全文获取类型
收费全文 | 29338篇 |
免费 | 2869篇 |
国内免费 | 27篇 |
出版年
2022年 | 270篇 |
2021年 | 451篇 |
2020年 | 262篇 |
2019年 | 364篇 |
2018年 | 406篇 |
2017年 | 399篇 |
2016年 | 579篇 |
2015年 | 947篇 |
2014年 | 983篇 |
2013年 | 1340篇 |
2012年 | 1540篇 |
2011年 | 1520篇 |
2010年 | 1119篇 |
2009年 | 879篇 |
2008年 | 1146篇 |
2007年 | 1186篇 |
2006年 | 1096篇 |
2005年 | 1090篇 |
2004年 | 1132篇 |
2003年 | 1048篇 |
2002年 | 1024篇 |
2001年 | 837篇 |
2000年 | 867篇 |
1999年 | 773篇 |
1998年 | 381篇 |
1997年 | 376篇 |
1996年 | 376篇 |
1995年 | 351篇 |
1994年 | 283篇 |
1993年 | 312篇 |
1992年 | 657篇 |
1991年 | 542篇 |
1990年 | 580篇 |
1989年 | 580篇 |
1988年 | 467篇 |
1987年 | 463篇 |
1986年 | 414篇 |
1985年 | 423篇 |
1984年 | 426篇 |
1983年 | 333篇 |
1982年 | 296篇 |
1981年 | 289篇 |
1980年 | 251篇 |
1979年 | 339篇 |
1978年 | 277篇 |
1977年 | 293篇 |
1976年 | 219篇 |
1975年 | 238篇 |
1974年 | 244篇 |
1973年 | 215篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
921.
922.
Cloning and molecular characterization of two genes encoding adhesion proteins involved in Trichomonas vaginalis cytoadherence 总被引:11,自引:2,他引:9
923.
A. R. Kleinig C. J. Mansell Q. D. Nguyen A. Badalyan A. P. J. Middelberg 《Biotechnology Techniques》1995,9(10):759-762
Summary The effect of cell concentration (5 to 150 g/L wet wt after broth dilution) on homogenizer disruption efficiency and homogenate viscosity is reported for E. coli. Broth dilution increases homogenizer efficiency and decreases feed and homogenate viscosity. However, this increase in disruption efficiency is not sufficient to warrant dilution of the broth prior to homogenization. The optimal feed concentration is the maximum possible that does not lead to practical handling difficulties due to high viscosity. 相似文献
924.
Rijcken W.Robert Pels; Ferwerda Wijnholt; Van den Eijnden Dirk H.; Overdijk Bernard 《Glycobiology》1995,5(5):495-502
Rat hepatocytes were incubated in the presence of a high concentrationof the hepatopathogenic agent D-galactosamine (GalN), and theeffect on the cellular concentrations of pyrimidine nucleotidesand nucleotide sugars was determined. The UTP pool became depleted.The pools of UMP and CMP in RNA decreased to 72%, indicativefor an inhibition of RNA synthesis. UDP-HexNAc (where HexNAcis GlcNAc + GalNAc) and UDP-HexN (where HexN is GlcN + GalN)levels increased, and those of UDP-hexose and UDP-GlcA (whereGlcA is glucuronic acid) decreased. The cellular concentrationof CTP did not change, whereas that of CMP-NeuAc (where NeuAcis N-acetylneuraminic add) showed a 2-fold increase. Labellingwith [14C]orotic acid and [3H]cytidine showed that the metabolicflow via the de novo pathway was not changed. The depletionof the so-called overflow pool of UTP [Pels Rijcken et al, Biochem.J., 293, 207213, 1993] caused a release of the feedbackinhibition by UTP and thus an increased flow through the salvagepathway. Finally, it appeared that GalN, when added to hepatocytes,gives rise to a pool of UDP-GlcNAc (where GlcNAc is N-acetylglueosamine)that is separate from the pool of UDP-GlcNAc that is derivedfrom GlcN. D-galactosamine glycosylation sugar nucleotide biosynthesis 相似文献
925.
The effect of acetaldehyde concentrations on the relative rates of formation of acetaldehyde-modified hemoglobins 总被引:1,自引:0,他引:1
L B Nguyen C M Peterson 《Proceedings of the Society for Experimental Biology and Medicine. Society for Experimental Biology and Medicine (New York, N.Y.)》1984,177(2):226-233
The effect of various concentrations of acetaldehyde (0, 0.05, 0.1, 0.25, 0.5, 1.0, and 5.0 mM) on the relative rates of formation of hemoglobin acetaldehyde adducts detected in fractions eluted from cation exchange high-pressure liquid chromatography (HPLC) was investigated. When the hemoglobin and acetaldehyde mixtures were incubated at 37 degrees C for various time intervals up to 24 hr, increased amounts of HbA1c could be observed after 2 hr incubation with 1 mM or greater concentrations of acetaldehyde, or after 4 hr incubation with at least 0.5 mM acetaldehyde. An increase in the HbA1a + b fraction was not observed with 4 hr incubation time until the acetaldehyde level reached 1 mM. The HPLC method detected no difference in minor hemoglobins from alcoholic and normal subjects. Incubation of red blood cells at 37 degrees C for 1 hr with six consecutive pulses of 0.05 mM [14C]acetaldehyde showed no differences in the amounts of minor hemoglobins determined chromatographically at various pulse intervals. However, the measure of the 14C-label incorporation into hemoglobin showed that adducts eluting in the HbA1a+b fraction were formed at a faster rate than those eluting in the HbA1c or HbA0 fraction, respectively. The specific activities of the HbA1a+b fractions at 2, 4, and 6 pulses were 34, 128, and 949 cpm/mg hemoglobin; those of the HbA1c fraction were 15, 58, and 174 cpm/mg hemoglobin. This evidence of modification of hemoglobin by physiological levels of acetaldehyde from 14C-label incorporation suggests that an assay more sensitive than chromatographic separation of adducts might be clinically useful in detecting alcoholism or monitoring alcohol detoxification programs. 相似文献
926.
