全文获取类型
收费全文 | 921069篇 |
免费 | 104049篇 |
国内免费 | 473篇 |
专业分类
1025591篇 |
出版年
2016年 | 10331篇 |
2015年 | 14548篇 |
2014年 | 17175篇 |
2013年 | 23996篇 |
2012年 | 27387篇 |
2011年 | 27852篇 |
2010年 | 19014篇 |
2009年 | 17801篇 |
2008年 | 25445篇 |
2007年 | 26315篇 |
2006年 | 24678篇 |
2005年 | 23669篇 |
2004年 | 23650篇 |
2003年 | 22926篇 |
2002年 | 22407篇 |
2001年 | 39813篇 |
2000年 | 40331篇 |
1999年 | 32104篇 |
1998年 | 12063篇 |
1997年 | 12633篇 |
1996年 | 12012篇 |
1995年 | 11402篇 |
1994年 | 11202篇 |
1993年 | 11092篇 |
1992年 | 27240篇 |
1991年 | 26734篇 |
1990年 | 26176篇 |
1989年 | 25585篇 |
1988年 | 23672篇 |
1987年 | 23099篇 |
1986年 | 21214篇 |
1985年 | 21429篇 |
1984年 | 17664篇 |
1983年 | 15448篇 |
1982年 | 11991篇 |
1981年 | 10667篇 |
1980年 | 10261篇 |
1979年 | 17042篇 |
1978年 | 13690篇 |
1977年 | 12401篇 |
1976年 | 11640篇 |
1975年 | 12881篇 |
1974年 | 14031篇 |
1973年 | 13711篇 |
1972年 | 12430篇 |
1971年 | 11504篇 |
1970年 | 9899篇 |
1969年 | 9400篇 |
1968年 | 8486篇 |
1967年 | 7691篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Nonintegrative transformation in the filamentous fungus Podospora anserina: stabilization of a linear vector by the chromosomal ends of Tetrahymena thermophila. 总被引:3,自引:0,他引:3 下载免费PDF全文
The effect of the chromosomal ends of Tetrahymena thermophila on the stability of linear transforming molecules in the filamentous fungus Podospora anserina was tested. A derivative of an integrative vector for this fungus has been constructed, so that after linearization, the ends of the plasmid are the telomeric sequences of T. thermophila. After transformation, this linear molecule was maintained as an extrachromosomal plasmid with no integrated copies in about 50% of the transformants. Under selective conditions, there was approximately one linear molecule per 5 to 10 nuclei, and these extrachromosomal molecules were rapidly lost under nonselective conditions. The circular plasmid carrying an inverted repeat of T. thermophila telomeres could be linearized and processed in vivo. 相似文献
112.
Protein metal-binding sites. 总被引:2,自引:0,他引:2
Metal ions have a role in a variety of important functions in proteins including protein folding, assembly, stability, conformational change, and catalysis. The presence or absence of a given metal ion is crucial to the conformation or activity of over one third of all proteins. Recent developments have been made in the understanding and design of metal-binding sites in proteins, an important and rapidly advancing area of protein engineering. 相似文献
113.
V K Khugaeva E D Klimenko P N Aleksandrov G F Leskova M I Reutov 《Biulleten' eksperimental'no? biologii i meditsiny》1985,100(7):15-19
Experiments were made on rabbit fed an atherogenic diet (0.5 g/kg cholesterol) singly for 15 and 24 h and repeatedly for 3, 9 and 30 days. At early stages of lipid metabolism distress the interrelationship was established between blood rheological disorders and microcirculatory abnormalities. The dependence of the initial reaction of some rheological characteristics on their initial level was marked. 相似文献
114.
Nitrogen fertigation of greenhouse-grown cucumber 总被引:2,自引:0,他引:2
I. Papadopoulos 《Plant and Soil》1986,93(1):87-93
Summary This greenhouse study investigated the response of trickle-irrigated cucumber (Cucumis sativa cv. ‘Petita’) to three N levels applied with every irrigation via the irrigation stream. The plants were grown in pots filled
with 12 kg of soil. Water containing 5.8, 11.8, or 17.8 mmol N/l, and uniformly supplied with 2.0 and 3.9 mmol/l of P and
K, respectively, was applied two to three times daily. In all treatments of 0.3 leaching fraction was allowed.
