全文获取类型
收费全文 | 617篇 |
免费 | 67篇 |
专业分类
684篇 |
出版年
2022年 | 7篇 |
2021年 | 9篇 |
2020年 | 8篇 |
2018年 | 11篇 |
2017年 | 6篇 |
2016年 | 11篇 |
2015年 | 19篇 |
2014年 | 17篇 |
2013年 | 18篇 |
2012年 | 30篇 |
2011年 | 35篇 |
2010年 | 15篇 |
2009年 | 20篇 |
2008年 | 17篇 |
2007年 | 23篇 |
2006年 | 16篇 |
2005年 | 17篇 |
2004年 | 23篇 |
2003年 | 24篇 |
2002年 | 18篇 |
2001年 | 17篇 |
2000年 | 14篇 |
1999年 | 13篇 |
1998年 | 11篇 |
1995年 | 8篇 |
1994年 | 8篇 |
1992年 | 6篇 |
1991年 | 10篇 |
1990年 | 10篇 |
1989年 | 13篇 |
1988年 | 13篇 |
1987年 | 5篇 |
1986年 | 14篇 |
1985年 | 9篇 |
1984年 | 12篇 |
1983年 | 11篇 |
1982年 | 7篇 |
1981年 | 12篇 |
1980年 | 6篇 |
1979年 | 8篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1975年 | 14篇 |
1974年 | 8篇 |
1973年 | 10篇 |
1972年 | 7篇 |
1970年 | 5篇 |
1969年 | 8篇 |
1968年 | 6篇 |
排序方式: 共有684条查询结果,搜索用时 0 毫秒
1.
One particular kind of structure offers possible explanations, for long-term memory, efficient consolidation of stored information
from the environment, clustering of data strings and multimodal functioning. It is a possible model for pieces of neural structure
and its use offers a uniform method for both studying and constructing an extensive class of mechanisms. 相似文献
2.
3.
4.
Plasmodium falciparum: gene structure and hydropathy profile of the major merozoite surface antigen (gp195) of the Uganda-Palo Alto isolate 总被引:13,自引:0,他引:13
S P Chang K J Kramer K M Yamaga A Kato S E Case W A Siddiqui 《Experimental parasitology》1988,67(1):1-11
The gene encoding the 195,000-Da major merozoite surface antigen (gp195) of the FUP (Uganda-Palo Alto) isolate of Plasmodium falciparum, a strain widely used for monkey vaccination experiments, has been cloned and sequenced. The translated amino acid sequence of the FUP gp195 protein is closely related to the sequences of corresponding proteins of the CAMP (Malaysia) and MAD-20 (Papua New Guinea) isolates and more distantly related to those of the Wellcome (West Africa) and K1 (Thailand) isolates, supporting the proposed allelic dimorphism of gp195 within the parasite population. The prevalence of dimorphic sequences within the gp195 protein suggests that many gp195 epitopes would be group-specific. Despite the extensive differences in amino acid sequence between gp195 proteins of these two groups, the hydropathy profiles of proteins representative of both groups are very similar. The conservation of overall secondary structure shown by the hydropathy profile comparison indicates that gp195 proteins of the various P. falciparum isolates are functionally equivalent. This information on the primary structure of the FUP gp195 protein will enable us to evaluate the possible roles of conserved, group-specific and variable epitopes in immunity to the blood stage of the malaria parasite. 相似文献
5.
The coexistence and coevolution of sexual and asexual species under resource competition are explored with three models: a nongenetic ecological model, a model including single locus genetics, and a quantitative-genetic model. The basic assumption underlying all three models is that genetic differences are translated into ecological differences. Hence if sexual species are genetically more variable, they will be ecologically more variable. Under classical competition theory, this increased ecological variability can, in many cases, be an advantage to individual sexual genotypes and to the sexual species as a whole. The purpose of this paper is to determine the conditions when this advantage will outway three disadvantages of sexuality: the costs of males, of segregation, and of the additive component of recombination. All three models reach similar conclusions. Although asexuality confers an advantage, it is much less than a two-fold advantage because minor increases in the overall species niche width of the sexual species will offset the reproductive advantage of the asexual species. This occurs for two reasons. First, an increase in species niche width increases the resource base of the sexual species. Second, to the extent that the increase in niche width is due to increased differences between individuals, a reduction in intraspecific competition will result. This is not to imply that the sexual species will always win. The prime conditions that enable sexual species to stably coexist with or even supplant an asexual sister species are:
- 相似文献
6.
R. M. Case D. I. Cook M. Hunter M. C. Steward J. A. Young 《The Journal of membrane biology》1985,84(3):239-248
Summary The characteristics of nonelectrolyte secretion by the rabbit mandibular salivary gland have been investigated in anin vitro perfused preparation. The concentrations of14C-labeled nonelectrolytes were measured in saliva samples collected over a range of flow rates during the secretory response of the gland to continuous acetylcholine infusion. Of the nine nonelectrolytes studied, the two particularly lipid-soluble molecules, ethanol and antipyrine, appeared in the saliva at approximately the same concentration as in the perfusate, regardless of the secretory flow rate. The more polar molecules (urea, ethanediol, thiourea, glycerol, erythritol, mannitol and sucrose) appeared at saliva/perfusate concentration ratios () which showed a strong dependence on flow. With the exception of thiourea, this could be attributed to the combined contributions of diffusion and solvent drag.For the polar nonelectrolytes, estimates have been obtained of both the permeability coefficients of the gland (P) and the solvent-drag filtration coefficients (1–). The relation between 1– and molecular radius suggests that small polar nonelectrolytes and the bulk of the secreted water cross the epithelium via aqueous channels that are approximately 0.8 nm in width. The location of the channels remains uncertain because tissue space measurements indicate that the nonelectrolytes most affected by solvent drag have access to both transcellular and paracellular pathways. 相似文献
7.
