全文获取类型
收费全文 | 4568篇 |
免费 | 637篇 |
国内免费 | 198篇 |
出版年
2021年 | 83篇 |
2020年 | 49篇 |
2019年 | 68篇 |
2018年 | 51篇 |
2017年 | 57篇 |
2016年 | 104篇 |
2015年 | 156篇 |
2014年 | 164篇 |
2013年 | 202篇 |
2012年 | 279篇 |
2011年 | 275篇 |
2010年 | 163篇 |
2009年 | 153篇 |
2008年 | 190篇 |
2007年 | 215篇 |
2006年 | 183篇 |
2005年 | 182篇 |
2004年 | 166篇 |
2003年 | 162篇 |
2002年 | 162篇 |
2001年 | 113篇 |
2000年 | 115篇 |
1999年 | 109篇 |
1998年 | 74篇 |
1997年 | 44篇 |
1996年 | 40篇 |
1995年 | 43篇 |
1994年 | 54篇 |
1992年 | 70篇 |
1991年 | 65篇 |
1990年 | 69篇 |
1989年 | 67篇 |
1988年 | 83篇 |
1987年 | 82篇 |
1986年 | 57篇 |
1985年 | 74篇 |
1984年 | 61篇 |
1983年 | 59篇 |
1981年 | 42篇 |
1979年 | 59篇 |
1978年 | 56篇 |
1977年 | 48篇 |
1976年 | 53篇 |
1974年 | 48篇 |
1973年 | 60篇 |
1972年 | 45篇 |
1971年 | 49篇 |
1970年 | 54篇 |
1969年 | 50篇 |
1968年 | 39篇 |
排序方式: 共有5403条查询结果,搜索用时 15 毫秒
51.
Local application of K+ via micropressure-ejection, coupled with in vivo electrochemical detection, was used to study stimulated release from monoaminergic nerve terminals in the striatum of anesthetized rats and mice. K+-evoked releases were reversible, reproducible, and dose-dependent. In contrast, releases of electroactive species could not be evoked by local ejection of Na+. The magnitudes and time courses of K+-evoked releases recorded from the caudate nucleus of mice were greater than those seen in rats. Local application of nomifensine, a putative catecholamine reuptake blocker, augmented the magnitudes and time courses of K+-evoked releases. Releases were also recorded from brain regions adjacent the striatum; these signals were always smaller than those seen in the caudate nucleus and had amplitudes that showed good correspondence to the relative degree of dopaminergic input to these areas. These data, taken together with other information in the literature, suggest that this new technique is well suited for in situ studies of monoamine release and reuptake in intact animals. 相似文献
52.
A recently discovered partial skeleton of the adapid Cantius trigonodus from the early Eocene Willwood Formation of the Bighorn Basin, Wyoming, documents substantial new information about the anatomy of the oldest lemuriform primates. It is very similar in all features to its descendant, middle Eocene Notharctus, and both exhibit numerous resemblances to certain extant Malagasy lemurs, particularly Lepilemur, Propithecus, Lemur, and Hapalemur griseus. Like these forms, Cantius had relatively long hind limbs and short forelimbs. Forelimb traits (prominent brachialis flange of the humerus, well-developed olecranon process of the ulna, and strong shafts of the ulna and radius) suggest active use of the forelimbs in progression. Specializations in the hind limb (e.g., expanded articular surface of the femoral head, narrow and elevated patellar trochlea and prominent lateral trochlear ridge, posteriorly oriented femoral and tibial condyles, narrow and elongate talus, and hallucal metatarsal with prominent peroneal tubercle) indicate capabilities for leaping and for powerful grasping with an opposable hallux. Cantius was presumably primitive in having a relatively long ischium and much more distal inferior tibial tuberosity than most extant lemurs--traits suggesting that powerful extension of the thigh and flexion at the knee were important in its locomotion and posture. We interpret Cantius as an active arboreal quadruped with a propensity for leaping. The existence of this skeletal structure in one of the oldest primates of modern aspect suggests that it represents the primitive lemuriform morphology. 相似文献
53.
Partition kinetics of ribulose-1,5-bisphosphate carboxylase from Rhodospirillum rubrum 总被引:1,自引:0,他引:1
When the enzymatically generated intermediate 2-carboxy-3-keto-D-arabinitol-1,5-bisphosphate (II) was used as a substrate with fresh enzyme, 70% reacted to produce 3-phosphoglycerate (3PGA). When a reaction mixture of enzyme plus [1-32P]ribulose 1,5-bisphosphate (RuBP) was quenched in the steady state with the tightly bound inhibitor 2-carboxyarabinitol-1,5-bisphosphate, 30% of the enzyme-bound species was released as 3PGA and 70% as RuBP. The major source for this partition was the ternary substrates Michaelis complex. The level of carboxylated intermediate in the steady state was determined to be 8% of active sites under the conditions of substrate saturation. No burst was seen in the appearance of product when 6.5 eq of [1-32P]RuBP was mixed with enzyme plus saturating CO2 and the reaction followed in the steady state. From these data plus the steady-state Vmax and Km of RuBP it is possible to derive the five bulk rate constants represented in the scheme ECO2 + RuBP in equilibrium ERuBPCO2 in equilibrium E X II----E + 2(3PGA). 相似文献
54.
