全文获取类型
收费全文 | 2367篇 |
免费 | 179篇 |
国内免费 | 2篇 |
出版年
2023年 | 14篇 |
2022年 | 27篇 |
2021年 | 48篇 |
2020年 | 42篇 |
2019年 | 48篇 |
2018年 | 54篇 |
2017年 | 44篇 |
2016年 | 70篇 |
2015年 | 106篇 |
2014年 | 86篇 |
2013年 | 160篇 |
2012年 | 176篇 |
2011年 | 204篇 |
2010年 | 131篇 |
2009年 | 115篇 |
2008年 | 158篇 |
2007年 | 160篇 |
2006年 | 140篇 |
2005年 | 134篇 |
2004年 | 121篇 |
2003年 | 95篇 |
2002年 | 100篇 |
2001年 | 16篇 |
2000年 | 16篇 |
1999年 | 14篇 |
1998年 | 18篇 |
1997年 | 18篇 |
1996年 | 9篇 |
1995年 | 8篇 |
1994年 | 8篇 |
1993年 | 16篇 |
1992年 | 13篇 |
1991年 | 9篇 |
1990年 | 15篇 |
1989年 | 16篇 |
1988年 | 7篇 |
1987年 | 13篇 |
1986年 | 7篇 |
1985年 | 13篇 |
1984年 | 11篇 |
1983年 | 4篇 |
1982年 | 12篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1979年 | 8篇 |
1978年 | 9篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1975年 | 6篇 |
1973年 | 9篇 |
排序方式: 共有2548条查询结果,搜索用时 15 毫秒
11.
Joakim Galli Urban Lendahl Gabrielle Paulsson Christer Ericsson Tomas Bergman Mats Carlquist Lars Wieslander 《Journal of molecular evolution》1990,31(1):40-50
Summary We describe the structure of a gene expressed in the salivary gland cells of the dipteranChironomus tentans and show that it encodes 1 of the approximately 15 secretory proteins exported by the gland cells. This sp115,140 gene consists of approximately 65 copies of a 42-bp sequence in a central uninterrupted core block, surrounded by short nonrepetitive regions. The repeats within the gene are highly similar to each other, but divergent repeats are present in a pattern which suggests that the repeat structure has been remodeled during evolution. The 42-bp repeat in the gene is a simple variant of the more complex repeat unit present in the Balbiani ring genes, encoding four of the other secretory proteins. The structure of the sp115,140 gene suggests that related repeat structures have evolved from a common origin and resulted in the set of genes whose secretory proteins interact in the assembly of the secreted protein fibers. 相似文献
12.
Tomas Prt 《Evolution; international journal of organic evolution》1996,50(4):1625-1630
Four year's data on collared flycatchers, Ficedula albicollis, breeding in a nestbox plot on the island of Gotland, Sweden, was used to investigate whether individuals avoid mating with close kin (i.e., parents or sibs). Only one case of close inbreeding (0.5% of all pairs) was observed during the years of study. The observed frequency of close inbreeding was compared to expected frequencies based on two different null models. Assuming no inbreeding avoidance behaviors (e.g., dispersal or kin recognition), but taking into account the fact that mortality, and different arrival and pairing times of individuals reduce the probability of mating with close kin, the expected frequency of close inbreeding is 10% and 15% for female and male recruits (i.e., born in the study plot), respectively. However, assuming mating to be random within the study plot reduced the expected frequency of close inbreeding to 1% or less for both males and females. Consequently, conclusions drawn concerning inbreeding avoidance depend on the null model used. Contrasting estimated costs of tolerating close inbreeding with those of avoiding it (by dispersal to other plots), however, suggests that the costs of avoiding close inbreeding are substantially greater than those of tolerating it. Therefore, although inbreeding avoidance cannot be rejected as a cause of dispersal of this species, it is not the primary cause, and particularly not for sex-biased dispersal. The general problems of investigating inbreeding avoidance are discussed. It is argued that all previous null models based on random mating in finite populations produce expected frequencies of close inbreeding that in fact include inbreeding avoidance, since they implicitly assume random dispersal within a finite population. Thus, comparisons between observed and expected frequencies of close inbreeding based on random mating are inadequate. The most promising method of investigating inbreeding avoidance is to experimentally study individual movements and mating preferences in the presence and absence of close kin. 相似文献
13.
