全文获取类型
收费全文 | 2238篇 |
免费 | 274篇 |
国内免费 | 4篇 |
出版年
2021年 | 27篇 |
2020年 | 25篇 |
2018年 | 23篇 |
2017年 | 24篇 |
2016年 | 44篇 |
2015年 | 59篇 |
2014年 | 60篇 |
2013年 | 86篇 |
2012年 | 106篇 |
2011年 | 97篇 |
2010年 | 59篇 |
2009年 | 54篇 |
2008年 | 69篇 |
2007年 | 76篇 |
2006年 | 76篇 |
2005年 | 81篇 |
2004年 | 67篇 |
2003年 | 83篇 |
2002年 | 64篇 |
2001年 | 70篇 |
2000年 | 56篇 |
1999年 | 67篇 |
1998年 | 39篇 |
1997年 | 28篇 |
1996年 | 33篇 |
1995年 | 30篇 |
1994年 | 30篇 |
1993年 | 24篇 |
1992年 | 45篇 |
1991年 | 46篇 |
1990年 | 37篇 |
1989年 | 51篇 |
1988年 | 36篇 |
1987年 | 33篇 |
1986年 | 29篇 |
1985年 | 34篇 |
1984年 | 33篇 |
1983年 | 23篇 |
1982年 | 29篇 |
1979年 | 32篇 |
1978年 | 31篇 |
1977年 | 28篇 |
1976年 | 33篇 |
1975年 | 28篇 |
1974年 | 37篇 |
1973年 | 38篇 |
1971年 | 21篇 |
1970年 | 23篇 |
1969年 | 27篇 |
1968年 | 29篇 |
排序方式: 共有2516条查询结果,搜索用时 15 毫秒
991.
992.
Marisa J. Stone Luke Shoo Nigel E. Stork Fran Sheldon Carla P. Catterall 《Biotropica》2020,52(2):230-241
Converting forest to pasture can alter the roles of biota in ecosystem functioning, while vegetation restoration should arguably assist functional recovery. Since tests of this are scarce, this study quantifies both litter decomposition rates and their association with decomposer invertebrates, across 25 sites representing different phases of deforestation and subsequent reforestation of rain forest. Open and closed (to exclude macro-invertebrates) mesh bags containing forest leaves were exposed in the field for up to eight months, and invertebrates were extracted from separate collections of ground surface litter. Sites spanned five vegetation categories (five sites in each): reference states of both old-growth forest and grazed pasture; unassisted woody regrowth aged 20–50 years on former pasture; and assisted regeneration aged 1–3 and 5–10 years after interventions were applied to similar regrowth. Decomposition rates in open bags were about 50% slower in pasture than old-growth forest, and abundances of macro- and meso-decomposer invertebrates were 95% and 77% lower, respectively. However, in all restoration site-types, decomposition rates had recovered to 83% of old-growth values, and abundances of invertebrate decomposers were similar in old-growth forest. Decomposer community composition at a broad taxonomic level differed strongly between pasture and all other vegetation types. Exclusion of macro-invertebrates decreased decomposition rates by only about 3.1%, but decomposition rates in open bags were significantly correlated (across sites) with abundances of both macro- and meso-decomposers, most strongly so for meso-decomposers. Drawing useful generalizations across studies is impeded by differing methodologies and because few include both agricultural and forest reference sites. 相似文献
993.
Patterns of storage protein and triacylglycerol accumulation during loblolly pine somatic embryo maturation 总被引:1,自引:1,他引:0
Disa L. Brownfield Christopher D. Todd Sandra L. Stone Michael K. Deyholos David J. Gifford 《Plant Cell, Tissue and Organ Culture》2007,88(2):217-223
Conifer somatic embryo germination and early seedling growth are fundamentally different than in their zygotic counterparts
in that the living maternal megagametophyte tissue surrounding the embryo is absent. The megagametophyte contains the majority
of the seed storage reserves in loblolly pine and the lack of the megagametophyte tissue poses a significant challenge to
somatic embryo germination and growth. We investigated the differences in seed storage reserves between loblolly pine mature
zygotic embryos and somatic embryos that were capable of germination and early seedling growth. Somatic embryos utilized in
this study contained significantly lower levels of triacylglycerol and higher levels of storage proteins relative to zygotic
embryos. A shift in the ratio of soluble to insoluble protein present was also observed. Mature zygotic embryos had roughly
a 3:2 ratio of soluble to insoluble protein whereas the somatic embryos contained over 5-fold more soluble protein compared
to insoluble protein. This indicates that the somatic embryos are not only producing more protein overall, but that this protein
is biased more heavily towards soluble protein, indicating possible differences in metabolic activity at the time of desiccation. 相似文献
994.
995.
Paula B. Deming Shirley L. Campbell Jamie B. Stone Robert L. Rivard Alison L. Mercier Alan K. Howe 《The Journal of biological chemistry》2015,290(9):5783-5796
Netrin-1, acting through its principal receptor DCC (deleted in colorectal cancer), serves as an axon guidance cue during neural development and also contributes to vascular morphogenesis, epithelial migration, and the pathogenesis of some tumors. Several lines of evidence suggest that netrin-DCC signaling can regulate and be regulated by the cAMP-dependent protein kinase, PKA, although the molecular details of this relationship are poorly understood. Specificity in PKA signaling is often achieved through differential subcellular localization of the enzyme by interaction with protein kinase A anchoring proteins (AKAPs). Here, we show that AKAP function is required for DCC-mediated activation of PKA and phosphorylation of cytoskeletal regulatory proteins of the Mena/VASP (vasodilator-stimulated phosphoprotein) family. Moreover, we show that DCC and PKA physically interact and that this association is mediated by the ezrin-radixin-moesin (ERM) family of plasma membrane-actin cytoskeleton cross-linking proteins. Silencing of ERM protein expression inhibits DCC-PKA interaction, DCC-mediated PKA activation, and phosphorylation of Mena/VASP proteins as well as growth cone morphology and neurite outgrowth. Finally, although expression of wild-type radixin partially rescued growth cone morphology and tropism toward netrin in ERM-knockdown cells, expression of an AKAP-deficient mutant of radixin did not fully rescue growth cone morphology and switched netrin tropism from attraction to repulsion. These data support a model in which ERM-mediated anchoring of PKA activity to DCC is required for proper netrin/DCC-mediated signaling. 相似文献
996.
