全文获取类型
收费全文 | 12299篇 |
免费 | 657篇 |
国内免费 | 3篇 |
出版年
2023年 | 19篇 |
2022年 | 47篇 |
2021年 | 130篇 |
2020年 | 82篇 |
2019年 | 111篇 |
2018年 | 156篇 |
2017年 | 145篇 |
2016年 | 239篇 |
2015年 | 380篇 |
2014年 | 447篇 |
2013年 | 886篇 |
2012年 | 810篇 |
2011年 | 828篇 |
2010年 | 543篇 |
2009年 | 502篇 |
2008年 | 824篇 |
2007年 | 873篇 |
2006年 | 778篇 |
2005年 | 811篇 |
2004年 | 800篇 |
2003年 | 736篇 |
2002年 | 697篇 |
2001年 | 110篇 |
2000年 | 100篇 |
1999年 | 166篇 |
1998年 | 184篇 |
1997年 | 121篇 |
1996年 | 119篇 |
1995年 | 96篇 |
1994年 | 94篇 |
1993年 | 99篇 |
1992年 | 100篇 |
1991年 | 69篇 |
1990年 | 65篇 |
1989年 | 67篇 |
1988年 | 39篇 |
1987年 | 52篇 |
1986年 | 53篇 |
1985年 | 48篇 |
1984年 | 55篇 |
1983年 | 44篇 |
1982年 | 63篇 |
1981年 | 52篇 |
1980年 | 56篇 |
1979年 | 24篇 |
1978年 | 26篇 |
1977年 | 31篇 |
1976年 | 32篇 |
1975年 | 35篇 |
1973年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Takashi Gojobori 《Genetics》1983,105(4):1011-1027
A mathematical model for codon substitution is presented, taking into account unequal mutation rates among different nucleotides and purifying selection. This model is constructed by using a 61 X 61 transition probability matrix for the 61 nonterminating codons. Under this model, a computer simulation is conducted to study the numbers of silent (synonymous) and amino acid-altering (nonsynonymous) nucleotide substitutions when the underlying mutation rates among the four kinds of nucleotides are not equal. It is assumed that the substitution rates are constant over evolutionary time, the codon frequencies being in equilibrium, and, thus, the numbers of synonymous and nonsynonymous substitutions both increase linearly with evolutionary time. It is shown that, when the mutation rates are not equal, the estimate of synonymous substitutions obtained by F. Perler, A. Efstratiadis, P. Lomedico, W. Gilbert, R. Kolodner and J. Dodgson's "Percent Corrected Divergence" method increases nonlinearly, although the true number of synonymous substitutions increases linearly. It is, therefore, possible that the "saturation" of synonymous substitutions observed by Perler et al. is due to the inefficiency of their method to detect all synonymous substitutions. 相似文献
72.
Estimates of Denitrification and Nitrification in Coastal and Estuarine Sediments 总被引:13,自引:5,他引:8
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Denitrification and nitrification in sediments of Tama Estuary and Odawa Bay, Japan, were investigated by the combined use of a continuous-flow sediment-water system and a 15N tracer technique. At Odawa Bay, the nitrification rate was comparable to the nitrate reduction rate, and 70% of the N2 evolved originated from nitrogenous oxides (nitrate and nitrite) which were produced by the action of nitrifying bacteria in the sediments. At Tama Estuary, the nitrate reduction rate was 11 to 17 times higher than the nitrification rate, and nitrogenous oxides derived from ammonium accounted for only 6 to 9% of the N2 evolution by denitrification. 相似文献
73.
The effects of photoperiod on the sex expression of cucumberflowers were examined in relation to the endogenous GA and ethyleneproduction with short day (SD)- and long day (LD)-sensitivecultivars under controlled environmental conditions. Growth of plants was greatly reduced under LD in both typesof cultivars. Reflecting this, GA activity was greater and ethyleneevolution less in plants grown under SD than in those underLD. However, the number of pistillate flowers increased underSD and LD in SD- and LD-sensitive cultivar, respectively, indicatingthat a change in sex expression did not coincide with hormonalchanges in the SD-sensitive cultivar. Thus, the effect of photoperiodon the sex expression was found to be separate, at least inpart, from the effect of GA or ethylene in cucumber. (Received September 16, 1982; Accepted December 7, 1982) 相似文献
74.
