全文获取类型
收费全文 | 5933篇 |
免费 | 477篇 |
出版年
2023年 | 20篇 |
2022年 | 51篇 |
2021年 | 104篇 |
2020年 | 61篇 |
2019年 | 84篇 |
2018年 | 95篇 |
2017年 | 104篇 |
2016年 | 127篇 |
2015年 | 249篇 |
2014年 | 271篇 |
2013年 | 407篇 |
2012年 | 472篇 |
2011年 | 468篇 |
2010年 | 330篇 |
2009年 | 279篇 |
2008年 | 402篇 |
2007年 | 406篇 |
2006年 | 380篇 |
2005年 | 333篇 |
2004年 | 328篇 |
2003年 | 283篇 |
2002年 | 284篇 |
2001年 | 76篇 |
2000年 | 63篇 |
1999年 | 68篇 |
1998年 | 68篇 |
1997年 | 51篇 |
1996年 | 39篇 |
1995年 | 36篇 |
1994年 | 28篇 |
1993年 | 23篇 |
1992年 | 42篇 |
1991年 | 25篇 |
1990年 | 38篇 |
1989年 | 24篇 |
1988年 | 34篇 |
1987年 | 17篇 |
1986年 | 17篇 |
1985年 | 20篇 |
1984年 | 17篇 |
1983年 | 13篇 |
1982年 | 14篇 |
1981年 | 11篇 |
1979年 | 11篇 |
1978年 | 15篇 |
1977年 | 12篇 |
1975年 | 10篇 |
1974年 | 10篇 |
1973年 | 12篇 |
1966年 | 8篇 |
排序方式: 共有6410条查询结果,搜索用时 15 毫秒
21.
Duplication of HRAS1, INS, and IGF2 is not a common event in Beckwith-Wiedemann syndrome 总被引:1,自引:0,他引:1
I Henry M Jeanpierre F Barichard J L Serre J Mallet C Turleau J de Grouchy C Junien 《Annales de génétique》1988,31(4):216-220
A few cases of Beckwith-Wiedemann syndrome (BWS) have in common a duplication of 11p15. Among the genes located in 11p15, c-Ha-ras 1 (HRAS1), insulin (INS), and insulin-like growth factor II (IGF2) may account for the clinical features and the increased risk for malignancy. Using eight 11p15 markers including HRAS1, INS and IGF2 we have studied eight sporadic and hereditary cases of BWS whether or not associated with a nephroblastoma. By gene dosage determination and family studies, we have shown the following: the eight patients examined had an apparent diploid representation of all of the eight markers studied, thus indicating that a microduplication of these markers or of the region characterized by these markers is not a common event in BWS; in a family with three affected sibs the genes for HRAS1 and INS/IGF2 did not cosegregate with BWS and therefore may not participate in the pathogenic processes here observed. 相似文献
22.
F.9 embryonal carcinoma cell calcitonin autocrine system: correlation between immunoreactive calcitonin secretion and calcitonin receptor number 总被引:2,自引:0,他引:2
Mouse teratocarcinoma cells in culture offer an in vitro system to study the initial steps of embryogenesis. It has been suggested that, at such early stages, cell functions are regulated by an autocrine process in which embryonic cells produce factors that in turn act on themselves. F.9 cells possess specific membrane receptors for calcitonin (CT) (120 fmol/mg of protein, Ka, = 3.5 X 10(8) M-1). These cells produce CT detected by heterologous radioimmunoassay in serum-free culture-conditioned medium (75 pg/10(7) cells/12 h). When F.9 cells are incubated in serum-free medium, CT binding and secretion concomitantly drop by 50% within the first 2 h, then increase progressively to an upper plateau after the sixth hour. Preincubation with 10(-6) M CT leads to disappearance of CT receptors and CT secretion in the culture medium up to 6 h. Avoiding accumulation of CT in the medium by a continuous flow rate for 6 h leads to a progressive decrease of CT receptors. In addition, retinoic acid treatment of cells induces a parallel progressive decrease of CT receptor number and of total CT synthesis. These results suggest a reciprocal regulation of CT receptors and CT secretion, or a close relationship between their regulations. 相似文献
23.