A second gene has been discovered at a previously studied Balbiani ring in Chironomus. Northern hybridizations demonstrated that cDNA clone pCt35 originated from a salivary gland specific 6.5-kilobase (kb) RNA that was abundant, nonribosomal, and apparently poly(A)+. pCt35 had a 120-base pair (bp) insert with 1.6 copies of a 75-bp sequence that contained two open reading frames. Southern hybridizations indicated that pCt35 was homologous to at least a 4-kb block of genomic DNA organized as a hierarchy of 150- and 300-bp tandem repeats. In situ hybridization localized these sequences to Balbiani ring 1. From these results we postulated that a 6.5-kb RNA gene may have evolved by stepwise duplication and amplification of a 75-bp ancestral sequence. 相似文献
8.
Revertants and Secondary arom-2 Mutants Induced in Non-Complementing Mutants in the arom Gene Cluster of Neurospora Crassa 总被引:7,自引:0,他引:7
下载免费PDF全文
![点击此处可从《Genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Extensive genetical and biochemical studies have been performed with revertants and secondary arom-2 mutants induced in two different primary non-complementing mutants which map within the arom gene cluster of Neurospora crassa. These studies indicate that mutant M54 but not M25 can revert by super-suppressor mutations in unlinked genes, thus confirming previous evidence that M54 contains a nonsense codon. At least three new super suppressors of M54 have been detected. All four super suppressors (including one previously detected) when combined with M54 result in high levels of all five of the arom enzymic activities in the form of arom multienzyme complexes very similar to (but not necessarily identical with) that in wild type (WT).-Evidence has also been obtained that the two non-complementing mutants can yield revertants which appear to result from true back mutations and produce arom aggregates essentially indistinguishable from that of WT. In addition, M25, but not M54, when plated on quinic acid yields revertants (secondary mutants) some of which are phenotypically indistinguishable from arom-2 primary mutants and others of which, although also mapping within the arom-2 gene, exhibit unusual properties. Genetic evidence indicates that the M25 secondary mutants are localized within the arom-2 gene, but that they arise from mutational events more complex than ones resulting in single base pair changes in the M25 codon.-The recovery of secondary arom-2 mutants as revertants of non-complementing arom mutants provides strong evidence, independent of earlier recombination data, that non-complementing arom mutants are located within the arom-2 structural gene of the arom gene cluster. In addition, the occurrence and characteristics of these secondary arom-2 mutants provide strong evidence, independent of the results with nonsense suppressors, that the arom gene cluster is transcribed, beginning with the arom-2 gene, as a single polycistronic messenger ribonucleic acid (mRNA) molecule which is subsequently translated into the arom multienzyme complex. 相似文献
9.
At relatively high currents, fused anion-cation membranes give rise to rectifying and reactive effects. The rectification becomes less pronounced with increasing frequency. This effect results from changes in the concentration profiles of the ions during the positive and negative phases of the AC cycle. With reduction of the current, the voltage-current response becomes linear. The reactive effect can then be separated from the rectifying effect. The former effect can be attributed essentially to two factors: (a) the presence of transition regions of fixed charge and (b) the diffusion mechanism of the ions in an AC field. The first factor is largely frequency-independent and the second, frequency-dependent. A first approximation equivalent circuit is described. This circuit involves frequency-dependent elements. 相似文献
10.
The mutation site in hemoglobin Rothschild (37 beta Trp----Arg) is located in the "hinge region" of the alpha 1 beta 2 interface, a region that is critical for normal hemoglobin function. The mutation results in greatly reduced cooperativity and an oxygen affinity similar to that of hemoglobin A [Gacon, G., Belkhodja, O., Wajcman, H., & Labie, D. (1977) FEBS Lett. 82, 243-246]. Crystal were grown under "low-salt" conditions [100 mM Cl- in 10 mM phosphate buffer at pH 7.0 with poly(ethylene glycol) as a precipitating agent]. The crystal structure of deoxyhemoglobin Rothschild and the isomorphous crystal structure of deoxyhemoglobin A were refined at resolutions of 2.0 and 1.9 A, respectively. The mutation-induced structural changes were partitioned into components of (1) tetramer rotation, (2) quaternary structure rearrangement, and (3) deformations of tertiary structure. The quaternary change involves a 1 degree rotation of the alpha subunit about the "switch region" of the alpha 1 beta 2 interface. The tertiary changes are confined to residues at the alpha 1 beta 2 interface, with the largest shifts (approximately 0.4 A) located across the interface from the mutation site at the alpha subunit FG corner-G helix boundary. Most surprising was the identification of a mutation-generated anion-binding site in the alpha 1 beta 2 interface. Chloride binds at this site as a counterion for Arg 37 beta. The requirement of a counterion implies that the solution properties of hemoglobin Rothschild, in particular the dimer-tetramer equilibrium, should be very dependent upon the concentration and type of anions present. 相似文献