全文获取类型
收费全文 | 1086篇 |
免费 | 80篇 |
专业分类
1166篇 |
出版年
2023年 | 4篇 |
2022年 | 17篇 |
2021年 | 32篇 |
2020年 | 16篇 |
2019年 | 14篇 |
2018年 | 24篇 |
2017年 | 20篇 |
2016年 | 29篇 |
2015年 | 55篇 |
2014年 | 56篇 |
2013年 | 69篇 |
2012年 | 74篇 |
2011年 | 90篇 |
2010年 | 42篇 |
2009年 | 44篇 |
2008年 | 62篇 |
2007年 | 76篇 |
2006年 | 49篇 |
2005年 | 47篇 |
2004年 | 46篇 |
2003年 | 32篇 |
2002年 | 48篇 |
2001年 | 9篇 |
2000年 | 7篇 |
1999年 | 11篇 |
1998年 | 7篇 |
1997年 | 5篇 |
1996年 | 6篇 |
1995年 | 5篇 |
1994年 | 3篇 |
1993年 | 9篇 |
1992年 | 6篇 |
1991年 | 7篇 |
1990年 | 10篇 |
1989年 | 8篇 |
1988年 | 3篇 |
1986年 | 4篇 |
1985年 | 8篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1977年 | 7篇 |
1976年 | 4篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1968年 | 3篇 |
1892年 | 3篇 |
排序方式: 共有1166条查询结果,搜索用时 0 毫秒
1.
The gorgoniid Eugorgia is exclusively an eastern Pacific genus. It has a wide geographic and bathymetric range of distribution, found from California to Perú and extends down to 65 m deep. Two new species are herein described. The morphological characters were analyzed and illustrated by light and scanning electron microscopy. Eugorgia beebei
sp. n. can be distinguished by its white, ascending, sparse colony growth. Eugorgia mutabilis
sp. n. can be distinguished by its white colony that changes color after collection, and the conspicuous sharp-crested disc sclerites. From a morphological point of view the new species are related to the daniana-group, the rubens-group and the siedenburgae-group of Eugorgia; their affiliations, and the proposal of a new group are discussed. These new species increases the number of species in the genus to 15, and contribute to the knowledge of the eastern Pacific octocoral biodiversity. 相似文献
2.
Theresa M. Bert Kevin J. McCarthy Hector Cruz-Lopez Steven Bogdanowicz 《Evolution; international journal of organic evolution》1996,50(2):655-671
Many investigators categorize individuals from hybrid zones to facilitate comparisons among genotypic classes (e.g., parental, F1, backcross) for comparative studies in which components of fitness or geographic variation are being analyzed. Frequently, multiple character sets representing genetically independent traits are used to classify these individuals and various methodologies are employed to combine the classifications obtained from the different character sets. We adapted the principles of total evidence and taxonomic congruence (two formalized approaches used by systematists in formulating phylogenetic hypotheses) to address the problem of discriminating hybridizing species and classifying individuals from hybrid zones. As our model, we used two morphological (coloration and morphometric) and two molecular (allozyme and mitochondrial DNA restriction-fragment-length polymorphism) character sets that differentiate two stone crab species (Menippe adina and M. mercenaria). Using principal-components analysis, we determined that combining character sets and eliminating characters or character sets that did not have large eigenvector coefficients for the principal component that best separated the two species yielded the highest level of discrimination between species and allowed us to classify a broad range of morpho-genotypes as hybrids. For the stone crabs, three diagnostic allozyme loci and five diagnostic coloration characters best separated the species. The two character sets were not completely congruent, but they agreed in their classification of 50% of the individuals from the hybrid zone and rarely strongly disagreed in their classifications. Classification discrepancies between the two character sets probably represent variation between traits in interspecific gene flow rather than intraspecific, ecologically mediated variation. Our results support the assertions of previous investigators who espoused the benefits associated with using multiple character sets to classify individuals from hybrid zones and demonstrate that, if character sets are reasonably congruent and numerically balanced, combining diagnostic characters from multiple character sets (a total-evidence approach) can enhance discriminatory power between species and facilitate the assignment of hybrid-zone individuals to genotypic classes. On the contrary, classifying hybrid-zone individuals using character sets separately (a taxonomic-congruence approach) provides the opportunity to compare levels of introgression between species and to assess reasons for discordance among the data sets. 相似文献
3.
4.
Rita Maria Ulloa Hector Norberto Torres Claudia M. Ochatt Maria Teresa Téllez-Iñón 《Molecular and cellular biochemistry》1991,102(2):155-163
DEAE-cellulose column chromatography of Neurospora crassa soluble mycelial extracts leads to the resolution of three major protein kinase activity peaks designated PKI, PKII, and PKIII.PKII activity is stimulated by Ca2+ and Neurospora or brain calmodulin. Maximal stimulation was observed at 2 µM-free Ca2+ and 1 µg/ml of the modulator. The stimulatory effect of the Ca2+-calmodulin complex was blocked by EGTA and by some calmodulin antagonists such as phenothiazine drugs or compound 48/80.PKII phosphorylates different proteins, among which histone II-A at a low concentration and CDPKS, the synthetic peptide specific for Ca2+-calmodulin dependent protein kinases, are the best substrates. Some phosphorylation can be detected in the absence of any exogenous acceptor. PKII activity assayed in the presence of histone II-A or in the absence of exogenous phosphate acceptor (autophosphorylation) co-elute in a DEAE-cellulose column at 0.28 M NaCl. As result of the autophosphorylation reaction of the purified enzyme a main phosphorylated component of 70 kDa was resolved by SDS-polyacrylamide gel electrophoresis. It is possible that this component is an active part of this enzyme. 相似文献
5.
