全文获取类型
收费全文 | 3117篇 |
免费 | 266篇 |
国内免费 | 4篇 |
出版年
2022年 | 13篇 |
2021年 | 29篇 |
2020年 | 13篇 |
2019年 | 23篇 |
2018年 | 43篇 |
2017年 | 27篇 |
2016年 | 37篇 |
2015年 | 61篇 |
2014年 | 57篇 |
2013年 | 166篇 |
2012年 | 133篇 |
2011年 | 136篇 |
2010年 | 93篇 |
2009年 | 89篇 |
2008年 | 163篇 |
2007年 | 143篇 |
2006年 | 154篇 |
2005年 | 155篇 |
2004年 | 159篇 |
2003年 | 159篇 |
2002年 | 130篇 |
2001年 | 91篇 |
2000年 | 137篇 |
1999年 | 101篇 |
1998年 | 51篇 |
1997年 | 45篇 |
1996年 | 45篇 |
1995年 | 40篇 |
1994年 | 38篇 |
1993年 | 47篇 |
1992年 | 101篇 |
1991年 | 85篇 |
1990年 | 78篇 |
1989年 | 67篇 |
1988年 | 74篇 |
1987年 | 45篇 |
1986年 | 48篇 |
1985年 | 60篇 |
1984年 | 38篇 |
1983年 | 36篇 |
1982年 | 17篇 |
1981年 | 25篇 |
1980年 | 15篇 |
1979年 | 18篇 |
1978年 | 14篇 |
1977年 | 19篇 |
1976年 | 13篇 |
1973年 | 8篇 |
1971年 | 8篇 |
1968年 | 8篇 |
排序方式: 共有3387条查询结果,搜索用时 4 毫秒
31.
Aqualysin I is an alkaline serine protease which is secreted into the culture medium by Thermus aquaticus YT-1, an extreme thermophile [Matsuzawa, H., Hamaoki, M. & Ohta, T. (1983) Agric. Biol. Chem. 47, 25-28]. The gene encoding aqualysin I was cloned into Escherichia coli using synthetic oligodeoxyribonucleotides as hybridization probes. The nucleotide sequence of the cloned DNA was determined. The primary structure of aqualysin I, deduced from the nucleotide sequence, agreed with the NH2-terminal sequence previously reported and the determined amino acid sequences, including the COOH-terminal sequence, of the tryptic peptides derived from aqualysin I. Aqualysin I comprised 281 amino acid residues and its molecular mass was determined to be 28,350. On alignment of the whole amino acid sequence, aqualysin I showed high sequence homology with the subtilisin-type serine proteases, and 43% identity with proteinase K, 37-39% with subtilisins and 34% with thermitase. Extremely high sequence identity was observed in the regions containing the active-site residues, corresponding to Asp32, His64 and Ser221 of subtilisin BPN'. The nucleotide sequence of the cloned DNA (1105 nucleotides) revealed that it contains the entire gene encoding aqualysin I and one open reading frame without a translational stop codon. Therefore, aqualysin I was considered to be produced as a large precursor, which contains a NH2-terminal portion, the protease and a COOH-terminal portion. The G + C content of the coding region for aqualysin I was 64.6%, which is lower than those of other Thermus genes (68-74%). The codon usage in the aqualysin I gene was rather random in comparison with that in other Thermus genes. 相似文献
32.
33.
Sequence and over-expression of subunits of adenosine triphosphate synthase in thermophilic bacterium PS3 总被引:8,自引:0,他引:8
S Ohta M Yohda M Ishizuka H Hirata T Hamamoto Y Otawara-Hamamoto K Matsuda Y Kagawa 《Biochimica et biophysica acta》1988,933(1):141-155
The primary structures of all the subunits of thermophilic ATP synthase were determined, and its alpha, beta and gamma subunits could be over-expressed in Escherichia coli, because these subunits were stable and reconstitutable. DNA of 7500 base pairs in length was found to contain a cluster of nine genes for subunits of ATP synthase. The order of their reading frames (size in base pairs) was: I(381): a(630): c(216): b(489): delta(537): alpha(1507): gamma(858): beta(1419): epsilon(396), I being a gene for a small hydrophobic, basic protein expressed in vitro. All the termini of TF0F1 subunits were confirmed by peptide sequencing. Large quantities of the overexpressed thermophilic alpha, beta and gamma subunits were prepared from the extract of E. coli, by a few purification steps. 相似文献
34.
