全文获取类型
收费全文 | 229615篇 |
免费 | 13515篇 |
国内免费 | 861篇 |
出版年
2022年 | 1018篇 |
2021年 | 2241篇 |
2020年 | 1339篇 |
2019年 | 1727篇 |
2018年 | 13478篇 |
2017年 | 12150篇 |
2016年 | 10410篇 |
2015年 | 6244篇 |
2014年 | 6485篇 |
2013年 | 7860篇 |
2012年 | 13760篇 |
2011年 | 21476篇 |
2010年 | 17404篇 |
2009年 | 13477篇 |
2008年 | 17099篇 |
2007年 | 18620篇 |
2006年 | 7531篇 |
2005年 | 7523篇 |
2004年 | 7711篇 |
2003年 | 7338篇 |
2002年 | 6810篇 |
2001年 | 2619篇 |
2000年 | 2277篇 |
1999年 | 2257篇 |
1998年 | 1969篇 |
1997年 | 1533篇 |
1996年 | 1429篇 |
1995年 | 1351篇 |
1994年 | 1200篇 |
1993年 | 1336篇 |
1992年 | 1826篇 |
1991年 | 1461篇 |
1990年 | 1467篇 |
1989年 | 1409篇 |
1988年 | 1272篇 |
1987年 | 1155篇 |
1986年 | 1133篇 |
1985年 | 1390篇 |
1984年 | 1293篇 |
1983年 | 1104篇 |
1982年 | 1109篇 |
1981年 | 998篇 |
1980年 | 950篇 |
1979年 | 821篇 |
1978年 | 838篇 |
1977年 | 677篇 |
1976年 | 655篇 |
1974年 | 675篇 |
1973年 | 633篇 |
1972年 | 679篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Flow cytometry (FCM) in conjunction with immunocytochemical-labeling was used to analyze and screen a population of Escherichia coli clones containing a genomic library from the oil-degrading microorganism Acinetobacter calcoaceticus RAG-1 for the isolation of clones which expressed specific RAG-1 surface antigens. Reconstruction experiments using mixed populations indicated that RAG-1 cells could be clearly distinguished at a ratio of one RAG-1 cell to 500 Escherichia coli cells. Using this technique two clones, WM143 and WM191, were isolated and shown by restriction endonuclease cleavage and Southern hybridization to contain plasmids carrying inserts of RAG-1 DNA of 9.4 and 9.8 kb respectively.Non-common abbreviations FCM
flow cytometry
- FITC
fluorescein-iso-thiocyanate
- LB
Luria broth
- MM
minimal salt medium
- PBS
phosphate buffered saline
- PMSF
phenylmethylsulfonyl fluoride 相似文献
942.
A degradation pathway for dl--phenylhydracrylic, phenylacetic, 3- and 4-hydroxyphenylacetic acid by a Flavobacterium is presented. Experiments with washed cells and enzyme studies revealed that dl--phenylhydracrylic acid in an initial reaction was oxidatively decarboxylated to phenylacetaldehyde. Whole cells oxidized both stereoisomers of phenylhydracrylic acid at different rates. The product phenylacetaldehyde in turn was oxidized to phenylacetic acid. No hydroxylation of phenylacetic acid was detected in cell extracts, but on the basis of experiments with washed cells it is assumed that phenylacetic acid is mainly metabolized via 3-hydroxyphenylacetic acid. This latter product was subsequently hydroxylated yielding the ring-cleavage substrate homogentisate. 4-Hydroxyphenylacetic acid was also degraded via homogentisate. Ringcleavage of homogentisate gave maleylacetoacetate which was further degraded through a glutathione-dependent pathway. Homoprotocatechuate was not an intermediate in the metabolism of dl-phenylhydracrylic acid, phenylacetic, 3- and 4-hydroxyphenylacetic acid metabolism, but it could be hydroxylated aspecifically to 2,4,5-trihydroxyphenylacetic acid by the action of the 3-hydroxyphenylacetic acid-6-hydroxylase.Abbreviations HPLC
high-performance liquid chromatography
- PHA
phenylhydracrylic acid
- PA
phenylacetic acid
- HPA
hyxdroxyphenylacetic acid
- PMS
phenazine methosulphate
- PMA
phenylmalonic acid
- GSH
glutathione 相似文献
943.
