全文获取类型
收费全文 | 2281篇 |
免费 | 155篇 |
国内免费 | 5篇 |
专业分类
2441篇 |
出版年
2023年 | 7篇 |
2022年 | 43篇 |
2021年 | 69篇 |
2020年 | 52篇 |
2019年 | 61篇 |
2018年 | 65篇 |
2017年 | 58篇 |
2016年 | 88篇 |
2015年 | 96篇 |
2014年 | 142篇 |
2013年 | 123篇 |
2012年 | 180篇 |
2011年 | 168篇 |
2010年 | 123篇 |
2009年 | 95篇 |
2008年 | 148篇 |
2007年 | 145篇 |
2006年 | 127篇 |
2005年 | 108篇 |
2004年 | 116篇 |
2003年 | 94篇 |
2002年 | 83篇 |
2001年 | 20篇 |
2000年 | 29篇 |
1999年 | 22篇 |
1998年 | 18篇 |
1997年 | 17篇 |
1996年 | 8篇 |
1995年 | 3篇 |
1994年 | 6篇 |
1993年 | 3篇 |
1992年 | 15篇 |
1991年 | 12篇 |
1990年 | 10篇 |
1989年 | 11篇 |
1988年 | 6篇 |
1987年 | 6篇 |
1986年 | 7篇 |
1985年 | 9篇 |
1984年 | 3篇 |
1983年 | 6篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 6篇 |
1974年 | 4篇 |
1972年 | 2篇 |
1971年 | 5篇 |
1970年 | 3篇 |
1968年 | 2篇 |
1966年 | 2篇 |
排序方式: 共有2441条查询结果,搜索用时 0 毫秒
1.
New material ofTrischizolagus dumitrescuae from Moldova and Ukraine is described. The variation of p3 inTrischizolagus shows the gradual shift of morphotype frequencies from the ‘Hypolagus’ pattern in Turolian through the mixture of three patterns (including ‘Nekrolagus’ morphotype) in Early Ruscinian to the dominant ‘Alilepus’ pattern in the Late Ruscinian samples. These transformations took place parallel to that of the North AmericanNekrolagus. Probably North AmericanSylvilagus, Brachylagus, andRomerolagus had an North American origin fromNekrolagus, whereas Eurasiatic and AfricanOryctolagus, Caprolagus, Nesolagus, andPoelagus could have originated in the Old World fromTrischizolagus. 相似文献
2.
David Houle Alexey S Kondrashov 《Proceedings. Biological sciences / The Royal Society》2002,269(1486):97-104
Females often choose their mates, instead of mating at random, even when a father contributes nothing but genes to his offspring. Costly female preferences for males with exaggerated traits that reduce viability, such as the peacock's tail, are particularly puzzling. Such preferences can evolve if directly favoured by natural selection or when the exaggerated trait, although maladaptive per se, indicates high overall quality of the male's genotype. Two recent analyses suggested that the advantage to mate choice based on genetic quality is too weak to explain extreme cases of exaggeration of display traits and the corresponding preferences. We studied coevolution of a female mate-preference function and a genotype-dependent male display function where mutation supplies variation in genotype quality and mate preference is costly. Preference readily evolves, often causing extreme exaggeration of the display. Mate choice and trait expression can approach an equilibrium, or a limit cycle, or exaggeration can proceed forever, eventually causing extinction. 相似文献
3.
Formaldehyde fixation of cells is routinely used to study DNA-protein interactions in vivo. In these studies, DNA is often analyzed using a polymerase chain reaction technique. Although it is known that formaldehyde can damage DNA, no studies have been performed so far to compare the efficiency of DNA amplification between normal and fixed cells. Here we show that formaldehyde fixation results in a 15% to 20% reduction in the ability to amplify cellular DNA. The loss of amplifiability is independent of the length of the amplification region and the degree to which DNA is compacted on packaging into chromatin. 相似文献
4.
5.