The resulting total N applications were 15.7, 31., and 47.2 g N/plant. The total amount of water applied was 1851/plant. Total
N and NO3-N, in lajinae and petioles, increased with increasing N level whereas P and K in generated decreased. Although different
NO3/NH4 ratios in the treatments may have influeced the response to N, it could be concluded that the highest yield was obtained
with 11.8 mmol N/1 due to increased number of fruit. In the root volume of this treatment the NO3-N concentration in the soil solution was aroun 7 mmol/1 for most of the growing season. The dry matter concentration of fruits
was not affected by the N levels.
It was concluded that 11.8 mmol N/1 applied with every irrigation via the irrigation stream is adequate to cover the needs
of greenhous-grown cucumber for higher yield (9.42 kg/plant over a harvesting period of 93 days). 相似文献
115.
116.
Synthesis of prenylquinones in chloroplasts 总被引:3,自引:0,他引:3
117.
Growth of fathead minnow yolk sac larvae was characterized from changes in dry weight and total content and concentrations of RNA, DNA and protein in fish exposed to a sublethal level of HCN (58 micrograms/l) and in age matched controls. Cyanide toxicosis occurred within 24 hr of exposure as evidenced by significant reductions in protein and RNA content and RNA/DNA ratio of larvae. After 96 hr exposure to HCN, larvae exhibited the same growth rate and protein synthetic rate (RNA/DNA) as control fish. HCN toxicosis and recovery is rapid and at least partial tolerance to HCN develops within 96 hr of exposure in larval fathead minnows. 相似文献
118.
An earlier report (1) of an adverse effect of high doses of oxytocin on human memory included results of studies on women receiving oxytocin as part of the treatment to induce 2nd trimester therapeutic abortion. These women served as their own controls. We have now been able to study a group of women who have been treated in all ways like the original group, with the exception that they did not receive oxytocin. The results from this external control corroborate the finding that oxytocin affected memory. 相似文献
119.
Purification and characterization of a membrane-bound protein kinase from spinach thylakoids 总被引:3,自引:0,他引:3
A protein kinase was isolated from spinach thylakoid membranes by solubilization with octyl glucoside and cholate. The enzyme was purified to apparent homogeneity by ammonium sulfate precipitation, gel filtration, and sucrose density centrifugation, followed by affinity chromatography on either Affi-Gel blue (yielding denatured enzyme) or on histone cross-linked to Sepharose (yielding active enzyme). Electrophoresis on denaturing polyacrylamide gels, followed by staining with silver, revealed the kinase as a single band corresponding to an apparent molecular mass of 64 kDa. The active enzyme underwent autophosphorylation and could be detected by autoradiography following incubation with [gamma-32P]ATP and Mg2+ ion. The specific phosphotransferase activity of purified kinase was approximately 30 nmol of phosphate min-1 (mg protein)-1 with lysine-rich histone (III-S or V-S) as substrate; casein was phosphorylated at approximately 30% of this rate. The physiological substrate for the kinase is presumed to be light-harvesting chlorophyll a/b protein complex. In solubilized form, this was phosphorylated at approximately 10% of the rate observed with histone III-S as substrate, or 10-100 times slower than the estimated rate of phosphorylation of the light-harvesting complex in situ. Possible reasons for this shortfall are considered. The kinase is proposed as the principal effector of thylakoid protein phosphorylation and associated State transition phenomena. 相似文献
120.
Several synthetic 7-hydroxyflavylium salts related to apigeninidin, a natural 3-deoxyanthocyanidin, have been studied in the Ames mutagenicity test using strain TA1537 of Salmonella typhimurium. Under the neutral pH conditions of the test, these flavylium salts are deprotonated through ionization of the C7-OH (pKa′ = 4.2–4.4) to form quinone methides. Only the quinone methides of 4-methyl-7-hydroxyflavylium chloride and 4′-methoxy-4-methyl-7-hydroxy-flavylium chloride showed mutagenicity. Responses of 4–8 times the background were observed at the higher doses (1000 μg/plate), both with and without metabolic activation. It was concluded that the induction of frameshift mutagenicity by this group of compounds is caused by those quinone methides that have non-ionic, stable polycyclic structures at neutral pH. 相似文献