全文获取类型
收费全文 | 3595篇 |
免费 | 342篇 |
国内免费 | 131篇 |
出版年
2023年 | 30篇 |
2022年 | 45篇 |
2021年 | 95篇 |
2020年 | 78篇 |
2019年 | 89篇 |
2018年 | 119篇 |
2017年 | 100篇 |
2016年 | 122篇 |
2015年 | 145篇 |
2014年 | 166篇 |
2013年 | 217篇 |
2012年 | 229篇 |
2011年 | 246篇 |
2010年 | 169篇 |
2009年 | 120篇 |
2008年 | 185篇 |
2007年 | 175篇 |
2006年 | 148篇 |
2005年 | 114篇 |
2004年 | 108篇 |
2003年 | 87篇 |
2002年 | 110篇 |
2001年 | 73篇 |
2000年 | 74篇 |
1999年 | 59篇 |
1998年 | 37篇 |
1997年 | 24篇 |
1992年 | 53篇 |
1991年 | 46篇 |
1990年 | 47篇 |
1989年 | 40篇 |
1988年 | 36篇 |
1987年 | 43篇 |
1986年 | 36篇 |
1985年 | 35篇 |
1984年 | 33篇 |
1983年 | 31篇 |
1982年 | 27篇 |
1981年 | 39篇 |
1980年 | 31篇 |
1979年 | 32篇 |
1978年 | 38篇 |
1977年 | 24篇 |
1976年 | 24篇 |
1975年 | 24篇 |
1973年 | 28篇 |
1972年 | 19篇 |
1970年 | 20篇 |
1969年 | 20篇 |
1968年 | 18篇 |
排序方式: 共有4068条查询结果,搜索用时 765 毫秒
21.
S.P. Pathak J.W. Bhattacherjee N. Kalra S. Chandra 《Journal of applied microbiology》1988,65(4):347-352
The seasonal distribution of Aeromonas hydrophila in water and recovery rate from live river fish was investigated. The highest isolation rates of A. hydrophila occurred in water during the late winter followed by a progressive decline in density during the summer and monsoon seasons. The organism was recovered from fish throughout the period from which it was concluded that they form a reservoir which is unrelated to their density in water. The enterotoxigenicity of some environmental strains was tested in suckling mice and rabbit ileal loop. 相似文献
22.
The effect of high supercoil densities on the melting characteristics of a supercoiled DNA has been studied. It is found that although the melting temperature increases abruptly on converting a linear DNA merely into the relaxed circular form, it falls back substantially at high supercoil densities. It is further predicted, in such cases, that the number of melted base pairs should be significantly enhanced even at the physiological temperature, which may facilitate the binding of other molecules to the highly supercoiled DNA. 相似文献
23.
Bhabesh Sen Franklin A. Graybill Naitee Ting 《Biometrical journal. Biometrische Zeitschrift》1992,34(3):259-274
The model considered in this article is the two-factor nested unbalanced variance component model: for p = 1, 2, …, P; q = 1, 2, …, Qp; and r = 1, 2, …, Rpq. The random variables Ypqr are observable. The constant μ is an unknown parameter, and Ap, Bpq and Cpqr are (unobservable) normal and independently distributed random variables with zero means and finite variances σ2A, σ2B, and σ2C, respectively. Approximate confidence intervals on ?A and ?B using unweighted means are derived, where The performance of these approximate confidence intervals are evaluated using computer simulation. The simulated results indicate that these proposed confidence intervals perform satisfactorily and can be used in applied problems. 相似文献
24.
Sixty-nine nonmetrical morphological variants of the cranium have been studied in six samples of non-tribal, state populations
in India, and their incidence reported. Using C.A.B. Smith’s angular transformation of frequencies, the multivariate Thetasquare
distances and their respective standard deviations have been presented. On the basis of nonmetrical cranial variation, it
is clear that the samples from Uttar Pradesh, Andhra Pradesh and Bihar are closer to each other but distant from Madhya Pradesh,
Karnataka and Maharashtra. On the other hand, Karnataka and Maharashtra samples are quite close to each other and both, in
turn, are comparatively closer to Madhya Pradesh than to Uttar Pradesh, Andhra Pradesh and Bihar. Madhya Pradesh sample emerges
as the most divergent group among the six population samples studied. This, in general, is in conformity with the picture
that emerges from various analysis of morphometric and other biological data on various populations of India. 相似文献
25.
