全文获取类型
收费全文 | 11809篇 |
免费 | 1136篇 |
国内免费 | 8篇 |
出版年
2021年 | 106篇 |
2018年 | 117篇 |
2017年 | 102篇 |
2016年 | 186篇 |
2015年 | 284篇 |
2014年 | 327篇 |
2013年 | 475篇 |
2012年 | 563篇 |
2011年 | 587篇 |
2010年 | 406篇 |
2009年 | 365篇 |
2008年 | 542篇 |
2007年 | 589篇 |
2006年 | 571篇 |
2005年 | 585篇 |
2004年 | 585篇 |
2003年 | 519篇 |
2002年 | 570篇 |
2001年 | 136篇 |
2000年 | 115篇 |
1999年 | 177篇 |
1998年 | 177篇 |
1997年 | 129篇 |
1996年 | 144篇 |
1995年 | 117篇 |
1994年 | 165篇 |
1993年 | 137篇 |
1992年 | 118篇 |
1991年 | 125篇 |
1990年 | 133篇 |
1989年 | 120篇 |
1988年 | 106篇 |
1987年 | 112篇 |
1986年 | 110篇 |
1985年 | 138篇 |
1984年 | 149篇 |
1983年 | 128篇 |
1982年 | 177篇 |
1981年 | 196篇 |
1980年 | 184篇 |
1979年 | 125篇 |
1978年 | 153篇 |
1977年 | 135篇 |
1976年 | 113篇 |
1975年 | 93篇 |
1974年 | 130篇 |
1973年 | 124篇 |
1972年 | 83篇 |
1970年 | 85篇 |
1969年 | 83篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
142.
Lawrence W. Nichol Alexander G. Ogston Donald J. Winzor William H. Sawyer 《The Biochemical journal》1974,143(2):435-443
Theoretical expressions are derived for affinity chromatography of systems comprising an acceptor A with one binding site for attachment to a functional group X on the column matrix and one site for interaction with a small ligand B that specifically affects its elution. From a general relationship covering all possible interactions between A, B and X simpler expressions are derived for affinity systems in which only two equilibria operate. Methods are suggested whereby these simpler systems may be characterized in terms of the two pertinent equilibrium constants and the concentration of matrix-bound constituent. The means by which the theory may be adapted to affinity chromatography of acceptors with multiple binding sites for ligand is also illustrated. Results of partition experiments on the Sephadex G-100-lysozyme-d-glucose system in acetate-chloride buffer (I=0.17m), pH5.4, are used to demonstrate the feasibility of evaluating quantitatively affinity-chromatography interactions. Values of 30m(-1) and 1.2x10(6)m(-1) are obtained for the equilibrium constants for the reactions of lysozyme with glucose and Sephadex respectively, there being only an occasional binding site in the polysaccharide matrix (approximately 1 in 10(5) glucose residues). In a second experimental study the phytohaemagglutinin from Ricinus communis is subjected to frontal chromatography on Sepharose 4B in the presence of different concentrations of d-galactose, the results illustrating some of the difficulties and limitations that are likely to be encountered in quantitative studies of affinity-chromatographic systems. 相似文献
143.
Isobel Braidman Mark Carroll Norman Dance Donald Robinson 《The Biochemical journal》1974,143(2):295-301
Hexosaminidase C was separated from human brain supernatant by immunoadsorption of the A and B forms on to a column of immobilized antibody followed by preparative starch-block electrophoresis. There were some differences in the properties of hexosaminidase C preparations after each of these stages, shown by comparison of their heat-inactivation characteristics and filtration through Bio-Gel P-200. The C form prepared by both separation steps had properties which differed markedly from those of the A and B isoenzymes; its molecular weight was much larger, greater than 200000, it had optimum activity between pH6 and 7 and could not be successfully eluted from DEAE-cellulose, even with high salt concentrations, or from Sephadex G-200. These results seem to support the proposal that the C form is under a separate genetic control from the others. 相似文献
144.
We present absorption and circular dichroism (CD) spectra for the synthetic polymers poly d(AAT):d(AAT) and poly r(AAU):r(AAU), in both native and heat-denatured forms. As a means of evaluating the first-neighbor hypothesis, the CD spectra are compared with approximations derived from spectra of other synthetic polymers containing the same first-neighbor sequences. This is the first instance where such a comparison has been possible using spectra of double-stranded RNA sequences, and the agreement between the measured and approximated spectra for poly r(AAU):r(AUU) is surprisingly good. We have also subjected the CD spectrum of poly d(AAT):d(AAT) to a previously published analytical procedure for obtaining estimates of first-neighbor frequencies. In this first independent test of the procedure, we find that the analysis does infer the existence of a majority (86%) of AA, TT, AT, and TA first neighbors but does not precisely indicate their relative proportions. 相似文献
145.
