全文获取类型
收费全文 | 20668篇 |
免费 | 1299篇 |
国内免费 | 6篇 |
出版年
2022年 | 128篇 |
2021年 | 290篇 |
2020年 | 186篇 |
2019年 | 240篇 |
2018年 | 347篇 |
2017年 | 270篇 |
2016年 | 470篇 |
2015年 | 620篇 |
2014年 | 719篇 |
2013年 | 1530篇 |
2012年 | 1121篇 |
2011年 | 1233篇 |
2010年 | 735篇 |
2009年 | 690篇 |
2008年 | 1118篇 |
2007年 | 1059篇 |
2006年 | 1067篇 |
2005年 | 1031篇 |
2004年 | 995篇 |
2003年 | 928篇 |
2002年 | 982篇 |
2001年 | 477篇 |
2000年 | 460篇 |
1999年 | 396篇 |
1998年 | 280篇 |
1997年 | 199篇 |
1996年 | 203篇 |
1995年 | 211篇 |
1994年 | 172篇 |
1993年 | 189篇 |
1992年 | 325篇 |
1991年 | 299篇 |
1990年 | 245篇 |
1989年 | 267篇 |
1988年 | 207篇 |
1987年 | 197篇 |
1986年 | 166篇 |
1985年 | 183篇 |
1984年 | 172篇 |
1983年 | 150篇 |
1982年 | 165篇 |
1981年 | 130篇 |
1980年 | 108篇 |
1979年 | 108篇 |
1978年 | 118篇 |
1977年 | 80篇 |
1976年 | 106篇 |
1975年 | 74篇 |
1973年 | 82篇 |
1972年 | 60篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
'Classical nitroreductase' is an enzyme involved in the intracellular metabolic activation of mutagenic nitroarenes. The nitroreductase gene of Salmonella typhimurium TA1538 was cloned into pBR322 and the plasmids harboring the gene were introduced into TA98 and TA100. The resulting strains (YG1021 and YG1026) had more than 50 times higher nitrofurazone-reductase activity than TA1538 containing pBR322, and were extremely sensitive to the mutagenic action of 2-nitrofluorene, 1-nitropyrene and 2-nitronaphthalene. These results indicate that the new strains permit the efficient detection of mutagenic nitroarenes. 相似文献
62.
Nishizono Hiromi; Watanabe Takemasa; Orii Tokiko; Suzuki Shizuo 《Plant & cell physiology》1989,30(4):565-569
A metal-tolerant fern, Athyrium yokoscense, is capable of growingin highly copper-contaminated soil, but cupric chloride inhibitedthe activities of some enzymes extracted from the fern. Thefunction in the detoxification of copper of two copper-bindingsubstances was investigated by examination of their effectson various enzymes assayed in vitro, i.e. acid phosphatase (orthophosphoric-monoesterphosphohydrolase [acid optimum], EC 3.1.3.2
[EC]
), glucose-6-phosphatedehydrogenase (D-glucose-6-phosphate: NADP+ 1-oxidoreductase,EC 1.1.1.49
[EC]
) and isocitrate dehydrogenase (threo-Ds-isocitrate:NADP+ oxidoreductase [decarboxylating], EC 1.1.1.42
[EC]
). The twocopper-binding substances, whose apparent molecular weightsare 9.5 kDa and 2 kDa, were previously obtained from the solublecytoplasmic fraction of the fern root. The 9.5-kDa substance,which is a cysteine-rich peptide induced as a result of exposureof the fern to copper, was found to suppress almost entirelythe inhibitory effects of the metal on the enzymes. The suppressoractivity of the peptide was nearly as effective as that of ethylenediaminetetraaceticacid. The 2-kDa substance, which is also found in fern thathas not been exposed to copper, had a more modest suppressoractivity. These results indicate that the 9.5-kDa substancemay contribute to the copper-tolerance of the fern growing incopper-contaminated soil. (Received August 26, 1988; Accepted March 17, 1989) 相似文献
63.
Komatsuna (Brassica campestris L. var. rapa) plants were grownhydroponically under various conditions with respect to thesupply of nitrate, and the variations in levels of natural 15Nabundance (15N) in nitrogenous fractions of leaf blades, petiolesplus midribs, and roots in these plants were analyzed. The fractionation of nitrogen isotopes during uptake of nitratewas null irrespective of the concentrations of nitrate in theculture medium. The roots had lower 15N values than that inthe nitrate applied to plants. The nitrate in the three tissuesexamined had higher 15N values than that in the nitrate applied:the values were highest in the leaf blades which were presumedto have highest activities in terms of reduction of nitrate.In contrast, the amino acids and residual fractions had lower15N values than those in the nitrate applied. These resultssuggest that reduction of nitrate is a critical step in thefractionation of nitrogen isotopes in plant tissues in vivo.