Cortical microtubules are oriented transversely to the cellaxis in leaf sheath cells of onion plants (Allium cepa L. cv.Osaka-Okute) that have not started bulb formation. As the bulbdevelops and the leaf sheath cells swell, the microtubules becomedisoriented and scattered and finally disappear. The microtubuleinhibitors colchicine and cremart [O-ethyl O-(3-methyl-6-nitrophenyl)N-sec-butylphosphorothioamidate]cause swelling of leaf sheath cells and make the basal partof the plant bulbous. The cortical microtubules may have animportant role in regulating bulb development in onion plants. (Received August 21, 1982; Accepted December 6, 1982) 相似文献
75.
12-O-Tetradecanoylphorbol-13-acetate (TPA) is a well-known tumor promoter in mouse-skin carcinogenesis. Its effects on mutagenesis in a soybean test system were examined, and the effects were judged from the appearance of spots of various colors on the leaves. When soybean seeds were treated with TPA plus 0.03% caffeine, the frequency of spots per leaf decreased significantly and in proportion to the concentration of TPA. TPA alone at concentrations of 1–20 μg/ml did not induce any mutations. Mutations induced by γ-rays were not affected by administration of TPA either before or after exposure to γ-rays. The mechanism of suppression by TPA of mutations induced by caffeine is discussed. 相似文献
76.
77.
The thoracic aorta and basilar artery, in which the incidence of atherosclerosis is known to be different, were examined to elucidate the correlation between the structure of the intercellular cleft junction between adjacent endothelial cells and its permeability to HRP. Tannic acid or HRP in the vessel lumen passed through the intercellular clefts of the thoracic aorta into the subendothelial space, whereas in the basilar artery they were unable to penetrate beyond the tight junction of the intercellular clefts. Freeze-fracture replicas revealed that the tight junctions of the thoracic aorta consisted of one to two junctional strands in most areas of the cleaved planes, with discontinuities in some places, whereas those of the basilar artery consisted of a continuous belt-like meshwork of six anastomosing junctional strands on average. These observations confirm that the structure of endothelial junctions in arteries has a close correlation with the permeability of the intercellular clefts to HRP. 相似文献
78.
79.
80.
Phenethyl Alcohol Resistance in ESCHERICHIA COLI. III. a Temperature-Sensitive Mutation (dnaP) Affecting DNA Replication 总被引:18,自引:1,他引:17
下载免费PDF全文
![点击此处可从《Genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A temperature-sensitive DNA replication mutant of E. coli K-12 was isolated among the mutants selected for phenethyl alcohol resistance at low temperatures. This mutation, designated as dnaP18, affects sensitivity of the cell to phenethyl alcohol, sodium deoxycholate and rifampicin, presumably due to an alteration in the membrane structure. At high temperatures (e.g., 42 degrees ), synthesis of DNA, but not RNA or protein, is arrested, leading to the formation of "filaments" in which no septum formation is apparent. Nucleoids observed under electron microscope seem to become dispersed and DNA fibrils less condensed, which may explain the loss of viability under these conditions. Genetic analyses, including reversion studies, indicate that a recessive dnaP mutation located between cya and metE on the chromosome is responsible for both alterations of the membrane properties and temperature sensitivity. The dnaP18 mutation does not affect growth of phage T4 or lambda under conditions where host DNA replication is completely inhibited. Kinetic studies of DNA replication and cell division in this mutant after the temperature shift from 30 to 42 degrees , and during the subsequent recovery at 30 degrees , accumulated evidence suggesting that DNA replication comes to a halt at 42 degrees upon completion of a cycle already initiated before the temperature shift. Since the recovery of DNA synthesis after exposure to 42 degrees does not depend on protein or RNA synthesis or other energy-requiring processes, the product of the mutant dnaP gene appears to be reversibly inactivated at 42 degrees . Taken together with the recessive nature of the present mutation, it was suggested that one of the membrane proteins involved in initiation of DNA replication is affected in this mutant. 相似文献