全文获取类型
收费全文 | 2576篇 |
免费 | 275篇 |
国内免费 | 110篇 |
专业分类
2961篇 |
出版年
2024年 | 6篇 |
2023年 | 44篇 |
2022年 | 55篇 |
2021年 | 89篇 |
2020年 | 102篇 |
2019年 | 144篇 |
2018年 | 127篇 |
2017年 | 98篇 |
2016年 | 113篇 |
2015年 | 106篇 |
2014年 | 158篇 |
2013年 | 214篇 |
2012年 | 116篇 |
2011年 | 153篇 |
2010年 | 120篇 |
2009年 | 127篇 |
2008年 | 133篇 |
2007年 | 139篇 |
2006年 | 108篇 |
2005年 | 87篇 |
2004年 | 89篇 |
2003年 | 63篇 |
2002年 | 75篇 |
2001年 | 42篇 |
2000年 | 59篇 |
1999年 | 39篇 |
1998年 | 50篇 |
1997年 | 27篇 |
1996年 | 46篇 |
1995年 | 35篇 |
1994年 | 37篇 |
1993年 | 28篇 |
1992年 | 25篇 |
1991年 | 18篇 |
1990年 | 11篇 |
1989年 | 16篇 |
1988年 | 9篇 |
1987年 | 10篇 |
1986年 | 8篇 |
1985年 | 6篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1973年 | 1篇 |
1972年 | 2篇 |
排序方式: 共有2961条查询结果,搜索用时 15 毫秒
11.
The nest site characteristics of the freshwater three-spined stickleback, Gasterosteus aculeatus (form leiurus ), were quantitatively investigated in springs and the main stream of the Yamayoke and the Tsuya River system, central Japan. Most nests (93·4%) were on a muddy or sandy substratum, at depths of 10–40 cm (84·3%), in water velocities less than 15 cm s−1 (76·2%) and in the temperature range of 14 to 16° C (82·7%), Spring-fed localities provided more of these conditions than the main stream channel and hence contained more potential nesting areas. Thus, they were utilized by male sticklebacks both temporally (prolonged breeding season) and spatially (more nest sites). 相似文献
12.
Information analysis by the paper wasp,Polistes fuscatus,during nest construction (Hymenoptera,Vespidae) 总被引:1,自引:0,他引:1
H. A. Downing 《Insectes Sociaux》1994,41(4):361-377
Summary The building decision process of the paper wasp,Polistes fuscatus, was studied by 1) analyzing the search pattern of the wasps prior to the addition of pulp to different areas of the nest, 2) comparing the pulp addition needs of the cell chosen for lengthening to those of other cells in the nest, and 3) presenting the wasps with eight types of dichotomous building choices, which provided information about the relative influence of different building cues. Wasps conduct a hierarchical search prior to pulp addition, which means that they search the comb face and petiole disproportionately more often and more thoroughly than the comb back and sides. Once a particular nest area triggers closer scrutiny, comparisons are made with adjoining areas. The most needy location is then chosen based on nest cues. When lengthening a cell, the development of the brood and relative cell length have a strong influence on which cell is chosen at all times, while distance of the brood from the cell mouth becomes important during the later stages of brood development. The results indicate that there is no simple hierarchical weighting of cues. The decision process involves comparisons among multiple cues, which for the most part have an additive influence when variation in relative cue strength is considered. 相似文献
13.
Summary Excavation of 18 nests ofHarpegnathos saltator from southern India revealed an unusually complex architecture for a ponerine ant. The inhabited chambers are not deep in the ground. The uppermost chamber is protected by a thick vaulted roof, on the outside of which is an intervening space serving as isolation from the surrounding soil. In large colonies, the vaulted roof is extended into a shell which encloses several superimposed chambers. Little openings, which may be encircled by moulded flanges, occur in the upper region of the shell. The inside of the chambers is partly or completely lined with strips of empty cocoons. A refuse chamber is always found deeper than the inhabited chambers; live dipteran larvae (family Milichiidae) are typically present. These elaborate nests represent a large energetic investment, and we speculate therefore that nest emigration is unlikely in this species. Consequently, colony fission may never occur, unlike other ants where gamergates reproduce. 相似文献
14.
Hiromi Maekawa Tomoko Nakagawa Yoko Uno Kenji Kitamura Chikashi Shimoda 《Molecular & general genetics : MGG》1994,244(5):456-464
When the fission yeastSchizosaccharomyces pombe is starved for nitrogen, the cells are arrested in the G1 phase, enter the G0 phase and initiate sexual development. Theste13 mutant, however, fails to undergo a G1 arrest when starved for nitrogen and since this mutant phenotype is not suppressed by a mutation in adenylyl cyclase (cyr1), it would appear thatste13
+ either acts independently of the decrease in the cellular cAMP level induced by starvation for nitrogen, or functions downstream of this controlling event. We have used functional complementation to clone theste13
+ gene from anS. pombe genomic library and show that its disruption is not lethal, indicating that, while the gene is required for sexual development, it is not essential for cell growth. Nucleotide sequencing predicts thatste13
+ should encode a protein of 485 amino acids in which the consensus motifs of ATP-dependent RNA helicases of the DEAD box family are completely conserved. Point mutations introduced into these consensus motifs abolished theste13
+ functions. The predicted Ste13 protein is 72% identical to theDrosophila melanogaster Me31B protein over a stretch of 391 amino acids. ME31B is a developmentally regulated gene that is expressed preferentially in the female germline and may be required for oogenesis. Expression of ME31B cDNA inS. pombe suppresses theste13 mutation. These two evolutionarily conserved genes encoding putative RNA helicases may play a pivotal role in sexual development. 相似文献
15.