1Permanent address: Fukuoka Agricultural Experiment Station,Yoshiki 587, Chikushino, 818 Japan. (Received March 10, 1989; Accepted July 10, 1989) 相似文献
64.
Quantitative Analysis of Endogenous Gibberellins in Normal and Dwarf Cultivars of Rice 总被引:7,自引:0,他引:7
Kobayashi Masatomo; Sakurai Akira; Saka Hitoshi; Takahashi Nobutaka 《Plant & cell physiology》1989,30(7):963-969
Endogenous levels of gibberellins in shoots and ears of twodwarf rice (Oryza sativa L.) cultivars, Tan-ginbozu (dx mutant)and Waito-C (dy mutant), were analyzed and compared with thoseof normal rice cultivar, Nihonbare. The endogenous levels of13-hydroxylated gibberellins in Tan-ginbozu were much lowerthan those in Nihonbare. In Waito-C, the levels of GA19 andGA20 in the shoots were higher but that of GA1 was lower thanthe levels of these gibberellins in Nihonbare. These resultssupport the hypothesis that the dy gene controls the 3ß-hydroxylationof GA20 to GA1 while the dx gene controls a much earlier stepin the gibberellin biosynthesis. Our results indicate that GA1is the active gibberellin that regulates the vegetative growthof rice. The endogenous levels of GA4 in the ears of the twodwarf cultivars of rice were higher than the level of GA4 inthe ears of the normal cultivar, Nihonbare suggesting that thebiosynthesis of gibberellin is not blocked in the anthers ofthe dwarf rice. (Received April 27, 1989; Accepted July 12, 1989) 相似文献
65.
The changes of dominance rank among female Japanese monkeys of the Koshima group over a period of 29 years from 1957 were
studied. The dominance rank order was relatively stable in the early population growing phase, while large scale-changes of
dominance rank order occurred successively in the phase of population decrease brought about by the severe control of artificial
feeding after 1972. Nevertheless, the rank order of several females of the highest status was stable. Furthermore, the reproductive
success of these highest status females was high (Mori, 1979a;Watanabe et al., in prep.). Divergence of the dominance rank order fromKawamura's rules (Kawamura, 1958) was observed in the following respects: (1) Some females significantly elevated their rank depending on the leader
males. (2) If mothers died when their daughters were still juveniles or nulliparous, the dominance rank of some of these offspring
females was significantly lower than the mother's one. However 55% of daughters which lost their mothers at a young age inherited
the mother's rank. (3) Dominance among sisters whose mother had died when at least one of the daughters was under 6 years
old followed the rule of youngest ascendancy in 60% (Kawamura, 1958), and in 80% when both of the daughters were nulliparous at the mother's death.
The mean rate of aggressive interactions for each female with subordinates to her was calculated by dividing the total aggressive
interactions between the female in question and her subordinates by the number of subordinate females to the female in question.
A female which showed a high rate of aggressive interactions with her subordinates was categorized as an “Attacker”, and a
female showing a lower rate was categorized as a “Non-attacker”. Similarly, categories of “Attacked”, and “Non-attacked” were
distinguished by using the rate of aggressive interactions with dominant females. Several females which were once categorized
in one category in a year were repeatedly categorized in the same category over different years. The “Attacked” tended to
be females of higher rank, and “Non-attackers” tended to be females of lower rank. “The second-higher-status females”, were
“Attacked”, and their rank was unstable. In particular, females of lower rank within the lineage of the highest rank suffered
this kind of severe status. Most of the daughters of these females showed a sharp drop of rank, and died when they were still
at a young age, i.e. “the second-higher-status females” displayed low fitness. “Non-attackers” were significantly “Non-attacked”;
i.e. they were females which showed a non-social attitude. Females which underwent a drop of rank tended to be “Non-attackers”.
The most important factor which determined the females' rank was the memory of their dominance relations under the influence
of their mother [dependent rank (Kawai, 1958)] in their early life during development. This finding corresponds well with the results in baboons obtained byWalter (1980); the target females of aggressive interactions by adolescent females were determined by the rank of the mothers when
these adolescent females were born. 相似文献
66.
