全文获取类型
收费全文 | 325篇 |
免费 | 48篇 |
出版年
2021年 | 3篇 |
2020年 | 3篇 |
2019年 | 4篇 |
2018年 | 12篇 |
2016年 | 7篇 |
2015年 | 18篇 |
2014年 | 9篇 |
2013年 | 14篇 |
2012年 | 11篇 |
2011年 | 17篇 |
2010年 | 5篇 |
2009年 | 7篇 |
2008年 | 16篇 |
2007年 | 13篇 |
2006年 | 13篇 |
2005年 | 4篇 |
2004年 | 13篇 |
2003年 | 10篇 |
2002年 | 12篇 |
2001年 | 7篇 |
2000年 | 9篇 |
1999年 | 9篇 |
1998年 | 6篇 |
1997年 | 6篇 |
1996年 | 6篇 |
1994年 | 4篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1989年 | 4篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1976年 | 10篇 |
1975年 | 6篇 |
1974年 | 11篇 |
1973年 | 3篇 |
1972年 | 3篇 |
1971年 | 8篇 |
1970年 | 3篇 |
1969年 | 4篇 |
1967年 | 5篇 |
1966年 | 4篇 |
1955年 | 3篇 |
排序方式: 共有373条查询结果,搜索用时 15 毫秒
41.
42.
43.
44.
45.
L-Lactate dehydrogenase (L-LDH, E.C. 1.1.1.27) is encoded by two or three
loci in all vertebrates examined, with the exception of lampreys, which
have a single LDH locus. Biochemical characterizations of LDH proteins have
suggested that a gene duplication early in vertebrate evolution gave rise
to Ldh-A and Ldh-B and that an additional locus, Ldh-C arose in a number of
lineages more recently. Although some phylogenetic studies of LDH protein
sequences have supported this pattern of gene duplication, others have
contradicted it. In particular, a number of studies have suggested that
Ldh-C represents the earliest divergence among vertebrate LDHs and that it
may have diverged from the other loci well before the origin of
vertebrates. Such hypotheses make explicit statements about the
relationship of vertebrate and invertebrate LDHs, but to date, no closely
related invertebrate LDH sequences have been available for comparison. We
have attempted to provide further data on the timing of gene duplications
leading to multiple vertebrate LDHs by determining the cDNA sequence of the
LDH of the tunicate Styela plicata. Phylogenetic analyses of this and other
LDH sequences provide strong support for the duplications giving rise to
multiple vertebrate LDHs having occurred after vertebrates diverged from
tunicates. The timing of these LDH duplications is consistent with data
from a number of other gene families suggesting widespread gene duplication
near the origin of vertebrates. With respect to the relationships among
vertebrate LDHs, our data are not consistent with previous claims that
Ldh-C represented the earliest divergence. However, the precise
relationships among some of the main lineages of vertebrate LDHs were not
resolved in our analyses.
相似文献
46.
R J Castellani P L Harris L M Sayre J Fujii N Taniguchi M P Vitek H Founds C S Atwood G Perry M A Smith 《Free radical biology & medicine》2001,31(2):175-180
Advanced glycation end products are a diverse class of posttranslational modifications, stemming from reactive aldehyde reactions, that have been implicated in the pathogenesis of a number of degenerative diseases. Because advanced glycation end products are accelerated by, and result in formation of, oxygen-derived free radicals, they represent an important component of the oxidative stress hypothesis of Alzheimer disease (AD). In this study, we used in situ techniques to assess N(epsilon)-(Carboxymethyl)lysine (CML), the predominant advanced glycation end product that accumulates in vivo, along with its glycation-specific precursor hexitol-lysine, in patients with AD as well as in young and aged-matched control cases. Both CML and hexitol-lysine were increased in neurons, especially those containing intracellular neurofibrillary pathology in cases of AD. The increase in hexitol-lysine and CML in AD suggests that glycation is an early event in disease pathogenesis. In addition, because CML can result from either lipid peroxidation or advanced glycation, while hexitol-lysine is solely a product of glycation, this study, together with studies demonstrating the presence of 4-hydroxy-2-nonenal adducts and pentosidine, provides evidence of two distinct oxidative processes acting in concert in AD neuropathology. Our findings support the notion that aldehyde-mediated modifications, together with oxyradical-mediated modifications, are critical pathogenic factors in AD. 相似文献
47.
