全文获取类型
收费全文 | 1582篇 |
免费 | 485篇 |
学科分类
生物科学 | 2067篇 |
出版年
2022年 | 17篇 |
2021年 | 15篇 |
2020年 | 17篇 |
2019年 | 9篇 |
2018年 | 14篇 |
2017年 | 21篇 |
2016年 | 41篇 |
2015年 | 54篇 |
2014年 | 67篇 |
2013年 | 84篇 |
2012年 | 92篇 |
2011年 | 76篇 |
2010年 | 49篇 |
2009年 | 39篇 |
2008年 | 65篇 |
2007年 | 66篇 |
2006年 | 54篇 |
2005年 | 51篇 |
2004年 | 56篇 |
2003年 | 43篇 |
2002年 | 36篇 |
2001年 | 41篇 |
2000年 | 30篇 |
1999年 | 45篇 |
1998年 | 31篇 |
1997年 | 35篇 |
1996年 | 35篇 |
1995年 | 45篇 |
1994年 | 56篇 |
1993年 | 43篇 |
1992年 | 65篇 |
1991年 | 59篇 |
1990年 | 46篇 |
1989年 | 57篇 |
1988年 | 42篇 |
1987年 | 48篇 |
1986年 | 38篇 |
1985年 | 43篇 |
1984年 | 46篇 |
1983年 | 25篇 |
1982年 | 24篇 |
1981年 | 34篇 |
1980年 | 23篇 |
1979年 | 32篇 |
1978年 | 21篇 |
1977年 | 16篇 |
1976年 | 16篇 |
1975年 | 15篇 |
1974年 | 10篇 |
1973年 | 12篇 |
排序方式: 共有2067条查询结果,搜索用时 15 毫秒
1.
We have used variations in the trypsin sensitivity of eukaryotic protein synthesis elongation factor 2 (eEF-2) to probe for structural alterations induced by phosphorylation, ribosomal binding, or guanosine nucleotides. We could not detect any nucleotide-related effect on the tryptic cleavage rate of Arg66. However, eEF-2 was protected from trypsin after ribosomal binding. Also, phosphorylation of eEF-2 led to a protection of Arg66. This indicates that phosphorylation leads to a structural rearrangement that could explain the reduced affinity of the phosphorylated factor for ribosomes (Carlberg, U., Nilsson, A., and Nyg?rd, O. (1990) Eur. J. Biochem. 191, 639-645). Cleavage of Arg66 led to a complete loss of the ability of the factor to be phosphorylated. Furthermore, ribosome-bound eEF-2 was found to be inaccessible for phosphorylation. Based on these findings and previously published data, we suggest that the region around the sites of phosphorylation and trypsin cleavage is vitally important for the factor function and ribosomal binding. 相似文献
2.
3.
A M Dunning R Houlston J Frosteg?rd J Revill J Nilsson A Hamsten P Talmud S Humphries 《Biochimica et biophysica acta》1991,1096(3):231-237
We have searched for sequence differences in the region of the apolipoprotein B (apo B) gene encoding amino acids 3130-3630 in eight individuals with reduced affinity of low density lipoprotein (LDL) for the normal LDL-receptor. All individuals were hypercholesterolaemic and were selected either on the basis of reduced fractional catabolic rate (FCR) of autologous LDL or substantially reduced binding of their LDL to normal LDL-receptors determined by an in vitro cell growth assay using the U937 macrophage-like cell line. Segments of the apo B gene were amplified by the polymerase chain reaction. Using a combination of cloning and sequencing the amplified fragment, together with chemical cleavage mismatch analysis, no sequence differences were identified in this region of the gene. We therefore conclude that variation outside the region of the apo B gene that codes for amino acids 3130-3630 must be responsible for the reduced LDL clearance in these patients. 相似文献
4.