We have used measurements of fluorescence and circular dichroism (CD) to compare chlorosome-membrane preparations derived from the green filamentous bacterium Chloroflexus aurantiacus grown in continuous culture at two different light-intensities. The cells grown under low light (6 mol m–2 s–1) had a higher ratio of bacteriochlorophyll (BChl) c to BChl a than cells grown at a tenfold higher light intensity; the high-light-grown cells had much more carotenoid per bacteriochlorophyll.The anisotropy of the QY band of BChl c was calculated from steady-state fluorescence excitation and emission spectra with polarized light. The results showed that the BChl c in the chlorosomes derived from cells grown under high light has a higher structural order than BChl c in chlorosomes from low-light-grown cells. In the central part of the BChl c fluorescence emission band, the average angles between the transition dipole moments for BChl c molecules and the symmetry axis of the chlorosome rod element were estimated as 25° and 17° in chlorosomes obtained from the low- and high-light-grown cells, respectively.This difference in BChl organization was confirmed by the decay associated spectra of the two samples obtained using picosecond single-photon-counting experiments and global analysis of the fluorescence decays. The shortest decay component obtained, which probably represents energy-transfer from the chlorosome bacteriochlorophylls to the BChl a in the baseplate, was 15 ps in the chlorosomes from high-light-grown cell but only 7 ps in the preparation from low-light grown cells. The CD spectra of the two preparations were very different: chlorosomes from low-light-grown cells had a type II spectrum, while those from high-light-grown cells was of type I (Griebenow et al. (1991) Biochim Biophys Acta 1058: 194–202). The different shapes of the CD spectra confirm the existence of a qualitatively different organization of the BChl c in the two types of chlorosome.Abbreviations BChl
bacteriochlorophyll
- CD
circular dichroism
- DAS
decay associated spectrum
- PMSF
phenylmethylsulfonyl fluoride 相似文献
14.
The light-induced oxidation of the accessory donor tyrosine-D (YD) has been studied by measurements of the EPR Signal IIslow at room temperature in the autotrophically and photoheterotrophically cultivated alga Chlamydobotrys stellata. After illumination and dark adaptation, YD Signal IIslow was observed only in autotrophic algae, i.e. under conditions of a linear photosynthetic electron transfer from water to NADP+. The addition of artificial electron acceptors phenyl-p-benzoquinone (PPQ) or dichloro-p-benzoquinone (DCQ) to the autotrophic cells caused an almost negligible increase of this signal. When photosynthetic electron flow and oxygen evolution were diminished by removal of the carbon source CO2 and addition of acetate (photoheterotrophy), a pronounced YD Signal IIslow was seen only in presence of DCQ or PPQ. Several possibilities are discussed to explain the absence of YD Signal IIslow in photoheterotrophic Chl. stellata such as the existence of a cyclic PS II electron flow very effectively reducing P680 and thereby preventing the possibility of YD oxidation. Artificial electron acceptors withdraw electrons from this cycle thus keeping the primary quinone acceptor, QA, oxidized and thereby diminishing the reduction of P680
+ by cyclic PSII. This leads to the appearance of the YD Signal IIslow also in the photoheterotrophically grown algae.Abbreviations A-band-
thermoluminescence band associated with S2QA
- charge recombination
- DCQ-
2,5-dichlorobenzoquinone
- D2-
structure protein of Photosystem II
- EPR-
electron paramagnetic resonance
- OEC-
oxygen evolving complex
- PPQ-
phenyl-p-benzoquinone
- PS II-
Photosystem II
- P680-
reaction center of Photosystem II
- Q-band-
thermoluminescence band associated with S2QA
- charge recombination
- Si-
oxidation levels of the OEC
- YD-
tyrosine-D accessory donor to P680
- YZ-
tyrosine-Z electron donor to P680
Dedicated to Prof. Dr E. Schnepf/Heidelberg. 相似文献
15.
Background: The base excision–repair pathway is the major cellular defence mechanism against spontaneous DNA damage. The enzymes involved have been highly conserved during evolution. Base excision–repair has been reproduced previously with crude cell-free extracts of bacterial or human origin. To further our understanding of base excision–repair, we have attempted to reconstitute the pathway in vitro using purified enzymes.Results We report here the successful reconstitution of the base excision–repair pathway with five purified enzymes from Escherichia coli: uracil-DNA glycosylase, a representative of the DNA glycosylases that remove various lesions from DNA; the AP endonuclease IV that specifically cleaves at abasic sites; RecJ protein which excises a 5′ terminal deoxyribose-phosphate residue; DNA polymerase I; and DNA ligase. The reaction proceeds with high efficiency in the absence of additional factors in the reconstituted system. Four of the enzymes are absolutely required for completion of the repair reaction. An unusual feature we have discovered is that the pathway branches after enzymatic incision at an abasic DNA site. RecJ protein is required for the major reaction, which involves replacement of only a single nucleotide at the damaged site; in its absence, an alternative pathway is observed, with generation of longer repair patches by the 5′ nuclease function of DNA polymerase I.Conclusion Repair of uracil in DNA is achieved by a very short-patch excision–repair process involving five different enzymes. No additional protein factors seem to be required. There is a minor, back-up pathway that uses replication factors to generate longer repair patches. 相似文献
16.
A study was performed to investigate possible interactions by magnetic fields (MF) with the processes of initiation and promotion of chemically induced preneoplastic lesions in rat liver. Male Sprague-Dawley rats were subjected to a 70% partial hepatectomy followed after 24 h by i.p. injection of diethylnitrosamine (DENA) as a tumour initiator. Starting one week after the DENA-treatment phenobarbital (PB) was given to promote growth of enzymatically altered foci of liver cells. MF was applied immediately after the partial hepatectomy and continued until sacrifice after 12 weeks of PB exposure. Homogenous horizontal AC magnetic fields with a frequency of 50 Hz and flux densities of 0.5 μT or 0.5 mT were used. The rats coexposed with MF and DENA plus PB did not gain weight as much as the rats exposed to the chemical agents only. The MF-exposure also resulted in a slight reduction in size and numbers of the focal lesions. The results suggest an interaction of MF with the processes of chemical carcinogenesis either as a result of stress or depending on effects on the proliferation of preneoplastic cells. © 1993 Wiley-Liss, Inc. 相似文献
17.
