全文获取类型
收费全文 | 8196篇 |
免费 | 874篇 |
国内免费 | 41篇 |
专业分类
9111篇 |
出版年
2023年 | 39篇 |
2022年 | 98篇 |
2021年 | 150篇 |
2020年 | 113篇 |
2019年 | 165篇 |
2018年 | 188篇 |
2017年 | 166篇 |
2016年 | 265篇 |
2015年 | 398篇 |
2014年 | 474篇 |
2013年 | 528篇 |
2012年 | 616篇 |
2011年 | 577篇 |
2010年 | 440篇 |
2009年 | 374篇 |
2008年 | 461篇 |
2007年 | 476篇 |
2006年 | 389篇 |
2005年 | 393篇 |
2004年 | 358篇 |
2003年 | 313篇 |
2002年 | 316篇 |
2001年 | 123篇 |
2000年 | 134篇 |
1999年 | 135篇 |
1998年 | 85篇 |
1997年 | 63篇 |
1996年 | 55篇 |
1995年 | 55篇 |
1994年 | 44篇 |
1993年 | 47篇 |
1992年 | 75篇 |
1991年 | 73篇 |
1990年 | 55篇 |
1989年 | 46篇 |
1988年 | 50篇 |
1987年 | 54篇 |
1986年 | 38篇 |
1985年 | 53篇 |
1984年 | 40篇 |
1983年 | 33篇 |
1982年 | 40篇 |
1981年 | 48篇 |
1979年 | 36篇 |
1978年 | 29篇 |
1977年 | 29篇 |
1974年 | 34篇 |
1973年 | 33篇 |
1970年 | 25篇 |
1968年 | 28篇 |
排序方式: 共有9111条查询结果,搜索用时 0 毫秒
11.
Distinct effects of climate warming on populations of silver fir (Abies alba) across Europe 下载免费PDF全文
12.
Characterization of FP22, a large streptomycete bacteriophage with DNA insensitive to cleavage by many restriction enzymes 总被引:1,自引:0,他引:1
Bacteriophage FP22 has a very broad host range within streptomycetes and appeared to form lysogens of Streptomyces ambofaciens ATCC 15154. FP22 shared strong cross-immunity and antibody cross-reactivity with bacteriophage P23, but not with seven other streptomycete bacteriophages. FP22 particles had a head diameter of 71 nm and a tail length of 307 nm. The FP22 genome was 131 kb, which is the largest bacteriophage genome reported for streptomycetes. The G + C content of the genome was 46 mol% and restriction mapping indicated that FP22 DNA had discrete ends. NaCl- and pyrophosphate-resistant deletion mutants were readily isolated and the extent of the deletions defined at least 23 kb of dispensable DNA in two regions of the genome. The DNA was not cleaved by most restriction endonucleases (or isoschizomers) which have been identified in the streptomycetes, including the tetranucleotide cutter MboI (GATC). 相似文献
13.
Aromatic L-amino acid decarboxylase (AADC) is responsible for the conversion of L-3,4-dihydroxyphenylalanine (L-DOPA) and L-5-hydroxytryptophan to dopamine and serotonin, respectively, which are important neurotransmitters. We characterized genomic clones derived from the rat AADC locus by Southern blot and nucleotide sequencing analyses to explore the exonal organization of the gene. Our results suggest that the rat AADC gene is relatively large, containing at least 12 exons and spanning at least 40 kb in the rat genome. In this study, nine exons corresponding to 71% of the published cDNA sequence were identified, the smallest of which was as short as 20 base pairs (bp). In the Drosophila dopa decarboxylase (DDC) gene, the sequences homologous to these nine exons are all present in the fourth exon. This implies that either multiple intron sequences have been added to the vertebrate AADC gene or alternatively, deleted from the invertebrate gene after the divergence of vertebrates and invertebrates during evolution. 相似文献
14.
Assay of DNA-RNA hybrids by S1 nuclease digestion and adsorption to DEAE-cellulose filters. 总被引:11,自引:1,他引:10 下载免费PDF全文
A fast and accurate assay procedure for DNA-RNA hybrids is described in which exhaustive digestion of unhybridized DNA with S1 nuclease is followed by binding of hybrids to filter discs of DEAE-cellulose. The digested DNA can be efficiently washed from the filters so that background levels of 0.1-0.2% of input tracer DNA can be achieved, in contrast to the much higher (approximately 1-5%) backgrounds obtained using TCA precipitation procedures. Short duplexes, as small as 36 nucleotides in length, which are inefficiently bound to hydroxyapatite, are quantitatively bound to the DEAE-cellulose filters. 相似文献
15.
