全文获取类型
收费全文 | 42498篇 |
免费 | 3026篇 |
国内免费 | 5711篇 |
出版年
2024年 | 189篇 |
2023年 | 913篇 |
2022年 | 1146篇 |
2021年 | 1410篇 |
2020年 | 1423篇 |
2019年 | 2004篇 |
2018年 | 1612篇 |
2017年 | 1450篇 |
2016年 | 1368篇 |
2015年 | 1323篇 |
2014年 | 2138篇 |
2013年 | 3086篇 |
2012年 | 1715篇 |
2011年 | 2015篇 |
2010年 | 1549篇 |
2009年 | 2022篇 |
2008年 | 1991篇 |
2007年 | 2186篇 |
2006年 | 1930篇 |
2005年 | 1761篇 |
2004年 | 1563篇 |
2003年 | 1417篇 |
2002年 | 1286篇 |
2001年 | 1015篇 |
2000年 | 898篇 |
1999年 | 821篇 |
1998年 | 802篇 |
1997年 | 703篇 |
1996年 | 598篇 |
1995年 | 570篇 |
1994年 | 546篇 |
1993年 | 498篇 |
1992年 | 487篇 |
1991年 | 466篇 |
1990年 | 355篇 |
1989年 | 347篇 |
1988年 | 354篇 |
1987年 | 282篇 |
1986年 | 316篇 |
1985年 | 541篇 |
1984年 | 637篇 |
1983年 | 351篇 |
1982年 | 510篇 |
1981年 | 496篇 |
1980年 | 428篇 |
1979年 | 332篇 |
1978年 | 241篇 |
1977年 | 253篇 |
1976年 | 217篇 |
1973年 | 188篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
Structure and expression of the overlapping ND4L and ND5 genes of Neurospora crassa mitochondria 总被引:1,自引:0,他引:1
Summary Genes homologous to the mammalian mitochondrial NADH dehydrogenase subunit genes ND4L and ND5 were identified in the mitochondrial genome of the filamentous fungus Neurospora crassa, and the structure and expression of these genes was examined. The ND4L gene (interrupted by one intervening sequence) potentially encodes an 89 residue long hydrophobic protein that shares about 26% homology (or 41% homology if conservative amino acid substitutions are allowed) with the analogous human mitochondrial protein. The ND5 gene (which contains two introns) encodes a 715 residue polypeptide that shares 23% homology with the human analogue; a 300 amino acid long region is highly conserved (50% homology) in the two ND5 proteins. The stop codon of the ND4L gene overlaps the initiation codon of the downstream ND5 gene, and the two genes are contranscribed and probably cotranslated. A presumed mature dicistronic (ND4L plus ND5) RNA was detected. The postulated mRNA (about 3.2 kb) contains 5 and 3 non-coding regions of about 86 and 730 nucleotides, respectively; this species is generated from very large precursor RNAs by a complex processing pathway. The ND4L and ND5 introns are all stable after their excision from the precursor species.Abbreviations bp
base pairs
- rRNA
ribosomal RNA
- ND
NADH dehydrogenase
- URF
unidentified reading frame
- kDal
kilodaltons; a.a., amino acid 相似文献
162.
L. Jouanin P. Guerche N. Pamboukdjian C. Tourneur F. Casse Delbart J. Tourneur 《Molecular & general genetics : MGG》1987,206(3):387-392
Summary The structure of the T-DNA in Ri-transformed plants of Brassica napus, Nicotiana plumbaginifolia and Nicotiana tabacum was analysed. All the plants studied present a particular phenotype with wrinkled leaves. The T-DNA is composed of two parts: TL and TR. The size of the TL-DNA (19–20 kb) seems to be almost constant, except in N. tabacum where it is shorter. The TR-DNA can be absent, and its size varies from about 5–28 kb, with two predominant lengths. The smaller size does not include the region homologous to the tms genes of the pTi T-DNA. The copy number varies from one to four copies per plant genome. TL and TR-DNA are not always present in the same copy number, but in some cases are linked together. 相似文献
163.
Ethylene production of habituated and auxin-requiring tobacco ( Nicotiana tabacum L. cv. Xanthi) callus cultures were compared. More ethylene was produced by auxinrequiring i.e. auxin-heterotrophic cultures than by habituated ones. Treatment with 2,4-dichlorophenoxyacetic acid increased the ethylene evolution of habituated cultures over the range 10−7 to 10−4 M , which suggests that the higher ethylene production of auxin-dependent callus is caused by the 2,4-D in the medium. The IAA levels depended on the age of both types of callus cultures. 相似文献
164.
In the developing peanut (Arachis hypogaea L.) kernels, the period between 15 and 35 days after podding (DAP) was identified as the active period of oil-filling. The
period of active oil-filling was associated with a decrease in the starch, soluble sugars and proteins so as to make available
the energy and carbon skeleton for the synthesis of oil. The oil content in the mature kernels decreased by 11, 12 and 25
per cent with Zn, S and Zn+S deficiency, respectively. In addition, proteins and starch content decreased significantly while
that of soluble sugars increased slightly. The activity of malate dehydrogenase and glucose-6-phosphate dehydrogenase also
decreased due to Zn as well as S deficiency. The deficiency treatments resulted in a decrease in phospholipids, free fatty
acids and triacylglycerols in mature kernels. Further the proportion of 16∶0 and 18∶2 decreased while that of 18∶1 increased
in developing kernels. 相似文献
165.
