全文获取类型
收费全文 | 10069篇 |
免费 | 1146篇 |
国内免费 | 9篇 |
出版年
2021年 | 114篇 |
2019年 | 108篇 |
2018年 | 97篇 |
2017年 | 121篇 |
2016年 | 194篇 |
2015年 | 305篇 |
2014年 | 336篇 |
2013年 | 394篇 |
2012年 | 491篇 |
2011年 | 478篇 |
2010年 | 316篇 |
2009年 | 268篇 |
2008年 | 435篇 |
2007年 | 414篇 |
2006年 | 386篇 |
2005年 | 395篇 |
2004年 | 345篇 |
2003年 | 337篇 |
2002年 | 377篇 |
2001年 | 291篇 |
2000年 | 273篇 |
1999年 | 260篇 |
1998年 | 148篇 |
1997年 | 123篇 |
1996年 | 104篇 |
1995年 | 107篇 |
1994年 | 91篇 |
1993年 | 100篇 |
1992年 | 217篇 |
1991年 | 173篇 |
1990年 | 154篇 |
1989年 | 178篇 |
1988年 | 164篇 |
1987年 | 140篇 |
1986年 | 145篇 |
1985年 | 141篇 |
1984年 | 114篇 |
1983年 | 110篇 |
1982年 | 104篇 |
1980年 | 99篇 |
1979年 | 117篇 |
1978年 | 95篇 |
1977年 | 95篇 |
1975年 | 100篇 |
1974年 | 134篇 |
1973年 | 106篇 |
1972年 | 100篇 |
1971年 | 86篇 |
1970年 | 95篇 |
1968年 | 83篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
82.
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
83.
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
84.
85.
Well differentiated and small cell neuroendocrine carcinomas of the lung. Two related but distinct clinicopathologic entities 总被引:2,自引:0,他引:2
W H Warren V A Memoli V E Gould 《Virchows Archiv. B, Cell pathology including molecular pathology》1988,55(5):299-310
Twenty-two resected pulmonary well differentiated neuroendocrine carcinomas (WDNC) were re-evaluated histologically as were 28 resected intermediate-small cell neuroendocrine carcinomas (IC-SCNC). WDNC were distinguishable from IC-SCNC by their consistently recognizable organoid architecture, and by the absence or limited extent of necrosis. Furthermore, WDNC could be subclassified into 3 subsets based upon the degrees of pleomorphism, local and vascular invasion, and stromal fibrosis, the mitotic count, and the extent of tumor necrosis. Whereas all those parameters were important in discriminating between WDNC and IC-SCNC, the quality of the organoid architecture, and the extent and pattern of necrosis emerged as the most significant. WDNC with the more aggressive histologic features (subset III) had, as a group, a distinctly worse clinical course that those displaying blander features (subsets I and II). Nevertheless, even subset III of WDNC had, as a group, a longer survival than similarly treated Stages I and II IC-SCNC. We conclude that the histologic spectrum of WDNC is broader than generally recognized. Moreover, 3 subsets of WDNC are definable based on conventional histologic criteria provided sufficient, well preserved samples are examined. Even the most aggressive subset of WDNC can be thus histologically discriminated from IC-SCNC, and, given comparable stages, has a better prognosis than the latter. 相似文献
86.
J Kok D Hill A J Haandrikman M J de Reuver H Laan G Venema 《Applied and environmental microbiology》1988,54(1):239-244
87.
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
88.
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
89.
Results of a survey of Nephrops norvegicus larvae conductedin the western Irish Sea during May 1984 are discussed. Thelarval distributions show a pronounced tongue of high numbersextending southward from the muddy area where they are hatched.The inferred density-driven circulation at the time of the surveyis consistent with the southward transport of larvae. BecauseN.norvegicus requires a muddy substrate for successful larvalsettlement, the processes which influence the circulation mayprovide an important control on the level of recruitment. 相似文献
90.
Studies were conducted to determine the effect of dietary iron (Fe) levels ranging from a deficiency to an excess on the toxicity
of cadmium (Cd) in chicks. In Fe-deficient animals, cadmium was found to be more toxic than in Fe supplemented animals as
measured by growth. The liver Cd burdens were increased significantly in the presence of dietary Fe supplementation, and there
was a significant Cd−Fe interaction in the Cd concentration of the kidney, indicating that iron deficiency increased the concentration
of Cd in the kidneys of those chicks receiving this element. Cd tended to reduce the Fe concentration in both the liver and
kidney. The absorption of Cd as measured by the amount of109Cd that disappeared from an isolated duodenal segment in one h was not affected by the Fe content of the diet, but the amount
of isotope appearing in the liver compared to the amount present in the blood was increased in the Fe supplemented chicks.
Separation of the Cd binding ligands by column chromatography revealed that more of the Cd in the liver, but not the kidney,
was associated with ligands which eluted in a column volume that contained metallothionein in those chicks receiving Fe than
in the livers from Fe deficient animals. The inverse relationship between the amount of Cd bound to the metallothionein containing
fraction and toxicity may be related causally.
Paper No. 10538 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7601. The use
of trade names in this publication does not imply endorsement by the NC Agricultural Research Service of the products named
nor criticism of similar ones not mentioned. 相似文献