全文获取类型
收费全文 | 108086篇 |
免费 | 8724篇 |
国内免费 | 49篇 |
出版年
2023年 | 536篇 |
2022年 | 1069篇 |
2021年 | 2085篇 |
2020年 | 1535篇 |
2019年 | 1791篇 |
2018年 | 2548篇 |
2017年 | 2209篇 |
2016年 | 3266篇 |
2015年 | 4623篇 |
2014年 | 4866篇 |
2013年 | 6045篇 |
2012年 | 7369篇 |
2011年 | 6965篇 |
2010年 | 4399篇 |
2009年 | 4075篇 |
2008年 | 5346篇 |
2007年 | 5204篇 |
2006年 | 4694篇 |
2005年 | 4683篇 |
2004年 | 4728篇 |
2003年 | 4009篇 |
2002年 | 3486篇 |
2001年 | 2665篇 |
2000年 | 2498篇 |
1999年 | 2176篇 |
1998年 | 1157篇 |
1997年 | 977篇 |
1996年 | 1008篇 |
1995年 | 867篇 |
1994年 | 795篇 |
1993年 | 761篇 |
1992年 | 1436篇 |
1991年 | 1296篇 |
1990年 | 1188篇 |
1989年 | 1165篇 |
1988年 | 1106篇 |
1987年 | 1032篇 |
1986年 | 944篇 |
1985年 | 927篇 |
1984年 | 816篇 |
1983年 | 683篇 |
1982年 | 535篇 |
1981年 | 493篇 |
1980年 | 473篇 |
1979年 | 641篇 |
1978年 | 528篇 |
1977年 | 432篇 |
1975年 | 443篇 |
1974年 | 478篇 |
1973年 | 460篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Evidence for a rate-limiting role of cysteinesulfinate decarboxylase activity in taurine biosynthesis in vivo 总被引:1,自引:0,他引:1
J de la Rosa M H Stipanuk 《Comparative biochemistry and physiology. B, Comparative biochemistry》1985,81(3):565-571
The relationship between activities of enzymes involved in cysteine oxidation and the apparent conversion of cysteine to taurine in vivo were investigated in the rat and cat. Both hepatic cysteinesulfinate decarboxylase activity and the oxidation in vivo of cysteine to taurine were lower in the kitten than in the adult female rat and lower in the latter than in the young male rat. Our data support the hypothesis that cysteinesulfinate decarboxylase plays a rate-limiting role in taurine biosynthesis. 相似文献
992.
993.
994.
Variants within the yeast Ty sequence family encode a class of structurally conserved proteins. 总被引:10,自引:3,他引:7
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)
A M Fulton J Mellor M J Dobson J Chester J R Warmington K J Indge S G Oliver P de la Paz W Wilson A J Kingsman 《Nucleic acids research》1985,13(11):4097-4112
The Ty transposable elements of Saccharomyces cerevisiae form a heterogeneous family within which two broad structural classes (I and II) exist. The two classes differ by two large substitutions and many restriction sites. We show that, like class I elements a class II element, Tyl-17, also appears to contain at least two major protein coding regions, designated TYA and TYB, and the organisational relationship of these regions has been conserved. The TYA genes of both classes encode proteins, designated p1 proteins, with an approximate molecular weight of 50 Kd and, despite considerable variation between the TYA regions at the DNA level, the structures of these proteins are remarkably similar. These observations strongly suggest that the p1 proteins of Ty elements are functionally significant and that they have been subject to selection. 相似文献
995.
Summary rRNA gene activity was evaluated by cytologic methods in cultured human cells from two different tissues grown under controlled experimental conditions. The modal and average numbers of silver positive nucleolus organizers (NOs) per cell as well as the distribution of cells with different numbers of silver positive NOs and different combinations of D-plus G-group silver stained chromosomes, were evaluated. Statistically significant differences in the average number of silver positive NOs per cell between leukocytes and fibroblasts grown under standard experimental conditions have been demonstrated. The observed differences became sharper in cells cultured under more restrictive conditions. Also, differences in the frequency of silver positivity of specific chromosomal NOs located on individually indentified chromosomes were observed in cells from the same tissue. Furthermore, differences in the frequency of activation of rDNA clusters located on the same chromosome were also observed between cells from the two tissues. The possible biologic meanings of these findings are discussed.This paper is dedicated to Professor G. Montalenti on the occasion of his 80th birthday 相似文献
996.
Dr. Jan A. M. de Bont Annie Scholten Will J. J. van den Tweel 《Current microbiology》1985,12(5):267-271
The microbial potential to degrade acetylenic compounds (alkynes) was investigated, and several fungi and bacteria were isolated on 2-propyn-1-ol, 3-butyn-1-ol, propynoic acid, and 2-butyne-1,4-diol. The results indicate that a wide variety of microorganisms may degrade alkynes in nature. 相似文献
997.