Gonadal size and the circulating concentrations of two pituitary hormones (luteinizing hormone and prolactin) and three gonadal steroids (testosterone, progesterone and oestradiol-17β) were measured in two closely related Diomedea albatrosses at South Georgia. The Grey-headed albatross D. chrysostoma , if successful in rearing a chick, usually breeds biennially, whilst the Black-browed albatross D. melanophris normally breeds annually. Direct examination (by laparoscopy) of the gonads showed that the testes of both species underwent annual cycles, whilst endocrine data confirmed that those male Grey-headed albatrosses at the colony in the pre-laying period but not breeding in that year (having bred successfully the previous year) were apparently in full reproductive condition with elevated testosterone levels typical of breeding birds. However, the females of the two species differed markedly. Grey-headed albatrosses, in a year following successful breeding, had undeveloped ovaries with low levels of circulating oestradiol but high levels of progesterone, whereas the Black-browed albatrosses showed a pattern consistent with annual ovarian development. The profiles of gonadal steroids through the breeding season were similar for the males of both species but differences existed between the females. In the female Grey-headed albatrosses, transient peaks of progesterone were present throughout chick rearing but these were absent from Black-browed albatrosses. Prolactin had a similar profile in both species, with uniformly high levels throughout incubation and a rapid fall near the end of the brood-guard period. It is suggested that Grey-headed, like Black-browed, albatrosses are intrinsically annual breeders. However, if a female Grey-headed albatross breeds successfully in one year, then nutritional factors operate to ensure that in the following year the female does not show ovarian development, although the ovary is active in terms of progesterone secretion. 相似文献
6.
Renny T. Franceschi Hector F.De Luca Donna L. Mercado 《Archives of biochemistry and biophysics》1983,222(2):504-517
The interaction of the 1α,25-dihydroxyvitamin D3 receptor with immobilized calf thymus DNA has been compared with its sedimentation properties on hypotonic sucrose gradients. Forty to sixty percent of total hormone:receptor complexes formed at 4 °C were retained by DNA-cellulose and could be eluted by 0.18 to 0.2 m KCl. In contrast, heating preparations to 25 °C rapidly and irreversibly converted receptor to a form which bound hormone and DEAE-cellulose normally, but was unable to associate with DNA. Similarly, the ability of receptor to aggregate to a 6 S species was labile at 25 °C. Stabilization of receptor in the DNA binding aggregating form was accomplished using Ca2+, Mg2+, Mn2+, or Na2MoO4 while several protease and phosphatase inhibitors were ineffective. An examination of DNA binding properties of aggregating and nonaggregating receptor forms revealed that only receptor competent to enter into aggregates could bind DNA suggesting that a functional nucleic acid binding site, and, hence, a nucleic acid interaction is necessary for aggregate formation. Consistent with this view, an RNA:receptor interaction appears to be involved in formation of the 6 S complex since removal of RNA by ribonuclease treatment or purification of receptor reduced aggregation, an effect that could be reversed by addition of purified RNA. 相似文献
7.
Chemical modification of Rhodospirillum rubrum chromatophores by 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) results in inactivation of photophosphorylation, Mg2+-ATPase, oxidative phosphorylation and ATP-driven transhydrogenase, with apparent first-order kinetics. Other energy-linked reactions such as light-driven transhydrogenase and light-dependent proton uptake were insensitive to NBD-Cl. The Ca2+-ATPase activity of the soluble coupling factor from chromatophores (R. rubrum F1) was inactivated by NBD-Cl with kinetics resembling those described for Mg2+-ATPase and photophosphorylation activities of chromatophores. Both NBD-chromatophores and NBD-R. rubrum F1 fully recovered their activities when subjected to thiolysis by dithioerythritol. Phosphoryl transfer reactions of chromatophores and Ca2+-ATPase activity of R. rubrum F1 were fully protected by 5 mM Pi against modification by NBD-Cl. ADP or ATP afforded partial protection. Analysis of the protection of Ca2+-ATPase activity by Pi indicated that NBD-Cl and Pi are mutually exclusive ligands. Spectroscopic studies revealed that tyrosine and sulfhydryl residues in R. rubrum F1 underwent modification by NBD-Cl. However, the inactivation was only related to the modification of tyrosine groups. 相似文献
8.
9.
Batdorj Enkhjin AlOgayil Najla Zhuang Qinwei Kim-wee Galvez Jose Hector Bauermeister Klara Nagata Kei Kimura Tohru Ward Monika A. Taketo Teruko Bourque Guillaume Naumova Anna K. 《Mammalian genome》2023,34(1):44-55
Mammalian Genome - Several lines of evidence suggest that the presence of the Y chromosome influences DNA methylation of autosomal loci. To better understand the impact of the Y chromosome on... 相似文献
10.
Postnatal Changes in the Activity Ratio of Specific and Nonspecific Cholinesterases from Neuronal Perikarya 总被引:3,自引:2,他引:1
Enrique L. M. Ochoa Alicia Brusco Victor P. Idoyaga-Vargas† Hector Carminatti† 《Journal of neurochemistry》1982,38(5):1236-1240
Abstract: A soluble fraction from rat brain neuronal perikarya was shown to contain both the specific and nonspecific forms of the enzyme acetylcholines-terase (EC's 3.1.1.7. and 3.1.1.8., respectively). The ratio of the enzyme activities varied along the course of brain development: the nonspecific form being predominant from 1 to 15 days of age and the specific one showing the pattern of rising activity from day 15 onward. We suggest a possible relationship between this changing in cholinesterase activities and the establishment of synapses within the rat cerebral cortex. 相似文献