全文获取类型
收费全文 | 2374篇 |
免费 | 151篇 |
国内免费 | 5篇 |
出版年
2022年 | 8篇 |
2021年 | 38篇 |
2020年 | 9篇 |
2019年 | 18篇 |
2018年 | 15篇 |
2017年 | 26篇 |
2016年 | 43篇 |
2015年 | 60篇 |
2014年 | 65篇 |
2013年 | 194篇 |
2012年 | 141篇 |
2011年 | 153篇 |
2010年 | 85篇 |
2009年 | 87篇 |
2008年 | 139篇 |
2007年 | 145篇 |
2006年 | 142篇 |
2005年 | 166篇 |
2004年 | 119篇 |
2003年 | 120篇 |
2002年 | 103篇 |
2001年 | 33篇 |
2000年 | 39篇 |
1999年 | 48篇 |
1998年 | 20篇 |
1997年 | 30篇 |
1996年 | 22篇 |
1995年 | 23篇 |
1994年 | 26篇 |
1993年 | 19篇 |
1992年 | 29篇 |
1991年 | 23篇 |
1990年 | 24篇 |
1989年 | 36篇 |
1988年 | 21篇 |
1987年 | 20篇 |
1986年 | 26篇 |
1985年 | 18篇 |
1984年 | 18篇 |
1983年 | 14篇 |
1982年 | 25篇 |
1981年 | 21篇 |
1980年 | 14篇 |
1979年 | 14篇 |
1978年 | 17篇 |
1977年 | 8篇 |
1976年 | 16篇 |
1975年 | 9篇 |
1973年 | 11篇 |
1972年 | 9篇 |
排序方式: 共有2530条查询结果,搜索用时 46 毫秒
91.
Absence of pantothenic acid in gramicidin S synthetase 2 obtained from some mutants of Bacillus brevis 总被引:2,自引:0,他引:2
The pantothenic acid content of gramicidin S synthetase 2(GS 2) was estimated microbiologically with enzymes obtained from the wild strain and gramicidin S-lacking mutant strains of Bacillus brevis. Four mutant enzymes from BI-4, C-3, E-1, and E-2 lacked pantothenic acid. Other mutant enzymes from BII-3, BI-3, BI-9, and BI-2 contained the same amount of pantothenic acid as the wild-type enzyme. Pantothenic acid-lacking GS 2 belonged to group V of mutant enzymes, which could activate all amino acids related to gramicidin S; their complementary enzyme, gramicidin S synthetase 1(GS 1), lacked racemizing activity. To ascertain whether 4'-phosphopantetheine is involved in the formation of D-phenylalanyl-L-prolyl diketopiperazine (DKP) and gramicidin S, combinations were tested of intact GS 1 from the wild strain with various mutant GS 2 either containing or lacking pantothenic acid. Only the combinations of wild-type GS 1 with mutant GS 2 containing pantothenic acid could synthesize DKP. Combinations with pantothenic acid-lacking GS 2 also failed to elongate peptide chains. Pantothenic acid-lacking GS 2 could bind the four amino acids which constitute gramicidin S as acyladenylates and thioesters, but the binding abilities were lower than those of the wild-type enzyme and other mutant enzymes containing the pantothenic group. 相似文献
92.
In vivo sister chromatid exchange (SCE) in mouse cells derived from various organs was studied by infusing BrdU from the tail vein. It was found that at BrdU concentrations ranging from 2.2–13.5 g/g/h, the SCE frequency in bone marrow cells seemed to stay at a constant level (1.5–2/cell/two cell cycles) whereas it started to rise as the BrdU dose exceeded this dose range. When BrdU within this dose range was infused continuously from the tail vein for appropriate hours to label chromosomes in various organs, the average SCE frequencies per cell were found to be 1.64 in bone marrow cells, 1.82 in spermatogonia, 1.99 in splenic cells, 2.89 in intestinal cells and 3.69 in cells from adjuvant stimulated lymph nodes. It is suggested that the spontaneous level of the in vivo SCE frequency might be about 1.5–2/cell/two cell cycles in the mouse. In cells derived from intestine and adjuvant stimulated lymph node, some unknown factors might work as a inducer of SCEs resulting in a significant increase in the SCE frequency in these organs. 相似文献
93.
Corelike structures, which were interpreted as straight, large cylinders containing ribosome-like particles surrounded by
an amorphous substance of low electron density, were found in a stable L-form ofStreptococcus pyogenes grown in the absence of antibiotics. 相似文献
94.
Carbon dioxide enhanced chloroplast regeneration in glucose-bleachedcells of Chlorella protothecoides in the presence of CMU inthe light. Both the formation of chlorophyll and the synthesesof RNA and protein were considerably enhanced. The CO2 metabolism of algal cells during greening was investigatedusing 14C-bicarbonate as the tracer. Radiocarbon was largelyincorporated into purine and pyrimidine bases in nucleic acidand the arginine in protein, specifically at the crabon atomsderived from carbamylphosphate.
1Part of this investigation was reported at the conference onthe "Autonomy and biogenesis of mitochondria and chloroplasts"held at Canberra in 1969 (4). (Received August 19, 1975; ) 相似文献
95.