Black rats are of outstanding interest in parasitology and infective disease analysis. We used chromosome paints from both
the mouse(Mus musculus) and the Norway rat(Rattus norvegicus) to characterize the genome of two Black rat subspecies from Italy. Both subspecies have two large metacentrics (n. 1, 4)
not present in the Norway rat (2n = 42).Rattus rattus rattus has a diploid number of 2n = 38, whileRattus rattus frugivorous has two small metacentric “supernumerary” or B chromosomes for a diploid number of 2n = 38 + 2B. The 21 mouse paints gave
38 signals on theR. r. rattus karyotype and 39 signals in theR. r. frugivorous karyotype. The two metacentrics, not present inR. norvegicus, were hybridized by mouse 16/1/17 and mouse 4/10/15. These chromosomes are homologous to: RRA1 = RNO 5/7, and RRA4 = RNO
9/11 and not “4/7” and “11/12” as previously reported. Furthermore, the synteny of Chr 13 of theR. r. frugivorous withR. norvegicus Chr 16 and mouse Chrs 8/14 is not complete, because there is a small pericentromeric insertion of RNO Chr 18 (mouse Chr 18).
If we consider only the two metacentrics, RRA1 and RRA4, the principal differences betweenR. norvegicus andR. rattus, then we can propose the derived synteny of 124 genes in the black rat. A comparison of the Z index between rats and mice
shows an acceleration of genomic evolution among genus, species, and subspecies. The chromosomal differences betweenR. r. rattus xR. r. frugivorous suggest that they may be classified as different species because hybrids would produce 50% unbalanced gametes. 相似文献
997.
The effects of polysaccharide structure and environment on the formation of fluorescent complexes between the polysaccharide and a fluorochrome (4,4′ - [carbonylbis (benzene-4,1-diyl) bis(imino)] bisbenzenesulfonic acid (Sirofluor) isolated from the triarylmethane dye, aniline blue, have been studied. Amongst the wide range of water-soluble polysaccharides tested, fluorescent complexes are formed only with glucans, the strongest fluorescence being obtained with linear (1 → 3)-β-d-glucans and with linear (1 → 3)-β-d-glucans bearing single glucose residues attached at the 6-position. The fluorescence of complexes formed with water-insoluble polysaccharides depends on the ionic environment as well as the polysaccharide structure. (1 → 3)-β-d-Glucans form strongly fluorescent complexes in the dry state and in the presence of water or phosphate buffer. Various cellulose ((1 → 4)-β-d-glucan) samples form strongly fluorescent complexes in the dry state and in the presence of phosphate buffer, but are significantly reduced in the presence of water alone. 相似文献
998.
Margaret E Kellogg Sandra Burkett Thomas R Dennis Gary Stone Brian A Gray Peter M McGuire Roberto T Zori Roscoe Stanyon 《BMC evolutionary biology》2007,7(1):6
Background
Sirenia (manatees, dugongs and Stellar's sea cow) have no evolutionary relationship with other marine mammals, despite similarities in adaptations and body shape. Recent phylogenomic results place Sirenia in Afrotheria and with elephants and rock hyraxes in Paenungulata. Sirenia and Hyracoidea are the two afrotherian orders as yet unstudied by comparative molecular cytogenetics. Here we report on the chromosome painting of the Florida manatee. 相似文献999.
Ellen Weisberg Chengcheng Meng Abigail E. Case Hong L. Tiv Prafulla C. Gokhale Sara J. Buhrlage Jing Yang Xiaoxi Liu Jinhua Wang Nathanael Gray Sophia Adamia Martin Sattler Richard Stone James D. Griffin 《Journal of cellular and molecular medicine》2020,24(5):2968-2980
Recently, several targeted agents have been developed for specific subsets of patients with acute myeloid leukaemia (AML), including midostaurin, the first FDA-approved FLT3 inhibitor for newly diagnosed patients with FLT3 mutations. However, in the initial Phase I/II clinical trials, some patients without FLT3 mutations had transient responses to midostaurin, suggesting that this multi-targeted kinase inhibitor might benefit AML patients more broadly. Here, we demonstrate submicromolar efficacy of midostaurin in vitro and efficacy in vivo against wild-type (wt) FLT3-expressing AML cell lines and primary cells, and we compare its effectiveness with that of other FLT3 inhibitors currently in clinical trials. Midostaurin was found to synergize with standard chemotherapeutic drugs and some targeted agents against AML cells without mutations in FLT3. The mechanism may involve, in part, the unique kinase profile of midostaurin that includes proteins implicated in AML transformation, such as SYK or KIT, or inhibition of ERK pathway or proviability signalling. Our findings support further investigation of midostaurin as a chemosensitizing agent in AML patients without FLT3 mutations. 相似文献
1000.