Structural requirements of a membrane-spanning domain for protein anchoring and cell surface transport 总被引:41,自引:0,他引:41
The membrane-spanning domain of the vesicular stomatitis virus glycoprotein (G) contains 20 uncharged and mostly hydrophobic amino acids. We created DNAs specifying G proteins with shortened transmembrane domains, by oligonucleotide-directed mutagenesis. Expression of these DNAs showed that G proteins containing 18, 16, or 14 amino acids of the original transmembrane domain assumed a transmembrane configuration and were transported to the cell surface. G proteins containing only 12 or 8 amino acids of this domain also spanned intracellular membranes, but their transport was blocked within a Golgi-like region in the cell. A G protein completely lacking the membrane-spanning domain accumulated in the endoplasmic reticulum and was secreted slowly. These experiments indicate that the size of the transmembrane domain is critical not only for membrane anchoring, but also for normal cell surface transport. 相似文献
55.
56.
S M Liang B Allet K Rose M Hirschi C M Liang D R Thatcher 《The Biochemical journal》1985,229(2):429-439
Interleukin 2 isolated from Escherichia coli cells expressing the human interleukin gene has been characterized. The observed properties of the protein have been compared with those properties which can be deduced from the DNA sequence alone and the published properties of natural human interleukin 2. The purified E. coli-derived interleukin 2 is a monomeric protein of Mr 15 000 with a sedimentation velocity of 1.86S. The amino acid composition of the protein and isoelectric point (7.7) are consistent with that part of the translated DNA sequence of the gene corresponding to the mature protein. A single disulphide bridge was identified between Cys-58 and Cys-105. C.d. suggested that interleukin 2 is predominantly alpha-helical in secondary structure. The E. coli-derived protein differed from natural interleukin 2 in the presence of N-terminal methionine and also in the absence of a carbohydrate moiety. Removal of the coding region for the first three amino acids of the natural interleukin 2 protein sequence (Ala-Pro-Thr) by site-specific mutagenesis resulted in a protein with N-terminal serine. The possibility that the specificity of the E. coli ribosomal methionine aminopeptidase may not recognize the sequence NH2-Met-Xaa-Pro is discussed (where Xaa is any amino acid residue). 相似文献
57.
Incorporation of a charged amino acid into the membrane-spanning domain blocks cell surface transport but not membrane anchoring of a viral glycoprotein. 总被引:30,自引:2,他引:28
下载免费PDF全文
![点击此处可从《Molecular and cellular biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The membrane-spanning domain of the vesicular stomatitis virus glycoprotein (G protein) consists of a continuous stretch of 20 uncharged and mostly hydrophobic amino acids. We examined the effects of two mutations which change the amino acid sequence in this domain. These mutations were generated by oligonucleotide-directed mutagenesis of a cDNA clone encoding the G protein, and the altered G proteins were then expressed in animal cells. Replacement of an isoleucine residue in the center of this domain with a strongly polar but uncharged amino acid (glutamine) had no effect on membrane anchoring or transport of the protein to the cell surface. Replacement of this same isoleucine residue with a charged amino acid (arginine) generated a G protein that still spanned intracellular membranes but was not transported efficiently to the cell surface. The protein accumulated in the Golgi region in about 50% of the cells, and about 20% of the cells had detectable protein levels in a punctate pattern on the cell surface. In the remaining cells the protein accumulated in a vesicular pattern throughout the cytoplasm. Models which might explain the abnormal behavior of this protein are discussed. 相似文献
58.
Measles virus polypeptides were separated by polyacrylamide gel electrophoresis and electroeluted from gel sections. The antigenicity of the polypeptides was determined by enzyme-linked immunosorbent assays. The ability of these measles virus antigens to stimulate lymphoproliferation was measured in both high- and low-responder individuals. In contrast to the low-responder lymphocytes which did not proliferate when stimulated with measles virus antigens, the high-responder lymphocytes proliferated when challenged with hemagglutinin, nucleocapsid-associated phosphoprotein, nucleocapsid protein, matrix protein, and fusion protein. 相似文献
59.
60.
College undergraduates classified as high (n = 25) and low (n = 25) on recent life stress participated in an experiment involving a novel laboratory stressor. Heart rate and pulse arrival time (PAT) were measured during baseline, anticipation, testing, and recovery periods of the experiment. The results did not replicate those obtained by Pardine and Napoli in that high and low life stress subjects did not show differential physiological reactions. In addition, regression analyses failed to demonstrate that physiological reactivity moderated the relationship between life stress and subsequent self-reported psychiatric or physical health symptomatology. The present findings demonstrated neither the stress-buffering effects of physiological reactivity nor a relationship between life stress and reactivity when the latter was conceptualized as an outcome. 相似文献