全文获取类型
收费全文 | 1415篇 |
免费 | 147篇 |
出版年
2022年 | 10篇 |
2021年 | 12篇 |
2020年 | 16篇 |
2019年 | 13篇 |
2018年 | 12篇 |
2017年 | 28篇 |
2016年 | 28篇 |
2015年 | 49篇 |
2014年 | 47篇 |
2013年 | 52篇 |
2012年 | 73篇 |
2011年 | 68篇 |
2010年 | 47篇 |
2009年 | 40篇 |
2008年 | 47篇 |
2007年 | 31篇 |
2006年 | 50篇 |
2005年 | 43篇 |
2004年 | 47篇 |
2003年 | 46篇 |
2002年 | 37篇 |
2001年 | 62篇 |
2000年 | 53篇 |
1999年 | 38篇 |
1998年 | 16篇 |
1996年 | 22篇 |
1995年 | 13篇 |
1994年 | 21篇 |
1993年 | 12篇 |
1992年 | 26篇 |
1991年 | 31篇 |
1990年 | 29篇 |
1989年 | 27篇 |
1988年 | 27篇 |
1987年 | 35篇 |
1986年 | 20篇 |
1985年 | 21篇 |
1984年 | 25篇 |
1983年 | 14篇 |
1982年 | 15篇 |
1981年 | 14篇 |
1979年 | 17篇 |
1978年 | 17篇 |
1977年 | 21篇 |
1975年 | 12篇 |
1974年 | 9篇 |
1973年 | 14篇 |
1971年 | 10篇 |
1967年 | 10篇 |
1966年 | 10篇 |
排序方式: 共有1562条查询结果,搜索用时 15 毫秒
31.
Gary M. Wessel Margaret R. Truschel Scott A. Chambers David R. McClay 《Molecular reproduction and development》1987,18(4):339-348
The ultrastructural localization of B-1,3-glucanase in three species of sea urchin eggs was determined using a monospecific antibody in an electronmicroscopic immunogold procedure. In all three species, Lytechinus variegatus, Strongylocentrotus purpuratus, and Arbacia punctulata, B-1,3-glucanase was localized specifically to the cortical granules. No other organelle within the egg contained significant label. During the fertilization reaction, B-1,3-glucanase was released from cortical granules into the perivitelline space and became associated with the hyaline layer. No significant label was found in association with the fertilization envelope. 相似文献
32.
C H Phoenix K C Chambers 《Proceedings of the Society for Experimental Biology and Medicine. Society for Experimental Biology and Medicine (New York, N.Y.)》1986,183(2):151-162
Old male rhesus macaques display less sexual behavior than young and middle-aged males. The decrease in sexual activity occurs without a statistically significant decline in gonadal hormones or change in diurnal patterns of serum T, DHT, or LH. Levels of sexual activity are not increased by administering T to old intact males. However, the hormone is effective in increasing sexual behavior in old long-term-castrated males. Performance can be increased to levels observed in equally old untreated intact males. Readily detectable physical disabilities of old age have been observed to impair sexual performance, but the observed general decline in sexual activity cannot be accounted for by known physical disabilities. Novelty, as represented by a change in female partner or by a change in environment, has not increased sexual performance in old rhesus males. Only when old males were paired with empirically selected preferred females is their sexual behavior increased to levels displayed by young males. Drugs reported to increase levels of sexual behavior in rats have thus far been less effective in old rhesus males than powdered rhinoceros horn has been in man. The probable absence of a placebo effect in rhesus males should increase their usefulness as an animal model for the study of sexual behavior in aging men. 相似文献
33.
H1 histone kinases from nuclei of Physarum polycephalum 总被引:1,自引:0,他引:1
34.
35.
