全文获取类型
收费全文 | 7367篇 |
免费 | 620篇 |
国内免费 | 455篇 |
专业分类
8442篇 |
出版年
2024年 | 14篇 |
2023年 | 161篇 |
2022年 | 279篇 |
2021年 | 285篇 |
2020年 | 257篇 |
2019年 | 329篇 |
2018年 | 282篇 |
2017年 | 260篇 |
2016年 | 218篇 |
2015年 | 254篇 |
2014年 | 460篇 |
2013年 | 496篇 |
2012年 | 302篇 |
2011年 | 342篇 |
2010年 | 316篇 |
2009年 | 355篇 |
2008年 | 378篇 |
2007年 | 368篇 |
2006年 | 349篇 |
2005年 | 296篇 |
2004年 | 283篇 |
2003年 | 259篇 |
2002年 | 188篇 |
2001年 | 131篇 |
2000年 | 118篇 |
1999年 | 142篇 |
1998年 | 111篇 |
1997年 | 110篇 |
1996年 | 70篇 |
1995年 | 100篇 |
1994年 | 107篇 |
1993年 | 86篇 |
1992年 | 91篇 |
1991年 | 58篇 |
1990年 | 71篇 |
1989年 | 45篇 |
1988年 | 37篇 |
1987年 | 45篇 |
1986年 | 23篇 |
1985年 | 67篇 |
1984年 | 73篇 |
1983年 | 62篇 |
1982年 | 55篇 |
1981年 | 31篇 |
1980年 | 23篇 |
1979年 | 22篇 |
1978年 | 15篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1975年 | 4篇 |
排序方式: 共有8442条查询结果,搜索用时 0 毫秒
41.
A computer-aided search for potential ribosome recognition sequences of mRNAs from tobacco chloroplasts shows that more than 90% of mRNA species contain sequences upstream of the respective initiator codons, which allow base pairing with 3′-terminal sequences of small subunit rRNA. This complementarity in several cases involves 16 S rRNA sequences between the canonical CCUCC sequence and the 3′-terminal stem/loop structure. The distances between potential ribosome recognition sequences and initiator codons can be up to 25 nucleotides which is much greater when compared to the spacing of 7±2 nucleotides observed for the classical Shine-Dalgarno sequences in bacterial mRNAs. 相似文献
42.
The electrophoretic patterns of the enzyme alcohol dehydrogenase (ADH) from Anastrepha fraterculus and A. obliqua were studied. Two loci were found to code for the enzyme in A. fraterculus, and three in A. obliqua. In both species, all isozymes were active in third-instar larvae. A cationic isozyme (Adh-1) was active mainly in the visceral fat body of both species. In A. fraterculus, the locus had an anionic polymorphic isozyme (Adh-3) that was detected in the parietal fat body. In addition to these two loci, a third locus for an anionic isozyme (Adh-2), which was active in the digestive tube of larvae, was present in A. obliqua and probably resulted from gene duplication. For both species, multiple forms of the isozymes are formed by binding of an NAD-carbonyl compound, as in Drosophila melanogaster. Both larvae and early pupae of A. obliqua had almost twice the specific ADH activity as A. fraterculus. The ethanol content of the host fruit infested with A. obliqua (red mombim) was also higher than that of the host fruit infested with A. fraterculus (guava).This research was supported by grants from Conselho Nacional de Desenvolvimento Científico e Tecnologico (CNPq-PIG 40.2486/82). 相似文献
43.
Claudio Nicolini Andrew S. Belmont Antonietta Martelli 《Cell biochemistry and biophysics》1986,8(2):103-117
Using HeLa S-3 cells synchronized by selective detachment, in this paper we report a parallel study of nuclear morphology
and autoradiography grain patterns between middle G1 and middle S phases: Our results show two distinct [3H]-thymidine labeling patterns. The first “peripheral” labeling pattern has a characteristic nuclear size distribution, in
contrast to the heterogeneous and varying size distributions of Feulgen-stained nuclei, and apparently is characteristic of
very early S phase. The sizes of the second labeling pattern—homogeneous or inhomogeneous grain distribution throughout the
nucleus—are equal or larger than the first and vary with S phase progression. Together, the corresponding nuclear sizes of
the labeled nuclei represent the larger extreme of nuclear areas, and the labeling index closely parallels the fraction of
nuclei with areas larger than the minimum size of the labeled nuclei. These results suggest a characteristic nuclear size
(reflecting unique intranuclear DNA distribution) as a necessary, if not sufficient, requirement for S phase initiation. Parallel
experimentation with rat liver cells—synchronized in vivo by partial hepatectomy and analyzed by thin section autoradiography—confirms
the existence of a peripheral labeling pattern in both the very early part and the very late part of S phase, which reconciles
our data with previous results and points to the fact that both initiation and termination sites for DNA replication are near
the nuclear periphery. 相似文献
44.
