全文获取类型
收费全文 | 6656篇 |
免费 | 513篇 |
专业分类
7169篇 |
出版年
2024年 | 8篇 |
2023年 | 56篇 |
2022年 | 104篇 |
2021年 | 204篇 |
2020年 | 107篇 |
2019年 | 137篇 |
2018年 | 185篇 |
2017年 | 187篇 |
2016年 | 226篇 |
2015年 | 394篇 |
2014年 | 392篇 |
2013年 | 525篇 |
2012年 | 643篇 |
2011年 | 558篇 |
2010年 | 346篇 |
2009年 | 296篇 |
2008年 | 379篇 |
2007年 | 377篇 |
2006年 | 314篇 |
2005年 | 293篇 |
2004年 | 268篇 |
2003年 | 247篇 |
2002年 | 235篇 |
2001年 | 51篇 |
2000年 | 32篇 |
1999年 | 47篇 |
1998年 | 67篇 |
1997年 | 40篇 |
1996年 | 44篇 |
1995年 | 35篇 |
1994年 | 23篇 |
1993年 | 19篇 |
1992年 | 17篇 |
1991年 | 16篇 |
1990年 | 47篇 |
1989年 | 9篇 |
1988年 | 12篇 |
1987年 | 15篇 |
1986年 | 13篇 |
1985年 | 13篇 |
1984年 | 23篇 |
1983年 | 20篇 |
1982年 | 14篇 |
1981年 | 21篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1975年 | 7篇 |
1974年 | 7篇 |
1973年 | 6篇 |
排序方式: 共有7169条查询结果,搜索用时 15 毫秒
21.
Genetic control of the mitochondrial form of superoxide dismutase in hexaploid wheat 总被引:3,自引:0,他引:3
Extracts of mature grains of a large number of aneuploid derivatives of Triticum aestivum cv. Chinese Spring and of the members of five wheat-alien chromosome addition series were subjected to isoelectric focusing in polyacrylamide gels in order to study the genetic control of superoxide dismutase (SOD). Evidence was obtained that homologous structural genes for the mitochondrial form of SOD are located in the long arms of the homoeologous group 2 chromosomes of Chinese Spring and in chromosome 2R of Secale cereale cv. Imperial. The SOD gene loci located in chromosomes 2A, 2B, 2D, and 2R were designated Sod-A1, Sod-B1, Sod-D1, and Sod-R1, respectively. Chromosome-arm pairing data indicate that 2DL is not homoeologous to either 2AS or 2BL. The results of this study suggest, however, that 2BL is partially homoeologous to both 2AL and 2DL.Technical article No. 21074 of the Texas Agricultural Experiment Station. This work was supported by USDA Grant 83-CRCR-1-1322 to GEH. 相似文献
22.
Cloning and Characterization of Yeast LEU4, One of Two Genes Responsible for α-Isopropylmalate Synthesis 总被引:4,自引:0,他引:4 下载免费PDF全文
Li-fen L. Chang Thomas S. Cunningham Paula R. Gatzek Wen-Ji Chen Gunter B. Kohlhaw 《Genetics》1984,108(1):91-106
By complementation of an alpha-isopropylmalate synthase-negative mutant of Saccharomyces cerevisiae (leu4 leu5), a plasmid was isolated that carried a structural gene for alpha-isopropylmalate synthase. Restriction mapping and subcloning showed that sequences sufficient for complementation of the leu4 leu5 strain were located within a 2.2-kilobase SalI-PvuII segment. Southern transfer hybridization indicated that the cloned DNA was derived intact from the yeast genome. The cloned gene was identified as LEU4 by integrative transformation that caused gene disruption at the LEU4 locus. When this transformation was performed with a LEU4fbr LEU5 strain, the resulting transformants had lost the 5',5',5'-trifluoro-D,L-leucine resistance of the recipient strain but were still Leu+. When it was performed with a LEU4 leu5 recipient, the resulting transformants were Leu-. The alpha-isopropylmalate synthase of a transformant that carried the LEU4 gene on a multicopy plasmid (in a leu5 background) was characterized biochemically. The transformant contained about 20 times as much alpha-isopropylmalate synthase as wild type. The enzyme was sensitive to inhibition by leucine and coenzyme A, was inactivated by antibody generated against alpha-isopropylmalate synthase purified from wild type and was largely confined to the mitochondria. The subunit molecular weight was 65,000-67,000. Limited proteolysis generated two fragments with molecular weights of about 45,000 and 23,000. Northern transfer hybridization showed that the transformant produced large amounts of LEU4-specific RNA with a length of about 2.1 kilonucleotides. The properties of the plasmid-encoded enzyme resemble those of a previously characterized alpha-isopropylmalate synthase that is predominant in wild-type cells. The existence in yeast of a second alpha-isopropylmalate synthase activity that depends on the presence of an intact LEU5 gene is discussed. 相似文献
23.