George V. Sharonov Eduard V. Bocharov Peter M. Kolosov Maria V. Astapova Alexander S. Arseniev Alexey V. Feofanov 《The Journal of biological chemistry》2014,289(21):14955-14964
The EphA2 receptor tyrosine kinase plays a central role in the regulation of cell adhesion and guidance in many human tissues. The activation of EphA2 occurs after proper dimerization/oligomerization in the plasma membrane, which occurs with the participation of extracellular and cytoplasmic domains. Our study revealed that the isolated transmembrane domain (TMD) of EphA2 embedded into the lipid bicelle dimerized via the heptad repeat motif L535X3G539X2A542X3V546X2L549 rather than through the alternative glycine zipper motif A536X3G540X3G544 (typical for TMD dimerization in many proteins). To evaluate the significance of TMD interactions for full-length EphA2, we substituted key residues in the heptad repeat motif (HR variant: G539I, A542I, G553I) or in the glycine zipper motif (GZ variant: G540I, G544I) and expressed YFP-tagged EphA2 (WT, HR, and GZ variants) in HEK293T cells. Confocal microscopy revealed a similar distribution of all EphA2-YFP variants in cells. The expression of EphA2-YFP variants and their kinase activity (phosphorylation of Tyr588 and/or Tyr594) and ephrin-A3 binding were analyzed with flow cytometry on a single cell basis. Activation of any EphA2 variant is found to occur even without ephrin stimulation when the EphA2 content in cells is sufficiently high. Ephrin-A3 binding is not affected in mutant variants. Mutations in the TMD have a significant effect on EphA2 activity. Both ligand-dependent and ligand-independent activities are enhanced for the HR variant and reduced for the GZ variant compared with the WT. These findings allow us to suggest TMD dimerization switching between the heptad repeat and glycine zipper motifs, corresponding to inactive and active receptor states, respectively, as a mechanism underlying EphA2 signal transduction. 相似文献
6.
Alexey F. Bochkov Vladimir N. Chernetsky Nikolay K. Kochetkov 《Carbohydrate research》1975,43(1):35-41
Cationic polymerisation of 3-O-acetyl-β-L-arabinofuranose 1,2,5-orthobenzoate initiated by either triphenylcarbonium tetrafluoroborate or benzoylium perchlorate has been studied. The existence of living chains was demonstrated by termination of polymerisation with tritium-labelled 1-butanol. The number of growing chains reached a maximum after ≈10 min and then decreased. 相似文献
7.
The study was concerned with investigation of the biochemical properties, antibiotic resistance and the effect of various doses of penicillin on the activity of metabolic enzymes in the bacterial cells of Staphylococcus epidermidis isolated from the pharynx mucosa of healthy subjects. The study revealed an increase in the aggressive properties of the representatives of the mouth normal microflora. It was also observed that the highest percentage of the staphylococcal isolates was resistant to beta-lactam antibiotics. The effect of various concentrations of penicillin on the microbial metabolism was shown to be directed at different targets. Penicillin intensified the aerobic processes and lipid synthesis in the bacterial cells. The reactions of the processes of the amino acid metabolism proceeded in different directions. Some concentrations of penicillin inhibited such processes. 相似文献
8.
CT and roentgenography were used for the investigation of 78 patients with the radicular syndrome. The state of the intervertebral disks, intervertebral joints and cerebrospinal canal in degenerative vertebral diseases was assessed. CT permits the detection of hernia, protrusion of the intervertebral disks, deformity of the intervertebral joints, and the narrowing of the cerebrospinal canal as a result of degenerative changes, as well as establishing the cause of the affection of neural structures in the cerebrospinal canal, radicular holes. CT possesses some advantages over roentgenography in the diagnosis of degenerative vertebral diseases and can be recommended as the principal method together with roentgenography for investigation of patients with lumbar pains. 相似文献
9.
10.
V G Savchenko V G Isaev E M Abakumov N D Khoroshko A G Turkina A N Sokolov M M Viadro S M Navashin 《Antibiotiki i khimioterapii͡a》1992,37(7):47-49
The clinical efficacy of aclarubicin, an anthracycline antibiotic, was studied in 48 patients with leukemia. The antibiotic was used in the following combinations with cytarabine: "7 + 7", "5 + 5" and "7 & 3". A complete remission was stated in 14 (42.4 per cent) out of 33 patients with acute nonlymphoid leukemia, 6 (43 per cent) out of the 14 patients having relapses. The combined therapy was effective in 4 out of 5 pre-resistant patients. The "7 + 3" scheme was the most beneficial. The most common adverse reactions were nausea and vomiting. 相似文献