W Ferro 《Mutation research》1985,149(3):399-408
We investigated larval sensitivity to UV and repair of UV- and X-ray-induced lesions in the DNA of the ebony strain compared to a wild-type strain (Canton S). The ebony strain was previously characterized as being more sensitive to UV-induced killing of embryos than Canton S. Also the ebony strain is more sensitive to X-rays for induction of larval killing, dominant lethals and recessive lethals. In this paper it is demonstrated that (1) ebony larvae are more sensitive to killing by UV and less proficient in photoreactivation (PR) ability than Canton S larvae; (2) the ebony strain has a defect in PR repair of endonuclease-sensitive sites induced in the DNA of primary cell cultures by UV irradiation; (3) the ebony strain has a defect in the repair of single-strand breaks induced in the DNA by X-rays (again in primary cell cultures), at least early on in the repair incubation. A rough localization of the UV sensitivity and the PR ability is presented and the possible relevance of the biochemical to the genetic results is discussed. 相似文献
45.
In field experiments, larvae of codling moth Cydia pomonella (L.) rarely acquired granulosis virus on hatching from the egg, but picked up most later, on the tree surface. Deposits of virus sprayed in 1.0% w/v skimmed milk did not affect neonate larval behaviour. Larvae died, usually in the first instar, after entering treated fruit, but they frequently entered via the calyx or near the base of the stalk or through cracks in the skin, where little feeding damage by first-and sometimes second-instar larvae was seen.
Résumé En verger, la pulvérisation d'oeufs de carpocapse avec du virus de la granulose en suspension dans l'eau (additionnée de lait écrémé dilué à 1%) n'a pas modifié la survie des chenilles avant pénétration dans le fruit; par contre la pulvérisation des arbres a provoqué une forte mortalité. Bien que des chenilles consommant des poils et la surface des feuilles aient été observées avant leur pénétration dans le fruit, ce qui aurait pu provoquer leur contamination par le virus, il semble que la contamination létale provienne des fruits seuls.La présence de produit n'a modifié ni le comportement larvaire, ni le taux de pénétration dans les fruits; la mortalité y a lieu ensuite, généralement au premier stade. Dans 74 à 78% des cas, les chenilles ont pénétré dans le fruit par le calice ou près de la base du pédoncule — aucun dégât provenant de larves du premier stade n'y était visible, de même que dans le calice pour les larves du deuxième stade. Par contre, toute pénétration par la surface du fruit était repérable dès le premier stade. Il est possible que la répartition des lieux de pénétration dans le fruit influe sur la létalité due au virus et explique les variations d'efficacité observées en verger. Un système de classification des dégâts, provoqués lors de la pénétration dans le fruit, de chenilles du premier au troisième stade est proposé pour évaluer l'efficacité des essais en verger.相似文献
46.
Variation in larval black fly populations at three sites in a stream system over five years (Diptera:Simuliidae) 总被引:1,自引:1,他引:0
M. H. Colbo 《Hydrobiologia》1985,121(1):77-82
Five years of monthly samples taken from three sites in the Broad Cove River, two at lake outlets and one a kilometer downstream from an outlet were analyzed. Considerable variations in the population of each species between sites and years were observed. Fluctuations in the populations were not always consistent from site to site or between species at the same site from one year to the next. As all sites were on the same stream and within 3 km of each other with only slight altitude differences climatic factors were not a major factor producing variations observed between sites. Also the previous years population level did not appear to be directly related to the following seasons population level. Apparently localized conditions at a site were exerting a marked influence on the larval simuliid population at each site. 相似文献
47.