Syringomycin, a wide-spectrum antibiotic produced by strains of Pseudomonas syringae which cause bacterial canker of peach, was able to bind to salmon sperm and calf thymus deoxyribonucleic acid but not to calf thymus histone; it also inhibited ribonucleic acid polymerase activity. These abilities to bind to deoxyribonucleic acid and to inhibit ribonucleic acid polymerase were inactivated when the phytotoxic and antibiotic properties of syringomycin were inactivated. 相似文献
146.
A survey to determine the occurrence of Histoplasma capsulatum and Cryptococcus neoformans in air-conditioners 总被引:1,自引:0,他引:1
Summary A survey of 84 dust samples from 42 air-conditioners and controls was conducted at Kansas State University, Manhattan. Isolations ofC. neoformans andH. capsulatum were attemped using various mycological procedures.
H. capsulatum was not recovered in this survey. One isolate ofC. neoformans was obtained. The contaminated air-conditioner yielding the organism contained considerable bird feces, feathers, and dust.Contribution No. 102, Department of Infectious Diseases, Kansas Agricultural Experiment Station, Manhattan, Kansas. 相似文献
147.
Summary Two distinct types of ribonucleoprotein containing structures are found in oocytes of the house cricket, Acheta domesticus, a large secondary or accessory nucleolus and many small primary nucleoli. The secondary nucleolus increases in size during oocyte development and is similar in appearance to the nucleolus of somatic cells. The primary nucleoli are intimately associated with a large, extrachromosomal DNA containing body. The DNA body is no longer visible in nuclei of late diplotene stage cells when the primary nucleoli are dispersed within the nucleoplasm. Both types of nucleoli contain cytochemically detectable RNA and acid protein, little or no DNA and basic protein, and particulate structures similar to but smaller than cytoplasmic ribosomes.The authors acknowledge the technical assistance of Miss Celeste Malinoski and Mrs. Marcia Andrews. This work was supported by a U.S.P.H.S. grant, number GM-16440-01 and grants number L-16 and J-1 from the Health Research Services Foundation. 相似文献
148.
Zusammenfassung In beiden Lobi der Pars distalis photosensitiver Zonotrichia leucophrys gambelii-Männchen bewirkt die photoperiodische Stimulation unter Langtagbedingungen das Auftreten von Acetylcholinesterase, die, solange die Vögel unter Kurztagbedingungen gehalten wurden, nicht oder nur in geringer Menge vorhanden war. Das Enzym ist auf PAS-positive Zellen beschränkt, doch nicht in allen von ihnen anzutreffen. Nach dem Erreichen eines Maximums zwischen Tag 31 und Tag 52 sinkt der AChE-Gehalt wieder und nähert sich nach 80 Tagen dem Ausgangsstadium. Der Parallelismus zwischen den Testesgewichten und der AChE-Aktivität läßt vermuten, daß die AChE-Zellen gonadotropes Hormon bilden.
Mit Unterstützung durch National Institutes of Health (Grant 5R01 NB 06187, Prof. Farner, Principal Investigator).
Institut für Haustierkunde der Universität Kiel (Direktor: Prof. Dr. Dr. h. c. Wolf herre). 相似文献
Acetylcholinesterase in the Pars distalis of Zonotrichia leucophrys gambelii (Aves)
Summary Photoperiodic stimulation of photosensitive male white-crowned sparrows, Zonotrichia leucophrys gambelii, causes the appearance of acetylcholinesterase activity in the cephalic and caudal lobes of the pars distalis. In photosensitive birds held on short daily photoperiods little or no activity can he demonstrated histochemically. The enzyme is restricted to PAS-positive cells. After attaining a maximum between 31 and 52 days after the beginning of photoperiodic stimulation (20 hours of light per day), AChE activity declines and after 80 days returns approximately to the initial level of short-day birds (eight hours of light per day). The correlation between the increase in weight of the testes and AChE activity, and other considerations, suggest that these AChE cells must produce gonadotropic hormones. Our observations are generally consistent with the results of the biochemical studies of Russell (1968) on the AChE activity in the pars distalis of the same species.
Mit Unterstützung durch National Institutes of Health (Grant 5R01 NB 06187, Prof. Farner, Principal Investigator).
Institut für Haustierkunde der Universität Kiel (Direktor: Prof. Dr. Dr. h. c. Wolf herre). 相似文献
149.
Summary Sporulating dikaryotic fruit-bodies of Schizophyllum commune (str. 699 A41B41 + str. 845 A51B51) were produced on glucose-asparagine medium and harvested at various stages of development for histological studies. Distinct patterns of cell organization were observed from basidiocarp initials to mature fructifications. Different cell types included basal bulbous cells, cylinders of parallel hyphae in the stipe, loosely knit tramal cells in the lamellae, specialized sub-hymenial cells and basidia. Large encrustations were observed protruding from the hymenium near the lamellar apex. Lateral hairs and crystalline matter were also seen in these genetically defined dikaryotic fruit-bodies which arose on simple medium in the laboratory. 相似文献
150.