Summary We have examined the 13C and 13C chemical shifts of a number of proteins and found that their values at the N-terminal end of a helix provide a good predictor for the presence of a capping box. A capping box consists of a hydrogen-bonded cycle of four amino acids in which the side chain of the N-cap residue forms a hydrogen bond with the backbone amide of the N3 residue, whose side chain in turn may accept a hydrogen bond from the amide of the N-cap residue. The N-cap residue exhibits characteristic values for its backbone torsion angles, with and clustering around 94±15° and 167±5°, respectively. This is manifested by a 1–2 ppm upfield shift of the 13C resonance and a 1–4 ppm downfield shift of the 13C resonance, relative to their random coil values, and is mainly associated with the unusually large value of . The residues following the N-cap residue exhibit downfield shifts of 1–3 ppm for the 13C resonances and small upfield shifts for the 13C ones, typical of an -helix. 相似文献
16.
17.
Piotr Orlewski Vassilios Tsikaris Maria Sakarellos-Daitsiotis Constantinos Sakarellos Ketty P. Soteriadou Michel Marraud Manh Thong Cung 《Letters in Peptide Science》1996,3(5):317-326
Summary The IASRYDQL synthetic octapeptide (250–257) of the Leishmania major surface glycoprotein gp63 efficiently inhibits parasite attachment to the macrophage receptors in in vitro experiments, and the SRYD-containing tetrapeptide mimics antigenically and functionally the RGDS sequence of fibronectin. The conformational properties of the octapeptide were investigated in dimethylsulfoxide (DMSO) with the combined use of NMR data (vicinal coupling constants, nuclear Overhauser effects (NOEs) and temperature coefficient values), molecular modeling by energy minimization and molecular dynamics. The structure is characterized by the high occurrence, exceeding 95%, of the Arg-Asp side-chain-side-chain ionic interaction, which plays a key role in the backbone folding through a distorted type-I -turn involving the Gln256-NH to Arg253-CO hydrogen bond. 相似文献
18.
Thomas Haaf A. Gregory Mater Johannes Wienberg David C. Ward 《Journal of molecular evolution》1995,41(4):487-491
CENP-B, a highly conserved centromere-associated protein, binds to -satellite DNA, the centromeric satellite of primate chromosomes, at a 17-bp sequence, the CENP-B box. By fluorescence in situ hybridization (FISH) with an oligomer specific for the CENP-B box sequence, we have demonstrated the abundance of CENP-B boxes on all chromosomes (except the Y) of humans, chimpanzee, pygmy chimpanzee, gorilla, and orangutan. This sequence motif was not detected in the genomes of other primates, including gibbons, Old and New World monkeys, and prosimians. Our results indicate that the CENP-B box containing subtype of -satellite DNA may have emerged recently in the evolution of the large-bodied hominoids, after divergence of the phylogenetic lines leading to gibbons and apes; the box is thus on the order of 15–25 million years of age. The rapid process of dispersal and fixation of the CENP-B box sequence throughout the human and great ape genomes is thought to be a consequence of concerted evolution of -satellite subsets on both homologous and nonhomologous chromosomes.Correspondence to: T. Haaf 相似文献
19.
Distribution of Pax6 protein during eye development suggests discrete roles in proliferative and differentiated visual cells 总被引:6,自引:0,他引:6
Although Pax6 is required during eye development in rodents and humans, little is known about the precise role of the protein
in this process. To aid in the interpretation of functional studies, we have determined the precise spatial and temporal distributions
of the Pax6 protein in the eye. We find that Pax6 is initially distributed contiguously throughout a large domain of the anterior
neural plate of zebrafish, including the presumptive eye fields and the dorsal diencephalon. After evagination of the optic
vesicle, Pax6 becomes restricted to all proliferating cells of the pigment epithelial and neural layers of the retina. Pax6
is downregulated in most cells concomitant with differentiation. However, it remains present in several mature cell types
of the eye including amacrine cells and the lens and corneal epithelia. This expression is conserved across diverse vertebrate
species and suggests that Pax6 has additional conserved functions in the mature eye.
Received: 27 August 1996 / Accepted: 21 October 1996 相似文献
20.
The use of a partition locus to increase stability of tryptophan-operon-bearing plasmids in Escherichia coli 总被引:6,自引:0,他引:6
The stability of different derivatives of plasmid vectors pBR322 and pACYC184 carrying the tryptophan operon of Escherichia coli was monitored in various media. It was found that in the absence of any special selective pressure, all plasmids were lost from the culture. The stability varied depending both on the orientation of the inserted tryptophan fragment and the growth media used. The pBR322::trp+ plasmids were lost at an average frequency of 0.3 to 0.8% per cell generation, while the pACYC184::trp+ plasmid was lost at a rate higher than 5%. In all cases the whole plasmid was lost at a rate higher than 5%. In all cases the whole plasmid was lost, indicating a high stability of the plasmid::cloned DNA as such. To increase the stability of the cloning vectors, the partition locus of plasmid pSC101 was added to both the pBR322::trp+ and pACYC184::trp+ plasmids. The addition of this gene increased the replicon stability at least 3- to 10-fold, with the pBR322::trp+-par+ plasmids being the most stable. Also in this case, the stability was dependent on the plasmid type and on the growth medium. In no case was there a discoordinate loss of the antibiotic-resistance and tryptophan genes from the vectors. 相似文献