全文获取类型
收费全文 | 6301篇 |
免费 | 533篇 |
国内免费 | 5篇 |
出版年
2023年 | 39篇 |
2022年 | 96篇 |
2021年 | 176篇 |
2020年 | 90篇 |
2019年 | 96篇 |
2018年 | 138篇 |
2017年 | 131篇 |
2016年 | 221篇 |
2015年 | 402篇 |
2014年 | 440篇 |
2013年 | 435篇 |
2012年 | 622篇 |
2011年 | 571篇 |
2010年 | 337篇 |
2009年 | 292篇 |
2008年 | 404篇 |
2007年 | 402篇 |
2006年 | 386篇 |
2005年 | 298篇 |
2004年 | 312篇 |
2003年 | 212篇 |
2002年 | 219篇 |
2001年 | 44篇 |
2000年 | 29篇 |
1999年 | 49篇 |
1998年 | 42篇 |
1997年 | 46篇 |
1996年 | 30篇 |
1995年 | 20篇 |
1994年 | 35篇 |
1993年 | 20篇 |
1992年 | 23篇 |
1991年 | 20篇 |
1990年 | 12篇 |
1989年 | 14篇 |
1988年 | 5篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 10篇 |
1983年 | 12篇 |
1982年 | 7篇 |
1981年 | 7篇 |
1980年 | 11篇 |
1978年 | 6篇 |
1977年 | 13篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 5篇 |
1970年 | 4篇 |
排序方式: 共有6839条查询结果,搜索用时 31 毫秒
51.
Three endocyclic sulfoximides of the 1-aryl- and 1-alkyl-3-oxo-benzo[d]-isothia (IV)-azole 1-oxide type (1-substituent = 2′-carboxyphenyl, 2′-carbethoxyphenyl, and octyl, respectively) were found to be well resolved on a chiral phase derived from bovine serum albumin (BSA). Selectivities (α) of 1.74, 1.12, and 1.44, respectively, were obtained. The retention behaviour of 1-octyl-3-oxo-benzo[d]isothia(IV)-azole 1-oxide was further investigated in some detail as a function of the mobile phase composition and the elution order was established from optically active material obtained from the enantiopure sulfoxide precursor. An enantiomeric excess of 85.4% was obtained in the cyclocondensation reaction of the octyl-substituted sulfoxide precursor with hydrazoic acid to the corresponding endocyclic sulfoximide. © 1995 Wiley-Liss, Inc. 相似文献
52.
Mobile gene cassettes and integrons: capture and spread of genes by site-specific recombination 总被引:38,自引:0,他引:38
An integron is a genetic unit that includes the determinants of the components of a site-specific recombination system capable of capturing and mobilizing genes that are contained in mobile elements called gene cassettes. An integron also provides a promoter for expression of the cassette genes, and integrons thus act both as natural cloning systems and as expression vectors. The essential components of an integron are an int gene encoding a site-specific recombinase belonging to the integrase family, an adjacent site, attl, that is recognized by the integrase and is the receptor site for the cassettes, and a promoter suitably oriented for expression of the cassette-encoded genes. The cassettes are mobile elements that include a gene (most commonly an antibiotic-resistance gene) and an integrase-specific recombination site that is a member of a family of sites known as 59-base elements. Cassettes can exist either free in a circularized form or integrated at the attl site, and only when integrated is a cassette formally part of an integron. A single site-specific recombination event involving the integron-associated attl site and a cassette-associated 59-base element leads to insertion of a free circular cassette into a recipient integron. Multiple cassette insertions can occur, and integrons containing several cassettes have been found in the wild. The integrase also catalyses excisive recombination events that can lead to loss of cassettes from an integron and generate free circular cassettes. Due to their ability to acquire new genes, integrons have a clear role in the evolution of the genomes of the plasmids and transposons that contain them. However, a more general role in evolution is also likely. Events involving recombination between a specific 59-base-element site and a nonspecific secondary site have recently been shown to occur. Such events should lead either to the insertion of cassettes at non-specific sites or to the formation of stable cointegrates between different plasmid molecules, and a cassette situated outside the integron context has recently been identified. 相似文献
53.
54.
Albert de la Chapelle Christina Kari Markku Nurminen Sven Hernberg 《Human genetics》1977,37(2):183-194
Summary An epidemic of agranulocytosis and granulocytopenia occurred in 1975 in conjunction with clozapine treatment of mental patients in Finland. An attempt was made to assess the epidemiologic and genetic factors contributing to the adverse drug effect. The estimated incidence rate in Finland was 2.1/1000 patient-months. This figure could not be compared with rates from other countries because of the inexact nature of the figures reported so far. All 16 cases occurred in seven hospitals in southwestern Finland, whereas the overall hospital net use of the drug was geographically evenly distributed. The difference between the observed and the proportionally expected incidence of cases amongst the hospitals where clozapine was used was statistically significant. The average consumption of the drug did not differ between the hospitals where cases occurred and those where no definite cases could be diagnosed. Six-generation pedigree analyses failed to reveal significant parental consanguinity or genetic kinship between probands. Neither did the birth places of the ancestors of the probands disclose a typical isolate pattern. In conclusion, the cases appeared to be confined to a few hospitals in southwestern Finland. Although a genetic factor is not excluded, we found no evidence in support of a genetic mechanism. 相似文献
55.
