全文获取类型
收费全文 | 535827篇 |
免费 | 49184篇 |
国内免费 | 304篇 |
出版年
2018年 | 9562篇 |
2017年 | 9294篇 |
2016年 | 9250篇 |
2015年 | 7392篇 |
2014年 | 8780篇 |
2013年 | 12486篇 |
2012年 | 16093篇 |
2011年 | 20601篇 |
2010年 | 14914篇 |
2009年 | 13870篇 |
2008年 | 17954篇 |
2007年 | 19939篇 |
2006年 | 12877篇 |
2005年 | 13083篇 |
2004年 | 12650篇 |
2003年 | 12523篇 |
2002年 | 12505篇 |
2001年 | 24838篇 |
2000年 | 25299篇 |
1999年 | 19139篇 |
1998年 | 5930篇 |
1997年 | 6237篇 |
1996年 | 5802篇 |
1995年 | 5402篇 |
1994年 | 5285篇 |
1993年 | 5231篇 |
1992年 | 15779篇 |
1991年 | 15548篇 |
1990年 | 14925篇 |
1989年 | 14553篇 |
1988年 | 13475篇 |
1987年 | 12658篇 |
1986年 | 11547篇 |
1985年 | 11624篇 |
1984年 | 9335篇 |
1983年 | 8093篇 |
1982年 | 5768篇 |
1981年 | 5115篇 |
1980年 | 4874篇 |
1979年 | 8881篇 |
1978年 | 6766篇 |
1977年 | 6115篇 |
1976年 | 5591篇 |
1975年 | 6572篇 |
1974年 | 7033篇 |
1973年 | 6900篇 |
1972年 | 6972篇 |
1971年 | 6118篇 |
1970年 | 5067篇 |
1969年 | 4782篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
I.D. Algranati 《Biochemical and biophysical research communications》1980,96(1):54-60
Polypeptide synthesis programmed by poly(U) and globin mRNA has been studied in cell-free extracts from wheat germ. A two-step reaction with a preincubation at high Mg++ levels followed by a second step carried out after a shift to a low Mg++ concentration and the addition of labeled amino acids is described. Under these conditions the initiation of polyphenylalanine synthesis can be blocked without affecting the elongation of polypeptide chains. This procedure allows the selective inhibition of polypeptide synthesis initiation without using any drug or antibiotic. 相似文献
32.
V A Berezin E N Zhmarev I A Brodskaia G M Shevchenko L S Zashko 《Biulleten' eksperimental'no? biologii i meditsiny》1985,100(7):68-70
The content of neurospecific proteins S-100, GFA and D2 was measured in malignant cerebral tumors by electrophoresis with the use of monospecific antisera. Concomitant measurement of proteins S-100 and GFA is a more reliable diagnostic criterion as to the tumor histogenesis than study of each protein alone. D2 protein appeared to be the most stable specific marker. 相似文献
33.
I T Norton D M Goodall S A Frangou E R Morris D A Rees 《Journal of molecular biology》1984,175(3):371-394
The thermally induced order-disorder transition of xanthan (extracellular bacterial polysaccharide from Xanthomonas campestris) has been investigated by optical rotation, differential scanning calorimetry, stopped-flow reaction kinetics and low-angle laser light scattering, and the results have been analysed in terms of Zimm -Bragg helix-coil transition theory. The reciprocal of the transition midpoint temperature (Tm) varies linearly with the logarithm of cation (K+) the salt dependence of Tm, is in agreement with Manning polyelectrolyte theory the ordered structure. The associated increase in cation binding, calculated from the salt dependence of tm, is in agreement with the Manning polyelectrolyte theory for one of the candidate structures from X-ray diffraction, a 5(1) single helix stabilized by packing of side-chains along the polymer backbone, but not for the alternative double-helix structure that has also been proposed. At each salt concentration, the two fundamental parameters of the Zimm -Bragg theory, s and sigma, were calculated. The equilibrium constant for growth of the ordered structure (s) is derived directly from calorimetric measurement of transition enthalpy (delta Hcal ), and sigma, which quantifies the relative instability of the helix nucleus, is derived from the ratio of delta Hcal to the apparent transition enthalpy (delta Happ ) obtained by van't Hoff analysis of the optical rotation data. The temperature course of conformational ordering calculated theoretically is in good quantitative agreement with experimental results from both optical rotation and scanning calorimetry. The calculated average length of stable, ordered chain-sequences increases with decreasing temperature, but equals or exceeds the total chain length from light scattering only at temperatures more than approximately equal to 70 K below Tm, suggesting that ordered and disordered regions may co-exist within the same xanthan molecule. Consistent with this interpretation, the observed rate of conformational ordering increases sharply under conditions where the starting solution for dynamic measurements is partially ordered, suggesting that ordered sequences within each chain may act as helix nuclei for adjacent disordered regions, so that helix growth, rather than the slower nucleation process, becomes rate limiting. 相似文献
34.
