全文获取类型
收费全文 | 1156篇 |
免费 | 62篇 |
专业分类
1218篇 |
出版年
2014年 | 16篇 |
2013年 | 23篇 |
2012年 | 20篇 |
2011年 | 19篇 |
2010年 | 33篇 |
2009年 | 24篇 |
2008年 | 37篇 |
2007年 | 48篇 |
2006年 | 26篇 |
2005年 | 24篇 |
2004年 | 23篇 |
2003年 | 12篇 |
2002年 | 22篇 |
2001年 | 15篇 |
2000年 | 11篇 |
1999年 | 32篇 |
1998年 | 18篇 |
1997年 | 17篇 |
1996年 | 14篇 |
1995年 | 15篇 |
1994年 | 18篇 |
1993年 | 22篇 |
1992年 | 32篇 |
1991年 | 28篇 |
1990年 | 21篇 |
1989年 | 21篇 |
1988年 | 17篇 |
1987年 | 26篇 |
1986年 | 21篇 |
1985年 | 21篇 |
1984年 | 20篇 |
1982年 | 16篇 |
1980年 | 17篇 |
1979年 | 15篇 |
1978年 | 11篇 |
1976年 | 15篇 |
1975年 | 13篇 |
1974年 | 18篇 |
1973年 | 21篇 |
1972年 | 17篇 |
1968年 | 11篇 |
1965年 | 36篇 |
1964年 | 19篇 |
1963年 | 32篇 |
1962年 | 23篇 |
1961年 | 33篇 |
1960年 | 21篇 |
1959年 | 27篇 |
1958年 | 27篇 |
1957年 | 12篇 |
排序方式: 共有1218条查询结果,搜索用时 0 毫秒
101.
102.
E. coli Rep protein is a 3' to 5' SF1 superfamily DNA helicase which is monomeric in the absence of DNA, but can dimerize upon binding either single-stranded or duplex DNA. A variety of biochemical studies have led to proposals that Rep dimerization is important for its helicase activity; however, recent structural studies of Bacillus stearothermophilus PcrA have led to suggestions that SF1 helicases, such as E. coli Rep and E. coli UvrD, function as monomeric helicases. We have examined the question of whether Rep oligomerization is important for its DNA helicase activity using pre-steady state stopped-flow and chemical quenched-flow kinetic studies of Rep-catalyzed DNA unwinding. The results from four independent experiments demonstrate that Rep oligomerization is required for initiation of DNA helicase activity in vitro. No DNA unwinding is observed when only a Rep monomer is bound to the DNA substrate, even when fluorescent DNA substrates are used that can detect partial unwinding of the first few base-pairs at the ss-ds-DNA junction. In fact, under these conditions, ATP hydrolysis causes dissociation of the Rep monomer from the DNA, rather than DNA unwinding. These studies demonstrate that wild-type Rep monomers are unable to initiate DNA unwinding in vitro, and that oligomerization is required. 相似文献
103.
104.
The Avian Shoulder: An Experimental Approach 总被引:2,自引:0,他引:2
GOSLOW G. E. JR.; DIAL K. P.; JENKINS F. A. JR. 《Integrative and comparative biology》1989,29(1):287-301
SYNOPSIS. This essay is in two parts. The first describes functionalstudies of the shoulder in modern vertebrates that led to theformulation of the hypotheses that motor patterns of homologousmuscles have been maintained during the evolution of the tetrapodshoulder, and that a primitive organization of the neural controlcomponents has persisted in derived groups. The second part of this essay focuses upon a longstanding questionin vertebrate evolution: what neuromuscular and musculoskeletalchanges in the tetrapod shoulder accompanied the evolution offlight in birds? The lack of empirical data on shoulder functionin extant birds limited our insight into this question, andprompted our initiation of experimental studies. Preliminarykinematics of the furcula and humerus of European starlings(Sturnus vulgaris) flying in a wind tunnel, as revealed by highspeed cineradiography, are presented. The two halves of thefurcula, which contact the coracoids dorsally, are bent laterallyduring downstroke and medially during upstroke by as much as60% of the intrafurcular resting distance. High speed film andelectromyographic studies of freeflying pigeons (Columba livia)reveal that the supracoracoideus muscle is strongly activatedduring wing elevation and, as predicted from studies of Varanusand Didelphis, an additional activation burst occurs at mid-downstrokein 48% of the recordings. 相似文献
105.
Abstract. An earlier study on the blowfly Phormia regina (Meigen) demonstrated that the injection of amphetamine (12 μg) depletes biogenic amine levels in the CNS. In the present study, P. regina females were injected with amphetamine (12 μg), each female was placed with three males and insemination success was determined. Amphetamine inhibited female insemination by 43.3% at 2–90 min post-injection and by 70% at 10–60 min post-injection. At 180–270 min post-injection, there was no significant inhibition of female insemination. This study indicates a possible role in insects for the biogenic amines in female insemination. 相似文献
106.
Jan A. Gossen Wiljo J. F. de Leeuw Aart Verwest Paul H. M. Lohman Jan Vijg 《Mutation research》1991,250(1-2):423-429
To study spontaneous and induced mutagenesis in vivo we recently constructed a series of transgenic mice harboring different numbers of bacteriophage lambda shuttle vectors, provided with a LacZ mutational target gene, integrated in their genome. The transgenic mice enabled analysis of spontaneous and induced mutation frequencies in postmitotic tissues like liver and brain. The obtained data indicated spontaneous mutation frequencies in the order of −5 - 10−6. Here we report a 25–100 times higher spontaneous mutation frequency in liver and brain DNA of mice from strain 35.5, with the lambda-gt10LacZ concatemer integrated on the X-chromosome. These results indicate the presence of a mutational ‘hot spot’ in the mammalian somatic genome in vivo. 相似文献
107.
