全文获取类型
收费全文 | 8998篇 |
免费 | 794篇 |
国内免费 | 1篇 |
专业分类
9793篇 |
出版年
2023年 | 92篇 |
2022年 | 184篇 |
2021年 | 337篇 |
2020年 | 192篇 |
2019年 | 242篇 |
2018年 | 284篇 |
2017年 | 233篇 |
2016年 | 374篇 |
2015年 | 577篇 |
2014年 | 563篇 |
2013年 | 683篇 |
2012年 | 799篇 |
2011年 | 740篇 |
2010年 | 479篇 |
2009年 | 368篇 |
2008年 | 515篇 |
2007年 | 453篇 |
2006年 | 396篇 |
2005年 | 361篇 |
2004年 | 335篇 |
2003年 | 289篇 |
2002年 | 254篇 |
2001年 | 57篇 |
2000年 | 45篇 |
1999年 | 62篇 |
1998年 | 75篇 |
1997年 | 43篇 |
1996年 | 34篇 |
1995年 | 30篇 |
1994年 | 28篇 |
1993年 | 34篇 |
1992年 | 29篇 |
1991年 | 35篇 |
1990年 | 40篇 |
1989年 | 23篇 |
1988年 | 31篇 |
1987年 | 29篇 |
1986年 | 28篇 |
1985年 | 36篇 |
1984年 | 38篇 |
1983年 | 27篇 |
1982年 | 24篇 |
1981年 | 21篇 |
1980年 | 19篇 |
1979年 | 27篇 |
1978年 | 21篇 |
1977年 | 14篇 |
1976年 | 18篇 |
1973年 | 18篇 |
1972年 | 17篇 |
排序方式: 共有9793条查询结果,搜索用时 0 毫秒
51.
Sara Brosh Oded Sperling Esther Dantziger Yechezkel Sidi 《Journal of neurochemistry》1992,58(4):1485-1490
The metabolic fate of guanine and of guanine ribonucleotides (GuRNs) in cultured rat neurons was studied using labeled guanine. 8-Aminoguanosine (8-AGuo), an inhibitor of purine nucleoside phosphorylase, was used to clarify the pathways of GMP degradation, and mycophenolic acid, an inhibitor of IMP dehydrogenase, was used to assess the flux from IMP to GMP and, indirectly, the activity of the guanine nucleotide cycle (GMP----IMP----XMP----GMP). The main metabolic fate of guanine in the neurons was deamination to xanthine, but significant incorporation of guanine into GuRNs, at a rate of approximately 8.5-13.1% of that of the deamination, was also demonstrated. The turnover rate of GuRNs was fast (loss of 80% of the radioactivity of the prelabeled pool in 22 h), reflecting synthesis of nucleic acids (32.8% of the loss in radioactivity) and degradation to xanthine, guanine, hypoxanthine, guanosine, and inosine (49.3, 4.3, 4.1, 1.1, and 0.5% of the loss, respectively). Of the radioactivity in GuRNs, 7.9% was shifted to adenine nucleotides. The accumulation of label in xanthine indicates (in the absence of xanthine oxidase) that the main degradative pathway from GMP is that to xanthine through guanosine and guanine. The use of 8-AGuo confirmed this pathway but indicated the operation of an additional, relatively slower degradative pathway, that from GMP through IMP to inosine and hypoxanthine. Hypoxanthine was incorporated mainly into adenine nucleotide (91.5%), but a significant proportion (6%) was found in GuRNs.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
52.
Communicated by R. N. Beachy 相似文献
53.
E V Kurenova B A Le?bovich I A Bass D V Bebikhov M N Pavlova O N Danilevskaia 《Genetika》1990,26(10):1701-1712
A mobile element (ME) having 91% homology with Dm1360 (Kholodilov et al., 1987) has been cloned from the Drosophila melanogaster genome and sequenced. The family of ME was designated hoppel. The members of this family are flanked by short inverted repeats likewise P, hobo and HB. The hoppel is hybridized with 10-30 euchromatic sites of polytene chromosomes of different Drosophila stocks. Abundant hybridization with heterochromatic regions of chromosomes-chromocenter, pericentric heterochromatin, the 4 chromosome and telomeres was observed in all stocks of D. melanogaster examined and in D. simulans. At least six genomic variants of ME differing in length of the central part were revealed. Hoppel possesses ARS activity similar to the P element. Two ME hoppel were shown to be arranged as a direct repeat in the recombinant phage. 相似文献
54.
