全文获取类型
收费全文 | 20274篇 |
免费 | 1675篇 |
国内免费 | 4篇 |
专业分类
21953篇 |
出版年
2023年 | 66篇 |
2022年 | 160篇 |
2021年 | 328篇 |
2020年 | 184篇 |
2019年 | 242篇 |
2018年 | 338篇 |
2017年 | 301篇 |
2016年 | 441篇 |
2015年 | 826篇 |
2014年 | 898篇 |
2013年 | 1154篇 |
2012年 | 1559篇 |
2011年 | 1528篇 |
2010年 | 927篇 |
2009年 | 924篇 |
2008年 | 1294篇 |
2007年 | 1302篇 |
2006年 | 1198篇 |
2005年 | 1229篇 |
2004年 | 1103篇 |
2003年 | 1005篇 |
2002年 | 1009篇 |
2001年 | 283篇 |
2000年 | 219篇 |
1999年 | 249篇 |
1998年 | 291篇 |
1997年 | 183篇 |
1996年 | 156篇 |
1995年 | 153篇 |
1994年 | 135篇 |
1993年 | 140篇 |
1992年 | 157篇 |
1991年 | 118篇 |
1990年 | 122篇 |
1989年 | 116篇 |
1988年 | 103篇 |
1987年 | 81篇 |
1986年 | 104篇 |
1985年 | 105篇 |
1984年 | 116篇 |
1983年 | 88篇 |
1982年 | 97篇 |
1981年 | 115篇 |
1980年 | 99篇 |
1979年 | 74篇 |
1978年 | 82篇 |
1977年 | 59篇 |
1976年 | 47篇 |
1975年 | 44篇 |
1974年 | 65篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
The difference in the type of codon-anticodon base pairing at the ribosomal P-site is one of the determinants of the translational rate 总被引:3,自引:0,他引:3
By utilizing an enzymatically reconstructed tRNA variant containing an altered anticodon sequence, we have examined the different biochemical behavior of translation between the Watson-Crick type and the wobble type base pair interactions at the first anticodon position. We have found that the Watson-Crick type base pair has an advantage in translation in contrast to the wobble type base pair by comparing the efficiency of transpeptidation of native tRNA(Phe) (anticodon; GmAA) with its variant tRNA (anticodon; AAA) in the poly(U)-programmed ribosome system. Thomas et al. [Proc. Natl. Acad. Sci. U.S. (1988) 85, 4242-4246] showed that the wobble codon at the ribosomal A-site accepted its cognate tRNA less efficiently than the Watson-Crick base pairing codon. We report here that the wobble interaction at the ribosomal P-site also affected the rate of translation. This variable translational rate may be a mechanism of gene regulation through preferential codon usage. 相似文献
82.
83.
A ninhydrin-positive, phosphorus-negative lipid from Paracoccus denitrificans ATCC 13543 has been isolated and purified by mild alkaline methanolysis followed by silicic acid column chromatography and preparative thin-layer chromatography. The lipid was identified as an ornithine-containing lipid. The major ester-linked fatty acid was cis vaccenic acid. Major amide-linked fatty acids were 3-OH-20:1 and 3-OH-18:0. Ornithine-containing lipid was a major lipid component of P. denitrificans. Phospholipids made up about 57% and ornithine-containing lipid about 14% of the weight of the total lipid of the organism. The ratios of lipid ornithine: lipid phosphorus were 0.23, 0.65 and 0.58 in cytoplasmic membrane, outer membrane, and an NaCl extract, which is thought to represent chiefly outer membrane, respectively. Thus ornithine-containing lipid appears to be present in larger amounts in outer membrane than cytoplasmic membrane. No substantial variations in lipid ornithine levels were noted in stationary phase versus exposnential phase organisms, organisms grown in complex medium versus organisms grown in minimal medium with and without amino acid supplements, or in organisms grown in low phosphate-containing medium.Non standard abbreviations TLC
thin-layer chromatography
- Tris-HCl
tris(hydroxymethyl)aminomethane hydrochloride
- TMS
trimethylsilyl
- TFA
triluoroacetyl
- NPPN
ninhydrin-positive, phosphorus-negative
- ECL
equivalent chain length 相似文献
84.
Oxygen is transported 30 mm into the sediment at an 8 m depth site in eutrophic Lough Neagh by the irrigational activities
of the benthic fauna. Faunal activity also mixes the upper 20 mm of sediment. Sediment oxygen uptake rate, redox potential-depth
profile and the chlorophylla concentration were measured in the upper sediment layers from February to November 1979. Chlorophylla input to the sediment, following the Spring phytoplankton maximum, remained in the 0–1 cm sediment layer but did cause the
redox potential profile to change from one with potentials around 400 mV in the upper 50 mm to one with a strong gradient
over the 0–30 mm region. The start of benthic faunal activity in May caused the chlorophylla to be mixed into the 1–2 cm layer and also caused oxygen to be transported into the sediment at a rate sufficient to change
the redox potential back to its initial state. The biodiffusion coefficient for solids in the upper 20 mm was estimated to
be 6 × 10−8 cm2 s−1. Oxygen transport in the pore, waters of the upper sediment layers was considered to be best described as advection, caused
by the irrigational activities of the benthic fauna. 相似文献
85.
