全文获取类型
收费全文 | 6059篇 |
免费 | 700篇 |
专业分类
6759篇 |
出版年
2021年 | 76篇 |
2018年 | 56篇 |
2016年 | 86篇 |
2015年 | 154篇 |
2014年 | 144篇 |
2013年 | 239篇 |
2012年 | 251篇 |
2011年 | 248篇 |
2010年 | 155篇 |
2009年 | 161篇 |
2008年 | 207篇 |
2007年 | 230篇 |
2006年 | 228篇 |
2005年 | 201篇 |
2004年 | 210篇 |
2003年 | 180篇 |
2002年 | 209篇 |
2001年 | 188篇 |
2000年 | 192篇 |
1999年 | 175篇 |
1998年 | 97篇 |
1997年 | 84篇 |
1995年 | 79篇 |
1994年 | 66篇 |
1993年 | 93篇 |
1992年 | 157篇 |
1991年 | 130篇 |
1990年 | 140篇 |
1989年 | 117篇 |
1988年 | 106篇 |
1987年 | 108篇 |
1986年 | 96篇 |
1985年 | 107篇 |
1984年 | 100篇 |
1983年 | 82篇 |
1982年 | 69篇 |
1981年 | 62篇 |
1980年 | 76篇 |
1979年 | 91篇 |
1978年 | 72篇 |
1977年 | 75篇 |
1976年 | 65篇 |
1975年 | 71篇 |
1974年 | 81篇 |
1973年 | 88篇 |
1972年 | 61篇 |
1971年 | 55篇 |
1970年 | 58篇 |
1968年 | 48篇 |
1967年 | 47篇 |
排序方式: 共有6759条查询结果,搜索用时 15 毫秒
81.
82.
Nucleotide sequence and genome organization of carnation mottle virus RNA. 总被引:13,自引:5,他引:8
下载免费PDF全文
![点击此处可从《Nucleic acids research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
H Guilley J C Carrington E Balzs G Jonard K Richards T J Morris 《Nucleic acids research》1985,13(18):6663-6677
83.
Escherichia coli DNA topoisomerase III: purification and characterization of a new type I enzyme 总被引:12,自引:0,他引:12
A new topoisomerase capable of relaxing negatively supercoiled DNA in Escherichia coli has been identified during chromatography on novobiocin-Sepharose. A simple and reproducible purification procedure is described to obtain this enzyme, called topoisomerase III (topo III), in a homogeneous form. The protein is a single polypeptide with a molecular weight of 74 000 +/- 2000 and is a type I topoisomerase, changing the linking number of DNA circles in steps of one. It is present in deletion strains lacking the topA gene and further differs from the well-studied topoisomerase I (omega protein; Eco topo I) in (1) its requirement for K+ in addition to Mg2+ to exhibit optimal activity and (2) its affinity to novobiocin-Sepharose. Positively supercoiled DNA is not relaxed during exposure to the enzyme. Topo III has no ATPase activity, and ATP does not show any discernible effect on the reduction of superhelical turns. The purified topoisomerase has no supercoiling activity and is unaffected by high concentrations of oxolinic acid and novobiocin in the relaxing reaction. Single-stranded DNA and spermidine strongly inhibit the topoisomerase activity. 相似文献
84.
Three experiments demonstrate that inescapable footshock delivered to unrestrained rats produces analgesia as well as performance deficits in subsequent one-way shuttle acquisition. Both the performance and the antinociceptive effects are prevented by pretreatment with as little as 0.1 mg/kg i.p. of the opiate antagonist, naltrexone. These studies suggest that both effects are mediated through opiate receptors with similar underlying naltrexone pharmacodynamics. 相似文献
85.
86.
A strain of Clostridium sporogenes, an anaerobic bacterium, isolated from sewage in New York City synthesizes two constitutive enzymes with action on steroid molecules: (i) an enzyme capable of selectively acetylating the 21-hydroxyl function of certain steroids and (ii) the corresponding esterase. Under our experimental conditions the enzymes have a strict structural requirement for 3-keto-4-ene and C-20-keto or 20 alpha-hydroxyl group and convert their respective substrates to a mixture of free and acetylated products. 相似文献
87.