Ty1 sequence with enhancer and mating-type-dependent regulatory activities. 总被引:17,自引:9,他引:8 下载免费PDF全文
Some insertion mutations in Saccharomyces cerevisiae activate the expression of adjacent structural genes. The CYC7-H2 mutation is a Ty1 insertion 5' to the iso-2-cytochrome c coding region of CYC7. The Ty1 insertion causes a 20-fold increase in CYC7 expression in a and alpha haploid cell types of S. cerevisiae. This activation is repressed in the a/alpha diploid cell type. Previous computer analysis of the CYC7-H2 Ty1 activator region identified two related sequences with homology both to mammalian enhancers and to a yeast a/alpha control site. A 112-base-pair (bp) DNA fragment encompassing one of these blocks of homology functioned as one component of the Ty1 activator. A 28-bp synthetic oligonucleotide with the wild-type homology block sequence was also functional. A single base pair mutation within the enhancer core of the synthetic 28-bp regulatory element reduced its activation ability to near background amounts. In addition, the 112-bp Ty1 fragment by itself functioned as a target for repression of adjacent gene expression in a/alpha diploid cells. 相似文献
5.
Expression of rat transforming growth factor alpha mRNA during development occurs predominantly in the maternal decidua. 总被引:11,自引:5,他引:6 下载免费PDF全文
V K Han E S Hunter rd R M Pratt J G Zendegui D C Lee 《Molecular and cellular biology》1987,7(7):2335-2343
Previous studies have shown that transforming growth factor alpha is expressed during rodent development. To establish the site(s) of transforming growth factor alpha mRNA expression during rat embryogensis, we performed in situ hybridization and Northern blot analyses on samples of embryonic and maternal tissues at various gestational ages. Our results indicate that the high levels of transforming growth factor alpha mRNA that are observed during early development are the result of expression in the maternal decidua and not in the embryo. Decidual expression appears to be induced after implantation, peaks at day 8, and then slowly declines through day 15 at which time the decidua is being resorbed. Expression of transforming growth factor alpha mRNA is highest in that region of the decidua adjacent to the embryo and is low or nondetectable in the uterus, placenta, and other maternal tissues. The developmentally regulated expression of transforming growth factor alpha mRNA in the decidua, together with the presence of epidermal growth factor receptors in this tissue, suggests that transforming growth factor alpha stimulates proliferation locally through an autocrine mechanism. Since epidermal growth factor receptors are present in the embryo and placenta, transforming growth factor alpha produced in the decidua may also act on these tissues through paracrine or endocrine mechanisms. 相似文献
6.
7.
N-linked oligosaccharide changes with oncogenic transformation require sialylation of multiantennae 总被引:1,自引:0,他引:1
U V Santer R DeSantis K J H?rd J A van Kuik J F Vliegenthart B Won M C Glick 《European journal of biochemistry》1989,181(1):249-260
Glycopeptides derived from NIH 3T3 fibroblasts and these cells transformed by transfection with human DNA containing oncogene H-ras were analyzed by 500-MHz 1H-NMR spectroscopy and binding to immobilized lectins. The cells were metabolically labeled with D-[3H]glucosamine or L-[3H]fucose and the glycopeptides included in Bio-Gel P-10 (Mr 5000-3500) were separated into neutral and charged fractions on DEAE-cellulose. The major portion (80%) of these [3H]fucose glycopeptides from the non-transformed NIH 3T3 fibroblasts were neutral or contained one or two charged residues, whereas 90% of the glycopeptides from the transformed cells contained two or more charged residues. The structure of the predominant neutral glycopeptide from the non-transformed NIH 3T3 cells was determined by 1H-NMR spectroscopy to be tetraantennary containing terminal Gal alpha 1----3. (formula; see text) This structure was verified by binding to the immobilized alpha-Gal-specific lectin, Griffonia simplicifolia