全文获取类型
收费全文 | 4631篇 |
免费 | 393篇 |
国内免费 | 4篇 |
出版年
2022年 | 20篇 |
2021年 | 56篇 |
2020年 | 30篇 |
2019年 | 53篇 |
2018年 | 49篇 |
2017年 | 59篇 |
2016年 | 106篇 |
2015年 | 172篇 |
2014年 | 207篇 |
2013年 | 283篇 |
2012年 | 340篇 |
2011年 | 324篇 |
2010年 | 191篇 |
2009年 | 198篇 |
2008年 | 277篇 |
2007年 | 300篇 |
2006年 | 264篇 |
2005年 | 273篇 |
2004年 | 260篇 |
2003年 | 252篇 |
2002年 | 228篇 |
2001年 | 45篇 |
2000年 | 35篇 |
1999年 | 55篇 |
1998年 | 65篇 |
1997年 | 39篇 |
1996年 | 44篇 |
1995年 | 44篇 |
1994年 | 37篇 |
1993年 | 36篇 |
1992年 | 49篇 |
1991年 | 34篇 |
1990年 | 35篇 |
1989年 | 31篇 |
1988年 | 31篇 |
1987年 | 33篇 |
1986年 | 24篇 |
1985年 | 29篇 |
1984年 | 44篇 |
1983年 | 20篇 |
1982年 | 35篇 |
1981年 | 20篇 |
1979年 | 21篇 |
1978年 | 24篇 |
1977年 | 17篇 |
1976年 | 14篇 |
1975年 | 24篇 |
1974年 | 14篇 |
1973年 | 14篇 |
1969年 | 13篇 |
排序方式: 共有5028条查询结果,搜索用时 658 毫秒
991.
Corinne Rocchini Roswitha M. Marx Joachim Schnorr von Carosfeld Harold E. Kasinsky Ellen Rosenberg Freya Sommer Juan Ausio 《Journal of molecular evolution》1996,42(2):240-246
We have analyzed the chromosomal protein composition of the sperm from several species belonging to three different classes (Hydrozoa, Scyphozoa, Anthozoa) of the phylum Cnidaria. In every instance, the sperm nuclear basic proteins (SNBPs) were found to consist of one to two major protein fractions that belong to the histone H1 family, as can be deduced from their amino acid composition and solubility in dilute perchloric acid, and the presence of a trypsin-resistant core. In those species where mature spawned sperm could be obtained, we were able to show that these proteins completely replace the somatic histones from the stem cells that are present at the onset of spermatogenesis. The presence of a highly specialized histone H1 molecule in the sperm of this phylum provides support for the idea that the protamine-like proteins (PL) from higher groups in the phylogenetic tree (and possibly protamines as well) may all have evolved from a primitive histone H1 ancestor. 相似文献
992.
Summary Many of the flagellates inhabiting the hindgut of lower termites are associated with ectobiotic, rod-like bacteria or spirochetes. Different types of attachment sites are present. Electron dense material underlies, e.g., the plasma membrane ofJoenia annectens at the contact site, whereas other attachment sites do not show any visible specializations. The host cell's glycocalyx may, however, be reduced at the attachment sites as it is the case inDevescovina glabra. The thick glycocalyx ofStephanonympha nelumbium is not changed at the sites where bacterial rods attach, but spirochetes penetrate to a certain extent. Bacteria which colonize the extracellular surface structures ofMicrorhopalodina multinucleata express their own glycocalyx to mediate a contact. In this study we focussed on the examination of one common mode of interaction between bacteria and their host cells, i.e., adhesion via lectins and sugars. The sugar composition was analysed by light and electron microscopic labelling experiments using the lectins Con A, WGA and SBA. In general, only the posterior body surface ofJoenia which is colonized with bacteria is labelled. The demonstrated sugars are found in fibrous glycocalyx portions surrounding the attachment sites of the bacteria. Such glycocalyx fibres in combination with the electron dense material supporting the attachment sites seem to be the prerequisites for bacterial attachment. InD. glabra, however, a role for sugars in mediating the attachment could not be demonstrated. Removal of the ectobiotes using antibiotics revealed that the specialized contact sites ofJoenia are present in the absence of bacteria and thus possibly serve to attract bacteria. Nothing, however, remains of the former attachment sites in bacteria-freeDevescovina cells. Attachment sites in this case could be induced by bacterial contact. There is not one general mechanism for bacterial attachment to termite flagellates; rather, adhesion seems to follow different strategies.Abbreviations Con
A concanavalin A
- DAB
3,3-diaminobenzidine tetrahydrochloride
- DAPI
4,6-diamidino-2-phenylindole
- DIC
differential interference contrast
- FA
formaldehyde
- FITC
fluorescein isothiocyanate
- GA
glutaraldehyde
- PB
Soerensen's phosphate buffer
- PC
phase contrast
- pen/strep
penicillin and streptomycin
- SBA
soybean agglutinin
- SEM
scanning electron microscope
- TBS
Tris buffer saline
- TEM
transmission electron microscope
- WGA
wheat germ agglutinin
Dedicated to Prof. Dr. Dr. h.c. Eberhard Schnepf on the occasion of his retirement 相似文献
993.