Amino acid sequence homologies between rabbit, rat, and human serum retinol-binding proteins 总被引:1,自引:0,他引:1
J Sundelin B C Laurent H Anundi L Tr?g?rdh D Larhammar L Bj?rck U Eriksson B Akerstr?m A Jones M Newcomer 《The Journal of biological chemistry》1985,260(10):6472-6480
The main transporting protein for vitamin A in rabbit serum, the retinol-binding protein (RBP), was isolated and its amino acid sequence determined. Rabbit RBP was found to be highly homologous to human RBP, whose amino acid sequence was elucidated earlier, and to rat RBP. The rat RBP sequence was obtained by combining information deduced from the nucleotide sequences of two overlapping cDNA clones with the NH2-terminal sequence of the isolated protein determined by automated Edman degradation. The identity between the three proteins is approximately 90%. The high degree of homology between RBP molecules from different species is probably explained by the fact that RBP participates in at least three types of molecular interactions: in the binding of prealbumin, in the interaction with retinol, and in the recognition of a specific cell surface receptor. All these interactions should lead to a conservation of RBP structure. The amino acid differences between rabbit, rat, and human RBP are discussed in light of the recent elucidation of the three-dimensional structure of human RBP. Hybridization of a probe isolated from a rat RBP cDNA clone to restriction enzyme-digested genomic DNA from rat and mouse suggests that RBP is encoded by a single gene. 相似文献
24.
Summary Ultrastructure, distribution and abundance of cell types were examined in the gills of two freshwater salmonid species, Salmo fario and Salmo gairdneri, in media of selected ion content. In plain hard water (PW) with high concentrations of Ca2+, Na+, and Cl-, gill chloride cells (CC) were confined to trailing edges and interlamellar regions of filaments whereas in mountain soft water (MW) with low concentrations of Ca2+, Na+, and Cl-, CC were more numerous on filaments and covered lamellae, particularly along trailing edges. CC also appeared on lamellae of PW trout acclimated to soft water in a pond. This proliferation was not alleviated when ambient Ca2+ levels were raised (MW + Ca2+) but regressed in elevated NaCl media (MW + NaCl). The regression process involved an initial covering of CC by pavement cells followed by cytolysis and then eventual disappearance of CC. In MW, mucous cells were distributed mainly on trailing edges and, to a lesser extent, leading edges of filaments; they were absent from lamellae regardless of external ion levels.The results of this study shed some light on the functional significance of CC in freshwater fish. It is suggested that proliferation of CC is an adaptive response to dilute freshwater (i.e. [NaCl]<0.1 mequiv·1-1). 相似文献
25.
26.
27.
Tissue uptake of circulating hyaluronic acid 总被引:5,自引:0,他引:5
J. Robert E. Fraser Lars-Erik Appelgren Prof. Torvard C. Laurent 《Cell and tissue research》1983,233(2):285-293
Previous work in the rabbit has shown that there is a significant flux of plasma hyaluronic acid (HA) which is taken up and degraded mainly in the liver but also concentrated in the spleen. Purified 14C-labelled HA of high average molecular wt prepared by biosynthesis from D-[U-14C] glucose was injected i.v. in mice and its tissue distribution was determined by whole-body autoradiography during the next 24 h. As blood levels declined, radioactivity was concentrated in the liver and spleen as found in the rabbit, and also in bone marrow and lymph nodes. Distribution was uniform in liver tissue, concentrated and relatively persistent in the periphery of lymph nodes, and distinctly nodular within the spleen. Analysis of an aqueous liver extract taken 4 h after injection identified 14C in HA, in a macromolecular fraction resistant to fungal hyaluronidase, and in metabolites of low molecular wt. These findings confirm and extend observations based on tissue extraction in rabbits. The pattern of distribution through the body and the restricted localization within spleen and lymph nodes further suggest that HA is absorbed from plasma and tissue fluids by elements of the reticuloendothelial system. 相似文献
28.
Differential size variations between transcriptionally active and inactive telomeres of Trypanosoma brucei 总被引:19,自引:6,他引:13 下载免费PDF全文
We have studied the genes coding for the variant-specific surface antigen (VSA) in a series of seven trypanosome clones derived from AnTat 1.1: 1.1 leads to 1.3 leads to 1.6 leads to 1.16 leads to 1.1C leads to 1.3B leads to 1.18 These genes are all telomeric (1-5), and their surrounding, although sometimes similar, differs in each case. The length between these antigen genes and the corresponding DNA end appears to increase at each antigenic switch, with however occasional sharp size reductions, often linked to the involvement of the telomere in gene expression. This increase is due to a constant "growth" of the telomeres, at a rate of about 28 bp per day in at least four cases and probably linked to chromosome duplication. The telomere harbouring the transcribed VSA gene is growing slightly faster (about 36 bp per day), and it is the only one whose size reduction is progressive, leading to a terminal length heterogeneity within a clone. As a result, the active VSA gene is found in a population of telomeres which, as the trypanosomes divide, becomes increasingly heterogeneous, with however a preferred discrete size class about 1.4 kb smaller. The fact that the "active" telomere is the only one in a chromatin conformation highly sensitive to DNAaseI (1-4, 6), suggests that chromatin structure influences the rate and extent of both size increase and shortening of telomeres. 相似文献
29.
30.
Conformational transitions of polynucleotides in polymer media 总被引:5,自引:0,他引:5