全文获取类型
收费全文 | 108015篇 |
免费 | 8380篇 |
国内免费 | 9025篇 |
专业分类
125420篇 |
出版年
2024年 | 240篇 |
2023年 | 1417篇 |
2022年 | 3260篇 |
2021年 | 5511篇 |
2020年 | 3777篇 |
2019年 | 4679篇 |
2018年 | 4428篇 |
2017年 | 3238篇 |
2016年 | 4591篇 |
2015年 | 6676篇 |
2014年 | 7847篇 |
2013年 | 8297篇 |
2012年 | 9979篇 |
2011年 | 8974篇 |
2010年 | 5546篇 |
2009年 | 4971篇 |
2008年 | 5710篇 |
2007年 | 5131篇 |
2006年 | 4454篇 |
2005年 | 3493篇 |
2004年 | 2968篇 |
2003年 | 2719篇 |
2002年 | 2272篇 |
2001年 | 1866篇 |
2000年 | 1694篇 |
1999年 | 1669篇 |
1998年 | 1035篇 |
1997年 | 1001篇 |
1996年 | 941篇 |
1995年 | 821篇 |
1994年 | 787篇 |
1993年 | 617篇 |
1992年 | 818篇 |
1991年 | 617篇 |
1990年 | 466篇 |
1989年 | 443篇 |
1988年 | 354篇 |
1987年 | 344篇 |
1986年 | 266篇 |
1985年 | 286篇 |
1984年 | 156篇 |
1983年 | 161篇 |
1982年 | 99篇 |
1981年 | 85篇 |
1980年 | 60篇 |
1979年 | 77篇 |
1977年 | 59篇 |
1975年 | 56篇 |
1974年 | 52篇 |
1973年 | 56篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
181.
Factors Affecting Plant Regeneration from Tissue Cultures of Chinese Leymus (Leymus chinensis) 总被引:1,自引:0,他引:1
Chinese leymus (Leymus chinensis Trin.) is a perennial grass of the Gramineae, which is widely distributed in China, Mongolia and in Russian-Siberian. In order to explore the potential of biotechnology for genetic improvement of this forage grass, an efficient tissue culture system was established and the factors affecting plant regeneration were evaluated. Immature inflorescence segments 3–5 mm in length from eight accessions were cultured on N6 medium supplemented with 2.26–22.60 µM 2,4-D. The callus induction frequency ranged from 72.11 to 82.19%. Shoots were differentiated from the calli on N6 medium containing 4.65 µM kinetin and 4.44 µM BA. Viable regenerants were developed on hormone-free medium. Normal plants were obtained after natural vernalization in the field. The plant regeneration frequency in Chinese leymus was associated with different genotypes and different combinations of growth regulators in medium. The concentration of 2,4-D in the callus induction medium had a strong effect on successive plant regeneration. Relatively higher concentrations of 2,4-D (i.e., 9.04 and 22.60 µM) were more favorable to the plant regeneration than lower ones (i.e., 2.26 and 4.52 µM). This is the first report on plant regeneration in vitro in L. chinensis. 相似文献
182.
The gene (alyVI) encoding an alginate lyase of marine bacterium Vibrio sp. QY101, which was isolated from a decaying thallus of Laminaria, was cloned using a strategy of combined degenerate PCR and long range-inverse PCR (LR-IPCR), then sequenced and expressed in Escherichia coli. Gene alyVI was composed of a 1014 bp open reading frame (ORF) encoding 338 amino acid residues. The calculated molecular mass of alyVI product is 38.4 kDa, but a signal peptide is cleaved off, leaving a mature protein of 34 kDa. AlyVI was purified from culture supernatants to electrophoretic homogeneity using affinity chromatography. AlyVI was most active at pH 7.5 and 40 degrees C in the presence of 1 mM ZnCl2. A nine-amino-acid consensus region (YXRESLREM), which was only found in polyguluronate lyases, was also observed in the amino-terminal region of AlyVI. However, AlyVI could degrade both M block and G block. These results indicate that a novel alginate lyase-encoding gene has been cloned. 相似文献
183.
The cauliflower (Brassica oleracea L. var. botrytis) Or gene is a semi-dominant, single-locus mutation that induces the accumulation of high levels of beta-carotene in various tissues of the plant, turning them orange. As part of a map-based cloning strategy, molecular mapping of the Or gene in the cauliflower genome was undertaken in a mapping population consisting of 195 F2 individuals. By using amplified fragment length polymorphism (AFLP) in conjunction with bulked segregant analysis, we identified 10 AFLP markers closely linked to the Or gene. Four of the most closely linked flanking markers were converted into restriction fragment length polymorphism (RFLP) markers. Mapping of these markers in the mapping population placed two of them at 0.5 cM from the Or locus on one side, while another marker flanked the Or gene at 1.6 cM on the other side. Three of these markers were also successfully converted into sequence-characterized amplified region (SCAR) markers. These PCR-based markers will be useful for a large-scale application in facilitating the positional cloning of the Or gene. 相似文献
184.