Andreas Janshoff Joachim Wegener Manfred Sieber H.-J. Galla 《European biophysics journal : EBJ》1996,25(2):93-103
The viscoelastic behavior of epithelial cells (MDCK-I and MDCK-II) grown on AT-cut quartz crystals with a fundamental resonance
at 5 MHz was investigated by impedance spectroscopy. Using the electromechanical model recently derived by Martin et al. [(1991)
Anal Chem 63: 2272 – 2281] for Newtonian liquids in contact with shear wave resonators we quantified the viscous damping arising
from the adherent cells by fitting the impedance data with a modified Butterworth-Van Dyke circuit in the region of the resonance
frequency. Impedance spectroscopy was additionally performed in the frequency range from 1 Hz to 1 MHz to scrutinize the passive
electrical properties of the epithelial cell layers using an additional platinum electrode. These data allow one to document
the cell layers' integrity as well as the electrode coverage. We were able to confirm that the presence of a cell-layer mainly
increases damping of the shear wave and does not exhibit a pure mass-load behavior. These findings were supported by the discovery
that the inductance L in the electromechanical model was less influenced by the cell-layer than the resistance R. The apparent
cell-viscosities determined by our method are 0.097 poise for MDCK-I and 0.142 poise for MDCK-II cell-layers. These low apparent
viscosities may be explained in terms of a considerable spacing between the cells immobilized via their focal contacts and
the quartz surface.
Received: 5 June 1996 / Accepted: 6 August 1996 相似文献
994.
Summary In this paper, a preliminary concept on the interplay of local, regional and global control factors of bryozoan diversity
and distribution pattern is introduced. Recent bryozoans from the Philippines, New Zealand and the Gulf of Aqaba are compared
to the selected fossil specimens from the Oxfordian and Santonian.
Reef bryozoan skeletons are studied in order to separate local control within the substrate-water interface from regional
control. The latter originate mainly from the transport function of the water column (e.g. sediment load, wave energy, vagile
predators). This is true especially for erect (tree-like) and massive (multilaminar) bryozoans which are subjected to the
dynamics of the water body in the littoral area. This regional control, affects simultaneously several structural and substrate
zones of a reef. Early life history of vertically growing reef bryozoans reflect local control, while older zoarial structures
reflect the signals of regional influence. Three types of multiserial nodular bryozoans are cited: self-overgrowing sheets
(‘S-Nodule’, derived from ‘S-Sheet’), circumlaterally budding colonies (‘C-Nodules’), and fungiform bryozoans.
Bryozoan growth form selection allows the separation of two types of regional controls, long range control which favours the
selection of specialized sheets, and slow rate control documented in the growth form modification of sheets changing into
nodules.
In the domain of local control epibiontic microorganisms, microbial mats and biofilms on hard substrata represent probably
the most important elements, aside from the limited substrate space. Symbiotic and/or competitive bryozoan-microorganism interrelationships
result in the distinct adaptations of bryozoan growth. It is apparent that bryozoan modifications of substrate micro-topography
influence the character of epibiontic microbial settlement. The peak occurrence of microbial settlement on prominent external
bryozoan skeleton parts is discussed as an effect of feeding currents. In contrast, smooth and unelevated exoskeletons are
less favourable for micro-epibiontic colonization. Due to the high level of order in the spatial zonation patterns of epibionts
on calcifying bryozoans, the term ‘bryozoan-microreef’ is introduced. Bryozoan-microreefs are locally controlled reefs. They
are e.g. characterized by higher competitive abilities than bryozoans without ‘reefdwellers’. Local control has a fast rate
of change and is reflected in morphologies of individual zooids and/or single zooid generations.
Regional control has a slower rate but a higher range. It is important for growth modifications of the whole zoaria. The very
slow rate of global control cannot be recorded within the life span of bryozoan zoaria unless it is expressed through regional
control (such as monsoons). Nevertheless, global control is paleoecologically important because it is traceable in bryozoan
(paleo-)biogeography. For practical purpose, we suggest to define those control factors of bryozoans as global which affect
simultaneously at least one tropical and one non-tropical bryozoan community.
This reportis dedicated to the memory of our friend and research colleague Mr. Joselito G. Santiago of the University of San
Carlos, Marine Biology Section. Tiago (as we fondly call him) has started within the scope of the Philippine Bryozoan Project
an applied research on fish abundance and productivity around a new artificial reef model. This prototype model was conceptualized
with him. His promising study remained unfinished; he died May 13, 1993, while sampling inside the reef cave off Marigondon. 相似文献
995.
996.
