全文获取类型
收费全文 | 18227篇 |
免费 | 1763篇 |
国内免费 | 9篇 |
出版年
2021年 | 221篇 |
2020年 | 127篇 |
2019年 | 177篇 |
2018年 | 212篇 |
2017年 | 201篇 |
2016年 | 348篇 |
2015年 | 567篇 |
2014年 | 634篇 |
2013年 | 780篇 |
2012年 | 1029篇 |
2011年 | 989篇 |
2010年 | 675篇 |
2009年 | 609篇 |
2008年 | 945篇 |
2007年 | 886篇 |
2006年 | 848篇 |
2005年 | 914篇 |
2004年 | 860篇 |
2003年 | 844篇 |
2002年 | 869篇 |
2001年 | 325篇 |
2000年 | 270篇 |
1999年 | 333篇 |
1998年 | 293篇 |
1997年 | 201篇 |
1996年 | 191篇 |
1995年 | 194篇 |
1994年 | 173篇 |
1993年 | 176篇 |
1992年 | 236篇 |
1991年 | 198篇 |
1990年 | 195篇 |
1989年 | 200篇 |
1988年 | 189篇 |
1987年 | 179篇 |
1986年 | 184篇 |
1985年 | 205篇 |
1984年 | 204篇 |
1983年 | 187篇 |
1982年 | 200篇 |
1981年 | 192篇 |
1980年 | 210篇 |
1979年 | 142篇 |
1978年 | 143篇 |
1977年 | 134篇 |
1976年 | 111篇 |
1975年 | 136篇 |
1974年 | 158篇 |
1973年 | 146篇 |
1972年 | 109篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
102.
Messenger RNA recognition in Escherichia coli: a possible second site of interaction with 16S ribosomal RNA. 总被引:28,自引:3,他引:25 下载免费PDF全文
Examination of the nucleotides following the ATG or GTG initiation codons of a file of 251 genes from Escherichia coli has shown that 247 (98.4%) of them contain a sequence of at least three and 168 (66.9%) of them a sequence of at least four consecutive nucleotides that is complementary to some part of the 16 nt at the 5' terminus of the bacterial 16S rRNA. It is proposed that this sequence, which falls within the first 24 nt coding for the genetic message, might be involved in mRNA recognition through a mechanism analogous to the well-established 'Shine--Dalgarno' interaction with the 3' terminus of the 16S rRNA. Comparison of these data with data derived from a file of 117 'false' gene starts that have a Shine--Dalgarno-like sequence followed by a suitably spaced ATG or GTG triplet but which are believed not to lie at the beginnings of genetic messages shows the association that we have found to be statistically significant at the 99.9% level. 相似文献
103.
Jonathan D. Hill 《Human ecology: an interdisciplinary journal》1988,17(1):1-25
This essay examines ritual and ceremonial activities among the Arawakspeaking Wakuénai of the Venezuelan Amazon as processes of constructing power relations in changing historical and ecological conditions. Ritual evocations of the vertical dimension of power relations between mythic ancestors and human descendants adapt local populations to conditions of relatively severe stress, such as epidemics and scarcity of fish in long wet seasons. Other rituals evoke the horizontal dimension of power relations between affinally-related groups as a way of expanding the local descent group in conditions of lowered stress. These two ways of exercising ritual power link human populations to specific natural habitats and provide flexibility needed to adjust to demographic and other historical changes. Through ritual performances, the Wakuénai transform the natural environment into a cultural landscape of socialized objects and, conversely, remember the history of political relations among peoples through spirit-naming of natural species, objects, places, and geographic landmarks. 相似文献
104.
John M. Beals Joseph Weber Paul Derwent Kenneth L. Grant Francis J. Castellino 《Journal of Protein Chemistry》1988,7(5):613-632
The secondary structures of bovine blood coagulation factors IX and X, as well as that of bovine prothrombin, were predicted on the basis of a computerized combination of the Chou-Fasman and Burgess algorithms. Refinements in the predictions were made after consideration of the content of various secondary structures, as determined by circular dichroism studies of these same proteins. The final turn assignments were in good agreement with those assigned with use of an algorithm involving pattern matching of -turns in proteins of known structure. 相似文献
105.
106.