V D Bokkenheuser J Winter G N Morris S Locascio 《Applied and environmental microbiology》1986,52(5):1153-1156
The synthesis of a steroid desmolase was demonstrated in two obligate anaerobes: a new bacterial species, Eubacterium desmolans, isolated from cat fecal flora, and Clostridium cadavaris, recovered from sewage of New York City. The enzyme cleaves the C-17-C-20 bond of corticoids possessing hydroxyl functions at C-17 and C-21. The conversion is quantitative, provided the substrate concentration is less than 100 micrograms/ml and the organisms are in the log phase. The velocity of transformation parallels the bacterial growth curve and in the log phase is higher for E. desmolans than for C. cadavaris. In addition, both organisms synthesize a 20 beta-hydroxysteroid dehydrogenase. 相似文献
88.
Genes of the major histocompatibility complex (MHC) in the mouse (H-2 complex) have been shown to be an important factor in determining the immune responsiveness of various strains of mice to isolated antigens (e. g., lysozyme). The role of MHC genes in controlling the responsiveness of mice to multiple alloantigens is less well-defined, and although non-MHC genes have been shown to be important in determining responsiveness in some systems (e. g., haptens), they have not been demonstrated as yet to influence the rejection of vascularized organ allografts. In this study, the responsiveness of mice to vascularized cardiac allografts transplanted across well-defined major (H-2) and minor (non-H-2) histocompatibility barriers was investigated using congenic mice in 32 different donor/recipient combinations. The results show that both H-2 and non-H-2 gene products can act as target alloantigens for rejection. At the responder level, they may interact to effect responsiveness of a recipient strain to multiple alloantigens. In no case in this study has any one gene or group of genes been found to confer universal high or low responder status. 相似文献
89.
Morris Wayman Sarad Parekh Esteban Chornet Ralph P. Overend 《Biotechnology letters》1986,8(10):749-752
Summary This paper reports studies of large scale, 1500 kg/h, SO2-catalysed prehydrolysis of coniferous wood chips, samples then being hydrolyzed by a wood-saccharifying enzyme system followed by fermentation to ethanol in the laboratory. Hemicellulose hydrolysis using SO2 catalyst (prehydrolysis) was found to be more effective than steam alone (autohydrolysis). Prehydrolysis time was 2 min, with steam pressure at 1.2 to 1.7 MPa (175 to 250 psig), and SO2 catalyst 2.0 to 2.6% on dry wood. The amount of sugars recovered upon enzyme saccharification of the prehydrolysed wood was about 70% of the weight of the wood. When these combined hemicellulose and cellulose sugars were fermented by a pentose-fermenting strain of yeast,Pichia stipitis R, 372 L ethanol/tonne of (dry) wood was obtained. 相似文献
90.
Stimulation of acid invertase activity by indol-3yl-acetic acid in tissues undergoing cell expansion
The soluble acid invertase activity of young, excised P. vulgaris internodal segments fell when they were incubated in water, and their elongation ceased within 6–7 h. IAA (10 M) promoted segment elongation and stimulated an increase in the specific activity of acid invertase to a level greater than that originally present. The rate of segment elongation in the presence of IAA was closely and positively correlated with the specific activity of the enzyme. Optimum concentration of IAA for both elongation and stimulation of invertase activity was 10 M. Concurrent protein synthesis was necessary for these responses to IAA. Segments cut from mature, fully-elongated internodes did not responsd to IAA.Inclusion of Ca2+, vanadate or mannitol in the incubation medium abolished IAA-induced segment elongation but did not inhibit the stimulation of acid invertase activity by IAA. Auxin-induced elongation and acid invertase activity were both substantially increased in the presence of up to 25 mM D-glucose or up to 50 mM sucrose. Inclusion of either sugar in the medium considerably increased tissue hexose concentrations. Under some circumstances cell growth and invertase synthesis may compete for available hexose substrate.It is concluded that IAA-induced promotion of acid invertase in P. vulgaris internodal segments is not simply an indirect consequence of removal of end-product (hexose) during IAA-induced cell growth and that a more direct action of IAA on enzyme turnover is involved. 相似文献