During regeneration of the neural ganglion in Ciona intestinalis, the pattern of reappearance of some peptidergic cells is similar to the ontogenetic patterns exhibited by these cell types during normal post-metamorphic development. Using a specific antiserum to gamma-aminobutyric acid (GABA), we describe here the appearance of GABA-ergic cells in Ciona during both post-metamorphic development and regeneration of the neural ganglion following total ablation. Post-metamorphic animals were divided into the categories: 1, 3–5, 6–10, 11–15 and 23–27 mm in body length. Regeneration was monitored at 12, 15, 18, 21, 28 and 56 days post ablation. The first appearance of GABA-like immunoreactive cells during normal development were at the 3 to 5-mm stage where they were seen as discrete cells, without processes, evenly distributed in the cortical region throughout the ganglion. Fibres were first seen at the 6 to 10-mm stage. As development proceeded, GABA-like immunoreactive cells became more concentrated near the nerve root exits and along the dorsal rind of the ganglion. In regenerating ganglia, GABA was first detected at 18–21 days post ablation, in cells that lacked any obvious processes and that were distributed in all regions of the ganglion. At 28 days post ablation, processes could be detected in the neuropile, and after 56 days GABA cells were found predominantly in the same regions as in the normally developing adult ganglion. Although the overall pattern reflects that in a normal adult, a few differences were detectable. For example, rather more GABAergic cells were concentrated ventrally in the ganglion close to the neural gland. 相似文献
18.
Laminin and Neuropeptide Y Are Increased by Synapsin Transfection in Cultured NG108-15 Neuroblastoma/Glioma Hybrid Cells 总被引:2,自引:0,他引:2
† Gabriel Fried Hui Quan Han ‡Björn Meister ‡Tomas Hökfelt Paul Greengard 《Journal of neurochemistry》1995,64(6):2674-2680
Abstract: We have investigated the presence and expression of laminin and neuropeptide Y (NPY) in several NG108-15 cell lines transfected with synapsin Ib, IIa, or IIb. The content of laminin, a basal membrane glycoprotein that promotes adhesion and induces neurite outgrowth and neuronal differentiation, was increased in all transfected cell lines examined. In cells that were chemically differentiated with prostaglandin E1 plus 3-isobutyl-1-methylxanthine, laminin levels were increased even further. The content of NPY, suggested to be a neurotransmitter/neuromodulator in peripheral sympathetic neurons as well as in central neurons, was also increased in all transfected cell lines examined. Immunohistochemical analysis combined with confocal laser microscopy showed that NPY staining was granular and very often enriched in neuritic varicosities. The distribution and the staining pattern of NPY were consistent with storage of NPY in large dense-cored vesicles. The results indicate that, in differentiated neurons, the synapsins increase the levels of a neuropeptide transmitter stored in large dense-cored vesicles and of an extracellular matrix protein associated with neuronal maturation. 相似文献
19.
20.
The development of SO42- influx in roots and sulfur transport to shoots was followed in 35S-tracer experiments for sulfur-deficient spring wheat (Triticum aestivum L. cv. Svenno) seedlings pretreated for various time periods (0–24 h) in nutrient solutions with SO42-. Effects of the metabolic inhibitor 2,4-dinitrophenol (DNP) and the protein synthesis inhibitor cycloheximide (CH) on SO42- influx were also evaluated. The SO42- influx appears feedback-regulated by the internal sulfur level of the roots. Regulation may be achieved solely by a rapidly changed SO42- carrier activity through an allosteric effect by the intracellular SO42- concentration of the roots, followed first by induction of carrier synthesis and then by repression of carrier synthesis after transfer of the roots from SO42--deficient nutrient solutions to solutions with SO42-. A Hill plot of the partly sigmoidal relationship between SO42- influx and intracellular sulfur concentration in the roots gave a Hill coefficient of -4.2, indicating negative cooperativity between a minimum number of four interacting allosteric binding sites for sulfur on each carrier entity. DNP-experiments showed that SO42- influx was mainly metabolic, especially after short pretreatment in SO42- at an external SO42- concentration of 0.1 mM. Pretreatment with CH rapidly prevented new SO42- carriers from being formed. Long CH pretreatment (24 h) and different SO42- pretreatments reduced SO42- influx below the non-metabolic level obtained by uptake experiments with DNP, indicating the existence of SO42- carriers mediating passive SO42- transport across the plasmalemma of the root cells. SO42- influx was further decreased for the CH pretreated (24 h) plants by the presence of both CH and DNP in the experimental nutrient solution. This probably indicates the diffusive part of the non-metabolic SO42- influx in the present experiments. Finally, it is suggested that there is a feedback signal between root and shoot, regulating sulfur transport upwards. 相似文献