All plant cells are provided with the necessary rigidity to withstand the turgor by an exterior cell wall. This wall is composed
of long crystalline cellulose microfibrils embedded in a matrix of other polysaccharides. The cellulose microfibrils are deposited
by mobile membrane bound protein complexes in remarkably ordered lamellar textures. The mechanism by which these ordered textures
arise, however, is still under debate. The geometrical model for cell wall deposition proposed by Emons and Mulder (Proc.
Natl. Acad. Sci. 95, 7215–7219, 1998) provides a detailed approach to the case of cell wall deposition in non-growing cells, where there is no evidence for the
direct influence of other cellular components such as microtubules. The model successfully reproduces even the so-called helicoidal
wall; the most intricate texture observed. However, a number of simplifying assumptions were made in the original calculations.
The present work addresses the issue of the robustness of the model to relaxation of these assumptions, by considering whether
the helicoidal solutions survive when three aspects of the model are varied. These are: (i) the shape of the insertion domain,
(ii) the distribution of lifetimes of individual CSCs, and (iii) fluctuations and overcrowding. Although details of the solutions
do change, we find that in all cases the overall character of the helicoidal solutions is preserved. 相似文献
35.
Hostile intercommunity relations, including attacking and killing extra-community infants of both sexes have occurred at most
wild chimpanzee sites. We describe three recent cases of intercommunity attacks on infants committed by members of the Ngogo
chimpanzee community in Kibale National Park, Uganda. Two of the attacks resulted in confirmed infanticides while a third
attack probably resulted in the infant's death. In common with previous accounts of chimpanzee infanticides, the attacks described
here occurred during boundary patrols outside the Ngogo community's usual range, adult and adolescent males were the main
participants, one infant was cannibalized after being killed, and the victims’ mothers did not accompany the attacking party
back to the Ngogo range. However, the patrol parties during each infanticide were larger than before and included females
from the Ngogo community. Our observations indirectly support both the range expansion and imbalance of power hypotheses,
which address why and under which conditions chimpanzee intercommunity encounters lead to aggression. These cases of intercommunity
infanticide add to the growing database of the phenomenon in wild chimpanzees. 相似文献
36.
Summary The results of a recent quantitative analysis of the Teorell membrane oscillator are utilized to explore its role as an excitability analogue. Special attention is paid to its role as a mechano-electric transducer. A membrane of exceptionally well-defined pore structure has been used in this study. The analogue properties arise from nonlinear coupling between water and salt fluxes. When the membrane is simultaneously subjected to controlled gradients of hydrostatic pressure, electrical potential and concentration, bi-stable stationary states can be produced. These arise from the opposing effects of pressure and electro-osmosis on the volume flow. Transitions between these states show hysteresis. The factors governing such transitions are analogous to certain types of stimuli encountered in the natural excitation process. The membrane system also shows oscillatory behavior when the hydrostatic pressure gradient is allowed to vary under constant current conditions. This property is related to the bi-stable stationary state phenomena and is compared to the regenerative behavior found in biologically excitable tissues. Particular emphasis is placed upon analogies between the membrane oscillator and certain natural tissues. The importance of the nonlinear nature of the force-flux coupling in the analogue is stressed, and its possible relevance to biological excitability indicated. Some consideration is also given to the role of electro-osmotic flux coupling in biological tissues. 相似文献
37.
Four myeloid cell lines (M1, WEHI-3B D+, FDC-P1, and 32D) were screened for the presence of J11d antigen. One of these cell lines, the myeloid leukemia M1, was found to express a high level of J11d antigen on the cell surface. Recombinant mouse leukemic inhibitory factor (rm-LIF), recombinant human LIF (rh-LIF), and steroids (hydrocortisone and dexamethasone) could induce M1 cells to undergo monocytic differentiation. The level of J11d antigen was greatly reduced after treatment of the cells with LIF or steroids. Western blotting revealed that the apparent molecular weight of the J11d antigen on M1 cells was 45-48 kDa. Furthermore, the level of J11d mRNA was also reduced during LIF-induced differentiation of M1 cells. 相似文献
38.
39.
40.