Receptor fractions were prepared from follicle-rich ovaries (for FSH), luteal cell-rich ovaries (for LH and PRL), and adrenals (for PRL) of rats. Divalent metal ions, Mg++, Ca++, and Mn++ showed inhibitory effects on the binding of LH and FSH to their receptors. The binding of the former was more sensitive to these ions than the latter. On the other hand they showed bell-shaped promotive effects on PRL-ovarian receptor binding, the maximal effects being observed at 10-20 mM. Besides these ions, Ba++ also had a promotive effect, while other divalent metal ions such as Zn++, Cd++, Ni++, and Co++ showed inhibitory effects on PRL-ovarian receptor binding at 5 mM. Mg++ and Ca++ also promoted PRL-adrenal receptor binding, while Mn++ promoted the binding at 10 mM but inhibited it at higher concentrations. Association constant (Ka) and binding capacity (Bmax) of PRL receptors of the ovary and the adrenal were significantly different (ovary: Ka = 0.69 X 10(10) M-1, Bmax = 62 fmol/mg protein, adrenal: Ka = 0.21 X 10(10) M-1, Bmax = 99 fmol/mg protein). Ka of the ovarian PRL receptor was not influenced by these divalent ions, while that of the adrenal receptor was doubled by Ca and Mn ions, Bmax of the latter was also increased. A cooperative effect of Mg and Ca ions was observed on Ka and Bmax of the adrenal receptor. The sizes of the PRL binding sites of these organs revealed by affinity labelling were 17K and 40K in the ovary, and 40K and 110K in the adrenal. These results indicate the different properties of receptors in these different target organs. 相似文献
35.
Tomoko Ohta 《Genetics》1986,113(1):145-159
A model of an expanding family of dispersed repetitive DNA was studied. Based on the previous result of the model of duplicative transposition, an approximate solution to give allelism and identify coefficients as functions of time was obtained, and theoretical predictions were verified by Monte Carlo experiments. The results show that, even if the copy number per genome increases very rapidly, allelism and identity coefficients may take a long time to reach equilibrium. The changes of allelism and allelic identity are similar to that of homozygosity at an ordinary single locus, whereas that of nonallelic identity can be much slower, particularly when the copy number per genome is large. Thus, many existing families of highly repetitive sequences may represent nonequilibrium states for nonallelic identity. The present model may be extended to include other evolutionary forces such as gene conversion or the recurrent insertion from normal gene copies. 相似文献
36.
Changes in lipoxygenase (LOX) activity were followed duringthe germination of rice seeds. The enzyme activity of 3-day-oldseedlings was 20 times higher than that of ungerminated seeds.Sixty per cent of the increased activity was found in shoots.The increase in LOX activity was mainly due to an increase inlipoxygenase-2 (LOX-2), a minor component in ungerminated seeds;this increase was inhibited by cycloheximide. LOX-2 was isolatedfrom the 3-day-old seedlings and compared for its enzymologicalproperties with rice lipoxygenase-3 (LOX-3), a major componentin ungerminated seeds. Both LOX-2 and LOX-3 were stable at pH5 to 8, but LOX-2 was more heatstable than LOX-3. Apparent Kmvalues of LOX-2 and LOX-3 for linoleic acid were 170 and 59µM, and those for linolenic acid were 5,300 and 88 µM,respectively. Both LOXs were inhibited by some metal ions andantioxidants. (Received February 5, 1986; Accepted May 9, 1986) 相似文献
37.
