全文获取类型
收费全文 | 3811篇 |
免费 | 195篇 |
国内免费 | 13篇 |
专业分类
4019篇 |
出版年
2018年 | 33篇 |
2016年 | 30篇 |
2015年 | 41篇 |
2014年 | 47篇 |
2013年 | 114篇 |
2012年 | 117篇 |
2011年 | 116篇 |
2010年 | 107篇 |
2009年 | 155篇 |
2008年 | 129篇 |
2007年 | 116篇 |
2006年 | 108篇 |
2005年 | 94篇 |
2004年 | 93篇 |
2003年 | 73篇 |
2002年 | 65篇 |
2001年 | 52篇 |
2000年 | 47篇 |
1999年 | 52篇 |
1998年 | 55篇 |
1997年 | 54篇 |
1996年 | 55篇 |
1995年 | 41篇 |
1994年 | 32篇 |
1993年 | 33篇 |
1992年 | 59篇 |
1991年 | 42篇 |
1990年 | 49篇 |
1989年 | 43篇 |
1988年 | 46篇 |
1987年 | 45篇 |
1986年 | 44篇 |
1985年 | 44篇 |
1984年 | 38篇 |
1983年 | 40篇 |
1980年 | 30篇 |
1979年 | 38篇 |
1975年 | 28篇 |
1972年 | 30篇 |
1971年 | 28篇 |
1959年 | 53篇 |
1958年 | 125篇 |
1957年 | 138篇 |
1956年 | 135篇 |
1955年 | 150篇 |
1954年 | 152篇 |
1953年 | 121篇 |
1952年 | 108篇 |
1951年 | 98篇 |
1950年 | 60篇 |
排序方式: 共有4019条查询结果,搜索用时 0 毫秒
1.
The revision of the antarctic–subantarctic species Orchomenopsis reducta Schellenberg, 1931, has led to its attribution to a new, highly apomorphic genus: Falklandia gen.n. A new definition of the uristid group is given and Falklandia with 36 other lysianassoid genera are attributed to this supposedly monophyletic group. 相似文献
2.
J Markussen I Diers A Engesgaard M T Hansen P Hougaard L Langkjaer K Norris U Ribel A R S?rensen E S?rensen 《Protein engineering》1987,1(3):215-223
It has previously been found that insulins, to which positive charge has been added by substitutions in position B30, thus raising the isoelectric point towards pH 7, had a prolonged action when injected as slightly acidic solutions because such derivatives crystallize very readily upon neutralization. Positive charge has now been added by substituting the B13 and A17 glutamic acid residues with glutamines and B27 threonine with lysine or arginine. These substitutions were introduced by site-specific mutagenesis in a gene coding for a single-chain insulin precursor. By tryptic transpeptidation the single-chain precursors were transformed to the double-chain insulin structure, concomitantly with incorporation of residue B30. Thus insulins combining B13 glutamine, A17 glutamine and B27 lysine or arginine with B30 threonine, threonine amide or lysine amide were synthesized. The time course of blood glucose lowering effect and the absorption were studied after subcutaneous injection in rabbits and pigs. The prolonged action of B30-substituted insulins was markedly enhanced by B27 lysine or arginine substitutions and by B13 glutamine. The B27 residue is located on the surface of the hexamer, so a basic residue in this position presumably promotes the packing of hexamers at neutral pH. The B13 residues cluster in the centre of the hexamer. When the electrostatic repulsive forces from six glutamic acid residues are abolished by substitution with glutamine, a stabilization of the hexamer can be envisaged.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
3.
Cyanide-Resistant Root Respiration and Tap Root Formation in Two Subspecies of Hypochaeris radicata 总被引:1,自引:0,他引:1
Root respiration of the tap root forming species Hypochaeris radicata L. was measured during tap root formation. A comparison was made of two subspecies: H. radicata L. ssp. radicata L., a subspecies from relatively rich soils, and H. radicata L. ssp. ericetorum Van Soest, a subspecies from poor acidic soils. Root respiration was high and to a large extent inhibited by hydroxamic acid (SHAM) before the start of the tap root formation, indicating a high activity of an alternative non-phosphorylative electron transport chain. The rate of root respiration was much lower and less sensitive to SHAM when a considerable tap root was present. However, root respiration was also cyanide-resistant when a tap root was present, indicating that the alternative pathway was still present. A decreased rate of root respiration coincided with an increase of the content of storage carbohydrates, mainly in the tap root. The level of reducing sugars was constant throughout the experimental period, and it was concluded that the activity of the alternative oxidative pathway was significant in oxidation of sugars that could not be utilized for purposes like energy production, the formation of intermediates for growth or for storage. Root respiration decreased after the formation of a tap root. This decrease could neither be attributed to a gradual disappearance of the alternative chain, nor to a decreased level of reducing sugars. No differences in respiratory metabolism between the two subspecies have been observed, suggesting that a high activity of the alternative oxidative pathway is not significant in adaptation of the present two subspecies to relatively nutrient-rich or poor soils. 相似文献
4.
