全文获取类型
收费全文 | 8462篇 |
免费 | 652篇 |
国内免费 | 3篇 |
出版年
2023年 | 55篇 |
2022年 | 28篇 |
2021年 | 88篇 |
2020年 | 79篇 |
2019年 | 88篇 |
2018年 | 224篇 |
2017年 | 175篇 |
2016年 | 261篇 |
2015年 | 373篇 |
2014年 | 388篇 |
2013年 | 545篇 |
2012年 | 702篇 |
2011年 | 683篇 |
2010年 | 411篇 |
2009年 | 312篇 |
2008年 | 582篇 |
2007年 | 556篇 |
2006年 | 532篇 |
2005年 | 520篇 |
2004年 | 445篇 |
2003年 | 403篇 |
2002年 | 368篇 |
2001年 | 104篇 |
2000年 | 121篇 |
1999年 | 84篇 |
1998年 | 59篇 |
1997年 | 43篇 |
1996年 | 41篇 |
1995年 | 32篇 |
1994年 | 28篇 |
1993年 | 34篇 |
1992年 | 47篇 |
1991年 | 55篇 |
1990年 | 57篇 |
1989年 | 43篇 |
1988年 | 54篇 |
1987年 | 41篇 |
1986年 | 52篇 |
1985年 | 36篇 |
1984年 | 35篇 |
1983年 | 25篇 |
1982年 | 19篇 |
1981年 | 17篇 |
1979年 | 27篇 |
1978年 | 25篇 |
1975年 | 18篇 |
1974年 | 18篇 |
1973年 | 22篇 |
1969年 | 18篇 |
1968年 | 17篇 |
排序方式: 共有9117条查询结果,搜索用时 15 毫秒
101.
Christophe Biju-Duval Hajer Ennafaa Nicole Dennebouy Monique Monnerot Françoise Mignotte Ramon C. Soriguer Amel El Gaaïed Ali El Hili Jean-Claude Mounolou 《Journal of molecular evolution》1991,33(1):92-102
Summary A characterization was conducted on mitochondrial DNA (mtDNA) molecules extracted separately from 107 European rabbits (Oryctolagus cuniculus) both wild and domestic, 13 European hares (Lepus capensis), and 1 eastern cottontail (Sylvilagus floridanus). Experimentally this study took into account restriction site polymorphism, overall length variation of the noncoding region, and numbers of repeated sequences. Nucleotide divergences indicate that the mtDNAs from the three species derived from a common ancestor some 6–8 million years (Myr) ago. Every animal appeared heteroplasmic for a set of molecules with various lengths of the noncoding region and variable numbers of repeated sequences that contribute to them. This systematic heteroplasmy, most probably generated by a rate of localized mtDNA rearrangements high enough to counterbalance the cellular segregation of rearranged molecules, is a shared derived character of leporids.The geographic distribution of mtDNA polymorphism among wild rabbit populations over the western European basin shows that two molecular lineages are represented, one in southern Spain, the second over northern Spain, France, and Tunisia. These two lineages derived from a common ancestor some 2 Myr ago. Their present geographical distribution may be correlated to the separation of rabbits into two stocks at the time of Mindel glaciation.Finally the distribution of mtDNA diversity exhibits a mosaic pattern both at inter- and intrapopulation levels. 相似文献
102.
In-situ gelation of semidilute xanthan solutions with trivalent chromium, aluminum or iron ions was studied by rheology and UV-spectroscopy. Measurements of the elastic modulus of xanthan gel cylinders prepared by dialysis against the complexing ion at pH values from 2 to 6 indicate that monomeric species of the ion are ineffective, whereas dimeric or higher oligomeric species are effective in crosslinking the polysaccharide. When chromium was used as the crosslinking species, the dependence of the gelation rate on the ionic concentration followed a power law with a coefficient of 1·7. The gelation time and the gelation rate were found to extrapolate to zero at 1 m
Cr for 2·5 mg/ml xanthan. The limiting concentration of xanthan needed for gelation with 5 m
Cr(III) at 20°C was estimated as 0·35 mg/ml. This critical xanthan concentration is close to the overlap concentration c* estimated from the experimentally determined intrinsic viscosity [η] using c* = 1·4/[η]. An apparent activation energy for crosslinking of xanthan was calculated as Ea = 42 kJ/mol and Ea = 108 kJ/mol for Cr and Al ions, respectively. The fractal dimensionality of xanthan-Cr at the sol-gel transition was estimated as 1·3 applying the Chambon-Winter criterion for gelation, thus indicating that this gelation criterion is applicable also to stiff-chain polysaccharides such as xanthan. 相似文献
103.
Jean-Frédéric Brun Colette Fons Michelle Fussellier Lucette Bardet André Orsetti 《Biological trace element research》1992,32(1-3):317-323
We investigated whether zincuria is associated with microalbuminuria in type I (insulin-dependent) diabetics (IDDM). In 169
IDDM, 215 overnight urine samples were collected for simultaneous assay of zinc and albumin. In 76 samples with excessive
microalbuminuria (>15 mg/L), zincuria was higher than in the 139 other samples (0.83±0.06 vs 0.58±0.03 mg/Lp<0.001), though zincuria and microalbuminuria were not significantly correlated. An exercise provocation test was performed
in 78 IDDM. Although microalbuminuria increased, zincuria did not change during the test. Another group of 83 IDDM underwent
urinary zinc determination over a period of 1 h of recumbency. The 48 patients who had a zincuria higher than the mean+2 SD
of control values had higher microalbuminuria at rest (48±16 μg/min vs 12±2p<0.01) and after exercise (111±33 vs 42±14p<0.02) than the remaining 35 subjects. Both subgroups did not differ for zinc intake and zincemia. Thus, incipient nephropathy
as detected by the measurement of microalbuminuria is associated with a highly significant increase in zinc excretion, which
is not proportional to albumin leakage, nor is it amplified during exercise. Hyperzincuria is not explained by an increase
in zinc intake and does not result in hypozincemia. 相似文献
104.