Christina M. Hansen Bay 《Journal of insect physiology》1978,24(2):141-149
The salivary glands of adult Calliphora contain enzymes which hydrolyze starch, sucrose and trehalose. Amylase and sucrase are shown to be secretory enzymes, while trehalase remains in the gland. Results of electrophoretic and ultrastructural studies suggest that protein secretion is confined to the abdominal region of the gland. Secretion of both fluid and protein occurs from a single type of cell. While a fly is feeding on solid sugar, amylase and sucrase are lost from the gland and appear in saliva, while the level of trehalase in the gland increases slightly. The mixture of food and saliva passes mainly to the crop where carbohydrate is digested by the salivary enzymes. 相似文献
56.
57.
Christina Skarpe 《植被学杂志》1992,3(3):293-300
Abstract. Theories concerning the factors involved in the dynamics of savannas, particularly the tree-grass interface, are reviewed. Emphasis is put on factors related to soil moisture, soil nutrients, fire and large herbivores. The distinction between external (independent) and internal (dependent, interactive) environment is discussed and it is explained how this distinction is affected by the scale of observation. 相似文献
58.
59.
Liver mitochondrial and microsomal DT-diaphorase have been purified from 3-methylcholanthrene-treated rats. A 1150-fold and 3500-fold purification of mitochondrial and microsomal DT-diaphorase, respectively, is achieved after solubilization of the membranes with deoxycholate followed by affinity chromatography on azodicoumarol Sepharose 6B and subsequent gel filtration on Sephadex G-100. From this purification procedure, 65–70% of mitochondrial DT-diaphorase is recovered and the purified enzyme has a specific activity comparable to that of cytosolic DT-diaphorase; i.e., 50.4 kat/kg protein. Microsomal DT-diaphorase is obtained with a yield of 45% and a specific activity of 15.5 kat/kg protein.Purified mitochondrial DT-diaphorase exhibits an absorption spectrum characteristic of a flavoprotein and very similar to that of the cytosolic enzyme. Purification of both mitochondrial and microsomal DT-diaphorase results in fractions enriched in a polypeptide with a molecular weight of 28,000 which comigrates with purified cytosolic DT-diaphorase on SDS-polyacrylamide gel electrophoresis. Employing antiserum raised against cytosolic DT-diaphorase, immunological identity between DT-diaphorase isolated from the three cell fractions is observed with both the Ouchterlony immunodiffusion technique and fused rocket immunoelectrophoresis. The latter method also reveals that DT-diaphorase isolated from mitochondria and microsomes contains several antigenic forms identical to those observed in purified cytosolic DT-diaphorase. Furthermore, this antiserum inhibits DT-diaphorase to about the same extent whether the enzyme is isolated from mitochondria, microsomes, or cytosol. In addition, this antiserum efficiently inhibits membrane-bound microsomal DT-diaphorase. 相似文献
60.
Individuals from natural populations of the leopard frog, Rana pipiens, were analyzed for electrophoretic differences in blood proteins and enzymes from an amputated digit. The proteins examined represent products of 72 loci. Presumptive heterozygotes at multiple loci were selected for experimental crosses. Mendelian inheritance of 18 protein variations were demonstrated in the offspring. Tests for linkage or independent assortment were performed for 75 locus pairs. Three linkage groups were established. Linkage group 1 contains two loci, aconitase-1 (Acon1) and serum albumin (Alb), with a 19% recombination frequency between them. Linkage group 2 contains four loci, glyoxalase (Gly), acid phosphatase-1 (Ap1), acid phosphatase-2 (AP2), and esterase-5 (Est5). The data show the relationships Gly-21.1%-AP1-0%-AP2-6.3%-Est5, and Gly-25.6%-Est5. Linkage group 3 consists of four closely linked esterase loci. The data, Est1-5.1%-Est6, Est6-1.8%-Est10-1.9%-Est4 and Est6-3.0%-Est4, do not establish a complete order but suggest that Est10 is between Est4 and Est6. These results, with data demonstrating apparent independent assortment of 67 other locus pairs, provide a foundation for establishing the frog genetic map.The project was supported by Grant No. RR-00572 from the Division of Research Resources, National Institutes of Health. This paper is contribution No. C-87 from the Amphibian Facility, George W. Nace, Director. 相似文献