全文获取类型
收费全文 | 3699篇 |
免费 | 438篇 |
专业分类
4137篇 |
出版年
2021年 | 50篇 |
2019年 | 38篇 |
2018年 | 49篇 |
2017年 | 40篇 |
2016年 | 61篇 |
2015年 | 97篇 |
2014年 | 120篇 |
2013年 | 125篇 |
2012年 | 158篇 |
2011年 | 165篇 |
2010年 | 113篇 |
2009年 | 89篇 |
2008年 | 136篇 |
2007年 | 134篇 |
2006年 | 114篇 |
2005年 | 110篇 |
2004年 | 113篇 |
2003年 | 115篇 |
2002年 | 121篇 |
2001年 | 83篇 |
2000年 | 98篇 |
1999年 | 80篇 |
1998年 | 50篇 |
1997年 | 43篇 |
1996年 | 38篇 |
1995年 | 45篇 |
1994年 | 36篇 |
1993年 | 44篇 |
1992年 | 77篇 |
1991年 | 62篇 |
1990年 | 78篇 |
1989年 | 73篇 |
1988年 | 76篇 |
1987年 | 64篇 |
1986年 | 56篇 |
1985年 | 57篇 |
1984年 | 56篇 |
1983年 | 44篇 |
1982年 | 51篇 |
1981年 | 49篇 |
1980年 | 32篇 |
1979年 | 57篇 |
1978年 | 43篇 |
1977年 | 32篇 |
1976年 | 49篇 |
1975年 | 51篇 |
1974年 | 58篇 |
1973年 | 78篇 |
1972年 | 54篇 |
1971年 | 29篇 |
排序方式: 共有4137条查询结果,搜索用时 15 毫秒
81.
We documented 13 behaviors associated with oviposition in unfertilized and fertilized spruce budworm moths, Choristoneura fumiferana, by videotaping active moths with a macro lens. Apart from resting, the most pronounced behaviors were probing, drumming + probing, and egg laying. Probing was a bending of the abdomen and extension of the ovipositor to touch the substrate in a rocking, back and forth motion. Drumming + probing involved tapping of the substrate with the pro- and mesothocacic legs with concurrent bending of the abdomen as above. Egg laying was the actual process of egg deposition onto the needle surface of a balsam fir twig. Both the frequency and the duration of these behavioral elements varied depending on the mating status of the female. Unfertilized females exhibited a higher frequency of probing and egg laying. The duration of probing was longer in unfertilized females, while drumming + probing and egg laying were longer for fertilized females. This study is the basis for future work on the chemosensilla associated with the perception of host-plant surface chemicals by ovipositing females. 相似文献
82.
Many attempts have been made to recognise divisions within Acropora, the most diverse reef building coral genus on modern reefs, but only subgenera Acropora and Isopora are currently recognised. In this paper, morphological and genetic analyses, and study of reproductive mode and anatomy, demonstrate
that an endemic Indonesian species A. (Acropora) togianensis, Wallace, 1997, belongs to Isopora. Despite the presence of a clear central axial corallite (indicating sub-genus Acropora), this species has supplementary axial corallites, broods planula larvae rather than broadcast-spawning for external fertilisation
and develops stalked ova: all characters in common with the type species of subgenus Isopora A. (Isopora) palifera and the other species for which such data are available, A. (I.) cuneata and A. (I.) brueggemanni. Phylogenies are based on the protein-coding genes, mitochondrial cytochrome b (cytb) and nuclear histone 2a and 2b (h2ab) also group A. togianensis with these Isoporans. High bootstrapping and Bayesian support in the major lineages of the family Acroporidae demonstrate
significant differences between Isopora (including A. togianensis) and Acropora. As the type species of both subgenera, A. (Acropora) muricata (Linneaus 1758) and A. (Isopora) palifera (Lamarck, 1816) are used in these analyses, elevation of Isopora Studer, 1878 to genus is formally proposed. 相似文献
83.
Yuhsi Matuq Pamela S. Adams Nozomu Nishi Hidetaro Yasumitsu John W. Crabb Robert J. Matusik Wallace L. McKeehan 《In vitro cellular & developmental biology. Plant》1989,25(6):581-584
Summary Rat prostate extracts contain an abundant 20–22 kilodalton heparin-binding protein with near identical chromatographic properties,
but only 0.2–1% of the mitogenic activity, of bovine brain heparin-binding growth factor-1 (acidic fibroblast growth factor).
Amino terminal amino acid sequence (met-met-thr-asp-lys-asn-leu-lys-lys-lys-ile-glu-gly-asn-trp-arg-thr-val-tyr-leu-ala-ala-ser-?-val-glu-lys-ile-asn-glu-gly-ser-pro)
and immunochemical analysis revealed that the protein is identical to the androgen-dependent protein “probasin”.
This work was supported in part by NCI grant CA37589 (W. L. M., J. W. C.) and the Medical Research Council of Canada (R. J.
M.). 相似文献
84.
