全文获取类型
收费全文 | 39352篇 |
免费 | 2858篇 |
国内免费 | 7篇 |
出版年
2023年 | 248篇 |
2022年 | 247篇 |
2021年 | 485篇 |
2020年 | 504篇 |
2019年 | 504篇 |
2018年 | 964篇 |
2017年 | 862篇 |
2016年 | 1374篇 |
2015年 | 2062篇 |
2014年 | 2014篇 |
2013年 | 2657篇 |
2012年 | 3542篇 |
2011年 | 3288篇 |
2010年 | 1829篇 |
2009年 | 1419篇 |
2008年 | 2564篇 |
2007年 | 2439篇 |
2006年 | 2394篇 |
2005年 | 2066篇 |
2004年 | 2046篇 |
2003年 | 1879篇 |
2002年 | 1731篇 |
2001年 | 726篇 |
2000年 | 827篇 |
1999年 | 468篇 |
1998年 | 267篇 |
1997年 | 149篇 |
1996年 | 190篇 |
1995年 | 190篇 |
1994年 | 164篇 |
1993年 | 150篇 |
1992年 | 162篇 |
1991年 | 129篇 |
1990年 | 120篇 |
1989年 | 119篇 |
1988年 | 87篇 |
1987年 | 62篇 |
1986年 | 68篇 |
1985年 | 93篇 |
1984年 | 126篇 |
1983年 | 70篇 |
1982年 | 93篇 |
1981年 | 59篇 |
1980年 | 64篇 |
1979年 | 64篇 |
1978年 | 53篇 |
1977年 | 45篇 |
1976年 | 68篇 |
1975年 | 52篇 |
1974年 | 50篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
H. Ben Slimen F. Suchentrunk A. Memmi H. Sert U. Kryger P.C. Alves A. Ben Ammar Elgaaied 《Journal of Zoological Systematics and Evolutionary Research》2006,44(1):88-99
Systematics and taxonomy of hares of the genus Lepus (Lagomorpha) are under contentious debate, and phylogenetic relationships among many taxa are not well understood. Here we study genetic differentiation and evolutionary relationships among North African hares, currently considered subspecies of Lepus capensis , cape hares ( L. capensis ) from the Cape province in South Africa, and brown hares ( L. europeaus ) from Europe and Anatolia, using maternally (mtDNA) and biparentally (allozymes) inherited markers. A polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis of a c. 1.8 kb long segment of the mitochondrial control region using eight hexanucleotide-recognizing restriction endonucleases yielded 28 haplotypes, and horizontal starch gel electrophoresis of proteins encoded by 25 structural gene loci revealed 52 alleles at 18 polymorphic loci. Diverse phylogenetic analyses (neighbor joining dendrogram, median joining network, multidimensional scaling of pairwise distances, AMOVA, F -statistics, hierarchical F -statistics) of genetic variants revealed marked substructuring of mtDNA into three phylogeographic groups, namely an African, a central European, and an Anatolian, but a somewhat less pronounced overall differentiation of the nuclear genome, despite a relatively high number of population-specific (private) alleles. However, all our results are not incongruent with Petter's (1959: Mammalia 23 , 41; 1961: Z. f. Säugetierkunde 26 , 30; 1972 : Société Des Sciences Naturelles et Physiques du Maroc 52 , 122) hypothesis that North African hares generally belong to L. capensis and that brown hares should be included in this species as well. 相似文献
52.
After c. 30 years without sighting or capture, Anthias salmopunctatus was rediscovered at the type locality, St Peter and St Paul's Rocks, Mid-Atlantic Ridge. To date, the known distribution is restricted to depths varying from 35 to 55 m in vertical drop offs around the perimeter of the islands ( c. 400 m). This may be the smallest geographic range known for a marine fish species. 相似文献
53.
54.
In the present paper we discuss the properties of a recently developed preparation of isolated neurosecretory nerve endings obtained from the rate neurohypophysis. These nerve terminals release two neurohormones, oxytocin and vasopressin, which are easily assayed by radioimmunoassay. Depolarization-induced secretion is dependent on the same parameters as those regulating release from the whole neural lobe. The isolated nerve endings can be permeabilized by means of digitonin; a treatment which gives direct access to the cytoplasm allowing the study of the minimal requirements for inducing neuropeptide release. Furthermore, some nerve endings are large enough to allow the use of the patch-clamp technique. In the present paper we present evidences which show that the isolated neurohypophysial nerve terminals represent a protent tool for studying the mechanism of stimulus-secretion. 相似文献
55.
Ronald E. Koes Cornelis E. Spelt Jos N. M. Mol Anton G. M. Gerats 《Plant molecular biology》1987,10(2):159-169
Chalcone synthase (CHS) genes in Petunia hybrida comprise a multigene family containing at least 7 complete members in the strain Violet 30 (V30). Based on a high sequence homology in both coding and non-coding sequence, a number of CHS genes can be placed into two subfamilies. By restriction fragment length polymorphism (RFLP) analysis it was shown that both chromosomes II and V carry one of these subfamilies, in addition to the other CHS genes identified so far. Members of a subfamily were found to be closely linked genetically. Analysis of the Petunia species that contributed to the hybrid nature of P. hybrida (P. axillaris, P. parodii, P. inflata and P. violacea) shows that none of the CHS gene clusters is specific for either one of the parents and therefore did not arise as a consequence of the hybridization. The number of CHS genes within a subfamily varies considerably among these Petunia species. From this we infer that the CHS subfamilies arose from very recent gene duplications. 相似文献
56.
