the GRM structure and GRM‐based heritability estimates. As expected, our results show that sequence coverage strongly affects the number of recovered loci, the genotyping error rate and the amount of missing data. Ultimately, this had little effect on heritability estimates and their standard errors, provided that the GRM was built from a minimum number of loci (above 7,000). Genomic relatedness matrix‐based heritability estimates thus appear robust to a moderate level of genotyping errors in the SNP data set. We also showed that quality filters, such as the removal of low‐frequency variants, affect the relatedness structure of the GRM, generating lower h2 estimates. Our work illustrates the huge potential of RAD‐sequencing for estimating GRM‐based heritability in virtually any natural population.  相似文献   
44.
45.
We searched for quantitative trait loci (QTL) underlying fitness-related traits in a free-living pedigree of 588 Soay sheep in which a genetic map using 251 markers with an average spacing of 15 cM had been established previously. Traits examined included birth date and weight, considered both as maternal and offspring traits, foreleg length, hindleg length, and body weight measured on animals in August and jaw length and metacarpal length measured on cleaned skeletal material. In some cases the data were split to consider different age classes separately, yielding a total of 15 traits studied. Genetic and environmental components of phenotypic variance were estimated for each trait and, for those traits showing nonzero heritability (N= 12), a QTL search was conducted by comparing a polygenic model with a model including a putative QTL. Support for a QTL at genome-wide significance was found on chromosome 11 for jaw length; suggestive QTL were found on chromosomes 2 and 5 (for birth date as a trait of the lamb), 8 (birth weight as a trait of the lamb), and 15 (adult hindleg length). We discuss the prospects for refining estimates of QTL position and effect size in the study population, and for QTL searches in free-living pedigrees in general.  相似文献   
46.
The membrane-bound bacterial D-alanyl- D-alanine peptidases or penicillin-binding proteins (PBPs) catalyze the final transpeptidation reaction of bacterial cell wall biosynthesis and are the targets of beta-lactam antibiotics. Rather surprisingly, the substrate specificity of these enzymes is not well understood. In this paper, we present measurements of the reactivity of typical examples of these enzymes with peptidoglycan-mimetic beta-lactams under in vivo conditions. The minimum inhibitory concentrations of beta-lactams with Escherichia coli-specific side chains were determined against E. coli cells. Analogous measurements were made with Streptococcus pneumoniae R6. The reactivity of the relevant beta-lactams with E. coli PBPs in membrane preparations was also determined. The results show that under none of the above protocols were beta-lactams with peptidoglycan-mimetic side chains more reactive than generic analogues. This suggests that in vivo, as in vitro, these enzymes do not specifically recognize elements of peptidoglycan structure local to the reaction center. Substrate recognition must thus involve extended structure.  相似文献   
47.
The Madagascar Jacana Actophilornis albinucha (Jacanidae) is an endemic shorebird found in the threatened wetlands of western Madagascar. This species is presumed to exhibit classical polyandry; however, few data are available to support that assumption. More generally, a lack of basic understanding of this species hinders conservation efforts. We conducted the most extensive study of the Madagascar Jacana to date, and report on its: 1) distribution, population size and density; 2) degree of sexual size dimorphism; and 3) phylogenetic position. The surveys were conducted at 54 lakes, between January and October in 2016. Madagascar Jacana were found at 22 lakes, and within these were distributed at a mean density of 3.5 ± 0.74 [SE] individuals per hectare of surveyed habitat. We estimate the global population size to be between 975 and 2 064 individuals, and habitat destruction appears to be the main threat to the species. Females were significantly larger than males, consistent with reports for other Jacanidae species. Using a mitochondrial DNA fragment, we expanded the Jacanidae genetic phylogeny, and confirmed that Madagascar Jacana is the sister species to the African Jacana Actophilornis africanus. Further studies are urgently needed to thoroughly re-assess the threat status and population trend of the Madagascar Jacana.  相似文献   
48.