Monoclonal antibodies generated by immunization with a plasma-membrane preparation from suspension-cultured cells of Nicotiana glutinosa L. were used in combination with fluoresceinor rhodamine-labeled goat anti-mouse immunoglobulins to identify heterokaryons in protoplast fusion procedures. Antibody labeling did not inhibit callus formation nor plantlet regeneration. The antibodies are non-invasive and surface labeling provides clear optical discrimination of true heterokaryons from unfused aggregates as well as from parental protoplasts and homokaryons. Labeling is stable throughout fusion and hence by pre-labeling parental protoplast populations the strategy is both versatile and of general applicability. 相似文献
16.
17.
18.
R Schmutzler J F van Uem K Elkind-Hirsch D Hahn J L McGuire D R Danforth G D Hodgen 《Journal of medical primatology》1988,17(5):281-285
Preliminary data indicate the potential utility of an implantable subcutaneous device that facilitates chronic intravenous infusion of pulsatile gonadotropin-releasing hormone (GnRH) for ovulation induction. GnRH distribution curves were congruent in control monkeys and those with implanted devices. Tissue tolerance was good in this brief trial. These findings suggest that use of this or a similar implantable device be considered for chronic GnRH administration in human pulse therapy. 相似文献
19.
D T Francois I M Katona C H June L M Wahl N Feuerstein K P Huang J J Mond 《Journal of immunology (Baltimore, Md. : 1950)》1988,140(10):3338-3343
Cross-linking of surface Ig has been shown to stimulate phosphatidylinositol hydrolysis in murine B cells, leading to increases in [Ca2+]i and activation of protein kinase C (PKC). Preliminary evidence suggests that a similar activation mechanism occurs in human B cells. We wished to examine whether anti-Ig antibody-stimulated human B cell proliferation is as dependent upon the presence of PKC as is anti-Ig-mediated murine B cell proliferation. Using highly purified, small, dense peripheral-blood B lymphocytes from healthy adult donors, we confirmed that PMA, a direct activator of PKC, is a potent mitogen for human B cells that synergizes with anti-mu antibody. Furthermore, we demonstrated that PMA treatment abolishes detectable cellular stores of immunoreactive PKC. However, after such depletion of cellular PKC, anti-mu antibody is still capable of delivering a proliferative signal to human B cells. It is unlikely that this signal occurs solely on the basis of increases in [Ca2+]i, because the calcium ionophore A23187 does not induce a proliferative response in PMA-treated B cells similar in magnitude to that seen with anti-mu. Additionally, the finding that pretreatment of B cells with PMA ablates the ability of anti-Ig antibody to mobilize intracellular and extracellular calcium also suggests that the ability of PMA to enhance anti-Ig mediated stimulation does not depend on elevations of [Ca2+]i induced by anti-Ig. Together, these observations suggest that anti-Ig signaling of human B cells may occur via other pathways in addition to the phosphatidylinositol system of calcium influx and PKC activation. 相似文献
20.
Allison A. Welder Tina Machu Steven W. Leslie Richard E. Wilcox June Bradlaw Daniel Acosta 《In vitro cellular & developmental biology. Plant》1988,24(8):771-777
Summary Primary mycolardial cell cultures and freshly isolated cardiac cells in suspension resprensent two isolated, whole cell models
for investigating cellular transsarcolemmal45Ca++ exchange in response to a receptor-coupled stimulus. Studies were performed to characterize beta-adrenergic receptor binding,
beta-adrenergic receptor mediated cellular calcium (45Ca++) exchange, and viability in purified primary myocardial cell cultures and freshly isolated cardiac cells in suspension obtained
from 3-to 3-d-old Sprague-Dawley rats. In addition, beta-adrenergic receptor binding was characterized in whole-heart crude
membrane preparations. All three preparations had saturable beta-adrenergic binding sites with the antagonist [125I]iodopindolol ([125I]IPIN). The suspensions had a significantly lower B
max
(42±6 fmol/mg protein) than the membranes and cultures (77±8 and 95±10 fmol/mg protein, respectively). The K
D
of the cultures (218±2.0 pM) was significantly higher than that for the suspensions (107 ±1.3 pM) and membranes (93±1.3 pM). Viability was significantly lower in the suspensions (57%) when compared to 94% viability in myocardial cell cultures after
3 h of incubation in Kreb's Henseleit buffer. Incubation of the cultures with 5.0×10−7
M isoproterenol resulted in a significant increase in45Ca++ exchange as early as 15 s. In contrast,45Ca++ exchange into the suspensions was not increased. Although both primary cell cultures and cardiac cells in suspension possess
saturable beta-adrenergic receptors, only the monolayer cultures exhibited functional beta-adrenergic receptor-mediated45Ca++ exchange. Of the two intact cell models investigated, these data suggest that primary myocardial cell cultures are more suitable
than cell suspensions for investigating beta-adrenergic receptor binding and functions in the postnatal rat heart.
This research was supported by The University of Texas Research Institute, a grant from the Texas Advanced Research Technology
Program awarded to S. W. Leslie and R. E. Wilcox, and contract 223-86-2109 from the Food and Drug Administration. 相似文献