全文获取类型
收费全文 | 7794篇 |
免费 | 615篇 |
出版年
2024年 | 8篇 |
2023年 | 41篇 |
2022年 | 55篇 |
2021年 | 234篇 |
2020年 | 141篇 |
2019年 | 187篇 |
2018年 | 209篇 |
2017年 | 180篇 |
2016年 | 304篇 |
2015年 | 443篇 |
2014年 | 467篇 |
2013年 | 563篇 |
2012年 | 658篇 |
2011年 | 599篇 |
2010年 | 389篇 |
2009年 | 361篇 |
2008年 | 465篇 |
2007年 | 471篇 |
2006年 | 359篇 |
2005年 | 325篇 |
2004年 | 326篇 |
2003年 | 303篇 |
2002年 | 281篇 |
2001年 | 112篇 |
2000年 | 86篇 |
1999年 | 85篇 |
1998年 | 80篇 |
1997年 | 71篇 |
1996年 | 33篇 |
1995年 | 40篇 |
1994年 | 46篇 |
1993年 | 34篇 |
1992年 | 62篇 |
1991年 | 41篇 |
1990年 | 31篇 |
1989年 | 26篇 |
1988年 | 37篇 |
1987年 | 30篇 |
1986年 | 27篇 |
1985年 | 33篇 |
1984年 | 24篇 |
1983年 | 16篇 |
1982年 | 18篇 |
1981年 | 11篇 |
1980年 | 9篇 |
1979年 | 18篇 |
1978年 | 10篇 |
1972年 | 9篇 |
1970年 | 6篇 |
1965年 | 5篇 |
排序方式: 共有8409条查询结果,搜索用时 156 毫秒
81.
Subnucleolar location of fibrillarin and variation in its levels during the cell cycle and during differentiation of plant cells 总被引:5,自引:0,他引:5
The nucleolar protein fibrillarin has been studied in onion cells; it is detected as an Mr 37,000 protein by immunoblotting using a human autoimmune serum. Quantitative immunoelectron microscopy showed that most fibrillarin is localized in the transition zone between the fibrillar center (FC) and the dense fibrillar component (DFC) as well as in the priximal zone of the DFC, where the labeling shows a gradual decrease out-ward until it reaches insignificant levels in the distal zone of the DFC. Thus, fibrillarin is not uniformly distributed throughout the DFC of plant cells. This result supports the hypothesis that the morphologically homogeneous DFC may not be uniform in function; it is also in agreement with the hypothesized vectorial flow of ribosome biogenesis through the same compartments. Data are also presented showing that the amount of fibrillarin increase when nucleolar activity increases in G2, and probably decreases when nucleolar activity decreases during differentiation. 相似文献
82.
Domain-swapping analysis of FtsI, FtsL, and FtsQ, bitopic membrane proteins essential for cell division in Escherichia coli. 总被引:5,自引:0,他引:5
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
FtsI, FtsL, and FtsQ are three membrane proteins required for assembly of the division septum in the bacterium Escherichia coli. Cells lacking any of these three proteins form long, aseptate filaments that eventually lyse. FtsI, FtsL, and FtsQ are not homologous but have similar overall structures: a small cytoplasmic domain, a single membrane-spanning segment (MSS), and a large periplasmic domain that probably encodes the primary functional activities of these proteins. The periplasmic domain of FtsI catalyzes transpeptidation and is involved in the synthesis of septal peptidoglycan. The precise functions of FtsL and FtsQ are not known. To ask whether the cytoplasmic domain and MSS of each protein serve only as a membrane anchor or have instead a more sophisticated function, we have used molecular genetic techniques to swap these domains among the three Fts proteins and one membrane protein not involved in cell division, MalF. In the cases of FtsI and FtsL, replacement of the cytoplasmic domain and/or MSS resulted in the loss of the ability to support cell division. For FtsQ, MSS swaps supported cell division but cytoplasmic domain swaps did not. We discuss several potential interpretations of these results, including that the essential domains of FtsI, FtsL, and FtsQ have a role in regulating the localization and/or activity of these proteins to ensure that septum formation occurs at the right place in the cell and at the right time during the division cycle. 相似文献
83.