Cytoplasmic dynein is the primary molecular motor responsible for transport of vesicles, organelles, proteins and RNA cargoes from the periphery of the cell towards the nucleus along the microtubule cytoskeleton of eukaryotic cells. Dynactin, a large multi-subunit activator of dynein, docks cargo to the motor and may enhance dynein processivity. Here, we show that individual fluorescently labelled dynein-dynactin complexes exhibit bidirectional and processive motility towards both the plus and minus ends of microtubules. The dependence of this activity on substrate ATP concentration, nucleotide analogues and inhibitors suggests that bidirectional motility is an active energy-transduction property of dynein-dynactin motor mechano-chemistry. The unique motility characteristics observed may reflect the flexibility of the dynein structure that leads to an enhanced ability to navigate around obstacles in the cell. 相似文献
85.
86.
The 5'-terminal cap structures of 32P-labeled oligo(uridylic acid)-containing messenger ribonucleic acid [oligo(U+)mRNA] isolated from HeLa cell polyadenylated [poly(A+)] mRNA were analyzed and compared to those of the poly(A+) mRNA. A method employing P1 nuclease, alkaline phosphatase, and adsorption to activated charcoal showed that the types of cap core (m7 GpppXm) in oligo(U+) mRNA were essentially identical with those in poly(A+) mRNA. Analysis of RNase T2 digestion products of oligo(U+) mRNA demonstrated the presence of both cap 1 (m7GpppXmpYp) and cap 2 (m7GpppXmpYmpZp) in this subpopulation, confirming its cytoplasmic location. The base compositions of these two types of caps were different from each other and nonrandom but did not differ significantly between oligo(U+) and poly(A+) m RNA. The only observed difference between the mRNA populations was a higher ratio of cap 1 and cap 2 in the former. Possible implications of these findings for the relationship between oligo(U+) mRNA and poly(A+) mRNA are discussed. 相似文献
87.
Edward C. Jones-López Soyeon Kim Geisa Fregona Patricia Marques-Rodrigues David Jamil Hadad Lucilia Pereira Dutra Molina Solange Vinhas Nancy Reilly Stephanie Moine Soumitesh Chakravorty Mary Gaeddert Rodrigo Ribeiro-Rodrigues Padmini Salgame Moises Palaci David Alland Jerrold J. Ellner Reynaldo Dietze 《PloS one》2014,9(7)
Rationale
The degree to which tuberculosis (TB) is transmitted between persons is variable. Identifying the factors that contribute to transmission could provide new opportunities for TB control. Transmission is influenced by host, bacterial and environmental factors. However, distinguishing their individual effects is problematic because measures of disease severity are tightly correlated, and assessing the virulence of Mycobacterium tuberculosis isolates is complicated by epidemiological and clinical confounders.Objectives
To overcome these problems, we investigated factors potentially associated with TB transmission within households.Methods
We evaluated patients with smear-positive (≥2+), pulmonary TB and classified M. tuberculosis strains into single nucleotide polymorphism genetic cluster groups (SCG). We recorded index case, household contact, and environmental characteristics and tested contacts with tuberculin skin test (TST) and interferon-gamma release assay. Households were classified as high (≥70% of contacts with TST≥10 mm) and low (≤40%) transmission. We used logistic regression to determine independent predictors.Result
From March 2008 to June 2012, we screened 293 TB patients to enroll 124 index cases and their 731 contacts. There were 23 low and 73 high transmission households. Index case factors associated with high transmission were severity of cough as measured by a visual analog cough scale (VACS) and the Leicester Cough Questionnaire (LCQ), and cavitation on chest radiograph. SCG 3b strains tended to be more prevalent in low (27.3%) than in high (12.5%) transmission households (p = 0.11). In adjusted models, only VACS (p<0.001) remained significant. SCG was associated with bilateral disease on chest radiograph (p = 0.002) and marginally associated with LCQ sores (p = 0.058), with group 3b patients having weaker cough.Conclusions
We found differential transmission among otherwise clinically similar patients with advanced TB disease. We propose that distinct strains may cause differing patterns of cough strength and cavitation in the host leading to diverging infectiousness. Larger studies are needed to verify this hypothesis. 相似文献88.
Courtney L. Jones Teena Bhatla Roy Blum Jinhua Wang Steven W. Paugh Xin Wen Wallace Bourgeois Danielle S. Bitterman Elizabeth A. Raetz Debra J. Morrison David T. Teachey William E. Evans Michael J. Garabedian William L. Carroll 《The Journal of biological chemistry》2014,289(30):20502-20515
Although great advances have been made in the treatment of pediatric acute lymphoblastic leukemia, up to one of five patients will relapse, and their prognosis thereafter is dismal. We have previously identified recurrent deletions in TBL1XR1, which encodes for an F-box like protein responsible for regulating the nuclear hormone repressor complex stability. Here we model TBL1XR1 deletions in B-precursor ALL cell lines and show that TBL1XR1 knockdown results in reduced glucocorticoid receptor recruitment to glucocorticoid responsive genes and ultimately decreased glucocorticoid signaling caused by increased levels of nuclear hormone repressor 1 and HDAC3. Reduction in glucocorticoid signaling in TBL1XR1-depleted lines resulted in resistance to glucocorticoid agonists, but not to other chemotherapeutic agents. Importantly, we show that treatment with the HDAC inhibitor SAHA restores sensitivity to prednisolone in TBL1XR1-depleted cells. Altogether, our data indicate that loss of TBL1XR1 is a novel driver of glucocorticoid resistance in ALL and that epigenetic therapy may have future application in restoring drug sensitivity at relapse. 相似文献
89.