k-Binding and Degradation of [3 H]Dynorphin A (1–8) and [3 H]Dynorphin A (1–9) in Suspensions of Guinea Pig Brain Membranes 总被引:3,自引:0,他引:3
Maureen G. C. Gillan Linda E. Robson Alexander T. McKnight Hans W. Kosterlitz 《Journal of neurochemistry》1985,45(4):1034-1042
Following incubation of [3H]dynorphin A (1-8) and [3H]dynorphin A (1-9) with suspensions of guinea pig brain membranes, analysis of the supernatants by HPLC has shown that both peptides are degraded at 25 degrees C and at 0 degrees C. Bestatin and captopril reduce degradation at 0 degrees C but for a similar degree of protection at 25 degrees C arginine-containing dipeptides are also required. The effects of these peptidase inhibitors on the degradation profiles indicate that [3H]dynorphin A (1-8) has three main sites of cleavage: the Tyr1-Gly2, Arg6-Arg7, and Leu5-Arg6 bonds. With [3H]dynorphin A (1-9) as substrate the Arg7-Ile8 and Ile8-Arg9 bonds are also liable to cleavage. In binding assays, in contrast to the effects of peptidase inhibitors on the degradation of unbound [3H]dynorphin A (1-8) and [3H]dynorphin A (1-9), bestatin and captopril have little effect on the binding characteristics of the tritiated dynorphin A fragments at the kappa-site at 0 degrees C. However, at 25 degrees C binding is low in the absence of peptidase inhibitors. When binding at mu- and delta-sites is prevented, the maximal binding capacities of [3H]dynorphin A (1-8), [3H]dynorphin A (1-9), and [3H](-)-bremazocine at the kappa-site are similar; [3H]dynorphin A (1-9) has 5-10 times higher affinity for the kappa-site than [3H]dynorphin A (1-8). Comparison of the effects of peptidase inhibitors on unbound dynorphin A fragments with their effects in binding assays suggests that the bound peptides are protected from the action of peptidases. 相似文献
48.
Samuel H. K. Ho Uttam Das Gupta John S. Rieske 《Journal of bioenergetics and biomembranes》1985,17(5):269-282
Deformamidoazidoantimycin A (DAA), a photoactive derivative of antimycin A containing an azido group substituting for the formamido group attached to the phenyl ring, was synthesized. The ultraviolet spectrum of DAA was almost identical to that of antimycin A, indicating little alteration of the electronic structure of the substituted phenyl ring by the azido substitution. However, the inhibitory effectiveness of DAA toward ubiquinol-cytochromec reductase (Complex III) purified from bovine heart (K
i
=ca. 0.5 µM) was considerably less than that of antimycin (K
i
3 pM), indicating a direct rather than a supporting role of the formamido group in the inhibitory activity of antimycin. Exposure of purified Complex III to [3H]DAA plus ultraviolet light caused a major labeling by tritium of SDS-PAGE band 7 (m=13 kDa by SDS-PAGE) and lesser but significant labeling of bands 3, 6, 8, and 9. Pretreatment of Complex III with antimycin greatly suppressed the labeling of bands 5, 6, and 7 but caused an apparent increased labeling of bands 8 and 9 by [3H]DAA, respectively. The labeling of band 7 by [3H]DAA also was strongly suppressed by reduction of Complex III by either sodium borohybride or ascorbate. Based on magnitude of labeling by [3H]DAA and the degree of suppression of labeling by antimycin, the protein of band 7 qualified as the principal component for specific binding of antimycin with the protein of band 6 (m=16 kDa) showing a lesser but significant amount of specific binding. 相似文献
49.
The pyr4 gene of Neurospora crassa, which codes for orotidine-5'-phosphate decarboxylase, is capable of transforming an Aspergillus nidulans pyrG mutant by chromosomal integration, despite low homology between the transforming DNA and the recipient genome. Integration of pFB6, a plasmid carrying pyr4 and capable of replication in Escherichia coli, was not observed at the pyrG locus. The efficiency of transformation was considerably enhanced (50-100 fold) by inclusion in the transforming vector of a 3.5-kb A.nidulans chromosomal sequence, ans1. Although this sequence was isolated on the basis of replicating activity in Saccharomyces cerevisiae, there was no evidence for such activity in A.nidulans. Part of the ans1 fragment appears to be reiterated in the A.nidulans genome, though it is not yet clear whether this is directly responsible for the high transformation frequency. The efficiency of transformation of A.nidulans by plasmids bearing ans1, using an improved protocol, was approx. 5 X 10(3) stable transformants per microgram of plasmid DNA. 相似文献
50.