Influence of different rol gene products on the chain length of Shigella dysenteriae type 1 lipopolysaccharide O antigen expressed by Shigella flexneri carrier strains.
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Introduction of the rol genes of Shigella dysenteriae 1 and Escherichia coli K-12 into Shigella flexneri carrier strains expressing the heterologous S. dysenteriae type 1 lipopolysaccharide resulted in the formation of longer chains of S. dysenteriae 1 O antigen. In bacteria producing both homologous and heterologous O antigen, this resulted in a reduction of the masking of heterologous O antigen by homologous lipopolysaccharide and an increased immune response induced by intraperitoneal immunization of mice by recombinant bacteria. The rol genes of S. dysenteriae 1 and E. coli K-12 were sequenced, and their gene products were compared with the S. flexneri Rol protein. The primary sequence of S. flexneri Rol differs from both E. coli K-12 and S. dysenteriae 1 Rol proteins only at positions 267 and 270, which suggests that this region may be responsible for the difference in biological activities. 相似文献
84.
Juan Cabezas-Herrera María Teresa Moral-Naranjo F. Javier Campoy Cecilio J. Vidal 《Journal of neurochemistry》1997,69(5):1964-1974
Abstract: The distribution and glycosylation of acetylcholinesterase (AChE) forms in vesicles derived from sarcoplasmic reticulum of normal muscle (NMV) were investigated and compared with those from dystrophic muscle vesicles (DMV). AChE activity was similar in NMV and DMV. Most of the AChE in NMV and half in DMV were released with Triton X-100. Asymmetric (A12 ) and globular hydrophilic and amphiphilic (GH 4 , GA 4 , GA 2 , and GA 1 ) AChE species occurred in NMV and DMV, the lighter forms being predominant. The percentage of GH 4 and GA 4 decreased in DMV. A fraction of the AChE that could not be extracted with detergent was detached with collagenase. Most of the detergent-released A12 AChE from NMV and nearly half in DMV failed to bind to Ricinus communis agglutinin (RCA-I). Conversely, the collagenase-detached isoforms bound to RCA, revealing that asymmetric AChE associated with internal membranes or basal lamina differed in glycosylation. Moreover, nearly half of GA 4 AChE in DMV and a few in NMV bound to RCA. Most of the RCA-unreactive GA 4 forms in NMV come from sarcolemma. The results indicate that dystrophy induces minor changes in the distribution and glycosylation of AChE forms in internal membranes of muscle. 相似文献
85.
Lynn R. Trusal Albert W. Guzman Carol J. Baker 《In vitro cellular & developmental biology. Plant》1984,20(4):353-364
Summary The pathophysiology of endothelial cells is important to a variety of vascular conditions including coagulation and hemostasis
resulting from clinical frostbite. Use of an in vitro model system demonstrated that when bovine endothelial cells were frozen
at 1°C or 20°C/min and thawed immediately (20°C/min), a variety of ultrastructural alterations occurred. Membraneous structures
were most extensively damaged, with mitochondria the most sensitive organelle. Low amplitude mitochondrial swelling, first
evident at 0°C, progressed to high amplitude swelling by −10°C (frozen). In addition, the rough endoplasmic reticulum was
dilated and formed large vesicles with a homogeneous matrix. Nuclear changes first occurred at −15°C. These included separation
and distortion of the nuclear membrane, changes in chromatin distribution, and disruption of the nucleolus. Scanning electron
microscopy revealed perforated plasma membranes in some cells at −10°C (frozen) and in most cells by −20°C. Cultures frozen
at 20°C/min revealed mostly the same ultrastructural damage noted at 1°C/min except a higher percentage of cells exhibited
alterations. Data from the recovery index and lactic dehydrogenase (LDH) release correlated well with observed ultrastructural
changes. Early swelling of mitochondria and dilation of rough endoplasmic reticulum was not lethal in the absence of freezing.
Increased swelling in cytoplasmic organelles coupled with nuclear alterations at −15°C resulted in a decreased survival rate
and release of significant quantities of LDH by −20°C. No unique morphological changes were temperature specific, but the
total number of cells that displayed alterations increased as temperature decreased.
