全文获取类型
收费全文 | 4181篇 |
免费 | 450篇 |
国内免费 | 5篇 |
专业分类
4636篇 |
出版年
2021年 | 43篇 |
2019年 | 29篇 |
2018年 | 48篇 |
2017年 | 31篇 |
2016年 | 69篇 |
2015年 | 103篇 |
2014年 | 137篇 |
2013年 | 156篇 |
2012年 | 229篇 |
2011年 | 195篇 |
2010年 | 143篇 |
2009年 | 124篇 |
2008年 | 211篇 |
2007年 | 190篇 |
2006年 | 187篇 |
2005年 | 201篇 |
2004年 | 197篇 |
2003年 | 179篇 |
2002年 | 167篇 |
2001年 | 73篇 |
2000年 | 58篇 |
1999年 | 70篇 |
1998年 | 50篇 |
1997年 | 47篇 |
1996年 | 39篇 |
1995年 | 40篇 |
1993年 | 42篇 |
1992年 | 47篇 |
1991年 | 58篇 |
1990年 | 53篇 |
1989年 | 47篇 |
1988年 | 40篇 |
1987年 | 32篇 |
1985年 | 30篇 |
1984年 | 36篇 |
1983年 | 51篇 |
1982年 | 44篇 |
1981年 | 50篇 |
1980年 | 44篇 |
1979年 | 50篇 |
1978年 | 48篇 |
1977年 | 30篇 |
1976年 | 43篇 |
1975年 | 39篇 |
1974年 | 49篇 |
1973年 | 32篇 |
1972年 | 32篇 |
1971年 | 28篇 |
1970年 | 33篇 |
1969年 | 31篇 |
排序方式: 共有4636条查询结果,搜索用时 0 毫秒
11.
Shio Jean Lin Christine Figueiredo Leonard J. Sciorra Ming-liang Lee 《Human genetics》1987,76(2):173-175
Summary The induction of fragile sites on human chromosomes has been demonstrated under various conditions that cause thymidylate stress, including exposure to uridine. In this study, we examined common fragile site expression by initially exposing peripheral lymphocytes to uridine, followed by repair of the fragile sites with media containing various concentrations of thymidine. Lymphocytes were cultured in medium 199 with 2 mM uridine. At 0.5, 1, 2, 3, 8, 10, 12, and 18 h before harvest, the uridine medium was removed and replaced by medium containing thymidine at various concentrations. Our results demonstrate that the effect of uridine on chromosome fragility can be reversed by low concentrations of thymidine (2 M up to 200 M) and the rescuing effect of thymidine can be achieved if the cells were treated prior to 2–3 h before harvest. No repair was found if thymidine was added to culture within 2 h prior to harvesting, suggesting that packing of chromosomes is also an important factor in the expression and repair of fragile sites. 相似文献
12.
Three-dimensional reconstruction of maltoporin from electron microscopy and image processing. 总被引:5,自引:0,他引:5 下载免费PDF全文
J Lepault B Dargent W Tichelaar J P Rosenbusch K Leonard F Pattus 《The EMBO journal》1988,7(1):261-268
Two dimensional crystals of maltoporin (or phage lambda receptor) were obtained by reconstitution of purified maltoporin trimers and Escherichia coli phospholipids by detergent dialysis. Two different trimer packing forms were observed. One was hexagonal (a = 7.8 nm) and one rectangular (a = 7.8 nm, b = 13.6 nm). In this paper we describe the three-dimensional structure of maltoporin, deduced from the study of the rectangular form by electron microscopy and image processing. At a resolution of approximately 2.5 nm, maltoporin trimers form aqueous channel triplets which appear to merge into a single outlet at the periplasmic surface of the outer membrane. The pore defined by maltoporin has a similar structure to that outlined by the matrix protein. From the results of functional studies by conductance measurement, it is concluded that the three channels defined by maltoporin act, contrary to those formed by the porin (OmpF protein), as a single conducting unit. A tentative outline of the maltoporin promoter is given. Maltoporin appears to be constituted by three different domains: a major rod-like domain spanning the membrane, a minor domain located near the periplasmic surface of the membrane and finally a central domain responsible for the splitting of the channel. 相似文献
13.
Chromosomal aberrations and sister-chromatid exchanges in lymphocytes from coke oven workers 总被引:2,自引:0,他引:2
To test whether coke oven workers, an occupational group known to be at increased cancer risk, manifest increased peripheral blood chromosomal aberration frequencies, we obtained samples from a group of 30 steelworker volunteers, who had worked several years at coke oven jobs. Exposure estimates were made using measurements of work place atmospheric coal tar pitch volatiles and work histories. No statistically significant positive regression of chromosomal aberrations on exposure estimates was found. The data from the coke oven workers were also compared with the obtained concurrently and employing precisely the same laboratory protocol from a group of male Brookhaven National Laboratory employees. The coke oven workers as a group were found to have statistically significantly elevated frequencies of chromatid aberrations and of sister-chromatid exchanges. 相似文献
14.
Nitrate transport in excised corn (Zea mays L.) roots was inhibited by phenylglyoxal, but not by 4,4′-diisothiocyano-2,2′-stilbene disulfonic acid (DIDS) or fluorescein isothiocyanate (FITC). Inhibition of nitrate uptake by a 1-hour treatment with 1 millimolar phenylglyoxal was reversed after 3 hours, which was similar to the time needed for induction of nitrate uptake. If induction of nitrate uptake occurs by de novo synthesis of a nitrate carrier, then the resumption of nitrate uptake in the inhibitor-treated roots may occur because of turnover of phenylglyoxal-inactivated nitrate carrier proteins. All three chemicals inhibited chloride uptake to varying degrees, with FITC being the strongest inhibitor. While inhibition due to DIDS was reversible within 30 minutes, both FITC and phenylglyoxal showed continued inhibition of chloride uptake for up to 3 hours after removal from the uptake solution. Assuming that the anion transporter polypeptide(s) carries a positive charge density at or near the transport site, the results indicate that the nitrate carrier does not carry any lysyl residues that are accessible to DIDS or FITC, whereas the chloride carrier does. Both chloride and nitrate carriers, however, seem to possess arginyl residues that are accessible to phenylglyoxal. 相似文献
15.
