全文获取类型
收费全文 | 7218篇 |
免费 | 353篇 |
国内免费 | 1篇 |
出版年
2023年 | 36篇 |
2022年 | 27篇 |
2021年 | 76篇 |
2020年 | 90篇 |
2019年 | 117篇 |
2018年 | 272篇 |
2017年 | 244篇 |
2016年 | 320篇 |
2015年 | 230篇 |
2014年 | 332篇 |
2013年 | 511篇 |
2012年 | 389篇 |
2011年 | 554篇 |
2010年 | 318篇 |
2009年 | 210篇 |
2008年 | 375篇 |
2007年 | 428篇 |
2006年 | 466篇 |
2005年 | 398篇 |
2004年 | 333篇 |
2003年 | 377篇 |
2002年 | 294篇 |
2001年 | 188篇 |
2000年 | 170篇 |
1999年 | 113篇 |
1998年 | 61篇 |
1997年 | 23篇 |
1996年 | 14篇 |
1995年 | 22篇 |
1994年 | 18篇 |
1993年 | 16篇 |
1992年 | 16篇 |
1991年 | 27篇 |
1990年 | 19篇 |
1988年 | 17篇 |
1987年 | 21篇 |
1986年 | 22篇 |
1985年 | 30篇 |
1984年 | 26篇 |
1983年 | 19篇 |
1982年 | 19篇 |
1979年 | 23篇 |
1978年 | 21篇 |
1977年 | 17篇 |
1975年 | 14篇 |
1974年 | 14篇 |
1953年 | 18篇 |
1952年 | 40篇 |
1888年 | 14篇 |
1887年 | 14篇 |
排序方式: 共有7572条查询结果,搜索用时 15 毫秒
51.
The activities of adenosylhomocysteinase (EC 3.3.1.1) and adenosine nucleosidase (EC 3.2.2.7) were assayed in extracts from yellow lupin (Lupinus luteus L.) cotyledons at different stages of seed formation and seedling development. Adenosylhomocysteinase activity was demonstrated in all the cotyledon extracts examined. Its lowest level was found in the dry seeds and the highest, in 4-day-old seedling cotyledons. Extracts from the cotyledons of maturating seeds, dry seeds, and seedlings up to the second day of growth exhibited no adenosine nucleosidase activity. Adenosine nucleosidase activity appeared in the cotyledons of 2-day-old seedlings and its highest level was reached in 4-to 5-day-old seedlings. There is no inhibitor of adenosine nucleosidase in the maturating and dry yellow lupin seeds. No activator of a possible zymogen form of adenosine nucleosidase from maturating or dry seeds occurs in the growing seedlings. 相似文献
52.
The effect of oleate, palmitate, and octanoate on glucose formation was studied with lactate or pyruvate as substrate. Octanoate was much more quickly oxidized and utilized for ketone body production than were oleate and palmitate. Among fatty acids studied, only octanoate resulted in a marked increase of the 3-hydroxybutyrate/acetoacetate (3-) ratio. Each of the fatty acids studied stimulated glucose synthesis from pyruvate. The enhancement of gluconeogenesis by long-chain fatty acids was abolished after the addition of ammonia. As concluded from the “crossover” plot, the stimulatory effect of fatty acids was due to: (i) a stimulation of pyruvate carboxylation, (ii) a provision of reducing equivalents for glyceraldehyde phosphate dehydrogenase, and (iii) an acceleration of flux through hexose diphosphatase. Moreover, palmitate and oleate resulted in an increased generation of mitochondrial phosphpenolpyruvate, while in the presence of octanoate, the activity of mitochondrial phosphoenolpyruvate carboxykinase was diminished. When lactate was used as the glucose precursor, palmitate and oleate increased glucose production by about 50% but did not affect the contribution of mitochondrial phosphoenolpyruvate carboxykinase to gluconeogenesis. In contrast, in spite of the stimulation of both pyruvate carboxylase and hexose diphosphatase, as judged from the crossover plot, the addition of octanoate resulted in a marked inhibition of both glucose formation and mitochondrial generation of phosphoenolpyruvate. The inhibitory effect of octanoate was reversed by ammonia. Results indicate that fatty acids and ammonia are potent regulatory factors of both the rate of glucose formation and the contribution of mitochondrial phosphoenolpyruvate carboxykinase to gluconeogenesis in hepatocytes of the fasted rabbit. 相似文献
53.
54.
The sequence of DNA replication was studied within the inactive X chromosome in human lymphocytes, by means of the FPG method. Several variants of the replication sequence were found. The number of variants in the cells of a single donor exceeded 2 in each of the 4 normal individuals studied. The phenomenon is discussed with respect to the regulation of DNA synthesis and to the cell differentiation process. 相似文献
55.