Summary In the family Uloboridae, web reduction is associated with changes in web monitoring posture and prosomal features. A spider must extend its first pair of legs directly forward to monitor the signal line of a reduced web. This posture is facilitated by shifts in prosomal musculature that cause reduced web uloborids to have a narrower anterior prosoma, a reduced or absent anterior eye row, and prominent posterior lateral eye tubercles. The eye tubercles and larger posterior eyes of these uloborids suggest that web reduction may also be accompanied by ocular changes that compensate for reduction of the anterior eyes by expanding the visual fields of the posterior eyes. A comparison of the visual fields of the eight-eyed, orb web species Octonoba octonaria and a four-eyed, reduced web Miagrammopes species was made to determine if this is true. Physical and optical measurements determined the visual angles of each species' eyes and the pattern of each species' visual surveillance. Despite loss of the anterior four eyes, the Miagrammopes species has a visual coverage similar to that of O. octonaria. This is due to (1) an increase in the visual field of each of the four remaining Miagrammopes eyes, accruing from an extension of the retina and an increase in the lens' rear radius of curvature, and (2) a ventral shift of each visual axis, associated with the development of an eye tubercle and an asymmetrical expansion of the retina. Miagrammopes monitor their simple webs from twigs or moss where they are vulnerable to predation. Therefore, maintenance of visual cover may enable them to detect predators in time to assume or maintain their characteristic, cryptic posture. It may also allow them to observe approaching prey and permit them to adjust web tension or prepare to jerk their webs when prey strikes. 相似文献
24.
25.
Paula K. Donnelly James A. Entry Don L. Crawford Kermit Cromack Jr 《Microbial ecology》1990,20(1):289-295
The concentration of lignin in plant tissue is a major factor controlling organic matter degradation rates in forest ecosystems.
Microbial biomass and lignin and cellulose decomposition were measured for six weeks in forest soil microcosms in order to
determine the influence of pH, moisture, and temperature on organic matter decomposition. Microbial biomass was determined
by chloroform fumigation; lignin and cellulose decomposition were measured radiometrically. The experiment was designed as
a Latin square with soils of pH of 4.5, 5.5, and 6.5 adjusted to 20, 40, or 60% moisture content, and incubated at temperatures
of 4, 12, or 24°C. Microbial biomass and lignin and cellulose decomposition were not significantly affected by soil acidity.
Microbial biomass was greater at higher soil moisture contents. Lignin and cellulose decomposition significantly increased
at higher soil temperatures and moisture contents. Soil moisture was more important in affecting microbial biomass than either
soil temperature or soil pH. 相似文献
26.
Paula C. Southwick Susan K. Yamagata Charles L. Echols Jr. Gail J. Higson Scott A. Neynaber Robert E. Parson William A. Munroe 《Journal of neurochemistry》1996,66(1):259-265
Abstract: The principal constituent of amyloid plaques found in the brains of individuals with Alzheimer's disease (AD) is a 39–42-amino-acid protein, amyloid β protein (Aβ). This study examined whether the measurement of Aβ levels in CSF has diagnostic value. There were 108 subjects enrolled in this prospective study: AD (n = 39), non-AD controls (dementing diseases/syndromes; n = 20), and other (n = 49). CSF was obtained by lumbar puncture, and Aβ concentrations were determined using a dual monoclonal antibody immunoradiometric sandwich assay. The mean Aβ value for the AD group (15.9 ± 6.8 ng/ml) was not significantly different from that for the non-AD control group (13.0 ± 7.1 ng/ml; p = 0.07), and substantial overlap in results were observed. Aβ values did not correlate with age ( r = −0.05, p = 0.59), severity of cognitive impairment ( r = 0.22, p = 0.21), or duration of AD symptoms ( r = 0.14, p = 0.45). These findings are in conflict with other reports in the literature; discrepant results could be due to the instability of Aβ in CSF. Aβ immunoreactivity decays rapidly under certain conditions, particularly multiple freeze/thaw cycles. Use of a stabilizing sample treatment buffer at the time of lumbar puncture allows storage of CSF without loss of Aβ reactivity. In conclusion, the total CSF Aβ level is not a useful marker for current diagnosis of AD. 相似文献
27.