Don G. Benson JR. 《Biotechnic & histochemistry》1966,41(3):155-158
Paraffin sections from tissue fixed 4-12 hr in 10% formalin containing 0.5% cetyl pyridinium chloride, and washed 2 hr, were stained as follows: (1) Hydrolyze in 5 N HCl at room temperature for 8.5-9 min, or use standard Feulgen hydrolysis at 60 C. (2) Stain in azure A-Schiff, 0.5% in bisulfite bleach (1 N HCl, 5; 10% Na2S2O5, 5; and distilled water 90—parts by volume) for 10 min. (3) Place in bisulfite bleach 2 changes, 2 min each; wash in water, 1-2 min. (4) Stain in Alcian blue (0.1% in 0.01 2V HCl, pH 2.0) for 10 min. (5) Place in 0.01 N HCl for 2-3 min; wash in water for 1-2 min. (6) Oxidize in 0.5% HIO4 for 5 min; wash in water, 1-2 min. (7) Stain in Schiff's leucofuchsiu, 10 min. (8) Treat with bisulfite bleach as in step 3; wash in running water, 10 min. (9) Stain in naphthol yellow S (0.01% in 1% acetic acid) for 1-2 min. (10) Place in 1% acetic acid for 2 min, dehydrate in tertiary butanol, clear and cover. Result: DNA is deep blue; acidic mucins are light blue; neutral polysaccharides, red to magenta; and proteins, yellow. Proper timing of the hydrolysis for the Feulgen reaction is the most critical step. Overhydrolysis results in green nuclei (staining by naphthol yellow S) whereas purplish nuclei are the results of insufficient hydrolysis. 相似文献
108.
109.
HEATHER C. TIPTON VICTORIA J. DREITZ PAUL F. DOHERTY JR. 《The Journal of wildlife management》2008,72(4):1001-1006
Abstract: Concern over the decline of grassland birds has spurred efforts to increase understanding of grassland bird-habitat relationships. Previous studies have suggested that black-tailed prairie dogs (Cynomys ludovicianus) provide important habitat for shortgrass prairie avifauna, such as mountain plover (Charadrius montanus) and western burrowing owl (Athene cunicularia hypugaea), although such studies are lacking in Colorado (USA). We used methods to estimate occupancy (ψ) of mountain plover and burrowing owl on prairie dog colonies and other shortgrass prairie habitats in eastern Colorado. Mountain plover occupancy was higher on prairie dog colonies (ψ = 0.50, 95% CI = 0.36–0.64) than on grassland (ψ = 0.07, 95% CI = 0.03–0.15) and dryland agriculture (ψ = 0.13, 95% CI = 0.07–0.23). Burrowing owl occupancy was higher on active prairie dog colonies (ψ = 0.80, 95% CI = 0.66–0.89) compared with inactive colonies (ψ = 0.23, 95% CI = 0.07–0.53), which in turn was much higher than on grassland (ψ = 0.01, 95% CI = 0.00–0.07) and dryland agriculture (ψ = 0.00, 95% CI ψ 0.00–0.00). Mountain plover occupancy also was positively correlated with increasing amounts of prairie dog colony in the landscape. Burrowing owl occupancy was negatively correlated with increasing amounts of prairie dog colony in the surrounding landscape. Our results suggest that actions to conserve mountain plovers and burrowing owls should incorporate land management to benefit prairie dogs. Because managing for specific colony attributes is difficult, alternative management that promotes heterogeneity may ensure that suitable habitat is available for the guild of grassland inhabitants. 相似文献
110.
SYNOPSIS. Quantitative analysis of low-level (mostly subspecific)geographic differentiation of aposematic color patterns in threegroups of forest butterflies (Heliconiini, Ithomiinae, and Troidini),in 1,550 half-degree quadrants throughout the Neotropics, indicatesfour principal species-endemicity regions, 44 principal subspecies-endemicitycenters, and 300 quadrants of subspecies hybridization. Theendemism centers are different in outline but nearly identicalin position for each of the three ecologically different groups.Basic data on present and past physical environments (climate,soils, geomorphology, and vegetation structures), when integratedinto maps, show 1) regions of favorable ecological conditionsfor forest organisms in the present, 2) regions of environmentalconformities and rapid transitions in the present, and 3) regionsof high probability for humid forest persistence during themajor landscape changes at the end of the last glacial period,13,00020,000 yr ago. Species diversity (a local phenomenon)is readily related to favorable modern ecological conditionsand environmental microheterogeneity, promoted by unpredictablemild disturbance. Biogeographical patterns of evolution at thelevel of regional subspecies correlate very well with presumedpaleoecological conditions. The biogeographical data also supportan hypothesis of differentiation resulting from areas of rapidenvironmental change; most of the hybridization quadrants fallwithin transition zones. There is lower correlation betweencenters of endemism and zones of en vironmental conformity,but some individual cases of differentiation in the butterfliesmay be adequately explained by modern ecological factors. Speciespresently undergoing differentiation in isolated habitat islands(Amazonian savannas and high-altitude Andean forests and paramos)may offer the best opportunities for observation of evolutionaryprocesses in the present. 相似文献