Characterization of the membrane-bound protein kinase C and its substrate proteins in canine cardiac sarcolemma 总被引:1,自引:0,他引:1
Cardiac sarcolemma was purified from canine ventricles. Enrichment of the sarcolemmal membranes was demonstrated by the high (Na+ + K+)-ATPase activity of 28.0 +/- 1.5 mumol Pi/mg protein per h and the high concentration of muscarinic receptors with the Bmax of 8.2 +/- 2.5 pmol/mg protein as determined by [3H]QNB binding. The purified sarcolemma also contains significant levels of a membrane-bound Ca2+ and phospholipid-dependent protein kinase (protein kinase C). To elucidate the protein kinase C activity in sarcolemma, a prior incubation of the membranes with EGTA and Triton X-100 was necessary. The specific activity of protein kinase C was found to be 131.4 pmol Pi/mg per min, in the presence of 6.25 micrograms phosphatidylserine and 0.5 mM CaCl2. Treatment of sarcolemma with 12-O-tetradecanoylphorbol 13-acetate (TPA) and phorbol 12,13-dibutyrate (PBu2) resulted in a concentration-dependent activation of protein kinase C activity. The effect of TPA and PBu2 on protein kinase C in sarcolemma was independent of exogenous Ca2+ and phosphatidylserine. Polymyxin B inhibited phorbol-ester-induced activation of protein kinase C activity. The distribution of protein kinase C in the cytosolic fraction was also examined. The specific activity of the kinase in the cytosolic fraction was 59.7 pmol Pi/mg per min. However, the total protein kinase C activity in the cytosol was 213500 pmol Pi/min, compared to that of 1025 pmol Pi/min in the sarcolemma isolated from approx. 100 g of canine ventricular muscle. Several endogenous proteins in cardiac sarcolemma were phosphorylated in the presence of Ca2+ and phosphatidylserine. The major substrates for protein kinase C were proteins of Mr 94 000, 87 000, 78 000, 51 000, 46 000, 11 500 and 10 000. Most of these substrate proteins have not been identified before. Other proteins of Mr 38 000, 31 000 and 15 000 were markedly phosphorylated in the presence of Ca2+ only. Phosphorylation of phospholamban (Mr 27 000 and 11 000) was also stimulated in the presence of Ca2+ and phosphatidylserine, but the low Mr form of phospholamban was distinct from two other low Mr substrate proteins for protein kinase C. Polymyxin B was more selective in inhibiting the protein kinase C dependent phosphorylation. On the other hand, trifluoperazine selectively inhibited the phosphorylation of phospholamban and Mr 15 000 protein. Although the exact function of this kinase is unknown, based on these observations, we believe that protein kinase C in the cardiac sarcolemma may play an important role in the cell-surface-signal regulated cardiac function. 相似文献
26.
Twenty years after its discovery, the beta-adrenergic blocking agent propranolol continues to interest pharmacologists and clinicians. Its therapeutic profile has extended to areas beyond the purview of the cardiovascular system, and its ocular and central nervous system effects have been well documented. In addition, it still remains a very good pharmacological tool to map out the adrenergic beta-receptors in the body, and stereoisomers of propranolol and other beta-blockers serve as valuable agents to distinguish between the effects related to beta-adrenoceptors and those which are not. The primary purpose of this review is to summarize the evidence indicating that beta-adrenergic blocking agents lack stereoselectivity in some of their effects, including several of considerable therapeutic importance. Because many pharmacological actions of propranolol followed a nonsteroselective pattern, the involvement of beta-adrenoceptors in them was questioned and this led to the search for alternate mechanisms to explain these effects. Studies with propranolol and some related drugs indicated the involvement of a cholinergic mechanism in their antiarrhythmic, ocular hypotensive and some central effects. Also, a presynaptic inhibitory effect at the skeletal neuromuscular junction has been suggested to explain the benefical effect of propranolol and other beta-blockers in tremor. Biochemical studies with these drugs revealed their inhibitory action on the cholinesterase enzyme in blood and other tissues like myocardium and brain. It is thus hypothesized that modulation of cholinergic neurotransmission by propranolol could explain some of its nonstereoselective actions and open new vistas in propranolol pharmacodynamics. 相似文献
27.