The effect of cyclic nucleotides on purine biosynthesis and the induction of PRPP synthetase during lymphocyte activation 总被引:5,自引:0,他引:5
The activation of lymphocytes has been used to study the regulation of mammalian gene expression. Concanavalin A (Con A) added to mouse spleen lymphocytes in serum-free medium leads to an increase in the rate of DNA synthesis as great as 1000 fold, commencing 20 hr after its addition. Prior to 20 hr, the rate of purine synthesis increases 10–100 fold as measured by accumulation of the purine intermediate, formyl glycineamide ribonucleotide (FGAR). Addition of dibutyryl cyclic GMP to the lymphocyte suspensions results in a 10 fold increase in the rate of DNA synthesis in the absence of Con A and enhances both purine synthesis and DNA synthesis in its presence. The activity of phosphoribosyl pyrophosphate synthetase (PRPP synthetase), an enzyme central to purine and pyrimidine biosynthesis, is increased 2–10 fold during the activation. The increase begins to appear 8 hr after Con A addition and requires concomitant protein synthesis. The induced PRPP synthetase activity is stimulated by the presence of cyclic GMP in the enzyme assay. Addition of dibutyryl cyclic AMP to Con A-stimulated lymphocytes inhibits FGAR production, the stimulation of DNA synthesis, and the appearance of cyclic GMP-sensitive PRPP synthetase. These studies suggest that cyclic nucleotides play a significant role in the molecular mechanism of lymphocyte activation, the regulation of purine biosynthesis, and of eucaryotic genetic expression. 相似文献
36.
37.
38.
The purine and pyrimidine metabolism of Tetrahymena pyriformis 总被引:1,自引:0,他引:1
The metabolism of purines and pyrimidines by the ciliated protozoan Tetrahymena was investigated with the use of enzymatic assays and radioactive tracers. A survey of enzymes involved in purine metabolism revealed that the activities of inosine and guanosine phosphorylase (purine nucleoside: orthophosphate ribosyltransferase, E.C. 2.4.2.1) were high, but adenosine phosphorylase activity could not be demonstrated. The apparent Km for guanosine in the system catalyzing its phosphorolysis was 4.1 ± 0.6 × 10?3 M. Pyrophosphorylase activities for IMP and GMP (GMP: pyrophosphate phosphoribosyltransferase, E.C. 2.4.2.8), AMP (AMP: pyrophosphate phosphoribosyltransferase, E.C. 2.4.2.7), and 6-mercaptopurine ribonucleotide were also found in this organism; but a number of purine and pyrimidine analogs did not function as substrates for these enzymes. The metabolism of labeled guanine and hypoxanthine by intact cells was consistent with the presence of the phosphorylases and pyrophosphorylases of purine metabolism found by enzymatic studies. Assays for adenosine kinase (ATP: adenosine 5'-phosphotransferase, E.C. 2.7.1.20) inosine kinase, guanosine kinase, xanthine oxidase (xanthine: O2 oxidoreductase, E.C. 1.2.3.2), and GMP reductase (reduced-NADP: GMP oxidoreductase [deaminating], E.C. 1.6.6.8) were all negative. In pyrimidine metabolism, cytidine-deoxycytidine deaminase (cytidine aminohydrolase, E.C. 3.5.4.5), thymidine phosphorylase (thymidine: orthophosphate ribosyltransferase, E.C. 2.4.2.4), and uridine-deoxyuridine phosphorylase (uridine: orthophosphate ribosyltransferase, E.C. 2.4.2.3) were active; but cytidine kinase, uridine kinase (ATP: uridine 5'-phosphotransferase, E.C. 2.7.1.48), and CMP pyrophosphorylase could not be demonstrated. 相似文献
39.
40.
Van Vleck Chambers 《The Western journal of medicine》1960,93(4):211-216
A study was carried out to determine whether the double diffusion gel test when applied to the serum of patients with clear-cut penicillin reactions of various types, might be useful for demonstrating the presence of precipitating antibody. Results did not demonstrate the antibody.The difference in results with this test obtained by various workers was not explained by the observations in this study.Other approaches to determination of the mechanism of the penicillin reaction are discussed, and it is noted that the hemagglutination test, newly applied to the penicillin reaction problem, may be useful after further investigation. 相似文献