Summary Ornithine decarboxylase activity was measured during organogenesis in rat embryos grown in utero and whole rat conceptuses
maintained in an in vitro culture system. Ornithine decarboxylase levels in vivo showed a distinct peak at embryonic age 10.5
d. Despite identical morphology, protein content, crown rump length and numbers of somites cultured embryos displayed a different
developmental pattern and possessed less than half the ODC activity of that in vivo. The data suggest that the normal embryonic
programming of ODC activity is significantly altered by the culture environment and that further biochemical comparisons of
embryos growing in utero and in vitro may be required to evaluate properly the applicability of this technique to detailed
studies of teratogenesis and developmental biology.
This work was supported by NIH-5-507-RR5359-17 and a 1980 Research Starter Grant from the Pharmaceutical Manufacturers Association
Foundation. 相似文献
86.
Yoshiteru Harada Kunio Tanaka Yasushiro Uchida Akinori Ueno Sachiko Oh-ishi Yamashita Kowa Masataka Ishibashi Hiroshi Miyazaki Makoto Katori 《Prostaglandins & other lipid mediators》1982,23(6)
Injection of γ-carrageenin into t he pleural cavity of rats caused the accumulation of the pleural exudate. When levels of prostaglandins (PGs) and thromboxane (TX) B2 were quantified by gas chromatography-mass spectrometry as their methyl ester (ME)-dimethyllisopropylsilyl (DMiPS) ether or ME-methoxine-DMiPS ether derivatives, 6-keto-PGF1α reached the maximum at 1 hr after carrageenin, then PGE2 and TXB2 showed peaks at 3 hr and waned off before 9 hr. he PGF2α level was kept low, but PGD2, PGE1 and PGF1α were not detected. Aspirin (100 mg/kg, i.p.) significantly decreased the PG and TXB2 levels and suppressed the rate of plasma exudation until 5 hr, but did not at 7 hr, when it was measured by the amount of exuded pontamine sky blue injected intravenously. OKY-025 (300 mg/kg, i.p.), a selective TXA synthetase inhibitor, and tranylcypromine (20 mg/kg, i.p.), a PGI synthetase inhibitor, could not extensively inhibit the accumulation of the exudate. These results suggest that the cyclooxygenase products of arachidonic acid, particularly PGE2, definitely play an important role in the exudation during the first 5 hr. 相似文献
87.
Brian P. Nichols Giuseppe F. Miozzari Magda van Cleemput George N. Bennett Charles Yanofsky 《Journal of molecular biology》1980,142(4):503-517
The nucleotide sequences of Serratia marcescens trpG and the corresponding regions of Escherichia coli, Shigella dysenteriae and Salmonella typhimurium trpD have been determined. Analysis of the nucleotide sequence divergence suggests the following evolutionary relationships: Serratia-[Salmonella, (Escherichia, Shigella)]. Partial reconstruction of ancestral nucleotide sequences and subsequent analysis of nucleotide substitutions show that the majority of nucleotide substitutions in the evolution of trp(G)D are transitions that result in a reduction of G + C content. Since most of the nucleotide substitutions are in the third position of codons, bias in synonymous codon usage also reflects G + C content. The trpE-trp(G)D junction in the four organisms is characterized by overlapping translation termination and initiation codons. The relative positions of trpE and trp(G)D thus became fixed in evolution before the fusion of trpG and trpD. Nucleotide sequences representing the fusion of trpG and trpD in Escherichia, Shigella and Salmonella are not more nor less divergent than other portions of the trp(G)D coding sequences. 相似文献
88.
Edward A. Johnson J. Brian Chapman J. Mailen Kootsey 《Journal of theoretical biology》1980,87(4):737-756
A previous paper described a kinetic model for electrogenic sodium-potassium transport in cardiac muscle, combining a thermodynamically-constrained transport model with simple passive permeabilities for sodium and potassium to generate a cardiac action potential (Chapman, Kootsey & Johnson, 1979). The present paper explores the extent to which this simplest of active-passive transport models can account (without further modification) for the electrophysiological behavior of cardiac muscle. The long term (several minutes) changes in the duration of the action potential observed following a change in stimulation rate are predicted by the model through a shift in the steady-state current-voltage relationship caused by small changes in inside ion concentrations. The diastolic hyperpolarization observed following an increase in rate is also predicted, including the linear relationship between the maximum diastolic depolarization and the rate of stimulation. Varying the outside potassium concentration in the model produces changes in the rest potential and current-voltage relationship similar to published data. Deviations from ideal potassium electrode behavior occur at both high and low concentrations because of effects on the pump. The model not only predicts the observed shift of the current-voltage curve in the depolarizing direction with increasing [K+]0, but also the crossing of the curve in normal [K +]0 without having to assume a variation in gK. Anoxia was introduced into the model by changing the concentrations of ATP and ADP, thereby enabling the model to account for the rapid diastolic depolarization observed in myocardial ischemia. 相似文献
89.
Morphological and functional specializations of the shell, musculature and pallial glands in the Lithophaginae (Mollusca: Bivalvia) 总被引:1,自引:0,他引:1
The structure of the pallial glands in limestone and coral boring species of Lithophaga (Bivalvia: Lithophaginae) is examined and related to evolutionary trends in habitat specialization. Boring glands of all species occur in the middle mantle fold but display a progressive degree of complexity from simple epithelial to ducted sub-epithelial structures coinciding with increasing specialization of habitat from natural limestone to species specific live coral dwellers. The size of this gland is relatively reduced anteriorly and along the entire length of the mantle in live coral borers. 相似文献
90.
Brian D Keighley 《BMJ (Clinical research ed.)》1980,281(6253):1506-1507