全文获取类型
收费全文 | 12080篇 |
免费 | 632篇 |
国内免费 | 3篇 |
专业分类
12715篇 |
出版年
2023年 | 18篇 |
2022年 | 46篇 |
2021年 | 129篇 |
2020年 | 81篇 |
2019年 | 110篇 |
2018年 | 154篇 |
2017年 | 144篇 |
2016年 | 234篇 |
2015年 | 376篇 |
2014年 | 443篇 |
2013年 | 876篇 |
2012年 | 799篇 |
2011年 | 810篇 |
2010年 | 530篇 |
2009年 | 493篇 |
2008年 | 812篇 |
2007年 | 858篇 |
2006年 | 769篇 |
2005年 | 803篇 |
2004年 | 789篇 |
2003年 | 727篇 |
2002年 | 687篇 |
2001年 | 99篇 |
2000年 | 88篇 |
1999年 | 156篇 |
1998年 | 181篇 |
1997年 | 119篇 |
1996年 | 117篇 |
1995年 | 95篇 |
1994年 | 91篇 |
1993年 | 95篇 |
1992年 | 94篇 |
1991年 | 64篇 |
1990年 | 64篇 |
1989年 | 67篇 |
1988年 | 38篇 |
1987年 | 52篇 |
1986年 | 50篇 |
1985年 | 46篇 |
1984年 | 54篇 |
1983年 | 43篇 |
1982年 | 62篇 |
1981年 | 51篇 |
1980年 | 55篇 |
1979年 | 20篇 |
1978年 | 23篇 |
1977年 | 29篇 |
1976年 | 32篇 |
1975年 | 33篇 |
1973年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
22.
Summary This study demonstrates that endocytosis in the oocyte of Drosophila melanogaster is reversibly blocked at the stage of pit formation by the temperature-sensitive, single-gene mutant, shibire
ts1. Uptake of horseradish peroxidase conjugated with wheat-germ agglutinin was observed to be normal in mutant oocytes at 19°C, but was blocked at 29°C. After 10 min at 29°C, there was a build-up of coated pits along invaginations of the plasma membrane. Also, the endosomal compartment consisting of tubules, bulbs, and small yolk spheres, disappeared. Lowering the temperature to 19°C after 10 min at 29°C released a synchronized wave of endocytosis into a cytoplasm cleared of uptake-related organelles. By observing this synchronized wave after exposure to 19°C for varying durations, we determined that endocytosis proceeds as follows: coated pits/vesiclestubulessmall yolk spheresmature yolk spheres. The observations suggest that these organelles transform one into another within this sequence. 相似文献
23.
Efficient integrative transformation of the phytopathogenic fungus Alternaria alternata mediated by the repetitive rDNA sequences 总被引:4,自引:0,他引:4
An attempt was made to transform Alternaria alternata protoplasts using a plasmid vector, pDH25, bearing the Escherichia coli hygromycin B (Hy) phosphotransferase gene (hph) under the control of the Aspergillus nidulans trpC promoter. Transformants arose on a selective medium containing 100 μg Hy/ml. There were two types of transformants, forming large and small colonies on the selective medium. Transformation with one μg of the vector produced an average of 4.5 large colonies and 600 small ones. In large-colony transformants, the vector often integrated into the recipient chromosome in the form of highly rearranged tandem arrays. To increase transformation efficiency, fragments of the highly repetitive ribosomal RNA gene cluster (rDNA) of A. alternata were used to construct four new vectors for homologous recombination system. Use of these vectors gave higher transformation efficiency than the original plasmid. The best vector, pDH25r1a, gave rise to large-colony transformants at a frequency 20 times higher than pDH25. Transformation events in A. alternata with pDH25r1a occured by homologous recombination as a single crossover between the plasmid-borne rDNA segment and its homologue in the chromosome, often giving rise to tandemly repeated vector DNA. 相似文献
24.
Tatsuo Nakahara Makoto Hirano Takashi Matsumoto Toshihide Kuroki Yoshinori Tatebayashi Tetsuyuki Tsutsumi Kouji Nishiyama Hiroaki Ooboshi Kaoru Nakamura Hiroshi Yao Akio Shiraishi Michinori Waki Hideyuki Uchimura 《Neurochemical research》1990,15(6):609-611
DNA and RNA contents in 20 brain regions or nuclei of the rat were determined by a highly sensitive method using high-performance liquid chromatography with electrochemical detection. The high DNA and RNA contents were found in the hypothalamic nuclei, especially the median eminence-arcuate nucleus. These results may be available for the preparation of nucleic acids as the regional control. 相似文献
25.
Dr. Takashi Yashiro Motonaka Arai Eiko Miyashita Kayoko Yamashita Takuro Suzuki 《Cell and tissue research》1988,251(2):249-255
Summary Snell dwarf mice display remarkable retardation of growth after birth and are known to lack prolactin (PRL), thyroid stimulating hormone (TSH) and growth hormone (GH). The aim of this study was to determine the reason for these hormonal deficiencies. We examined the fine structure of the gland and its immunohistochemical staining pattern with respect to antisera raised against PRL, TSH, GH, adrenocorticotrophic hormone (ACTH) and luteinizing hormone (LH). The gland of control mice reacted immunohistochemically against all antisera used, whereas only ACTH-producing cells (ACTH cells) and LH-producing cells (LH cells) were distinguished in the dwarf mice. ACTH cells in dwarf mice varied in cell shape, although they were similar in size to those of controls. The distribution of secretory granules in the cytoplasm varied from cell to cell. LH cells in the dwarf mice showed immature features, having poorly developed rough endoplasmic reticulum and Golgi apparatus. The cells were about half the size of controls, and secretory granules were smaller. In dwarf mice, non-granulated cells were encountered in addition to granulated ACTH and LH cells. Some of them formed small clusters, characteristic cell junctions being found between the cells; they thus appeared to be follicular cells. The above results suggest that hormone deficiency in Snell dwarf mice is a result of a defect in the hormoneproducing cells in the gland. 相似文献
26.