Peptidoglycan synthetic activities in membranes of Escherichia coli caused by overproduction of penicillin-binding protein 2 and rodA protein 总被引:27,自引:0,他引:27
F Ishino W Park S Tomioka S Tamaki I Takase K Kunugita H Matsuzawa S Asoh T Ohta B G Spratt 《The Journal of biological chemistry》1986,261(15):7024-7031
Penicillin-binding protein (PBP)-2 and the RodA protein are known to function in determining the rod shape of Escherichia coli cells. Peptidoglycan biosynthetic reactions that required these two proteins were demonstrated in the membrane fraction prepared from an E. coli strain that overproduced both of these two proteins and which lacked PBP-1B activity (the major peptidoglycan synthetase activity in the normal E. coli membranes). The cross-linked peptidoglycan was synthesized from UDP-N-acetylmuramylpentapeptide and UDP-N-acetylglucosamine in the presence of a high concentration of cefmetazole that inhibited all of PBPs except PBP-2. The peptidoglycan was synthesized via a lipid intermediate and showed up to 30% cross-linking. The cross-linking reaction was strongly inhibited by the amidinopenicillin, mecillinam, and by other beta-lactam antibiotics that have a high affinity for PBP-2, but not by beta-lactams that had very low affinity for PBP-2. The formation of peptidoglycan required the presence of high levels of both PBP-2 and the RodA protein in the membranes, but it is unclear which of the two proteins was primarily responsible for the extension of the glycan chains (transglycosylation). However, the sensitivity of the cross-linking reaction to specific beta-lactam antibiotics strongly suggested that it was catalyzed by PBP-2. The transglycosylase activity of the membranes was sensitive to enramycin and vancomycin and was unusual in being stimulated greatly by a high concentration of a chelating agent. 相似文献
38.
Chromosomal localization of the Epstein-Barr virus (EBV) genome in Bloom's syndrome B-lymphoblastoid cell lines transformed with EBV 总被引:3,自引:0,他引:3
The localization of the Epstein-Barr virus (EBV) genome in chromosomes of human B-lymphoblastoid cell lines (LCLs) transformed with EBV, and the effect of EBV DNA on the level of sister chromatid exchange (SCE) in Bloom's syndrome (BS) B-LCLs, were examined with chromosomal in situ hybridization techniques using a 3H-EBV DNA probe. EBV DNA was detected in chromosomes 1–5 and 13–15 at specific G band regions in BS as well as in normal B-LCLs, regardless of SCE. Several chromosomal sites (1p31, 1q31, 4q22–24, 5q21, 13q21, 14q21) carrying EBV DNA seemed to be very characteristic in normal as well as in BS B-LCLs. There was no statistically significant difference in silver grain counts due to EBV DNA and their distribution in different chromosomes or groups among normal and BS B-LCLs with normal and high SCE. These findings strongly indicate that EBV infection did not introduce a correcting factor for BS SCE. 相似文献
39.
An actin-depolymerizing protein (destrin) from porcine kidney. Its action on F-actin containing or lacking tropomyosin 总被引:17,自引:0,他引:17
An Mr 19 000 protein (destrin) that has the ability to rapidly depolymerize F-actin in a stoichiometric manner was purified from porcine kidney by sequential chromatography on DNase I-agarose, hydroxyapatite, and Sephadex G-75. Its actin-depolymerizing activity is reversibly controlled by changes in KCl concentration but is insensitive to Ca2+ concentration. The rate of depolymerization of F-actin by destrin is much faster than that of spontaneous depolymerization induced by dilution and is not markedly decreased by the addition of end-blocking reagents such as cytochalasin B. These results suggest that destrin depolymerizes F-actin by interacting directly with F-actin protomers. Binding of muscle tropomyosin to F-actin slows down the rate of destrin-induced depolymerization of F-actin by about 30-fold. The data suggest that the destrin-induced depolymerization occurs from the ends of F-actin when F-actin is complexed with tropomyosin, but it takes place from the entire length of F-actin in the absence of tropomyosin. 相似文献
40.
Visualization method for sulfolipids and its application to the determination of nanomole quantities of lipid sulfur 总被引:1,自引:0,他引:1
A simple and sensitive visualization method for sulfolipids on a thin-layer chromatogram is described. By spraying with an acidic solution of azure A, a complex was formed between an anionic sulfolipid and a blue cationic compound. After the unbound dye was washed out by brief soaking in methanol, sulfolipids were visualized as clear dark-blue bands on a light-blue background. As little as 0.5 nmol could be detected. Sulfolipid-dye complex was estimated by densitometry or colorimetric measurement after extraction with chloroform/methanol. For the quantitative determination of sulfolipids having long sugar chains, it is necessary to treat thin-layer chromatography plates with acetic anhydride before color development. Of the other tissue lipids not containing sulfuric acid ester that were tested none were stained significantly. A linearity of quantitative determination was observed over the range of 1-8 nmol. 相似文献