Claudia Gaspar Iscia Lopes-Cendes Anita L. DeStefano Patrícia Maciel Isabel Silveira Paula Coutinho Patrick MacLeod Jorge Sequeiros Lindsay A. Farrer G. A. Rouleau 《Human genetics》1996,98(5):620-624
Machado-Joseph disease (MJD) is an autosomal dominant spinocerebellar degeneration originally described in families of Portuguese-Azorean
ancestry. The hypothesis that its present world distribution could result from the spread of an original founder mutation
has been raised. To test this possibility we have conducted a linkage disequilibrium study of markers segregating with the
MJD1 locus in a total of 64 unrelated families of different geographical origins. Significant association was detected between
the MJD1 locus and marker alleles at loci D14S280, D14S1050 and D14S81. All affected individuals, except one Chinese family, had allele
3 (237 bp) at D14S280. This finding is consistent with a founder effect in our MJD population. However, distinct haplotypes
were observed in patients originating from the two Azorean islands showing the highest disease prevalence; therefore, the
possible existence of more than one founder mutation can not be excluded with the markers currently available.
Received: 27 February 1996 / Revised: 4 June 1996 相似文献
28.
Paula P. Chee 《Plant cell reports》1996,16(3-4):184-187
Summary Taxusbrevifolia is the source of paclitaxel (Taxol®), an anticancer drug. A method for regeneration ofTaxus brevifolia from immature zygotic embryos via somatic embryogenesis is described. Embryogenic callus tissues were obtained by culturing immature zygotic embryos on Lloyd and McCown medium (MCM) supplemented with 160 M 2,4-dichlorophenoxyacetic acid (2,4-D) + 5 M benzylaminopurine (BA) + 5 M naphthaleneacetic acid (NAA) for 4 weeks. Putative embryoids were obtained following transfer of cultures to MCM medium supplemented with 4 M BA + 5 M kinetin + 1 M NAA for 6 to 8 weeks. Conversion of embryos was obtained on MCM medium supplemented with 40 M abscisic acid (ABA) + 1% activated charcoal. Development of bipolar structures with recognizable shoot and root apices was observed in somatic embryos. Five percent of somatic embryos were regenerated into plantlets on half-strength growth regulator-free MCM medium. 相似文献
29.
David H. Lewis Garry K. Burge David M. Schmierer Paula E. Jameson 《Physiologia plantarum》1996,98(1):179-186
The cytokinin content in fruit tissue of the kiwifruit ( Actinidia deliciosa [A. Chev.] C. F. Liang et A. R. Ferguson var. deliciosa cv. Hayward) was monitored during fruit development to identify which cytokinins were present and if they were linked with specific stages of fruit growth. Cytokinins were isolated and purified by column chromatography and high-performance liquid chromatography and quantified by radioimmunoassay. A novel HPLC step utilising an amine column was successfully introduced as a preparative step in the separation of the O - and 9-glucosides from the free bases and ribosides. The radioimmunoassay results were validated, and the different cytokinins identified, by gas chromatography-mass spectrometry. Cytokinins detected in fruit included the cytokinin free bases, zeatin and isopentenyladenine, their ribosides, nucleotides and both O - and 9-glucosides. Both qualitative and quantitative changes of the cytokinins occurred during fruit development. A decrease in cytokinin concentration occurred after anthesis (from 342 pmol g−1 fresh weight at anthesis to 41 pmol g−1 fresh weight 27 days after anthesis). A large increase in cytokinin concentration and content per fruit occurred as the fruit reached commercial maturity (to 1900 pmol g−1 fresh weight). Individual cytokinins showed quite different patterns. Zeatin, in particular, showed a peak in concentration (13 pmol g−1 fresh weight) 11 days after anthesis that correlated with the beginning of the cell division phase of fruit growth. The accumulation of cytokinin (mostly zeatin riboside or zeatin nucleotide) in mature fruit may be of significance for the postharvest storage of kiwifruit fruit. 相似文献
30.
Graciela C. Theiler Yanina C. Marcos Edgardo Kolkowski Nancy Lindel Mónica Capucchio Paula Barrionuevo Francisco R. Carnese M. Leonardo Satz 《Immunogenetics》1996,43(6):398-399
The nucleotide sequence data reported in this paper have been submitted to the GenBank nucleotide sequence database and have been assigned the accession number U17107. The nameB*3509 was officially assigned by the WHO Nomenclature Committee in December 1994 相似文献