全文获取类型
收费全文 | 23314篇 |
免费 | 2261篇 |
国内免费 | 2524篇 |
出版年
2024年 | 21篇 |
2023年 | 317篇 |
2022年 | 406篇 |
2021年 | 1235篇 |
2020年 | 904篇 |
2019年 | 1104篇 |
2018年 | 1011篇 |
2017年 | 778篇 |
2016年 | 1041篇 |
2015年 | 1513篇 |
2014年 | 1758篇 |
2013年 | 1825篇 |
2012年 | 2216篇 |
2011年 | 1982篇 |
2010年 | 1257篇 |
2009年 | 1153篇 |
2008年 | 1307篇 |
2007年 | 1172篇 |
2006年 | 1084篇 |
2005年 | 925篇 |
2004年 | 817篇 |
2003年 | 793篇 |
2002年 | 683篇 |
2001年 | 437篇 |
2000年 | 361篇 |
1999年 | 319篇 |
1998年 | 201篇 |
1997年 | 193篇 |
1996年 | 170篇 |
1995年 | 137篇 |
1994年 | 181篇 |
1993年 | 113篇 |
1992年 | 126篇 |
1991年 | 105篇 |
1990年 | 99篇 |
1989年 | 66篇 |
1988年 | 65篇 |
1987年 | 47篇 |
1986年 | 37篇 |
1985年 | 55篇 |
1984年 | 22篇 |
1983年 | 26篇 |
1982年 | 16篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1973年 | 2篇 |
1971年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
Summary The production of 9-cis-1, 18-octadecenedioic acid from different substrates by the mutant S76 of Candida tropicalis was studied. It was found that the mutant could convert oleyl alcohol, oleic acid, and methyl oleate to 9-cis-1, 18-octadecenedioic acid through either the diterminal oxidation of the alcohol or the -oxidation of the terminal methyl group of the mono acid. The geometric configuration of the product was identified by different analytical methods. The results demonstrated that no change in the geometric configuration of molecular structure happened during the bioconversion of such unsaturated substrates to 9-dioic acid. To enhance the production of this dioic acid in the medium, calcium carbonate was added as a trapping agent. The IR spectrum, 1H NMR and 13C-satellite spectra of this compound were presented.Dedicated to Professor Dr. K. Esser on the occasion of his 65th birthday 相似文献
22.
鼎突多刺蚁群体结构和生活史的研究 总被引:5,自引:0,他引:5
本文报告了鼎突多刺蚁(Polyrhachis vicina Roger)的群体结构和生活史。通过研究表明,鼎突多刺蚁一年发生一代,以蚁后、雄蚁、工蚁、幼虫和卵越冬。卵和幼虫在冬天发育停滞(或极慢)。到了春天,随着气温的上升,卵和幼虫又恢复正常发育。据室内人工饲养观察,卵的发育历期为23.8±2.5天(平均温度26℃)幼虫为20.4±4.4天(26℃),工蚁蛹为19.8±5.5天(27℃)。成长工蚁在5—11月出现;8—11月,雄蚁从蛹中羽化;10月,雌蚁从蛹中羽化。雌蚁分飞交尾后,进入邻近蚁巢或回到原巢,脱翅成为蚁后。蚁巢内存在着多后现象。 相似文献
23.
Beta-thalassemia mutations in Indonesia and their linkage to beta haplotypes. 总被引:2,自引:1,他引:1 下载免费PDF全文
L E Lie-Injo S P Cai I Wahidijat S Moeslichan M L Lim L Evangelista M Doherty Y W Kan 《American journal of human genetics》1989,45(6):971-975
A total of 72 chromosomes from 36 Indonesian patients, 23 with beta-thalassemia major and 13 with Hb E-beta-thalassemia, were analyzed by specific oligonucleotide hybridization after DNA amplification. Thirteen had the beta E mutation (codon 26 GAG----AAG). Of the 59-beta-thalassemic chromosomes, 32 were of the variant IVS-1 nt5 (G----C). Seven had the mutation IVS-2 nt654 (C----T), one had the mutation codon 41/42 (deletion CTTT), and one had the mutation codon 17 (AAG----TAG). Another six with the mutation IVS-1 nt1 (G----T), one with the mutation IVS-1 nt1 (G----A), four with the mutation codon 15 (TGG----TAG), one with a mutation codon 30 (AGG----ACG), and one with a mutation codon 35 (deletion C) were first identified by direct sequencing of a patient's genomic DNA followed by further hybridizing other patients' DNA with the appropriate oligonucleotide probes. Five did not carry the common mutations previously described in Asian populations. The four most prevalent mutations encountered made up 83% of the total number of beta-thalassemic chromosomes studied. The most common mutation, IVS-1 nt5 (G----C), was mostly associated with two different haplotypes. 相似文献
24.
