全文获取类型
收费全文 | 33263篇 |
免费 | 1556篇 |
国内免费 | 1579篇 |
专业分类
36398篇 |
出版年
2024年 | 42篇 |
2023年 | 360篇 |
2022年 | 486篇 |
2021年 | 689篇 |
2020年 | 691篇 |
2019年 | 878篇 |
2018年 | 766篇 |
2017年 | 743篇 |
2016年 | 837篇 |
2015年 | 1092篇 |
2014年 | 1853篇 |
2013年 | 2748篇 |
2012年 | 1460篇 |
2011年 | 1454篇 |
2010年 | 1256篇 |
2009年 | 1444篇 |
2008年 | 1509篇 |
2007年 | 1524篇 |
2006年 | 1449篇 |
2005年 | 1372篇 |
2004年 | 1239篇 |
2003年 | 1171篇 |
2002年 | 1074篇 |
2001年 | 799篇 |
2000年 | 785篇 |
1999年 | 733篇 |
1998年 | 724篇 |
1997年 | 646篇 |
1996年 | 581篇 |
1995年 | 619篇 |
1994年 | 606篇 |
1993年 | 482篇 |
1992年 | 488篇 |
1991年 | 381篇 |
1990年 | 387篇 |
1989年 | 295篇 |
1988年 | 345篇 |
1987年 | 269篇 |
1986年 | 231篇 |
1985年 | 336篇 |
1984年 | 391篇 |
1983年 | 256篇 |
1982年 | 287篇 |
1981年 | 140篇 |
1980年 | 121篇 |
1979年 | 118篇 |
1978年 | 71篇 |
1977年 | 38篇 |
1976年 | 41篇 |
1973年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Isolation and characterization of a human interleukin 2 gene 总被引:1,自引:0,他引:1
S Mita S Maeda K Obaru N Nishino K Shimada T Hirano K Onoue T Ogawa H Ogawa 《Biochemical and biophysical research communications》1983,117(1):114-121
An interleukin 2 (IL-2) gene was isolated from a Charon 4A human gene library. Electron microscopic examination of 15 heteroduplexes formed between the genomic DNAs and the IL-2 cDNAs demonstrated that the size of the IL-2 gene is about 5.1 +/- 0.5 kb and that there are at least two introns in this gene. Nucleotide sequence of the 5' flanking region of the IL-2 gene showed a homology with that of the corresponding region of the human immune interferon gene. 相似文献
72.
The structure of [3H]thymidine pulse-labeled chromatin in lymphocytes differs from that of non-replicating chromatin by several operational criteria which are related to the higher nuclease sensitivity of replicating chromatin. These structural features of replicating chromatin rapidly disappear when the [3H]thymidine pulse is followed by a chase in the presence of an excess of non-radioactive thymidine. However, when the rate of DNA replication is reduced, as in cycloheximide-treated lymphocytes, chromatin maturation is retarded. No chromatin maturation is observed when nuclei from pulse-labeled lymphocytes are incubated in vitro in the absence of DNA precursors. In contrast, when these nuclei are incubated under conditions known to be optimal for DNA replication, the structure of replicating chromatin is efficiently converted to that of 'mature', non-replicating chromatin. We conclude that the properties of nascent DNA and/or the distance from the replication fork are important factors in chromatin maturation. 相似文献
73.
The effects of nine drugs on the CD spectra of a synthetic calcium binding analog of site III of rabbit skeletal troponin C, can generally be divided into 3 groups: (1) that consisting of haloperidol, benperidol, molindone and promethazine, had no effect on the CD spectrum or calcium sensitivity of the apopeptide; (2) that composed of structurally rigid thioxanthenes, induced CD-detectable structural change in the apopeptide but prevented Ca2+-induced structural change; (3) that consisting of chlorpromazine, trifluoperazine and fluphenazine, induced structural change in the peptide but had no effect on the Ca2+-induced structural change. 相似文献
74.
A method for the ligation of DNA following isolation from low melting temperature agarose 总被引:14,自引:0,他引:14
A rapid, simple, and reliable method is presented for the isolation and subsequent ligation of DNA from agarose gels. The technique involves the use of low melting temperature agarose, but with the inclusion of bovine serum albumin or gelatin to the ligation reaction. 相似文献
75.