Conformational change of bovine serum albumin by heat treatment 总被引:1,自引:0,他引:1
Kunio Takeda Akira Wada Kazuo Yamamoto Yoshiko Moriyama Koichiro Aoki 《Journal of Protein Chemistry》1989,8(5):653-659
The thermal denaturation of bovine serum albumin (BSA) was studied at pH 2.8 and 7.0 in the range of 2–65°C. The relative proportions of -helix, -structure, and disordered structure in the protein conformation were determined as a function of temperature, by the curve-fitting method of circular dichroism spectra. With the rise of temperature at pH 7.0, the proportion of -helix decreased above 30°C and those of -structure and disordered structure increased in the same temperature range. The structural change was reversible in the temperature range below 45°C. However, the structural change was partially reversible upon cooling to room temperature subsequent to heating at 65°C. On the other hand, the structural change of BSA at pH 2.3 was completely reversible in the temperature range of 2–65°C, probably because the interactions between domains and between subdomains might disappear due to the acid expansion. The secondary structure of disulfide bridges-cleaved BSA remained unchanged during the heat treatment up to 65°C at pH 2.8 and 7.0. 相似文献
67.
Kenzo Yamanaka Akira Hasegawa Ryoji Sawamura Shoji Okada 《Biological trace element research》1989,21(1):413-417
DNA damage induced by administration of dimethylarsinic acid (DMAA) to rats and mice was investigated. At 12 h after administration of DMAA, DNA single-strand breaks were induced markedly in lung. The majority of dimethylarsine, one of the main metabolites, in the expired air was excreted within 6–18 h after administration of DMAA to rats. In vitro experiments using nuclei isolated from lung of mice indicated that DNA strand breaks were caused by dimethylarsine. Furthermore, the strand breaks after exposure to dimethylarsine were reduced in the presence of catalase and/or superoxide dismutase. These results strongly suggest that the strand breaks are induced not by dimethylarsine itself but by active oxygen, e.g., O 2 ? and ·OH, produced both by dimethylarsine and molecular oxygen. When DNA was exposed to dimethylarsine, thiobarbituric acid (TBA)-reactive intermediates andcis-thymine glycol were produced. Dimethylarsine appears to induce DNA damage by the mechanism similar to the damage produced by ionizing radiation. 相似文献
68.
Purification and characterization of two serine isoacceptor tRNAs from bovine mitochondria by using a hybridization assay method. 总被引:8,自引:8,他引:0
下载免费PDF全文
![点击此处可从《Nucleic acids research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
For large scale preparation of mitochondrial tRNAs, a new hybridization assay method using synthetic oligodeoxyribonucleotide probes (16-17mer) complementary to individual tRNA sequences was developed and applied for the purification of two serine isoacceptor tRNAs (tRNASerAGY and tRNASerUCN) from bovine mitochondria. It is about 100 times more sensitive than the conventional aminoacylation assay method. 2-4 A260 units each of both tRNASer isoacceptors were purified from 17.5 kg of bovine liver, and they were characterized by means of nuclease digestion, melting profiles and aminoacylation activity. It is suggested that tRNASerUCN possesses the D loop/T loop interaction like usual L-shaped tRNAs, and that tRNASerAGY lacking almost an entire D arm is aminoacylated with an efficiency not very much lower than that of tRNASerUCN. 相似文献
69.
70.
Several on-line optimizing control strategies were proposed and tested by computer simulation for the efficient operation of bioreactors. The control task was divided into two, one of which was to search for the optimal operating point and passed the set point to the lower layer of which task was to make the process output follow the set point as soon as possible. It was shown to be effective for the upper layer to express the objective function as a polynomial with respect to the measurement variable and to make use of it for finding the optimum point. Noting that the major dynamic characteristics of bioreactor system is the time-varying and nonlinear nature, the adaptive type control system is in evitable. It was shown to be quite effective to use discrete type self-tuning PID controller and the optimal controller compensated for the interaction between the control loops.Application was made to the cell recycle system for the production of lactic acid and baker's yeast cultivation. I was found from the former application that the control quality can be significantly improved by incorporating the decoupling strategy into the lower layer closed-loop system. It was also found from the latter application that the initial startup period can be significantly reduced by making use of the rough mathematical model. 相似文献