K Hirayama S Homma K Kanda Y Nakajima S Watanabe 《Archives italiennes de biologie》1976,114(3):213-227
Neuromuscular unit (NMU) spikes activated during the tonic vibration reflex (TVR) in man were studied by means of the cross-correlogram test. Those NMUs which preferredly fired at some definite interval with good correlation with vibration were called "locked" spikes. The amount above zero correlation level seen in the cross-correlogram was expressed as ipsilateral "excitatory frequency", Ef. TVR contraction of the ipsilateral quadriceps femoris muscle was stimulated by voluntary contraction of the contralateral quadriceps femoris. I. Cross-correlogram test of the contralateral voluntary NMU spikes with ipsilateral vibration revealed "contralateral inhibitory modulation". Such inhibitory modulation was observed in 24% of the contralateral NMUs during voluntary tracking contraction with visual feed-back and in 22% of NMUs during blind simulated contraction without visual feed-back. The "subtracted" amount, i.e., contralateral inhibitory modulation, was also expressed as the contralateral "inhibitory frequency", If, which was calculated from the indentations of the cross-correlogram. 2. When varied vibratory frequency was applied to the muscle both Ef and If attained their maximum value around an "optimal frequency" of vibration, i.e., 80-95 HZ. 3. The ratio Ef/If was proposed as the reciprocal co-activation ratio which was fairly independent of the change of vibratory frequency. An average value of Ef/If ratio was calculated at 2.2 +/- 0.9 from 19 experiments. 相似文献
96.
Dielectric relaxation of DNA in aqueous solutions. 总被引:1,自引:0,他引:1
Using a four-electrode cell and a new electronic system for direct detection of the frequency differences specturm of solution impedance, the complex dielectric constant of calf thymus DNA (Mr = 4 × 106) in aqueous NaCl at 10°C is measured at frequencies ranging from 0.2 Hz to 30 kHz. The DNA concentrations are Cp = 0.01% and 0.05%, and the NaCl concentrations are varied from Cs = 10?4 M to 10?3 M. A single relaxation regions is found in this frequency range, the relaxation frequency being 10 Hz at Cp = 0.01% and Cs = 10?3 M. At Cp = 0.05% it is evidenced that the DNA chains have appreciable intermolecular interactions. The dielectric relaxaton time τd at Cp = 0.01% agrees well with the rotational relaxation time estimated from the reduced visocisty on the assumption that the DNA is not representable as a rigid rod but a coiled chain. It is concluded that the dielectric relaxiatioinis ascribed to the rotation of the molecule. Observed values of dielectric increment and other experimental findings are reasonably explained by assuming that the dipole moment of DNA results from the slow counterion fluctuation which has a longer relaxation time than τd. 相似文献
97.
Hiromu Ohno Eiji Nakanishi Takeichiro Takamatsu 《Biotechnology and bioengineering》1976,18(6):847-864
This work is concerned with the optimization study of the semibatch fermentation by which an amino acid is produced. The particular fermentation studied is the synthesis of lysine by the auxotrophic mutant. Applying Green's theorem to the maximization problem was proposed, and it succeeded in determining the feed rate of the substrate that maximized the production rate of the desired product. 相似文献
98.
The titration curve of ultrasonic absorption at 2.82 MHz in aqueous solutions of lysozyme measured by Zana and Lang [J. Phys. Chem., 74 , 2734 (1970)] is theoretically analyzed. The maxima at pH 3 and pH 11 are describable with proton-transfer reactions of dissociable carboxyl and amino groups by assuming that volume changes due to the reactions are 2.3 and 5.2 cm3/mole, respectively, which are appreciably smaller than those of simple amino acids. The remaining, pH-independent excess absorption over solvent is measured at frequencies ranging from 3 to 150 MHz. The absorption is ascribed to the internal loss of protein. The complex compressibility β′p ? iβ″p of lysozyme molecule is evaluated as β′p = 7.2 × 10?12 cm2/dyne and β″P = 4.3 × 10?14 cm2/dyne from the increments over solvent in absorption as well as in sound velocity. 相似文献
99.
Eiji Hayase Takehiko Shibata Tadahiko Ando 《Biochemical and biophysical research communications》1975,62(4):849-855
An enzyme activity specific for UV-DNA1 was found in the extract of (Marburg 168). The enzyme preparation obtained from the extract by ammonium sulfate precipitation acts on UV-DNA endonucleolytically and induces single strand breaks. The number of single strand breaks introduced in DNA is proportional to UV dose. 相似文献
100.
Chloroplast-nucleus interactions were examined in cells of Euglenagracilis Z synchronized under photoautotrophic conditions. Thechloroplasts were localized near the cell periphery. At an earlystage of the cell cycle, however, some chloroplasts were transientlylocated in the inner space close to the nucleus. Electron microscopyusing serial cell sections revealed that the chloroplast formedprotrusions at several sites, which became associated with thenucleus. The outer membrane of the chloroplast envelope wasin contact, or at least continuous in part, with the outer membraneof the nuclear envelope at the sites of association, and densematerial was present in the chloroplast membrane. A chromosomewas close to each site of the association between these twoorganelles. Most of the chloroplasts including those in associationwith the nucleus were connected by fine bridges. The 4',6-diamidino-2-phenylindole-stainednucleoids in the chloroplast associated with the nucleus appearedto have a thread-like shape. There was another type of chloroplast-nucleusconnection, in which an intervening membranous body was in contactwith the outer part of the nuclear envelope on one side andwith the chloroplast envelope on the other side.
1 This work was reported at the 48th Annual Meeting of the BotanicalSociety of Japan, Kyoto, October, 1983. (Received June 5, 1984; Accepted November 20, 1984) 相似文献