A nerve growth factor-regulated messenger RNA encodes a new intermediate filament protein 总被引:23,自引:7,他引:16 下载免费PDF全文
Differential screening of a cDNA library from the PC12 rat pheochromocytoma cell line previously revealed a clone, clone 73, whose corresponding mRNA is induced by nerve growth factor (NGF). Induction parallels NGF-stimulated PC12 differentiation from a chromaffinlike phenotype to a sympathetic neuronlike phenotype. We report that DNA sequence analysis reveals that clone 73 mRNA encodes an intermediate filament (IF) protein whose predicted amino acid sequence is distinct from the known sequences of other members of the IF protein family. The sequence has highest homology with desmin and vimentin and includes the highly conserved central alpha-helical rod domain with the characteristic heptad repeat of hydrophobic residues, but has lower homology in the amino-terminal head and carboxyl-terminal tail domains. The head domain contains a large number of serine residues which are potential phosphorylation sites. The expression of clone 73 in vivo in the nervous system of the adult rat was investigated by in situ hybridization of clone 73 probes to tissue sections. The mRNA is expressed at high levels in ganglia of the peripheral nervous system, including the superior cervical ganglion (sympathetic), ciliary ganglion (parasympathetic), and dorsal root ganglion (sensory). In the central nervous system, motor nuclei of cranial nerves III, IV, V, VI, VII, X, and XII as well as ventral horn motor neurons and a restricted set of other central nervous system nuclei express the clone 73 mRNA. Tissues apart from those of the nervous system did not in general express the mRNA, with only very low levels detected in adrenal gland. We discuss the implications of these results for the mechanism of NGF-induced PC12 cell differentiation, the pathways of neuronal development in vivo, and the possible function of the clone 73 IF protein and its relationship to other IF proteins. 相似文献
16.
To assess the nature and extent of behavioural, biochemical and histological changes induced by trimethyltin (TMT), rats were treated with a single injection of TMT over a dose range of 6, 7 and 8 mg/kg i.p. Behavioural observations were performed at a minimum of 21 days after the administration of TMT. The behavioural consequences of TMT were hyperactivity in the open-field test, increased locomotor activity and deficits in passive and active avoidance behaviour, T-maze alternation and Morris Water Maze behaviour. The behavioural changes were dose dependent and were accompanied by a degree of pathological damage to the hippocampal pyramidal cells which was particularly apparent at the highest dose. The main biochemical effects of TMT involved deficits in the serotonergic and GABA-ergic systems and a decrease in M1 and M2 binding sites in the hippocampus. These results suggest that the toxic interaction of TMT with the hippocampus and other limbic brain regions may be responsible for its effect on learning and memory. 相似文献
17.
18.
Summary pSE211 fromSaccharopolyspora erythraea integrates site-specifically into the chromosome through conservative recombination betweenattP andattB, the plasmid and chromosomal attachment sites. Integration depends on the presence ofint, an open reading frame (ORF) that lies adjacent toattP and encodes the putative integrase. Immediately upstream ofint liesxis (formerly calledorf2) which encodes a basic protein that is thought to exhibit DNA binding.xis andint were cloned in various combinations in pUC18 and expressed constitutively inEscherichia coli from thelac promoter.attP andattB were cloned inStreptomyces orE. coli plasmids containing kanamycin resistance (KmR) or chloramphenicol resistance (CmR) markers. Stable KmR CmR cointegrates formed byattP ×attB orattP ×attP recombination (integration) were obtained inE. coli hosts that expressedint. Co-integrates were not found in hosts expressingint+xis. Excision (intraplasmidatt site recombination) was examined by constructing plasmids carryingattL andattR or twoattP sites separating CmR from KmR and by following segregation of the markers in various hosts. BothattL ×attR andattP ×attP excision depended on bothxis andint inE. coli. pSE211att site integration and excision were not affected by a deletion inhimA, the gene encoding a subunit of integration host factor. 相似文献
19.
20.
Drosophila projectin: relatedness to titin and twitchin and correlation with lethal(4) 102 CDa and bent-dominant mutants. 总被引:8,自引:0,他引:8
C C Fyrberg S Labeit B Bullard K Leonard E Fyrberg 《Proceedings. Biological sciences / The Royal Society》1992,249(1324):33-40
We have investigated projectin, a large protein of insect muscles, in Drosophila melanogaster. The 5.3 kilobases of coding sequence reported here contains Class I and Class II motifs characteristic of titin and twitchin, arranged in a three domain ... [II-I-I] [II-I-I] ... pattern. Two mutants mapped to the location of the projectin gene in the 102C subdivision of chromosome 4, lethal(4) 102 CDa and bent-Dominant, have DNA rearrangements within their projectin gene. The lethal(4) 102 CDa mutant has a 141 nucleotide insertion containing stop codons in all three reading frames within an exon sequence, showing that it cannot synthesize normal projectin. Both bent-Dominant and lethal(4) 102 CDa homozygotes die at the completion of embryogenesis because they are unable to escape the egg vitelline membrane. We propose that this hatching failure is due to muscle weakness caused by projectin defects. 相似文献