Summary Expression of the anchorage-independent transformed phenotype in BHK 21/13 cells generally behaves as a recessive trait. When
chemically induced and spontaneously arising transformants are fused to the nontransformed parent line, transformation is
initially suppressed, reappearing after extended growth of the hybrids. In this paper, complementation for the expression
of anchorage independence was sought among a large group of such transformants, all independently derived from BHK 21/13 cells.
Tumorigenicity studies on selected hybrids and parental lines indicated that the in vitro trait of anchorage independence
is an accurate indicator of in vivo neoplasia for these cells. Seventeen of the 18 clones tested did not complement one or
more of three tester strains. This result indicates that anchorage independence arose in these clones as a result of lesions
in the same genetic function and suggests that the final step in the progressive changes of carcinogenesis may frequently
be restricted to lesions at a single locus.
This investigation was supported by National Institutes of Health grant CA27306. 相似文献
998.
Expression of the gene encoding protein A in Staphylococcus aureus and coagulase-negative staphylococci 总被引:13,自引:4,他引:9
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Two shuttle vectors containing the gene for protein A (spa) from Staphylococcus aureus have been constructed to study expression of the gene in various strains of S. aureus and in the coagulase-negative species Staphylococcus epidermidis, Staphylococcus capitis, and Staphylococcus xylosus. One plasmid, pSPA15, contains the complete structural gene for protein A, which binds to the cell wall in various Staphylococcus species. The other plasmid, pSPA16, codes for a truncated protein A lacking the C-terminal part called region X. The latter is exclusively extracellular in all Staphylococcus species tested, which confirms the importance of region X for cell wall binding. The expression of the plasmid-coded protein A in various strains of S. aureus is strongly correlated to the expression of the chromosomal spa gene. The coagulase-negative species expressing plasmid-encoded protein A produce 12 to 30% of the amount coded by the chromosomal spa gene in S. aureus strains Cowan I and A676. 相似文献
999.
B. Cesca M. C. Manca de Nadra A. M. Strasser de Saad A. Pesce de Ruiz Holgado G. Oliver 《Folia microbiologica》1984,29(4):288-294
The activity of β-D-galactosidase was studied in 13 strains of lactobacilli (groupsStreptobacterium, Thermobacterium andBetabacterium). Using 2-nitrophenyl galactopyranoside as substrate, the enzyme activity varied with the strain. The values found in theThermobacterium group were superior to those in theStreptobacterium group. The optimum pH for the species belonging to theThermobacterium group was uniform, in contrast to the ph for those from theStreptobacterium which varied according to the species. The optimum temperature was quite uniform within each group and higher in theStreptobacterium. Lactose acted as a competitive inhibitor. MgCl2 protected the enzyme from thermal denaturation. The calcium ions inhibited the activity in all cases. The behaviour of the
protectors of the SH groups varied according to the strain. 6-Phospho-β-D-galactosidase activity was also determined, levels lower than β-D-galactosidase were found, except inLactobacillus plantarum ATCC 8014 and 14917. 相似文献
1000.
Excretion of superoxide by phagocytes measured with cytochrome c entrapped in resealed erythrocyte ghosts 总被引:2,自引:0,他引:2
D Roos C M Eckmann M Yazdanbakhsh M N Hamers M de Boer 《The Journal of biological chemistry》1984,259(3):1770-1775
Resealed erythrocyte membranes (ghosts) filled with (Fe3+)cytochrome c were used as an assay system to measure the release of superoxide (O-2) from human phagocytes into the incubation medium. Neutrophils, activated by either opsonized zymosan particles or the soluble stimulus phorbol myristate acetate, released O-2, which subsequently entered the ghosts and reduced (Fe3+)cytochrome c. This reaction was dependent on the time of incubation, the concentration of neutrophils, the concentration of stimulus, and the concentration of ghosts. The reaction was completely inhibited by superoxide dismutase and by 4,4'-diisothiocyano-2,2'-disulfonic acid, a specific blocker of anion channels in membranes. The reduction of (Fe3+)cytochrome c free in solution was about four times as fast as the reduction of (Fe3+)cytochrome c in the ghosts. Human eosinophils stimulated by phorbol myristate acetate reacted similarly to human neutrophils; the rate of O-2 production/cell was about twice as high for eosinophils as for neutrophils. In contrast, eosinophils stimulated with opsonized zymosan particles only reduced (Fe3+)cytochrome c free in solution, but not (Fe3+)cytochrome c in ghosts. This lack of reaction was not due to production of an inhibitor or below threshold generation of O-2 for the ghost assay. These results indicate: 1) activated human neutrophils and eosinophils can release O-2 or a similar product into the incubation medium; and 2) reduction of (Fe3+)cytochrome c free in solution is no proof for O-2 excretion by phagocytes. 相似文献