全文获取类型
收费全文 | 16202篇 |
免费 | 42篇 |
国内免费 | 41篇 |
专业分类
系统科学 | 70篇 |
丛书文集 | 108篇 |
教育与普及 | 28篇 |
理论与方法论 | 56篇 |
现状及发展 | 5794篇 |
研究方法 | 756篇 |
综合类 | 9155篇 |
自然研究 | 318篇 |
出版年
2013年 | 186篇 |
2012年 | 231篇 |
2011年 | 564篇 |
2010年 | 101篇 |
2008年 | 259篇 |
2007年 | 334篇 |
2006年 | 335篇 |
2005年 | 309篇 |
2004年 | 300篇 |
2003年 | 279篇 |
2002年 | 287篇 |
2001年 | 690篇 |
2000年 | 675篇 |
1999年 | 370篇 |
1994年 | 47篇 |
1992年 | 340篇 |
1991年 | 272篇 |
1990年 | 297篇 |
1989年 | 316篇 |
1988年 | 285篇 |
1987年 | 268篇 |
1986年 | 290篇 |
1985年 | 311篇 |
1984年 | 270篇 |
1983年 | 245篇 |
1982年 | 203篇 |
1981年 | 188篇 |
1980年 | 211篇 |
1979年 | 502篇 |
1978年 | 391篇 |
1977年 | 365篇 |
1976年 | 302篇 |
1975年 | 348篇 |
1974年 | 496篇 |
1973年 | 376篇 |
1972年 | 386篇 |
1971年 | 491篇 |
1970年 | 607篇 |
1969年 | 413篇 |
1968年 | 448篇 |
1967年 | 377篇 |
1966年 | 384篇 |
1965年 | 259篇 |
1959年 | 135篇 |
1958年 | 225篇 |
1957年 | 162篇 |
1956年 | 115篇 |
1955年 | 115篇 |
1954年 | 110篇 |
1948年 | 89篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
Androgen effect on genetic and goldthioglucose-induced obesity 总被引:1,自引:0,他引:1
63.
Growth of the bacterial cell 总被引:56,自引:0,他引:56
64.
Abnormal electroretinogram from a Drosophila mutant 总被引:21,自引:0,他引:21
65.
Vertebrate blood oxygen capacity and body weight 总被引:1,自引:0,他引:1
66.
Effects of a change in the level of inbreeding on the genetic load 总被引:10,自引:0,他引:10
"The effects of inbreeding may not be as noticeable in the first generation as the invigoration immediately apparent after crossing". This statement, published in 1919, has received little attention, and has apparently never been tested empirically, although the reduction of the genetic load of populations by inbreeding is well known in theoretical terms. Because inbreeding increases homozygosity, and hence the effectiveness of selection against recessive or partially recessive detrimental alleles, changes in levels of inbreeding can lead to a reduction in the frequencies of such mutant alleles. This results in equilibration at higher population mean fitness and is referred to as 'purging' populations of their genetic load. Severe inbreeding can also reduce genetic load due to overdominant alleles, provided selection coefficients are not symmetrical at all loci, because alleles giving lower fitness will be reduced in frequency at equilibrium. With either fitness model, however, reduction in genetic load takes time, and the initial effect of an increase in inbreeding is reduced fitness due to homozygosity. There are few data relating to the extent to which fitness is reduced during inbreeding in a set of lines and to how long the reduction lasts before increasing again to the initial level, or higher. Inbreeding experiments involving sib mating in mice and Drosophila subobscura, and successive bottlenecks in house flies have yielded some evidence consistent with the purging hypothesis. Here, we report results of an experiment demonstrating a prolonged time-course of recovery of mean fitness under self-fertilization of a naturally outcrossing plant, and also compare our results with expectations derived by computer calculations. Our results show that the genetic load present in an outcrossing population can be explained only with a high mutation rate to partially recessive deleterious alleles, and that inbreeding purges the population of mutant alleles. 相似文献
67.
68.
A. Alkofahi J. K. Rupprecht Y. -M. Liu C. -J. Chang D. L. Smith J. L. McLaughlin 《Cellular and molecular life sciences : CMLS》1990,46(5):539-541
Summary Gigantecin (I), a novel tetrahydroxy-di-tetrahydrofuran fatty acid -lactone (acetogenin), was isolated from an ethanolic extract of the stem bark ofGoniothalamus giganteus Hook. f., Thomas (Annonaceae), by means of activity-directed fractionation (brine shrimp lethality test). This new compound is extremely cytotoxic to human tumor cells, inhibits crown gall tumors on potato discs, and is active in an assay designed to detect antimitotic agents (9 ASK). 相似文献
69.
It has been suggested that amino acids and other organic compounds found in carbonaceous meteorites formed by aqueous alteration in the meteorite parent bodies. Observations of carbonaceous material in interstellar grains and interplanetary dust particles indicate that condensed organic compounds may have been present in meteorite parent bodies at the time of aqueous alteration. One group of compounds thought to be representative of this carbonaceous material is the polycyclic aromatic hydrocarbons (PAHs). Recently it was proposed that PAHs condense on SiC grains in the molecular envelopes of carbon-rich red-giant stars, which would allow for their subsequent incorporation into meteorite parent bodies during accretion. This incorporation mechanism is supported by the identification of SiC grains in carbonaceous chondrites. The possibility therefore exists that PAHs, and/or other condensed organic compounds, represent the starting material for aqueous alteration which leads to the formation of amino acids and other water-soluble organic compounds. Here we present calculations of the distribution of aqueous organic compounds in metastable equilibrium with representative PAHs as functions of the fugacities of O2, CO2 and NH3. The results reported here for pyrene and fluoranthene, two PAHs with different structures but the same stoichiometry, differ greatly but indicate that the formation of amino and carboxylic acids is energetically favourable at probable parent-body alteration conditions. The actual reaction mechanisms involved could be revealed by consideration of isotope data for PAHs, amino acids, other organic compounds and carbonates in carbonaceous chondrites. 相似文献
70.
The in vivo expression of the globin genes of theβ cistron in γ-,δ-, andδβ-thalassemia heterozygotes
A. J. Dimovski A. D. Adekile T. H. J. Huisman 《Cellular and molecular life sciences : CMLS》1994,50(2):167-170
There is considerable evidence suggesting that the switch from to and chain production after birth is due, in part, to silencing of the genes by stage-specific factors which bind to their promoters and to the competition from the adult ( and ) genes for a common enhancer element located in the locus control region. As a consequence one can expect that the increased Hb F production in adults with hereditary persistence of fetal hemoglobin or -thalassemia is directed mainly by -globin genes in cis to the deletion(s) responsible for these conditions. Here we review data on heterozygotes with -, -, or -thalassemia, who also had anAT mutation, in cis or in trans, which was used as a marker of gene expression. The results show that a deletion affecting adult genes favors the expression of genes in cis, while the deletion of a single gene does not affect the expression of the gene in cis but leads to a faster switch postnatally. 相似文献