Effect of chemical modification on the binding of gossypol by gossypin (11S protein) and congossypin (7S protein) of cottonseed 总被引:1,自引:0,他引:1
Binding of gossypol by gossypin and congossypin and their succinylated and sulfhydryl group-blocked derivatives has been measured.
The binding by gossypin and congossypin is characterized by weak interaction. Succinylation of gossypin decreases the binding
affinity whereas that of congossypin increases it. Blocking of sulfhydryl groups of both the proteins does not significantly
affect gossypol binding, Succinylation dissociates gossypin and causes conformational changes whereas it does not dissociate
congossypin but causes conformational changes. Sulfhydryl group blocking does not dissociate gossypin or congossypin, nor
does it cause any conformational changes. 相似文献
166.
The galactose-binding lectin of human Placenta has been Purified to homogeneity by affinity chromatograPhy on asialo-fetuin
column. The Protein, extractable from the tissue only with lactose is aPParently membrane-bound. Molecular weight determination
of native Protein and subunit indicated a dimer of l3.4 kDa subunits. Inhibition of haemagglutination with various saccharides
indicate that thiodigalactoside is the best inhibitor followed by lactose. However,P-nitroPhenyl-and 1-O-methyl derivatives of galactose showed that α-anomers inhibited slightly better than β-anomer. Modification
of amino acid residues indicated involvement of arginine, lysine and histidine residues at the saccharidebinding site. Cysteine
residue modificatioin also abolished haemagglutinating activity. Amino acid comPosition of the lectin is also Presented. 相似文献
167.
The analysis of planktonic rotifer populations: A plea for long-term investigations 总被引:20,自引:10,他引:10
Alois Herzig 《Hydrobiologia》1987,147(1):163-180
Short-term species succession, and long-term community development, of planktonic rotifer populations of the temperate zone and factors influencing species' abundance (ie., physical and chemical limitations, food and exploitative competition, mechanical interference competition, predation, parasitism) are described and discussed. The long-term development of plankton communities in three European lakes is described and the major events are discussed in relation to ecological interactions. Frequently, the shortcomings of our knowledge about population ecology and ecophysiology of rotifers prevent explanations of short-term and, especially, of long-term developments of these plankton organisms. The need for qualitative and quantitative observations in the field and in the laboratory over long periods becomes obvious. 相似文献
168.
Summary Physical parameters of membrane bilayers were studied for their effect on the binding of hematoporphyrin derivative (Hpd), which is used as a sensitizer in photodynamic therapy of cancerous tissues. The purpose of this study was to clarify which parameters were relevant, under physiological conditions, to the selectivity of Hpd binding to cancer cells. Fluorescence spectroscopy was used to measure the relative partitioning of the dye between the lipid and aqueous media. Increasing the microviscosity of the liposomes' membranes by various bilayer additives results in a strong reduction of Hpd binding, to an extent independent of the specific additive. The effect of temperature near the physiological value as well as the effect of cross membrane potential are small. Surface potential does not affect the binding constant, indicating that the binding species does not carry a net electric charge. 相似文献
169.
Michael F. Hammer James W. Schilling Ellen M. Prager Allan C. Wilson 《Journal of molecular evolution》1987,24(3):272-279
Summary Two major types of lysozymec (M and P) occur in the mouse genus,Mus, and have been purified from an inbred laboratory strain (C58/J) ofM. domesticus. They differ in physical, catalytic, and antigenic properties as well as by amino acid replacements at 6 of 49 positions in the amino-terminal sequence. Comparisons with four other mammalian lysozymesc of known sequence suggest that M and P are related by a gene duplication that took place before the divergence of the rat and mouse lineages. M lysozyme is present in most tissues; achieves its highest concentration in the kidney, lung, and spleen; and corresponds to the lysozyme partially sequenced before from another strain ofM. domesticus. InM. domesticus and several related species, P lysozyme was detected chiefly in the small intestine, where it is probably produced mainly by Paneth cells. A survey of M and P levels in 22 species of muroid rodents (fromMus and six other genera) of known phylogenetic relationships suggests that a mutation that derepressed the P enzyme arose about 4 million years ago in the ancestor of the housemouse group of species. Additional regulatory shifts affecting M and P levels have taken place along lineages leading to other muroid species. Our survey of 187 individuals of wild house mice and their closest allies reveals a correlation between latitude of origin and level of intestinal lysozyme. 相似文献
170.
Alvaro García-Tejedor A. Raul Castaño Federico Morán Francisco Montero 《Journal of molecular evolution》1987,26(4):294-300
Summary The most relevant properties of hypercycles were previously studied mainly from a theoretical point of view. We have developed a Monte Carlo method simulating hypercyclic organization to obtain information about the dynamics of this prebiotic organization. Nucleation, growth, and selective properties have been tested and the results obtained are in good agreement with those of the theoretical predictions. The influence of hypercyclic organization of the error threshold has also been studied. As a consequence of the emergence of a hypercycle, the value of this threshold decreases. The amount of this decrease depends on the population size. Moreover, for some interval of quality factor values, either the hypercycle organization or an error catastrophe can be produced, depending on the initial conditions. The influence of these phenomena on both the dynamic behavior and evolutionary advantages of the hypercycle, as well as their decisive roles on genome size, are discussed.Presented at the FEBS Symposium on Genome Organization and Evolution, held in Crete, Greece, September 1–5, 1986 相似文献