5.
Antibodies to human amyloid precursor protein (APP695) and beta‐amyloid peptide (Aβ1‐42) were used to determine timing of amyloidosis in the brain of kokanee salmon (Oncorhynchus nerka kennerlyi) in one of four reproductive stages: immature (IM), maturing (MA), sexually mature (SM), and spawning (SP), representing a range of aging from somatically mature but sexually immature to spawning and somatic senescence. In IM fish, immunoreactive (ir) intracellular APP occurred in 18 of 23 brain regions. During sexual maturation and aging, the number of neurons expressing APP increased in 11 of these APP‐ir regions. Aβ‐ir was absent in IM fish, present in seven regions in MA fish, moderately abundant in 15 regions in SM fish, and was most abundant in all brain regions of SP fish exhibiting Aβ‐ir. Intracellular APP‐ir was observed in brain regions involved in sensory integration, olfaction, vision, stress responses, reproduction, and coordination. Intra‐ and extracellular Aβ1‐42 immunoreactivity (Aβ‐ir) was present in all APP‐ir regions except the nucleus lateralis tuberis (hypothalamus) and Purkinje cells (cerebellum). APP‐ir and Aβ deposition increase during aging. APP‐ir is present in IM fish; Aβ‐ir usually appears first in MA or SM fish and increases in SM fish as does APP‐ir. Extracellular Aβ deposition dramatically increases between SM and SP stages (1–2 weeks) in all fish, indicating an extremely rapid and synchronized process. Rapid senescence observed in pacific salmon could make them a useful model to investigate timing of amyloidosis and neurodegeneration during brain aging. © 2002 Wiley Periodicals, Inc. J Neurobiol 53: 11–20, 2002 相似文献
6.
7.
8.
9.
Several leafhopper variants of the Circulifer tenellus complex were collected in “citrus stubborn” affected areas in Israel. Two of these variants transmitted the Spiroplasma citri to Matthiola incana after being injected with the disease agent. The variant from Atriplex halimus was designated Circulifer tenellus-A (CTA) and the variant from Portulaca oleracea was designated Circulifer tenellus-? (CTP). Transmission characteristics were determined for both leafhoppers. A high rate of transmission (43.3%) was obtained by single CTA leafhoppers that were injected with the Amiad S. citri isolate from the Upper Galilee, compared with 7% transmission obtained with the CTP leafhoppers. The Gilgal S. citri isolate from the Jordan Valley, was not transmitted by either. Injection was more effective than acquisition access feeding to render the leafhopper infective for both CTA and CTP. The minimum acquisition access period needed for the CTA variant to transmit the Amiad isolate was 1 h. Longer AAPs did not necessarily result in a higher rate of transmission. The minimum incubation period was 6 days and the maximum was 32 days. The LP50 calculated from the logarithmic curve y = 45.74Ln(x)–53.68 was 9.64 days. The minimum inoculation access period (IAP) was lh. The same transmission parameters for the CTP variant could not be determined, as no transmission was obtained even when groups of five-six insects were placed on a single plant. 相似文献
10.
The two gene pairs encoding H2A and H2B play different roles in the Saccharomyces cerevisiae life cycle. 总被引:14,自引:7,他引:7 下载免费PDF全文
We have isolated Saccharomyces cerevisiae mutants bearing deletions of one or the other of the two divergently transcribed gene pairs encoding H2A and H2B. The deletions produced diverse effects on the yeast life cycle. Deletion of TRT1, one of the H2A-H2B gene pair sets, affected mitotic growth, sporulation, spore germination, the heat shock response, and exit from the stationary phase; deletion of TRT2, the other H2A-H2B gene pair set, had negligible effects on these same processes. Using a genetic complementation assay, we found that the differential effects of the deletions could be attributed to two features of the gene sets: first, the expression of the TRT1 gene pair, but not the TRT2 gene pair, could compensate for the absence of its partner; second, the protein subtypes encoded by the two gene pairs appear to have different functions in the heat shock response. 相似文献