Chloroplast DNA (cpDNA) and mitochondrial DNA (mtDNA) of malesterile tobacco plants obtained by fusion of Nicotiana tabacumprotoplasts and X-irradiated N. debneyi protoplasts were analyzed.Digestion of cpDNA isolated from ten male sterile lines withfour restriction endonucleases (EcoRI, XhoI, SmaI and HindIII)indicated that these lines possessed either one or the otherparental chloroplast genome. Neither mixture of two types ofcpDNA nor unique restriction fragments were detected in anyof the cases examined. The genetic constitution of chloroplastgenomes identified by restriction analysis of cpDNA showed goodagreement with that based on isoelectric focusing of the largesubunit of the Fraction I protein. The mtDNA from five fusion-derivedmale sterile plants showed banding patterns quite differentfrom each other and from the parental plants. Each plant exhibitednew restriction fragments not found in the parental species.These findings indicate that recombinational events in the mitochondrialgenomes take place rather frequently in the mixed cytoplasmsafter protoplast fusion, whereas the mixed chloroplasts becomesegregated to homogeneity. (Received June 19, 1987; Accepted October 5, 1987) 相似文献
27.
Summary In order to demonstrate the presence of a Ca2+-activated Cl–-channel in theNitellopsis plasmalemma, tonoplast-free cells were prepared and their intracellular Ca2+ concentration was modified by internal perfusion. An increase in the Ca2+ concentration caused a large Cl– efflux with a concomitant depolarization of the membrane potential. These changes were for the most part reversible. The critical Ca2+ concentration was about 4.0 m. Neither the Cl– efflux nor the membrane depolarization showed a time-dependent inactivation. A Cl–-channel blocker, A-9-C (9-anthracenecarboxylic acid) reduced both the Cl– efflux and the magnitude of the membrane potential depolarization. A small increase in the intracellular Ca2+ concentration, which is caused by membrane excitation of tonoplast-free cells is not sufficient to activate this Ca2+-dependent Cl–-channel. 相似文献
28.
29.
Complementation study of peroxisome-deficient disorders by immunofluorescence staining and characterization of fused cells 总被引:5,自引:0,他引:5
Shigehiro Yajima Yasuyuki Suzuki Nobuyuki Shimozawa Seiji Yamaguchi Tadao Orii Yukio Fujiki Takashi Osumi Takashi Hashimoto Hugo W. Moser 《Human genetics》1992,88(5):491-499
Summary Genetic heterogeneity in peroxisome-deficient disorders, including Zellweger's cerebrohepatorenal syndrome, neonatal adrenoleukodystrophy and infantile Refsum disease, was investigated. Fibroblasts from 17 patients were fused using polyethylene glycol, cultivated on cover slips, and the formation of peroxisomes in the fused cells was visualized by immunofluorescence staining, using anti-human catalase IgG. Two distinct staining patterns were observed: (1) peroxisomes appeared in the majority of multinucleated cells, and (2) practically no peroxisomes were identified. Single step 12-(1-pyrene) dodecanoic acid/ultraviolet (P12/UV)-selection confirmed that the former groups were resistant to this selection, most of the surviving cells contained abundant peroxisomes, and the latter cells died. In the complementary matching, [1-14C]lignoceric acid oxidation and the biosynthesis of peroxisomal proteins were also normalized. Five complementation groups were identified. Group A: Zellweger syndrome and infantile Refsum disease; Groups B, C and D: Zellweger syndrome; Group E: Zellweger syndrome, neonatal adrenoleukodystrophy and infantile Refsum disease. We compared these groupings with those of Roscher and identified eight complementation groups. There was no obvious relation between complementation groups and clinical phenotypes. These results indicate that the transport, intracellular processing and function of peroxisomal proteins were normalized in the complementary matching and that at least eight different genes are involved in the formation of normal peroxisomes and in the transport of peroxisomal enzymes. 相似文献
30.
Ken-ichi Takita Akira Tanigami Takashi Tokino Carol Jones Yusuke Nakamura 《Genomics》1992,13(4):1296-1299
Fifty-four clones containing human inserts were selected from a cosmid library constructed from a somatic cell hybrid containing chromosome 11p15.3-p15.5 as its only human complement. In 32 of these clones, 63 polymorphic systems were identified with a panel of restriction enzymes: 57 conventional RFLP systems and 6 highly polymorphic VNTR systems. Although we examined the cosmid with only seven enzymes, 18 clones (including 6 VNTRs) were polymorphic with three or more enzymes. The results suggested that DNA sequences on the peritelomeric region of chromosome 11p tend to be highly variable. Because these markers are highly informative, they will be excellent resources for investigations of hereditary diseases and tumor suppressor genes in this region of chromosome 11. 相似文献