Jorge Zorzopulos Sara DeLong Virginia Chapman Lloyd M. Kozloff 《Journal of cellular biochemistry》1982,18(3):363-375
The characteristics of pure preparations of short-tail fibers of bacteriophage T4 have been studied in the optical and electron microscope. Three main structures were observed: 1) spheres of 8.1 nm diameter; 2) fibers 43 nm long and 3.8 nm thick; and 3) fibers 54 nm long and 3.2 nm thick. Both types of fibers exhibited a regular beaded appearance. The 43-nm fibers were the most abundant structure. During the process of purification of the short-tail fibers, the formation of aggregates was observed each time the material containing the short-tail fibers was dialyzed against saline solutions. These aggregates became increasingly fibrous (as observed in the optical microscope) as the material used was increasingly enriched in short-tail fibers. Finally, most of the aggregates were of the fibrous type when they were formed from a purified preparation of short-tail fibers. In the electron microscope, it was found that the filamentous aggregates were organized in well-defined bundles. The amino acid composition of the highly purified short-tail fibers was also determined. Among the known fibrous proteins, the ones that most resemble the amino acid composition of the short-tail fibers are actin and fibrinogen. These observations are discussed in relation to the T4 short-tail fiber structure and their localization on the hexagonal baseplate of the T4 tail structure. 相似文献
55.
Lipoxygenbase-derived products of arachidonic acid mediate stimulation of hexose uptake in human polymorphonuclear leukocytes 总被引:3,自引:0,他引:3
D A Bass M J Thomas E J Goetzl L R DeChatelet C E McCall 《Biochemical and biophysical research communications》1981,100(1):1-7
The titration of metal-freed bovine α-lactalbumin with Mg2+ ions causes a two-stepped decrease in the tryptophan fluorescence quantum yield and a pronounced spectral shift towards shorter wavelengths, which seems to be a result of the binding of two magnesium ions to the protein molecule. The magnesium binding constants evaluated from the fluorimetric Mg2+-titration are 2·103 and 2·102 M?1. Mg2+ ions in millimolar concentrations almost do not influence the binding of Ca2+ ions to the protein. 相似文献
56.
57.
Marker rescue transformation by linear plasmid DNA in Bacillus subtilis 总被引:21,自引:0,他引:21
Although plasmid-free Bacillus subtilis cannot be transformed for markers carried by linear or nicked plasmid DNA, a resident plasmid can rescue a marker on such damaged DNA under certain conditions. Linearized chimeric plasmid DNA has been used to transform cultures carrying a resident plasmid which is homologous with a portion of the donor. This system has revealed the following properties of the marker rescue process: (1) It is recE dependent. (2) It requires the presence in the resident plasmid of sequences which are homologous to the donor. (3) When the selected marker is on a nonhomologous segment it must be flanked by segments which are homologous to the resident plasmid. (4) The efficiency of rescue varies in a regular way with the position of the linearizing cut. (5) Marker rescue is first order with respect to DNA concentration. These properties and other data are interpreted as providing a strong indication that marker rescue occurs by recombination, although an alternative explanation involving recE-dependent recircularization of the donor plasmid has not been eliminated. Our results also suggest that if the major pathway of marker rescue is by recombination, an average of 0.15 Mdal (single strand) must be removed from each donor DNA molecule or otherwise rendered unavailable for recombination and that the exchange frequency during transformational recombination is approximately 0.2 to 0.5 Mdal−1. 相似文献
58.
59.
A two-year-old girl has the following features of the cri du chat syndrome: microcephaly, hypertelorism, downward slanting of the palpebral fissures, psychomotor retardation and a cat-like cry. She is only of five patients having the cat cry syndrome with 45 chromosomes. Her karyotype is 45,XX, -5, -14, +t(5; 14)(5qter leads to 5p11: : 14q11 leads to 14qter) with the translocation inherited from her mother and maternal grandmother, each of whom is the carrier of a balanced translocation 46,XX,t(5;14)(p11q11). Normal plasma activity for hexosaminidase B suggests the locus for this enzyme is not located in the delected segment of 5 p. 相似文献
60.
Since the report of (Simulium) tredecimatum from “the stomach of a trout, England” no specimens of the genus Metacnephia have been reported from the British Isles. A full account of the morphological features of the larvae, pupae and adults of a new species, Metacnephia amphora is given together with a brief account of its larval growth and presence on a wide range of substrata in a small, temporary stream. 相似文献