The Saccharomyces cerevisiae PAH1-encoded phosphatidate (PA) phosphatase, which catalyzes the dephosphorylation of PA to produce diacylglycerol, controls the bifurcation of PA into triacylglycerol synthesis and phospholipid synthesis. Pah1 is inactive in the cytosol as a phosphorylated form and becomes active on the membrane as a dephosphorylated form by the Nem1–Spo7 protein phosphatase. We show that the conserved Trp-637 residue of Pah1, located in the intrinsically disordered region, is required for normal synthesis of membrane phospholipids, sterols, triacylglycerol, and the formation of lipid droplets. Analysis of mutant Pah1-W637A showed that the tryptophan residue is involved in the phosphorylation-mediated/dephosphorylation-mediated membrane association of the enzyme and its catalytic activity. The endogenous phosphorylation of Pah1-W637A was increased at the sites of the N-terminal region but was decreased at the sites of the C-terminal region. The altered phosphorylation correlated with an increase in its membrane association. In addition, membrane-associated PA phosphatase activity in vitro was elevated in cells expressing Pah1-W637A as a result of the increased membrane association of the mutant enzyme. However, the inherent catalytic function of Pah1 was not affected by the W637A mutation. Prediction of Pah1 structure by AlphaFold shows that Trp-637 and the catalytic residues Asp-398 and Asp-400 in the haloacid dehalogenase-like domain almost lie in the same plane, suggesting that these residues are important to properly position the enzyme for substrate recognition at the membrane surface. These findings underscore the importance of Trp-637 in Pah1 regulation by phosphorylation, membrane association of the enzyme, and its function in lipid synthesis.  相似文献   
49.
The adenovirus type 5 mutant dl1520 was engineered previously to be completely defective for E1B-55K functions. Recently, this mutant (also known as ONYX-015) has been suggested to replicate preferentially in p53(-) and some p53(+) tumor cell lines but to be attenuated in primary cultured cells (C. Heise, A. Sampson-Johannes, A. Williams, F. McCormick, D. D. F. Hoff, and D. H. Kirn, Nat. Med. 3:639-645, 1997). It has been suggested that dl1520 might be used as a "magic bullet" that could selectively lyse tumor cells without harm to normal tissues. However, we report here that dl1520 replication is independent of p53 genotype and occurs efficiently in some primary cultured human cells, indicating that the mutant virus does not possess a tumor selectivity. Although it was not the sole host range determinant, p53 function did reduce dl1520 replication when analyzed in a cell line expressing temperature-sensitive p53 (H1299-tsp53) (K. L. Fries, W. E. Miller, and N. Raab-Traub, J. Virol. 70:8653-8659, 1996). As found earlier for other E1B-55K mutants in HeLa cells (Y. Ho, R. Galos, and J. Williams, Virology 122:109-124, 1982), dl1520 replication was temperature dependent in H1299 cells. When p53 function was restored at low temperature in H1299-tsp53 cells, it imposed a modest defect in viral DNA replication and accumulation of late viral cytoplasmic mRNA. However, in both H1299 and H1299-tsp53 cells, the defect in late viral protein synthesis appeared to be much greater than could be accounted for by the modest defects in late viral mRNA levels. We therefore propose that in addition to countering p53 function and modulating viral and cellular mRNA nuclear transport, E1B-55K also stimulates late viral mRNA translation.  相似文献   
50.
This study assessed the effectiveness of operant conditioning in training three species of captive callitrichid primates (Leontopithecus rosalia, Callithrix geoffroyi, and Saguinus imperator) to urinate on demand. There were three goals to the study: 1) to develop a system for quantitatively assessing positive reinforcement training; 2) to ascertain whether or not positive reinforcement techniques can be used to train callitrichid monkeys to urinate on demand, and if so, how many training sessions are required; and 3) to determine the effect on urination behavior of the trainer entering the cage to collect a urine sample. Positive reinforcement with a continuous reinforcement schedule was used to capture a natural behavior: urination. Training sessions (30 min each) were conducted at dawn thrice weekly during five consecutive phases: habituation, control, training (animals were rewarded for urinating), maintenance (animals had reached a defined training criteria and continued to be rewarded for urinating), and collection (animals were rewarded for urinating, and the trainer entered the cage to collect the sample). The numbers of 30-min training sessions required to train the three monkey species (L. rosalia, C. geoffroyi, and S. imperator) were five, six, and eight, respectively. For the three species, the mean number of urinations per animal was significantly greater during the training, maintenance, and collection phases compared to the control phase. However, the three species differed significantly in the manner in which the rates of urination changed across the five phases. A higher proportion of subjects urinated during the training, maintenance, and collection phases compared to the control phase. Latency to first urination varied significantly across the five phases, with significantly reduced latencies to urinate during the training, maintenance, and collection phases compared to the control phase. The entry of the trainer into the cage to collect the urine sample did not appear to alter urination behavior. We demonstrate that operant conditioning techniques, which typically incur minimal cost, time investment, and disturbance, can be used to increase the quantity of urine samples collected for physiological analysis, the proportion of animals that urinate, and the speed of sample collection.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号