全文获取类型
收费全文 | 4254篇 |
免费 | 459篇 |
国内免费 | 4篇 |
出版年
2021年 | 42篇 |
2019年 | 29篇 |
2018年 | 49篇 |
2017年 | 31篇 |
2016年 | 69篇 |
2015年 | 105篇 |
2014年 | 139篇 |
2013年 | 157篇 |
2012年 | 234篇 |
2011年 | 200篇 |
2010年 | 145篇 |
2009年 | 127篇 |
2008年 | 213篇 |
2007年 | 197篇 |
2006年 | 189篇 |
2005年 | 203篇 |
2004年 | 200篇 |
2003年 | 183篇 |
2002年 | 169篇 |
2001年 | 76篇 |
2000年 | 63篇 |
1999年 | 71篇 |
1998年 | 50篇 |
1997年 | 47篇 |
1996年 | 40篇 |
1995年 | 40篇 |
1993年 | 43篇 |
1992年 | 48篇 |
1991年 | 60篇 |
1990年 | 55篇 |
1989年 | 48篇 |
1988年 | 46篇 |
1987年 | 34篇 |
1985年 | 33篇 |
1984年 | 37篇 |
1983年 | 51篇 |
1982年 | 45篇 |
1981年 | 51篇 |
1980年 | 45篇 |
1979年 | 50篇 |
1978年 | 48篇 |
1977年 | 30篇 |
1976年 | 46篇 |
1975年 | 39篇 |
1974年 | 49篇 |
1973年 | 32篇 |
1972年 | 32篇 |
1971年 | 28篇 |
1970年 | 33篇 |
1969年 | 31篇 |
排序方式: 共有4717条查询结果,搜索用时 15 毫秒
141.
Jeffry Reidler Vernon T. Oi William Carlsen T.Minh Vuong Israel Pecht Leonard A. Herzenberg Lubert Stryer 《Journal of molecular biology》1982,158(4):739-746
The rotational motions of monoclonal mouse anti-dansyl immunoglobulins were studied by nanosecond fluorescence emission anisotropic spectroscopy using a mode-locked argon-ion laser as the pulsed excitation source. Three homogeneous antibodies of the immunoglobulin Gl (IgGl) subclass containing different V regions were prepared. The fluorescence emission maxima of these antibodies (designated as DNS1, DNS2 and DNS3) are at 515, 480 and 500 nm, respectively. Their mean rotational correlation times, 〈φ〉, are 84, 109 and 96 ns, respectively. The binding of protein A or a monoclonal anti-allotype antibody to the Fc unit of DNS1 increased 〈φ〉 to 142 and 150 ns, respectively, whereas reduction of the disulfide bond between the heavy chains decreased 〈φ〉 to 48 ns. These nanosecond measurements show that the rotational motion of the Fab arms in mouse IgGl is restricted. 相似文献
142.
Summary A determination was made of the differences in the utilization of energy by laboratory reared larval Choristoneura fumiferana fed either balsam fir or white spruce foliage. This enabled us to quantitatively measure the quality of these foliages vis a vis the spruce budworm.The larval strategy was to feed rapidly and develop quickly. Development time was longer on spruce than on fir. Total consumption was virtually identical on both foliage types though production was ca 20% greater on a white spruce diet. Calories/gram increased with insect development and with the development of balsam fir foliage but declined over time in white spruce. Assimilation efficiencies (A/C) were 33.9% to 40.2% while gross production efficiencies (P/C) were 9.5% to 13.3% and net production efficiencies (P/A) were 26.1% to 38.8%. The spruce fed insects gave higher values than those on fir, but all values were at the low end of the range for lepidopterous defoliators (possibly due to characteristics of conifer foliage).The enhanced growth found in spruce fed larvae contrasts the relationship between balsam fir and high insect densities. This is discussed together with the influence of spruce budworm on forest composition and the possible competitive benefits of spruce budworm induced tree mortality.Parts of this paper appeared in a Master's thesis (Koller 1978) submitted to the University of Maine 相似文献
143.
William H. Frey II Susan E. Senogles Leonard L. Heston Vicente B. Tuason Susan E. Nicol 《Journal of neurochemistry》1980,35(6):1418-1430
Abstract: Partial purification of soluble guanylate cyclase on DEAE-Sephacel yields two separate peaks of guanylate cyclase activity. After 10-fold purification of the soluble enzyme, guanylate cyclase is markedly inhibited by micromolar concentrations of dopamine (I50= 0.2 μm). Dopamine inhibition is observed whether the reaction is conducted with Mn21 or with Mg2+, under atmosphere or N2(g), and using enzyme from either peak from the DEAESephacel column. Other catecholamines also inhibit partially purified guanylate cyclase with an order of potency at 1 μm of: dopamine =l -DOPA > norepinephrine = isoproterenol = adrenochrome > epinephrine. The structural requirements for inhibition are two free hydroxyl groups on the phenyl ring and an ethylamine side chain. Dopamine also inhibits the Triton X-100-solubilized microsomal guanylate cyclase after partial purification on DEAESephacel. Neither chlorpromazine, propranolol, nor phentolamine at 20 μm effectively block the dopamine inhibition of partially purified soluble guanylate cyclase. Micromolar concentrations of the reducing agents dithiothreitol and glutathione also inhibit partially purified guanylate cyclase, but unlike these agents, catecholamines can inhibit whether added in the reduced or the oxidized forms. Inhibition of enzyme activity by micromolar concentrations of dopamine, adrenochrome, or dithiothreitol is rapidly reversed by dilution and the dopamine inhibition is competitive with MgGTP. Inhibition does not appear to involve covalent binding or to result from the ability of catecholamines to reduce the concentrations of oxygen or free radicals in solution. 相似文献
144.