Anna Maria Tagliasacchi Laura Maria Costantina Forino Manuela Frediani Silvana Avanzi 《Protoplasma》1983,115(2-3):95-103
Summary The pattern of DNA and RNA puffs in pair VII of polytene chromosomes has been investigated in the suspensor ofPhaseolus coccineus during early embryo development. The pattern of3H-TdR and3H-U incorporation has been also detected. Collected data indicate that: 1. both heterochromatic regions, p11 and q(111+112), of chromosome pair VII, organize large DNA puffs; 2. DNA puffs of both regions are specific of different embryo differentiation steps; 3. a seasonal influence on the DNA puffing seems also to be present, as demonstrated by the comparison of the results collected in two different crops; 4. the incorporation experiment by3H-TdR evidences that not all DNA puffs show clustered labeling; 5. the RNA puffing of the two regions seems also to be specific of determined embryo stages. 相似文献
76.
A simple method for electrophoretic elution of nucleic acids from gel slices is described. The procedure utilizes a standard tube gel system and can be completed in as little as one hour. Nucleic acids are recovered in a small volume with almost 100% efficiency. The procedure is applicable equally to acrylamide and agarose gels, and small as well as large RNA and DNA molecules. The eluted nucleic acids are essentially undegraded and are suitable for a variety of structural and biological analyses. 相似文献
77.
Nedialka G. Markova George G. Markov 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1983,741(1):7-14
Isopicnic centrifugation in Cs2SO4-Ag+ gradients at pH 7.0 reveals that the genome of the marine snail Rapana thomasiana Grosse (Gastropoda) contains an AT-rich satellite fraction comprising 5% of the DNA. Restriction enzyme analysis shows that the satellite DNA is composed of a number of related subsets arranged in tandem arrays. They have evolved from the segmental amplification of an 1460 bp long monomer unit with a complex inner organization. Most probably, the present basic repeat originates from an ancestral 400–500 bp long sequence in which some insertions and/or deletions have occurred. 相似文献
78.
L.H.F. Mullenders A.A. Van Zeeland A.T. Natarajan 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1983,740(4):428-435
The distribution of ultraviolet-induced repair patches along DNA loops attached to the nuclear matrix, was investigated by digestion with DNA-degrading enzymes and neutral sucrose gradient centrifugation. When DNA was gradually removed by DNAase 1, pulse label incorporated by ultraviolet-irradiated cells during 10 min in the presence of hydroxyurea or hydroxyurea/arabinosylcytosine showed similar degradation kinetics as prelabelled DNA. No preferential association of pulse label with the nuclear matrix was observed, neither within 30 min nor 13 h after irradiation. When the pulse label was incorporated by replicative synthesis under the same conditions, a preferential association of newly-synthesized DNA with the nuclear matrix was observed. Single-strand specific digestion with nuclease S1 of nuclear lysates from ultraviolet-irradiated cells, pulse labelled in the presence of hydroxyurea/arabinosylcytosine, caused a release of about 70% of the prelabelled DNA and 90% of the pulse-labelled DNA from the rapidly sedimenting material in sucrose gradients. The results suggest no specific involvement of the nuclear matrix in repair synthesis, a random distribution of repair patches along the DNA loops, and simultaneously multiple incision events per DNA loop. 相似文献
79.
G. Stauder H. Riesemann W.M. Joester K.E. Joester 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1983,741(3)
A third DNA polymerase ‘C’ with low molecular weight was isolated and purified 3700-fold from ground hyphae of Neurospora crassa WT 74 A, which shows similarities to β- and γ-polymerases from higher eukaryotes: preference for poly(rA)(dT) as a template/primer, inhibition by p-chloromercuribenzoate, resistance against N-ethylmaleimide up to 10 mmol/l, and molecular weight of about 40 000. This polymerase elutes as a distinct peak from DEAE-cellulose at 0.60 mol/l KCl and has an optimum for K+ at 2–20 mmol/l, for Mn2+ at 0.8 mmol/l, for Mg2+ at 4.0 mmol/l, the pH optimum is 8.0. Its Km is 1.5 μmol/l using dTTP as substrate. The enzyme activity described here is free of endonuclease but contains detectable amounts of exonuclease. 相似文献
80.
The irreversible binding of [2,3-14C]acrylonitrile (VCN) to proteins, RNA and DNA of various tissues of male Sprague-Dawley rats after a single oral dose of 46.5 mg/kg (0.5 LD50) has been studied. Proteins were isolated by chloroform-isoamyl alcohol-phenol extraction. RNA and DNA were separated by hydroxyapatite chromatography. Binding of VCN to proteins was extensive and was time dependent. Radioactivity in nucleic acids was registered in the liver and the target organs, stomach and brain. DNA alkylation, which increased by time, was significantly higher in the target organs, brain and stomach (119 and 81 pmol/mg, respectively, at 24 h) than that in the liver. The covalent binding indices for the liver, stomach and brain at 24 h after dosing were, 5.9, 51.9 and 65.3, respectively. These results suggest that VCN is able to act as a multipotent carcinogen by alkylation of DNA in the extrahepatic target tissues, stomach and brain. 相似文献