25.
Genetic variation within and the relationship betweenStellaria longipes
Goldie andS. longifolia
Muhl. were studied. Ten enzyme systems were assessed in eight natural populations ofS. longipes (25 loci) and three ofS. longifolia (20 loci) using starch and polyacrylamide gel electrophoresis. Patterns of population differentiation corresponded to geographic distance. There was no evidence that polyploidS. longipes had greater electrophoretic variability than diploidS. longipes. The isozyme data confirmed extensive population differentiation in these species and, within that context, a relatively close relationship betweenS. longipes andS. longifolia. It was postulated that diploids of these two species might be the progenitors of tetraploidS. longipes. 相似文献
26.
猕猴桃属美味猕猴桃和毛花猕猴桃种间杂交的胚胎学和胚援救 总被引:7,自引:0,他引:7
本文报道美味猕猴桃(Actintdia delictosa cv.Hayward)(6x)和毛花猕猴桃(A.eria-ntha)(2x)杂交当代种子的胚胎学分析和胚援救的结果:1.根据杂交种子外部形态和胚胎发育程度,区分为正常的和败育的两类,其比率接近1:1。正常种子的胚发育分化正常,且含有适量胚乳。败育种子中,除少数不含胚和胚乳外,绝大多数种子的胚发育终止于球形、心形和早子叶阶段,胚体畸形。胚乳处于消失或完全解体。2.在被试8组培养基中,最适合胚萌发和生长的是:MS+IAA0.5ppm+GA_30.5ppm,MS+2ip2ppm+IAA0.5ppm+GA_30.5ppm和 MS+2ip2ppm+GA_30.5ppm。适于实生苗生长的培养基为 MS+GA_30.5ppm 和 MS 培养基。在 MS+BAP2ppm 和 MS+IAA0.5ppm+GA_30.5ppm 培养基上,一些正常种子和少数不正常种子胚的下胚轴直接形成了许多不定芽,从而诱导产生了更多的杂种植株。但是在MS十BAP2ppm+GA_30.5ppm,MS+BAP2ppm+IAA0.5ppm 和 MS+2ip2ppm+GA_30.5ppm培养基上,虽然胚产生了愈伤组织,但都未分化出器官。杂种实生苗根尖细胞近半数的含有近4倍性染色体数(4x=116),约半数为其他倍性。 相似文献
27.
Gene splicing by overlap extension: tailor-made genes using the polymerase chain reaction 总被引:83,自引:0,他引:83
Gene Splicing by Overlap Extension or "gene SOEing" is a PCR-based method of recombining DNA sequences without reliance on restriction sites and of directly generating mutated DNA fragments in vitro. By modifying the sequences incorporated into the 5'-ends of the primers, any pair of polymerase chain reaction products can be made to share a common sequence at one end. Under polymerase chain reaction conditions, the common sequence allows strands from two different fragments to hybridize to one another, forming an overlap. Extension of this overlap by DNA polymerase yields a recombinant molecule. This powerful and technically simple approach offers many advantages over conventional approaches for manipulating gene sequences. 相似文献
28.
29.
The carboxy-terminal 41 amino acids of herpes simplex virus type 1 glycoprotein B are not essential for production of infectious virus particles. 总被引:6,自引:6,他引:0 下载免费PDF全文
Glycoprotein B (gB) is a virally encoded protein that is found in the envelope of herpes simplex virus type 1 and membranes of cells infected with herpes simplex virus type 1. It is essential for the production of infectious virus particles. An amber mutation was introduced into the gB gene by oligonucleotide-directed mutagenesis at the codon for amino acid 863 of the protein. Virus carrying this mutation should synthesize gB molecules lacking the last 41 amino acids of the cytoplasmic domain. Immunoprecipitation of infected cell extracts demonstrated the synthesis of appropriately truncated gB molecules. Characterization of the mutant virus indicated that the loss of the carboxy-terminal 41 amino acids has little effect on gB function. 相似文献
30.
从“湖北光敏感核不育水稻”的未受精子房和花药培养出单倍体植株 总被引:10,自引:0,他引:10
采用10种诱导培养基,培养湖北光敏感核不育水稻农垦58品种的未受精子房和花药。共培养未受精子房2790个,获得胚囊愈伤组织17块,最高诱导频率达3.33%,其中2块分化出绿苗。培养花药16740个,获得花药愈伤组织15块,最高诱导频率为0.92%,其中3块分化出苗,2丛白苗,1株绿苗。胚囊植株和花粉植株经根尖染色体检查为单倍体,2n=x=12。实验证明,液体培养、2,4-D0.2-0.5 mg/1、低温预处理对诱导胚囊愈伤组织及花粉愈伤组织的形成具良好效果。 相似文献