The views, opinions or findings, or both, contained in this report are those of the authros and should not be construed as
indicative of an official Department of the Army position, policy, or decision unless so designated by other official documentation. 相似文献
86.
87.
We found that chronic lithium diet affects the sensitivity of neuroleptic receptors and the content of amino acids in the brain, and that the changes in adult animals differ from those in young rats. Pregnant rats were kept on lithium diet (pellets with 0.21% Li2CO3 and 0.21% NaCl) during the gestation period and the offspring were kept on lithium for six weeks after delivery. Control rats were kept on normal diet under identical conditions. In corpus striatum and cerebral cortex of lithium-treated young rats a reduction in apparent dissociation constant and no change in (3H)spiperone total binding sites were found, suggesting a sensitization of the neuroleptic receptor; this result was unlike that obtained with adult lithium-treated rats, where the total number of binding sites was decreased. The lithium content of brain was very high (2.32 meq/kg of wet weight), whereas in the serum only 0.75 meq/l was recorded. K+ and Na+ levels increased by 20% and 9% respectively in the brain and remained at normal levels in the serum. Analysis of free amino acids in the cerebral cortex, midbrain, and cerebellum showed increases in GABA and glycine levels in all three regions, a significant increase in taurine in midbrain, and an increase in lysine in cerebral cortex and cerebellum. The results indicate that the effect of chronic dietary lithium given during pregnancy on the neuroleptic receptor in young rats is different from that in adult animals. It produces an increase in the number of the neuroleptic receptor sites instead of the decline in the number of binding sites found in adult rats. It remains to be established whether this effect is related more to the age of the animal tested or to the stage of development of the CNS at which the lithium was administered. 相似文献
88.
Wiedemann-Beckwith syndrome: clinical, cytogenetical and radiological observations in 39 new cases. 总被引:3,自引:0,他引:3
R Martínez y Martínez R Martínez-Carboney R Ocampo-Campos H Rivera J Gómez Plascencia y Castillo A Cuevas M C Martín Manrique 《Genetic counseling (Geneva, Switzerland)》1992,3(2):67-76
Thirty-nine patients (82% under 1 year of age) with Wiedemann-Beckwith syndrome (WBS) were prospectively studied. To evaluate the somatometric data the normal range was set out at mean +/- 2 SD. The relevant physical findings were a characteristic face, non increased mean height and weight, normal head circumference, defective abdominal wall, a predominance of the upper segment, and tibial bowing. Mental retardation was documented in 5 cases but in only 1 it was related to hypoglycemia. The 32 cases karyotyped were normal. Since neonatal hypoglycemia is frequent (34.3% in our series) and potentially deleterious for the CNS we propose to monitor the glycemia every 6 h during the first 3 days in WBS newborns in order to correct glycemia below of 2.6 mmol/l (46.8 mg/dl) according to recent studies. The comparison with previous large series enabled us to precise the frequency, onset and evolution of the main stigmata. 相似文献
89.
90.
Javier Turnay Nieves Olmo José G. Gavilanes María A. Lizarbe 《In vitro cellular & developmental biology. Animal》1991,27(6):447-452
Summary Fibroblastlike primary cells have been obtained from human colon adenocarcinoma explants. Such cells disappear during cell
culture and thus have not been previously studied. These cells have a number of altered phenotypic characteristics: a) morphology;
b) growth behavior and adherence to culture substrate (they required 3 h for 90% attachment and only presented a flattened
morphology 40 h after platting); and c) collagen metabolism. Increased protein biosynthesis (about double than control colon-derived
fibroblasts) and maintained ability for collagen biosynthesis have been observed for the tumor-associated fibroblastlike cells.
Thus, the collagen to noncollagenous proteins ratio was decreased for these cells. They exhibited an altered type I:type III
collagen (5:1 instead of 3:1 in colon fibroblasts) and procollagen (2:1 against 5:1 in colon fibroblasts) ratios as well as
a decreased secretion of collagen with an abnormal deposition of procollagens in the cell layer. These studies show a permanent
phenotypic alteration in the tumor-associated fibroblastlike cells. 相似文献