927.
Use of histone antibodies for studying chromatin topography and the phosphorylation of chromatin subunits 总被引:2,自引:0,他引:2 下载免费PDF全文
Polyclonal and monoclonal antibodies specific for histones as well as sera directed against synthetic peptides of histones were used to probe the topography of chromatin subunits. In native chromatin, the regions corresponding to residues 130-135 of H3 and 6-18 of H2B were found to be exposed and able to interact with antibodies whereas the regions 26-35 and 36-43 of H2B and 80-89 and 85-102 of H4 were not. In vitro phosphorylation of H3 and H5 in native chromatin or of H3 in H1/H5-depleted chromatin led to a marked drop in the binding of antibodies specific for residues 130-135 of H3 and 6-18 of H2B. Phosphorylation of H1/H5-depleted chromatin also altered the degree of exposure of certain H2A epitopes but it did not affect the surface accessibility of residues 1-11 of H2B. 相似文献
928.
De Block M Herrera-Estrella L Van Montagu M Schell J Zambryski P 《The EMBO journal》1984,3(8):1681-1689
Chimeric genes comprised of the nopaline synthase promoter and bacterial coding sequences specifying resistance to kanamycin, chloramphenicol or methotrexate, were inserted into the non-oncogenic Ti plasmid vector pGV3850 by recombination (through homologous pBR322 sequences present in the chimeric gene constructs and pGV3850). These co-integrates in Agrobacterium were used to infect single plant protoplasts of Nicotiana by co-cultivation. The resistance traits allowed the selection of transformed calli in tissue culture in the presence of the appropriate antibiotic. Furthermore, as a non-oncogenic Ti plasmid was used for the protoplast transformation, phenotypically normal and fertile plants could be regenerated from the resistant calli. We have shown that these fully differentiated plant tissues exhibit functional expression of resistance traits (KmR and CmR). All plants carrying the chimeric genes developed normally, flowered, and set seeds. The inheritance of several of these resistance traits was analyzed and shown to be Mendelian. These results are model experiments to demonstrate that genes of interest can be systematically transferred to the genome of plants using non-oncogenic Ti plasmid derivatives; and that transformed plants are capable of normal growth and differentiation, thus providing a natural environment for the study of gene expression and development of plant cells. 相似文献
929.
Intervening sequences in the ribosomal RNA genes of Ascaris lumbricoides: DNA sequences at junctions and genomic organization 总被引:13,自引:1,他引:12 下载免费PDF全文
E Back E Van Meir F Müller D Schaller H Neuhaus P Aeby H Tobler 《The EMBO journal》1984,3(11):2523-2529
An rDNA size class in the genome of the nematode Ascaris lumbricoides is described which is interrupted by a 4.5-kb long intervening sequence located in the 26S coding region. This molecular form occurs in approximately 15 copies per haploid genome and amounts to approximately 5% of the total nuclear rDNA. Intervening sequences are present only in the 8.8-kb rDNA, but not in the 8.4-kb rDNA repeating units of A. lumbricoides. Cloning of the interrupted rDNA units revealed, in addition to the main 4.5-kb insertion, shorter intervening sequences of 4-kb and 119-bp length. Both shorter rDNA forms are present in the single copy range of the haploid genome. Sequence analyses of the intervening sequence/rDNA junctions show an identical right-hand junction for all of the three different rDNA forms. The two shorter intervening sequences are a coterminal subset of the right-hand end of the main 4.5-kb insertion, whereas all three insertions have a different left-hand junction with the coding region of rDNA. Each intervening sequence is flanked by a short direct repeat of variable length, being only once present in the uninterrupted rDNA. The intervening sequences of A. lumbricoides show striking similarity to the organization of type I insertion family in dipteran flies, even though they are inserted at different positions in the 26S coding region. Additional rDNA intervening sequences may be present outside of the rDNA cluster, but in not more than 15-20 homologous copies per haploid genome. 相似文献
930.
Binding of human blood-coagulation Factors IXa and X to phospholipid membranes. 总被引:4,自引:3,他引:1 下载免费PDF全文
A simple centrifugation technique has been developed to study the interaction of human coagulation Factors IXa and X with phospholipid membranes. In the presence of Ca2+, equimolar phosphatidylserine/phosphatidylcholine membranes form tight complexes with Factor X (KD = 2.8 X 10(-8) M); the KD is independent of the phospholipid concentration. Binding sites are available for about 2 mmol of Factor X/mol of phospholipid. Factor IXa has a slightly higher affinity for the phospholipid membrane (KD = 1.2 X 10(-8)M), and competes with Factor X for binding. The experimentally observed competition between Factor X and Factor IXa is in agreement with a model that describes the binding of two distinct ligands to a single class of independent binding sites. 相似文献