小麦Kr基因在小麦与玉米或鸭茅状摩擦禾杂交中的失活 总被引:2,自引:0,他引:2
用37个小麦(Triticumaestivum)品种(系)为母本,分别与黑麦(Secalecereale)、球茎大麦(Hordeumbulbosum)、玉米(Zeamays)和鸭茅状摩擦禾(Tripsacumdactyloides)杂交,比较其亲和性,小麦和玉米或鸭茅状摩擦禾杂交比小麦与黑麦或球茎大麦杂交的亲和性显著提高。携带着显性Kr1和Kr2基因的小麦品种Hope与黑麦杂交,不能形成胚,而与玉米及鸭茅状摩擦禾杂交时,成胚率分别达16.00%和32.50%。表明控制小麦与黑麦及球茎大麦杂交亲和性的Kr基因系统在小麦与玉米及小麦与鸭茅状摩擦禾属间杂交中失活。讨论了还存在有其它控制小麦属间杂交亲和性的遗传调控系统的可能性。 相似文献
185.
186.
Construction and characterization of two rice bacterial artificial chromosome libraries from the parents of a permanent recombinant inbred mapping population 总被引:13,自引:0,他引:13
Hong-Bin Zhang Sangdun Choi Sung-Sick Woo Zhikang Li Rod A. Wing 《Molecular breeding : new strategies in plant improvement》1996,2(1):11-24
Rice is a leading grain crop and the staple food for over half of the world population. Rice is also an ideal species for genetic and biological studies of cereal crops and other monocotyledonous plants because of its small genome and well developed genetic system. To facilitate rice genome analysis leading to physical mapping, the identification of molecular markers closely linked to economic traits, and map-based cloning, we have constructed two rice bacterial artificial chromosome (BAC) libraries from the parents of a permanent mapping population (Lemont and Teqing) consisting of 400 F9 recombinant inbred lines (RILs). Lemont (japonica) and Teqing (indica) represent the two major genomes of cultivated rice, both are leading commercial varieties and widely used germplasm in rice breeding programs. The Lemont library contains 7296 clones with an average insert size of 150 kb, which represents 2.6 rice haploid genome equivalents. The Teqing library contains 14208 clones with an average insert size of 130 kb, which represents 4.4. rice haploid genome equivalents. Three single-copy DNA probes were used to screen the libraries and at least two overlapping BAC clones were isolated with each probe from each library, ranging from 45 to 260 kb in insert size. Hybridization of BAC clones with chloroplast DNA probes and fluorescent in situ hybridization using BAC DNA as probes demonstrated that both libraries contain very few clones of chloroplast DNA origin and are likely free of chimeric clones. These data indicate that both BAC libraries should be suitable for map-based cloning of rice genes and physical mapping of the rice genome. 相似文献
187.
S. R. Pearce D. Li A. J. Flavell G. Harrison J. S. Heslop-Harrison A. Kumar 《Molecular genetics and genomics : MGG》1996,250(3):305-315
We present an in-depth study of theTy1-copia group of retrotransposons within the plant genusVicia, which contains species with widely differing genome sizes. We have compared the numbers and sequence heterogeneities of these genetic elements in three diploidVicia species chosen to represent large (V. faba, 1C=13.3 pg), medium (V. melanops, 1C=11.5 pg) and small (V. sativa, 1C=2.3 pg) genomes within the genus. The copy numbers of the retrotransposons are all high but vary greatly, withV. faba containing approximately 106 copies,V. melanops about 1000 copies andV. sativa 5000 copies. The degree of sequence heterogeneity ofTy1-copia group elements correlates with their copy number within each genome, but neither heterogeneity nor copy number are related to the genome size of the host. In situ hybridization to metaphase chromosomes shows that the retrotransposons inV. faba are distributed throughout all chromosomes but are much less abundant in certain heterochromatic regions. These results are discussed in the context of plant retrotransposon evolution. 相似文献
188.
Insights into the local residual entropy of proteins provided by NMR relaxation. 总被引:4,自引:3,他引:1 下载免费PDF全文
Z. Li S. Raychaudhuri A. J. Wand 《Protein science : a publication of the Protein Society》1996,5(12):2647-2650
A simple model is used to illustrate the relationship between the dynamics measured by NMR relaxation methods and the local residual entropy of proteins. The expected local dynamic behavior of well-packed extended amino acid side chains are described by employing a one-dimensional vibrator that encapsulates both the spatial and temporal character of the motion. This model is then related to entropy and to the generalized order parameter of the popular "model-free" treatment often used in the analysis of NMR relaxation data. Simulations indicate that order parameters observed for the methyl symmetry axes in, for example, human ubiquitin correspond to significant local entropies. These observations have obvious significance for the issue of the physical basis of protein structure, dynamics, and stability. 相似文献
189.
190.
DISCOVERY OF VERTEBRATE FOSSILS AND PALEOLITHIC ARTIFACTS IN DANJIANG SUBMERGING AREA AND ITS IMPLICATIONS 总被引:1,自引:1,他引:0
IntheareatobefloodedinthesecondengineeringstagefortheDanjiangreservoir,wediscovered16vertebratefossillocalitiesand52Paleolithicsitesin1994,andcollected603artifactsandmanyfossils.Ofthel6new1yfoundvertebratefossillocalities,threearereptilesitesrepresentedbydinosaureggsandlimbbones,andl3producemamma1s,including4Pale0gene,2Ne0geneand7Quaternarysites'ThemammalianlocalitiesareofPale0cene,Eocene,EarlyMiocene,andPli0ceneorEarlyPleistocene,MiddleandIntePleistocene,respectively.Theyfillinsomestrat… 相似文献