944.
945.
946.
947.
948.
Uno Lindberg Clarence E. Schutt Eva Hellsten Ann-Christine Tjder Thomas Hult 《Biochimica et Biophysica Acta (BBA)/General Subjects》1988,967(3)
In the purification of proline hydroxylase by affinity chromatography on poly(L-proline)-Sepharose it was found earlier that two other components, profilin and the complex profilin-actin, also bind with high affinity to this matrix. We have exploited this observation to develop a rapid procedure for the isolation of profilin and profilin-actin complexes in high yields directly from high-speed supernatants of crude tissue-extracts. Through an extensive search for elution conditions, avoiding poly(L-proline) as the desorbant, we have found that active proteins can be recovered from the affinity column with a buffer containing 30% dimethyl sulphoxide. Subsequent chromatography on hydroxylapatite separates free profilin and the two isoforms of profilactin, profilin-actinβ and profilin-actinγ. The profilin-actin complexes produced this way have high specific activities in the DNAase-inhibition assay, give rise to filaments on addition of Mg2+, and can be crystallized. From the isolated profilin-actin complexes the β- and γ-actin isoforms of non-muscle cells can easily be prepared in a polymerization competent form. Pure profilin is either obtained from an excess pool present in some extracts or by dissociation of profilin-actin complexes and removal of the actin. 相似文献
949.
Cannibalism as a life boat mechanism 总被引:2,自引:0,他引:2
Under certain conditions a cannibalistic population can survive when food for the adults is too scarce to support a non-cannibalistic population. Cannibalism can have this lifeboat effect if (i) the juveniles feed on a resource inaccessible to the adults; and (ii) the adults are cannibalistic and thus incorporate indirectly the inaccessible resource. Using a simple model we conclude that the mechanism works when, at low population densities, the average yield, in terms of new offspring, due to the energy provided by one cannibalized juvenile is larger than one. 相似文献
950.
D Valerio H van der Putten F M Botteri P M Hoogerbrugge 《Nucleic acids research》1988,16(21):10083-10097
The promoter of the human gene for adenosine deaminase (ADA) is extremely G/C-rich, contains several G/C-box motifs (GGGCGGG) and lacks any apparent TATA or CAAT boxes. These features are commonly found in promoters of genes that lack a strong tissue specificity, and are referred to as "housekeeping genes". Like other housekeeping genes, the ADA gene is expressed in all tissues. However, there is a considerable variation in the levels of expression of the ADA protein in different tissues. In order to study the activity of the ADA promoter, transgenic mice were generated that harbor a chimeric gene composed of the ADA promoter linked to a reporter gene encoding the bacterial enzyme Chloramphenicol Acetyl Transferase (CAT). These mice reproducibly showed CAT expression in all tissues examined, including the hemopoietic organs (spleen, thymus and bone marrow). However, examination of the actual cell types expressing the CAT gene revealed the ADA promoter to be inactive in the hemopoietic cells. This was substantiated by a transplantation experiment in which bone marrow from ADA-CAT transgenic mice was used to reconstitute the hemopoietic compartment of lethally irradiated mice. The engrafted recipients revealed strongly reduced CAT activity in their hemopoietic organs. The lack of expression in hemopoietic cells was further shown to be correlated with a hypermethylated state of the transgene. Combined, our data suggest that the ADA promoter sequences tested can direct expression in a wide variety of tissues as expected for a regular housekeeping gene promoter. However, the activity of the ADA promoter fragment did not reflect the tissue-specific variations in expression levels of the endogenous ADA gene. Additionally, regulatory elements are needed for expression in the hemopoietic cells. 相似文献