The magnitude and kinetics of β-glucuronidase induction in mouse kidney are determined by a cis-acting regulatory gene, Gus-r, that is closely linked to the enzyme structural gene. The accumulation of β-glucuronidase mRNA during induction is much slower than the turnover time of the mRNA, suggesting progressive acquisition of mRNA synthesizing capacity during induction. Counts of the numbers of induced cells present at various times of induction in strains carrying three different alleles of Gus-r show that all potentially responsive cells respond immediately. The level of induction is progressive in individual cells and does not involve continued recruitment of new cells into the induced population. It appears that during induction each chromosome becomes progressively more active in directing the synthesis of β-glucuronidase. 相似文献
56.
Przemysław Szafrański 《Journal of molecular evolution》1992,34(5):465-467
Summary The existence of internal sequence homologies between the N-terminal halves of the gram-negative bacterial major sigma factors and their C-terminal halves, which correspond to minor factors, is reported. In the case of Escherichia-Salmonella sigma-70, an apparent homology was even found between the C-terminal helix-turn-helix DNA-binding motif and the corresponding region of the peptide N half, which, however, is not directly engaged in promoter recognition. It is proposed that major sigma factors may have originated by duplication and fusion of a DNA unit related to the ancestral gene for the whole sigma family. Coevolution of major sigma structures and complex promoters is suggested. 相似文献
57.
Jan Pałyga 《Biochemical genetics》1991,29(9-10):431-445
Histone H1 from erythrocytes of Japanese quail was resolved in a sodium dodecyl sulfate (SDS)-polyacrylamide gel into five fractions differing in apparent molecular weights. A polymorphism of histone H1.1, H1.2, and H1.3 bands was detected among quail individuals. While some birds possessed either a high (phenotype .3+) or a low (phenotype .3+/.3-) level of H1.3, at least half of the quail population lacked this H1 band (phenotype .3-). Appropriate genetic crosses demonstrated that H1.3 behaved as though it was coded by a gene with two codominant alleles at an autosomal locus. Using two-dimensional polyacrylamide gel electrophoresis (acid-urea followed by SDS gels), it was found that birds .3+ contained polypeptides H1.b1 and H1.b'1; birds .3-, polypeptides H1.b2 and H1.b'2 with lower apparent molecular weights; and birds .3+/.3-, both types of polypeptides in equal proportions. The H1.b2 + H1.b'2 complement was not discernible in SDS gels, for it migrated together with H1.c' within band H1.4. It was found that a small number of birds lacking the H1.2 band in SDS gels failed to express histone H1.a. Since birds with phenotype .2- with a defective allele of the gene H1.a were simultaneously lacking the H1.3 band, it seems that the imperfect allele of the H1.a gene might be closely linked to the alleles producing H1.b2 + H1.b'2. 相似文献
58.
R.D. Nolan G.J. Dusting J. Jakubowski T.J. Martin 《Prostaglandins & other lipid mediators》1982,24(6):887-902
Cyclo-oxygenase products of arachidonic acid metabolism formed by the pericardium and epicardial surface of dog heart were identified and quantitated by radioimmunoassay after separation by high-pressure liquid chromatography. Pieces of pariental pericardium, of dog, ox and rat, when incubated
produced mainly 6-keto-PGF1α, with lesser amounts of PGE2, PGF2α and thromboxane B2. Biosynthesis of all prostanoids increased during incubation of the pariental pericardium of each species with arachidonic acid, but 6-keto-PGF1α was still the major metabolite. When slices of dog heart were incubated with arachidonic acid (1 μg/ml) the rates of 6-keto-PGF1α formation by the pariental pericardium was much greater than that of the myocardium and endocardium. Epicardial slices appeared to be intermediate in 6-keto-PGF1α formation. The hearts of anesthetized dogs were also irrigated
with Krebs' solution, and during the first 5 min of epicardial irrigation the pericardial fluid leaving the heart again contained high levels of 6-keto-PGF1α, with lesser amounts of the other prostanoids. Addition of arachidonic acid (3 μg/ml) to the irrigating fluid caused an increase in all measured prostanoid levels, although 6-keto-PGF1α remained the predominant metabolite. In contrast, intravenous infusion of isoproterenol selectively increased the release of 6-keto-PGF1α from the irrigated heart. It is concluded that the pericardium and epicardium continuously release prostacyclin into the pericardial fluid, and that the increased release of this substance observed when cardiac workload increases derives mainly from these membranous sources. This raises the interesting possibility that pericardial prostacyclin might influence coronary vascular tone and chemoreflexes which arise from the epicardium during myocardial ischemia. 相似文献
59.
60.
The effective synthesis of 4,4,14α-trimethyl-19 (10 → 9β) abeo-steroids (iv), (v), and (Vl) with two- and five-carbon side chains from lanosterol is described. Their structures were proved on the basis of spectral data. The title compounds are the first synthetic synthons for the preparation of 4,4,14α-trimethyl-steroids with an unnatural configuration. 相似文献