Photosynthetically active vesicles prepared from Chlamydomonas reinhardtii retained a light-dependent glutamate synthase activity which was highly specific for 2-oxoglutarate (Km=2.1 mM) and L-glutamine (Km=0.9 mM) as amido group acceptor and donor respectively. This activity was inhibited by azaserine, p-hydroxymercuribenzoate and 3-(p-chlorophenyl)-1,1-dimethyl urea.Light-dependent synthesis of glutamate was also obtained by coupling Chlamydomonas photosynthetic particles to purified ferredoxin-glutamate synthase, using ascorbate and 2,6-dichlorophenol-indophenol as electron donor. This system was also specific for 2-oxoglutarate (Km=1 mM) and L-glutamine (Km=0.8 mM) as substrates, and was stimulated by dithioerythritol. Azaserine and p-hydroxymercuribenzoate, but not 3-(p-chlorophenyl)-1,1-dimethyl urea, inhibited the reconstituted activity; high concentrations of 2-oxoglutarate were inhibitory.Abbreviations A
Absorbance
- CCP
p-Trichlorometoxi-carbonylcyanide-phenylhydrazone
- Chl
Chlorophyll
- CMU
3-(p-Chlorophenyl)-1,1-dimethyl urea
- DPIP
2,6-Dichlorophenol-indophenol
- DTE
Dithioerythritol
- MSX
L-Methionine, D-L, sulfoximine
- MV
Methyl viologen 相似文献
57.
Selective emergence and survival of early polypeptides in water 总被引:2,自引:0,他引:2
André Brack 《Origins of life and evolution of the biosphere》1987,17(3-4):367-379
Oligopeptides essential to primitive cells could not be obtained just by raising the background noise of organic compounds
produced by a prebiotic chemistry working at random. Selection pathways were required. Experimental evidence is given for
selective condensation of amino acids in water as well as for selective resistance to degradation. It is shown that N-carboxyanhydrides
are good candidates for chemical selection in water. They are formed when active esters of amino acids are left in the presence
of bicarbonate ions or when N,N'-carbonyldiimidazole is used as condensing agent. Polymerization of a mixture of proteinaceous
and non-proteinaceous amino acids leads to an enrichment in the proteinaceous ones plusα-aminobutyric acid. Selective resistance toward degradation ofβ-pleated sheet conformation is used to exemplify a possible accumulation of homochiral sequences made of hydrophilic and strong
hydrophobic residues. Amino acids with branched aliphatic side-chains are selected but those having short linear aliphatic
side-chains such asα-aminobutyric acid or norvaline are not. 相似文献
58.
Marco L. F. Giuseppin Hendrikus M. J. van Eijk Marja Hellendoorn José W. van Almkerk 《Applied microbiology and biotechnology》1987,27(1):31-36
Summary The changes in cell wall strength of Hansenula polymorpha have been investigated in continuous cultures with respect to the recovery of methanol oxidase (MOX). Cultures grown on several substrate mixtures that enable induction of MOX have been compared with cultures grown on methanol as the sole inducer. The effects of dilution rate (D) on lysis properties have been studied. The cell wall strength was consistently influenced by growth media and D. Media containing glycerol/methanol showed the slowest lysis kinetics, with a large fraction of non-degradable cell wall material. In continuous cultures grown on a mixture of glucose and methanol both the resistance to zymolyase and the mean cell wall thickness increased at D<0.1 h–1. The yield of MOX by zymolyase lysis is reproducible and up to 100% higher than that of the standard ultrasonic treatment. The lysis kinetics indicated that zymolyase punctures the cell wall; since the release rate of MOX is lower than that of protein, the cell contents will leak through. At D-values>0.2 h–1, both protein and MOX release rates increase, reflecting a change in lysis mechanism due to the increased fraction of thin daughter cells. Kinetic analysis of zymolyase lysis using both physical and enzymatic methods provides information for achieving optimal recovery of MOX.Abbreviations and symbols
C
MOX
MOX activity [MOX units·g X–1]
-
D
dilution rate [h–1]
- MOX
methanol oxidase
- kc
decay rate constant of A 610 nm [min–1]
- kd
decay constant of MOX activity [min–1]
- kdis
dissociation rate constant [min–1]
- kMOX
release rate constant of MOX activity [min–1]
- kp
release rate constant of protein [min–1]
- R
recovery efficiency of enzyme [-]
-
St
stability of enzyme [-] 相似文献
59.
Summary The regulation of extracellular amylase production by the basidiomycetous yeast Filobasidium capsuligenum CCY 64-5-1 was characterized using growing and resting cells. A basal level of amylolytic activity was produced with various carbon sources including glucose. Amylase secretion was repressed by glucose and, more severely, by 2-deoxy-d-glucose, whereas compounds with -1,4-linked glucose, such as methyl glucoside, maltose, -cyclodextrin and soluble starch, served as inducers. Repression was not relieved by exogenously added cAMP. The effects of several metabolic inhibitors on amylase secretion were studied. Following UV-mutagenesis a mutant strain (FC-5) capable of growing in a 2-deoxy-d-glucose supplemented corn starch medium was selected for further characterization. This strain produced more amylase, had acquired an increased resistance against repression by glucose, and retained a growth rate comparable to the wild type. FC-5 was also characterized by a reduced glucokinase activity and an increased hexokinase activity. 相似文献
60.