X. Zhu Y. Wang O. Ogawa H. G. Lee A. K. Raina H. Fujioka S. Shimohama C. S. Atwood R. B. Petersen G. Perry M. A. Smith 《Journal of neurochemistry》2002,81(Z1):76-76
The mechanisms underlying neuronal degeneration in Alzheimer's disease (AD) are very controversial and none more so than whether apoptosis plays a role. Although neurons in AD face a wide assortment of apoptogenic stimuli, the temporal dichotomy between the acuteness of apoptosis vs. the chronicity of AD suggests that apoptosis should be extremely rare in AD. In this regard, survival factor(s) must be involved. In this study, we investigated Bcl‐w, a pro‐survival member of the Bcl‐2 family. Although expressed at low levels in brains of control cases, Bcl‐w is significantly up‐regulated in AD as shown by both immunocytochemistry and immunoblot analysis. Astonishingly, increased Bcl‐w was found to be associated with neurofibrillary pathologies in AD, which was further demonstrated by an EM study. Since neuronal death in AD is thought to be triggered by increased production of amyloid‐β (Aβ), it was interesting to find that exposure of human M17 neuroblastoma cells to Aβ1–42 (1 nm ?10 μm ) dramatically up‐regulates Bcl‐w protein levels. Such increases may be a protective response that attenuates apoptotic processes. Consistent with this, transfected M17 cells overexpressing Bcl‐w were protected from both STS‐induced and Aβ‐induced apoptosis compared to vector‐transfected controls. Notably, both tau phosphorylation and p38 is inhibited in Bcl‐w transfected cells which may contribute to the neuroprotective role of Bcl‐w. Taken together, these set of in vitro and in vivo results suggest that Bcl‐w plays an important protective role in neurons in the AD brain. 相似文献
48.
Correlated Electrophysiological and Ultrastructural Studies of a Crustacean Motor Unit 总被引:8,自引:1,他引:7
Structural and functional interrelationships between the pre- and postsynaptic elements of a singly motor innervated crab muscle (stretcher of Hyas araneus L.) were examined using electrophysiological and electron microscopic techniques. Excitatory postsynaptic potential (EPSP) amplitude at 1 Hz was found to be inversely related to the extent of facilitation, and directly related both to the amount of transmitter released at 1 Hz and the muscle fiber input resistance (Rin). The extent of facilitation (Fe), taken as the ratio of the EPSP amplitude at 10 Hz to that 1 Hz, was inversely related to muscle fiber Rin, τm, and sarcomere length. Sarcomere length was directly related to Rin and τm. The excitatory nerve terminals of low Fe muscle fibers had larger neuromuscular synapses than did those of high Fe fibers. Inhibitory axo-axonal synapses were more often found in low Fe muscle fibers. These structural features may account for the greater release of transmitter at low frequencies from the low Fe nerve terminals as well as provide for a greater amount of presynaptic inhibition of low Fe muscle fibers. The implications of these findings for the development and physiological performance of the crustacean motor unit are discussed. It is proposed that both nerve and muscle fiber properties may be determined by the developmental pattern of nerve growth. 相似文献
49.
50.
Russell J. Garwood Jason A. Dunlop Paul A. Selden Alan R. T. Spencer Robert C. Atwood Nghia T. Vo Michael Drakopoulos 《Proceedings. Biological sciences / The Royal Society》2016,283(1827)
Spiders are an important animal group, with a long history. Details of their origins remain limited, with little knowledge of their stem group, and no insights into the sequence of character acquisition during spider evolution. We describe a new fossil arachnid, Idmonarachne brasieri gen. et sp. nov. from the Late Carboniferous (Stephanian, ca 305–299 Ma) of Montceau-les-Mines, France. It is three-dimensionally preserved within a siderite concretion, allowing both laboratory- and synchrotron-based phase-contrast computed tomography reconstruction. The latter is a first for siderite-hosted fossils and has allowed us to investigate fine anatomical details. Although distinctly spider-like in habitus, this remarkable fossil lacks a key diagnostic character of Araneae: spinnerets on the underside of the opisthosoma. It also lacks a flagelliform telson found in the recently recognized, spider-related, Devonian–Permian Uraraneida. Cladistic analysis resolves our new fossil as sister group to the spiders: the spider stem-group comprises the uraraneids and I. brasieri. While we are unable to demonstrate the presence of spigots in this fossil, the recovered phylogeny suggests the earliest character to evolve on the spider stem-group is the secretion of silk. This would have been followed by the loss of a flagelliform telson, and then the ability to spin silk using spinnerets. This last innovation defines the true spiders, significantly post-dates the origins of silk, and may be a key to the group''s success. The Montceau-les-Mines locality has previously yielded a mesothele spider (with spinnerets). Evidently, Late Palaeozoic spiders lived alongside Palaeozoic arachnid grades which approached the spider condition, but did not express the full suite of crown-group autapomorphies. 相似文献