Thomas W. Becker Thomas S. Templeman Jean-Frédéric Viret Lawrence Bogorad 《Plant molecular biology》1992,20(1):49-60
Southern blot analysis has revealed the existence in maize of perhaps 12 members of the nuclearcab multigene family encoding the chlorophylla- andb-binding proteins of the photosystem II light-harvesting complex. Hybridization with 3 probes derived from unsequenced cDNA clones showed that six members of this family differ from one another with respect to expression in mesophyll and/or bundle sheath cells and regulation by light. An additional member of this family, designatedcab-m7, that encodes a 28 kDa primary translation product has now been identified. It has been cloned from a maize genomic library and sequenced to begin to define the bases for differences in the expression of these genes. Thiscab gene is shown to be strongly preferentially expressed in the mesophyll (vs. bundle sheath) cells of maize. Furthermore, the gene is photo-responsive; although small amounts ofcab-m7 mRNA are present in etiolated leaves, the mRNA pool is 8-fold larger after six hours of illumination. DNA sequences upstream of thecab-m7 gene resemble those found in the 5-flanking regions of some other plant genes. 相似文献
105.
R V Gnutova I I Sibiriakova Iu L Radavski? L Iarvekiul'g L S Za?tseva 《Nauchnye doklady vysshe? shkoly. Biologicheskie nauki》1991,(11):35-46
The antigenic properties of capsid proteins of potato virus Y (PVY) strains have been studied, the most wide antigenic specificity of necrotic group strains has been marked. Several antigen-active strains of necrotic group strains (PVY-N Far East, PVY-N Leningrad, PVY-N Moscow) and common group (PVY-O-3 Moscow, PVY-O Far East) have been revealed. Antisera against these strains reacted with any PVY strain. Virus specific surface epitope, corresponding to position 198-208 of polypeptide chain has been located, which is a group-specific epitope of potyviruses, identified in the Far East. 相似文献
106.
To achieve inhibition of proliferation of alloantigen-induced T-lymphocytes in mixed lymphocyte culture by specific suppressor T cells (SSTC), identity of SSTC and responder in MHC class II antigens is required; either in IC or in IA + IE. By shielding of the SSTC with antiserum to ICd product (without complement), it is demonstrated that ICd product is expressed on the SSTC surface only, rather than on the surface of both stimulator and responder cells (native or preactivated with the alloantigen). Pretreatment of SSTC with anti-ICd antibodies in the absence of complement does not prevent specific SSTC interaction with the alloantigen, but prevents the SSTC function in reversible fashion. Because part of responders preactivated with an alloantigen acquired a capacity to adhere to the syngeneic SSTC monolayer, it is supposed that a receptor to the syngeneic Ia-molecule of SSTC membrane arises on the surface of preactivated responder T cells, which results in direct interaction between these two cells, reflecting the "interactional restriction" mechanism of SSTC function. 相似文献
107.
This study identifies the types and quantities of metals retained by Nereis diversicolor and Scrobicularia plana in a predominantly arid estuary environment.Certain abiotic parameters and a number of physiological processes linked to metabolic and reproductive functions play a decisive role in the seasonal influence of metal levels on the tissues of these organisms.Given the deposivoric diet of the organisms, the sedimentary bed would appear to be the principal source of metal accumulation.The following classification illustrates the bioaccumulation of metals: 209-1The possibility that these metals may be carried into the estuary by sewage water from the cities of Rabat and Sále should certainly not be ignored. 相似文献
108.
From strains carrying two different F-prime factors, we recovered F' derivatives that acquired the trp chromosomal region. These F'trp plasmids can be isolated at a frequency of 10-5 to 10-6. They were characterized genetically by looking at the size of the trp segment they acquired and at the location of that segment in the parental F' plasmid. Results are discussed in relationship to possible transposition mechanisms. 相似文献
109.
Cloning and expression of bacteriophage SP02 DNAZ polymerase gene L in Bacillus subtilis, using the Staphylococcus aureus plasmid pC194 总被引:2,自引:1,他引:1
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
HindIII restriction endonuclease fragments of DNA from temperate Bacillus subtilis bacteriophage SP02 were cloned in B. subtilis by using the plasmid pC194. Three hybrid plasmids which permit growth of the mutant SP02 susL244 in suppressor-negative bacteria were isolated. SP02 gene L is thought to code for a DNA polymerase essential for autonomous replication of SP02 DNA. Extracts of bacteria carrying one of these hybrid plasmids, pC194-96, had 10- to 30-fold increased DNA polymerase activity. The plasmid-induced DNA polymerase activity differed from that of the known B. subtilis DNA polymerases in several respects. The results of the experiments support the idea that phage SP02 codes for a new DNA polymerase. 相似文献
110.