An amino-terminal polypeptide fragment of the influenza virus NS1 protein possesses specific RNA-binding activity and largely helical backbone structure. 总被引:4,自引:0,他引:4
下载免费PDF全文
![点击此处可从《RNA (New York, N.Y.)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The NS1 protein of influenza A virus has the unique property of binding to three apparently different RNAs: poly A; a stem-bulge in U6 small nuclear RNA; and double-stranded RNA. One of our major goals is to determine how the NS1 protein recognizes and binds to its several RNA targets. As the first step for conducting structural studies, we have succeeded in identifying a fragment of the NS1 protein that possesses all the RNA-binding activities of the full-length protein. The RNA-binding fragment consists of the 73 amino-terminal amino acids of the protein. We have developed procedures for obtaining large amounts of the polypeptide in pure form. This has enabled us to establish the RNA-binding properties of this polypeptide and to demonstrate that it retains the ability to dimerize exhibited by the full-length protein. In addition, far-UV CD spectroscopy indicates that this RNA-binding polypeptide is largely (approximately 80%) helical, suggesting that the mode of dimerization of the NS1 protein and of its interaction with RNA is mediated, at least in part, by helices. 相似文献
997.
Dieter Kreusch Johannes von Lintig Joachim Schröder 《Molecular genetics and genomics : MGG》1995,249(1):102-110
The occ and noc regions in octopine and nopaline Ti plasmids, respectively, are responsible for the catabolism of octopine and nopaline in Agrobacterium. The functions are activated in the presence of the opines by OccR and NocR, two related regulatory proteins, and the promoters contain common sequence motifs. We have investigated heterologous interactions between the regulators and the promoters. Previous experiments using all possible heterologous combinations of opines, regulators, and promoters in vivo had demonstrated that only the combination of nopalme, NocR, and the occ promoter led to limited promoter activation. We now show that OccR and NocR bind to the heterologous promoters in vitro and in vivo. The weak or non-existent promoter activation actually observed could be explained by the assumption that OccR and NocR use different activation mechanisms; we investigated protein-induced DNA bending because of reports that the two regulators differ in this respect. Analysis with a bending vector showed that both OccR and NocR induced a DNA bend that is relaxed in the presence of the respective opine. The data suggest that subtle differences in regulator/promoter interactions are responsible for the inactivity of the heterologous combinations. Investigations with a chimeric NocR/OccR protein indicated that it induced a DNA bend in both promoters. No opine-induced relaxation was detectable with the hybrid, and the inducible promoter was not activated. These findings suggest that bend relaxation may be an integral part of promoter activation. 相似文献
998.
Frank M. Dautzenberg Konstanze Dietrich Monika R. Palchaudhuri Joachim Spiess 《Journal of neurochemistry》1997,69(4):1640-1649
Abstract: Two cDNA clones encoding distinct members of the corticotropin-releasing factor (CRF) receptor family have been isolated from Xenopus laevis with PCR-based approaches. The first full-length cDNA amplified from Xenopus brain encoded a 415-amino acid protein with ∼80% identity to mammalian CRF receptor type 1 (CRF-R1). The second full-length cDNA isolated from Xenopus brain and heart encoded a 413-amino acid protein with ∼81% identity to the α-variant of mammalian CRF receptor, type 2 (CRF-R2). No evidence could be obtained that the β-variant of CRF-R2 existed in Xenopus laevis . Binding studies using human embryonic kidney 293 (HEK 293) cells stably transfected with xenopus CRF-R2 showed that the CRF analogues urotensin I, urocortin, and sauvagine were bound with higher affinities than human/rat CRF, xenopus CRF, and ovine CRF. In contrast to human CRF-R1, xenopus CRF-R1 (xCRF-R1) was very selective for different CRF ligands. Urotensin I, urocortin, human/rat CRF, and xenopus CRF were bound with significantly (10–22-fold) higher affinities than ovine CRF ( K D = 31.7 n M ) and sauvagine ( K D = 51.4 n M ). In agreement with these binding data, EC50 values of 39.7 and 1.1 n M were found for sauvagine and for human/rat CRF or xenopus CRF, respectively, when the cyclic AMP production in HEK 293 cells stably transfected with xCRF-R1 was determined. 相似文献
999.
1000.
Dipl. Geol. Volker Thiel Dr. Martina Merz-Preiß Prof. Dr. Joachim Reitner PD Dr. Walter Michaelis 《Facies》1997,36(1):163-172
Summary Biomarker investigations are applied to the free lipid fractions of a naturally grown freshwater microbial mat, constructed
by calcifying cyanobacteria (Scytonema sp. andSchizothrix sp.). The absolute and relative concentrations of hydrocarbons, free alcohols and carboxylic acids are studied and their
probable biological precursors are discussed. A significant signal of cyanobacterial lipids is recognized by the strong predominance
ofn-heptadecane (C17),n-heptadecene, two monomethyl-heptadecanes, and the pentacyclic triterpenoid diploptene. Their occurrences parallel the lipid
distributions found in pure cultured cyanobacteria and in recent cyanobacterial mats grown in particular environments (hypersaline,
lagoonal, hot spring). The observed compound signature appears to be a suitable reference for environments, where cyanobacteria
are directly associated with theloci of carbonate precipitation and thus, rock formation. In the studied material, a significant contribution of organic matter
from other sources, especially higher plants is characterized by the occurrence of several specific marker compounds, namely
lup-20(29)-ene-3-ol, high molecular weightn-alkanes and carboxylic acids. Although these components comprise a notably high portion of the sample’s lipid inventory,
they are shown to be distinguished easily from the signal left by the predominant mat building organisms. 相似文献