I and leukoagglutinating phytohemagglutinin from Phaseolus vulgaris (L-PHA), which binds certain tri- or tetraantennary glycopeptides. In contrast, the structure derived by NMR spectroscopy of one of the predominant charged glycopeptides from the transformed cells was triantennary containing terminal NeuNAc alpha 2----3 in addition to Gal alpha 1----3. (formula; see text) In attempting to verify this structure by lectin-binding properties it was found that removal of NeuNAc alpha 2----3 reduced the affinity to L-PHA - agarose. The other major glycopeptides of the transformed cells which were more charged also cotained NeuNAc alpha 2----3 but no NeuNAc alpha 2----6 or Gal alpha 1----3. A tentative structure was proposed for the major glycopeptide of the first charged class from NIH 3T3 cells on the basis of lectin-binding properties and the NMR spectrum which showed, in addition to NeuNAc alpha 2----3, the presence of NeuNAc alpha 2----6 and Gal alpha 1----3. On the basis of the NMR spectrum and other results, it is concluded that the presence of tetraantennary oligosaccharides are not sufficient for the transformed oligosaccharide phenotype. Rather, the tri- or tetraantennae must be sialylated in alpha 2----3 linkage, on more than one antennae, when properties of transformation are expressed in NIH 3T3 cells. Prior to transformation the tetraantennary oligosaccharides of these cells are terminated in alpha-Gal residues, whereas after transformation alpha-Gal residues appear to be replaced by NeuNAc alpha 2----3.(ABSTRACT TRUNCATED AT 400 WORDS) 相似文献
8.
A glutathione transferase from human mononuclear leukocytes with a high activity towardtrans-stilbene oxide (GT-tSBO) has been studied in liver and blood from fetus and adults and in blood from neonates. Using starch gel electrophoresis, different phenotypes of GST1 have been determined, GST1 0, GST1 1, and GST1 2. As judged from activity measurements and the fact that only those individuals who express the null allele of GST1, the GST1 0, which has a low activity towardtrans-stilbene oxide, it is concluded that the hepatic transferase GST1 is identical to GT-tSBO, as well as to hepatic transferase μ. In addition, it has been shown that the different genotypes of GST1 1 (GST1 1-1, GST1 1-0) and GST1 2 (GST1 2-2, GST1 2-0) can be separated by measuring the GT-tSBO activity in whole blood from the same individual. It is also demonstrated that GT-tSBO activity is much lower in fetal liver, approximately 10 times, compared with adult liver, while this activity seems to be unchanged in the blood from fetus and adults, as well as in neonates. 相似文献
9.
Structure and function of extracellular matrix proteoglycans 总被引:1,自引:0,他引:1
10.
Filtration rate capacities in 6 species of European freshwater bivalves 总被引:12,自引:0,他引:12
Summary Filtration rate capacities in undisturbed freshwater bivalves were determined by means of two different methods (indirect clearancerdquo" align="MIDDLE" BORDER="0"> and suctionrdquo" align="MIDDLE" BORDER="0"> methods) in Anodonta anatina (L.), Unio tumidus Philipsson, Unio pictorum (L.), Unio crassus Philipsson, Dreissena polymorpha (Pallas) and Sphaerium corneum (L.). In A. anatina, D. polymorpha, and S. corneum the filtration rate (FR, 1 h-1) at 19–20°C as a function of dry tissue weight (DW, g) or ash-free dry weight (AFDW, g) could be expressed by the equations: 1.10 DW0.78, 6.82 DW0.88, and 2.14 AFDW0.92, respectively. In U. tumidus, U. pictorum, and U. crassus filtration rates were comparable with those of A. anatina. In D. polymorpha the b value of the corresponding regression of gill area on dry weight was 0.87. The rates of water transport in freshwater bivalves are 2–8 times lower than in marine bivalves of comparable size. A corresponding difference in the filtration rate per gill area unit is found. The measured filtration rates in undisturbed bivalves are substantially higher (at least 4 times) than previously reported. This indicates that the impact of bivalve water processing on freshwater ecosystems is greater than hitherto suggested. 相似文献