Predicting genotypes at loci for autosomal recessive disorders using linked genetic markers: application to Wilson's disease 总被引:4,自引:0,他引:4
Lindsay A. Farrer Batsheva Bonne-Tamir Moshe Frydman Abraham Magazanik Kenneth K. Kidd Anne M. Bowcock Luigi L. Cavalli-Sforza 《Human genetics》1988,79(2):109-117
Summary Recently, the Wilson's disease locus (WND) has been mapped to the long arm of chromosome 13. We have analyzed segregation of serveral chromosome 13 markers flanking the WND locus and used multipoint linkage analysis to determine the most likely WND genotype of each of 57 unaffected individuals in 5 Wilson's disease families. Approximately 46% of these could be classified as carrier (heterozygote), homozygous normal, or homozygous affected (not yet symptomatic) with a probability of at least 90%, while 77% could be classified with a probability of at least 80%. Our results demonstrate that even though there is a significant decrease on average in serum copper concentration in Wilson's disease heterozygotes compared to normal homozygotes, other sources of variation in serum copper concentration are much greater and preclude use of serum copper to detect heterozygotes for Wilson's disease. Subsequent analyses showed that a familial component, independent of WND genotype, is the major factor accounting for variation in ceruloplasmin levels among unaffected individuals; age is another factor accounting for more variation in copper levels among unaffected individuals than WND genotype. 相似文献
107.
J Kok D Hill A J Haandrikman M J de Reuver H Laan G Venema 《Applied and environmental microbiology》1988,54(1):239-244
108.
Summary The effects of a non-ionic surfactant, Pluronic F-68, on growth of microbial cell cultures have been studied. Growth ofSaccharomyces cerevisiae at 30°C or 37°C as measured by viable cell counts was unaffected by culture with pluronic. However, corresponding absorbance measurements forS. cerevisiae incubated with 5–10% pluronic were lower than controls at both temperatures. Absorbance ofE. coli cultures was also significantly reduced by incubation with 5.0–10.0% pluronic at both temperatures although viable counts again revealed no significant inhibition of growth. 相似文献
109.
Cloning and characterization of the tetracycline resistance determinant of and several promoters from within the conjugative transposon Tn919 总被引:7,自引:0,他引:7
Tn919 is a 15- to 16-kilobase (kb) tetracycline resistance conjugative transposon that was originally isolated from Streptococcus sanguis FC1. The tetracycline resistance determinant (tet) was found on a 4.2-kb HindII fragment by in vitro deletion analysis. This fragment was subcloned to a pWV01 origin capable of directing replication in Escherichia coli, Bacillus subtilis, and Streptococcus lactis, and expression was observed in all three genera. In all cases, expression was weaker when only the 4.2-kb cloned fragment rather than the full transposon was present. The resistance gene is of the streptococcal tetM class and codes for a protein of approximately 70 kilodaltons. The restriction map resembles that of the tetM gene of Tn1545 (P. Martin, P. Trieu-Cuot, and P. Courvalin, Nucleic Acids Res. 14:7047-7058, 1986), which codes for a protein of 72.5 kilodaltons. A number of transposon-derived promoter-bearing fragments were also cloned and sequenced. These closely resemble the consensus sequence of E. coli and B. subtilis promoters. Fusion experiments with a truncated lacZ gene indicate the possibility of an open reading frame for one of the promoters. 相似文献
110.
E J Gibbs M C Maurer J H Zhang W M Reiff D T Hill M Malicka-Blaszkiewicz R E McKinnie H Q Liu R F Pasternack 《Journal of inorganic biochemistry》1988,32(1):39-65
Studies of the solution properties of gold(III)tetrakis(4-N-methylpyridyl) porphine and its DNA binding characteristics have been conducted utilizing uv/vis absorption spectroscopy, circular dichroism (CD), Mossbauer spectroscopy, and temperature-jump relaxation techniques. These studies indicate that over the concentration range considered this water soluble gold(III) porphyrin does not aggregate, binds axial ligands only weakly with a preference for soft Lewis bases, and is capable of intercalation into nucleic acids of appropriate base pair content. The interaction of this and several other porphyrins with the synthetic polynucleotide poly(dA-dC).poly(dT-dG) has been studied. Spectroscopic signatures for intercalation were found for those derivatives not having axial ligands. Intercalation into chromatin in vitro can also occur with those porphyrins and metalloporphyrins which do not have axial ligands. Finally, studies utilizing microinjection techniques indicate that once within the cell, tetrakis(4-N-methylpyridyl)porphine tends to localize in the nucleus. 相似文献