DNA amplification in multidrug, cross-resistant Chinese hamster ovary cells: molecular characterization and cytogenetic localization of the amplified DNA 总被引:2,自引:0,他引:2 下载免费PDF全文
Vincristine-resistant (VCR) Chinese hamster ovary (CHO) cells have been established by stepwise selection in increasing concentrations of vincristine. These cells exhibit multidrug cross-resistance to a number of drugs that have no structural or functional similarities. Cytogenetic analyses of resistant cells revealed the presence of double minutes and expanded chromosomal segments, thus implicating gene amplification as a possible mechanism of resistance. An amplified DNA segment isolated from other multidrug cross-resistant CHO cell lines (Roninson, I. B., H. T. Abelson, D. E. Housman, N. Howell, and A. Varshavsky, 1984, Nature (Lond.), 309:626-628) is also amplified in our VCR lines. This DNA segment was used as a probe to screen a cosmid library of VCR genomic DNA, and overlapping clones were retrieved. All of these segments, totaling approximately 45 kilobases (kb), were amplified in VCR cells. Using in situ hybridization, we localized the amplification domain to the long arm of CHO chromosome 1 or Z1. Northern hybridization analysis revealed that a 4.3-kb mRNA was encoded by this amplified DNA domain and was over-produced in the VCR cells. Suggestions for the involvement of these amplified DNA segments in the acquisition of multidrug cross-resistance in animal cells are also presented. 相似文献
28.
P Chattopadhyay S K Banerjee K Sen P Chakrabarti 《Canadian journal of microbiology》1985,31(4):352-355
A comparative study of the mycelial lipid composition of a wild strain (V35) and one unsaturated fatty acid auxotroph (UFA2) of Aspergillus niger has been performed. The lipid composition of both strains are qualitatively the same but quantitatively different. All the strains contain the following phospholipids: cardiolipin, phosphatidylethanolamine, phosphatidylcholine, lysophosphatidylethanolamine, lysophosphatidylcholine, and phosphatidylserine; and triglycerides, diglycerides, monoglycerides, ergosterol, and sterol esters as the neutral lipids; mono- and di-galactosyl diglyceride as the major glycolipids along with small amounts of the corresponding mannose analogs. Phosphatidylethanolamine and phosphatidylcholine constitute the bulk of the phospholipids. The mutant (UFA2) contains a higher level of glycerides and lower levels of sterol (both free and esterified form), phospholipids, and glycolipids than the wild type. Aspergillus niger contains C16 to C18 saturated and unsaturated fatty acids. Small amounts of long-chain (C20 to C24) and short-chain (C10 to C14) saturated and unsaturated acids are also present. Linoleic, oleic, and palmitic are the major acids, stearic and linolenic acids being minor ones. UFA2 grows only in the presence of unsaturated fatty acid (C16 or C18) and accumulates a higher concentration of supplemented acid which influences its fatty acid profile. 相似文献
29.
Cytophotometric estimation of DNA values from human buccal mucosa and lymphocyte culture nuclei shows a difference related to different X chromosomal abnormalities, namely, del X, XO, XXX and XXY. The values in the buccal mucosa in all cases except XXX were similar to the normal XX and XY. In lymphocytes nuclei, however, a steady increase in the DNA content at a significant level could be related to an increase in the number of X chromosomes. The similarity in the DNA values of normal XX and XY controls may be attributed to asynchrony in the replication patterns of X and Y chromosomes. 相似文献
30.
Application of a modified immunofluorescence technique using an anti-kinetochore serum enables cytogeneticists to obtain quality metaphase spreads and to localize kinetochores. In a patient with a 45, XX, -9, -11, tdic (9p;11p) constitution, we found that the dicentric marker chromosome has an intensely fluorescent kinetochore (no. 11), the functional centromere, and a less intensely fluorescent kinetochore (no. 9), the inactive centromere. The data suggest that in the process of tandem fusion (telomere-telomere between 11p and 9p), the centromere of chromosome 9 was not deleted, but, rather, inactivated. 相似文献