Natural control agents of the satin moth,Leucoma salicis (L.) were examined in 2Populus grandidentata Michaux stands. Highest mortality occurred in overwintering larvae, with the causal agents 2 fungi,Paecilomyces sp. andHirsutella gigantea Petch, a factor causing death symptomatic of a pathogen, the parasiteEupteromalis hemipterus (Walker) and death from unknown causes. Mortality fromPaecilomyces sp. andE. hemipterus was reduced where overwintering sites were covered with burlap cloth. Parasites recovered from larvae and pupae included the braconidsApanteles melanoscelus (Ratzeburg) andMeteorus versicolor (Wesmeal), the ichneumonidCoccygomimus pedalis (Cresson), the tachinidsCompsilura concinnata (Meigen),Carcelia laxifrons Villeneuve,Tachinomyia variata Curran, and the sarcophagidsSarcophaga aldrichi Parker andAgria housei Shewell. Larval and pupal predators included the carabidCalosoma frigidum Kirby, pentatomids, and birds, particularly black-billed cuckoos,Coccyzus erythrophthaimus (Wilson). Eggs were parasitized by the scelionidTelenomus prob.californicus Ashmead and the trichogrammidTrichogramma minutum (Riley). Predators of adult satin moths included the hermit thrush,Hylocichla guttata (Pallas), and pentatomid bugs. 相似文献
145.
Evelyn Ralston Leonard M. Hjelmeland Richard D. Klausner John N. Weinstein Robert Blumenthal 《生物化学与生物物理学报:生物膜》1981,649(1):133-137
Impurities in 5(6)-carboxyfluorescein can affect phospholipid vesicle stability and apparent rates of carboxyfluorescein transfer into cells. Thorough purification and characterization of the dye are thus important to many applications with vesicles and/or cells. The dye can be purified by adsorption chromatography on a hydrophobic gel, following treatment with activated charcoal and precipitation from ethanol-water. The 5- and 6-carboxy-isomers can be separated from each other (though for most purposes it is not necessary to do so) by synthesis, crystallization, and hydrolysis of the diacetate derivatives. Purification is monitored by thin-layer and high pressure chromatography. 相似文献
146.
147.
Genetically Induced Mitotic Exchange in the Heterochromatin of DROSOPHILA MELANOGASTER 总被引:5,自引:4,他引:1
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)
Leonard G. Robbins 《Genetics》1981,99(3-4):443-459
Multiple copies of the 18S and 28S ribosomal RNA cistrons are present in both the X and Y chromosomes of Drosophila melanogaster. Data are presented here that identify a locus, Rex, that causes exchange-like events between duplicated ribosomal complexes at the ends of an attached-XY chromosome. Rex: (1) is close to or in the basal heterochromatin of the X chromosome; (2) is semidominant and (its effect) is temperature sensitive; (3) acts maternally; and (4) affects behavior of paternally derived attached-XY chromosomes shortly after fertilization. Though, at this point, the existence of Rex is known only from its effects on behavior of a particular compound chromosome, it presents intriguing possibilities for understanding regulation of chromosome behavior and organization of the ribosomal cistrons. 相似文献
148.
Elizabeth Witkoski Mary F. Schuler Richard C. Feldhoff Samson I. Jacob Leonard S. Jefferson Allan Lipton 《Experimental cell research》1979,124(2)
Isolated rat hepatocytes release an acidic glycoprotein(s) that can selectively promote the growth of transformed cells. This factor has a molecular weight of 60 000–70 000 D. Liver microsomal and cytosol fractions contain two species of stimulatory activity—44 000 and 3 500 D. Mitochondrial and nuclear fractions contain only the lower molecular weight factor. 相似文献
149.
Jonathan R. Seals Jay M. McDonald David Bruns Leonard Jarett 《Analytical biochemistry》1978,90(2):785-795
A manual ATPase assay which measures the release of 32Pi from [γ-32P]ATP is described. Sodium dodecyl sulfate is used to terminate the enzyme reaction and extraction of the phophomolybdate complex into xylene: isobutanol is used to separate 32Pi from [γ-32P]ATP for quantitation by scintillation counting. The three-step assay is rapid (75–90 samples/h) and minimizes hydrolysis of ATP due to exposure to acidie conditions. The extraction procedure separates 10−15 to 10−7 mol of 32Pi from aqueous solution with an efficiency of 100,7 ± 0.62%. Less than 1% of unhydrolyzed [γ-32P]ATP is extracted. Extraction efficiency is not affected by protein or salts commonly present in enzyme incubation mixtures. Results obtained with this assay are precise, with an intraassay coefficient of variation of 0.6% and an interassay coefficient of variation of 1.8%. The results are comparable to results obtained with a spectrophotometric assay, with a correlation coefficient of 0,996, though assay performance and sensitivity are greatly improved with the isotopic assay. 相似文献
150.
Features of mitral stenosis and systolic anterior motion of the mitral valve were observed on the echocardiogram of a patient with a large posterior pericardial effusion. The abnormal findings disappeared with resolution of the fluid. Because pericardial effusion may result in the echocardiographic alteration of cardiac structures, repeat study is necessary after the effusion has cleared. 相似文献