全文获取类型
收费全文 | 2670篇 |
免费 | 283篇 |
国内免费 | 1篇 |
专业分类
2954篇 |
出版年
2023年 | 9篇 |
2022年 | 24篇 |
2021年 | 70篇 |
2020年 | 28篇 |
2019年 | 36篇 |
2018年 | 42篇 |
2017年 | 36篇 |
2016年 | 71篇 |
2015年 | 144篇 |
2014年 | 133篇 |
2013年 | 166篇 |
2012年 | 216篇 |
2011年 | 196篇 |
2010年 | 110篇 |
2009年 | 93篇 |
2008年 | 151篇 |
2007年 | 133篇 |
2006年 | 136篇 |
2005年 | 104篇 |
2004年 | 105篇 |
2003年 | 110篇 |
2002年 | 91篇 |
2001年 | 64篇 |
2000年 | 63篇 |
1999年 | 60篇 |
1998年 | 27篇 |
1997年 | 25篇 |
1996年 | 20篇 |
1995年 | 22篇 |
1994年 | 31篇 |
1993年 | 22篇 |
1992年 | 28篇 |
1991年 | 25篇 |
1990年 | 33篇 |
1989年 | 29篇 |
1988年 | 29篇 |
1987年 | 21篇 |
1986年 | 16篇 |
1985年 | 19篇 |
1984年 | 18篇 |
1983年 | 16篇 |
1982年 | 11篇 |
1981年 | 17篇 |
1979年 | 21篇 |
1978年 | 17篇 |
1977年 | 15篇 |
1976年 | 12篇 |
1974年 | 9篇 |
1973年 | 12篇 |
1972年 | 8篇 |
排序方式: 共有2954条查询结果,搜索用时 15 毫秒
1.
I. J. Pickering Graham N. George Verena Van Fleet-Stalder Thomas G. Chasteen Roger C. Prince 《Journal of biological inorganic chemistry》1999,4(6):791-794
Received: 2 April 1999 / Accepted: 17 September 1999 相似文献
2.
3.
4.
Jake V Bailey Verena Salman Gregory W Rouse Heide N Schulz-Vogt Lisa A Levin Victoria J Orphan 《The ISME journal》2011,5(12):1926-1935
We present evidence for a dimorphic life cycle in the vacuolate sulfide-oxidizing bacteria that appears to involve the attachment of a spherical Thiomargarita-like cell to the exteriors of invertebrate integuments and other benthic substrates at methane seeps. The attached cell elongates to produce a stalk-like form before budding off spherical daughter cells resembling free-living Thiomargarita that are abundant in surrounding sulfidic seep sediments. The relationship between the attached parent cell and free-living daughter cell is reminiscent of the dimorphic life modes of the prosthecate Alphaproteobacteria, but on a grand scale, with individual elongate cells reaching nearly a millimeter in length. Abundant growth of attached Thiomargarita-like bacteria on the integuments of gastropods and other seep fauna provides not only a novel ecological niche for these giant bacteria, but also for animals that may benefit from epibiont colonization. 相似文献
5.
Except at very low levels, uncorrected photometric determination of bacterial cell densities showed a decreasing proportionally to actual cell density or dry weight. A standard curve was prepared to convert photometric readings to truly proportional optical density values. With one dry weight determination, optical density values may be converted to absolute dry weight values. 相似文献
6.
Binding of a nuclear factor to a consensus sequence in the 5' flanking region of zein genes from maize 总被引:26,自引:6,他引:20 下载免费PDF全文
The genomic organization of the zein structural genes and of regulatory loci influencing their expression suggests that control of zein gene expression will involve interactions between cis elements in the flanking DNA sequences and products from trans-acting genes. The interaction between fragments from the 5' flanking region of a zein gene and specific, double-stranded oligonucleotides with crude nuclear extracts from maize endosperm have been studied by nitrocellulose filter binding, gel retention and DNase I footprinting assays. Specific binding of a nuclear factor was observed and the exact position of the protein binding site was determined. The 22-nt binding site included 14 bp of a 15-bp sequence conserved in all zein genes. 相似文献
7.
W Maier 《Gegenbaurs morphologisches Jahrbuch》1987,133(1):123-161
In most marsupials, the angular process is inflected medially. By using an ontogenetic series of Monodelphis domestica, the development of this characteristic structure has been described. In contrast with the eutherian mammals, in marsupials there is retained a close connection between the dentale and the tympanicum and goniale; it is well known that these 2 elements of the middle ear are derived from the angulare and prearticulare of the reptilian lower jaw. At the neonatal stage, the dentale and tympanicum are both relatively vertically orientated; during the following 2 weeks, they take an increasingly oblique position, which is primarily caused by the rapid growth of the braincase. Only after the eruption of the first teeth, the ascending ramus of the dentale takes a more and more vertical position, whereas the angular process remains with its tip near the medioventral floor of the tympanic bulla. The bulla shows at this place a rectangular fenestra which is covered by a membrane of loose connective tissue; the tip of the angular process, which is always free of muscular insertions, maintains contacts with this fenestra throughout life. During juvenile and adult life stages, the process becomes somewhat removed from the fenestra for obvious reasons, but at a gape of about 40 to 50 degrees it inevitably must touch the "inferior tympanic membrane" and possibly also the tympanic ring. It is speculated that the relationship between the angular process and the tympanic bulla represents a specific form-function complex for sound transmission, which may be a modified retention from archaic mammalian conditions. Further details of the ontogenetic development of the tympanic region have been described which may be of some relevance for the evolutionary morphology of mammals: The tympanic process of the petrosal, which fixes the posterior end of the tympanic ring, is formed by 'Zuwachsknochen' (additional bone) but not by cartilage. The styloid process remains cartilaginous throughout life: its free tip ends in the lateral wall of the tympanic cavity and it is closely connected with the collum mallei and the posterior end of the tympanicum; it guides the chorda tympani and may therefore be homologous with the cartilage of Spence. The cartilage of Paauw is interpreted in terms of functional morphology. A model of evolutionary transformation of the dentale-tympanicum complex in mesozoic mammals in outlined on the basis of the ontogenetic findings in Monodelphis and other didelphid and dasyurid marsupials. 相似文献
8.
M E Kugler-Steigmeier U Friederich U Graf W K Lutz P Maier C Schlatter 《Mutation research》1989,211(2):279-289
Various substituted aniline derivatives were tested for genotoxicity in several short-term tests in order to examine the hypothesis that a substitution at both ortho positions (2,6-disubstitution) could prevent genotoxicity due to steric hindrance of an enzymatic activation to electrophilic intermediates. In the Salmonella/microsome assay, 2,6-dialkylsubstituted anilines and 2,4,6-trimethylaniline (2,4,6-TMA) were weakly mutagenic in strain TA100 when 20% S9 mix was used, although effects were small compared to those of 2,4-dimethylaniline and 2,4,5-trimethylaniline (2,4,5-TMA). In Drosophila melanogaster, however, 2,4,6-TMA and 2,4,6-trichloroaniline (TCA) were mutagenic in the wing spot test at 2-3 times lower doses than 2,4,5-TMA. In the 6-thioguanine resistance test in cultured fibroblasts, 2,4,6-TMA was again mutagenic at lower doses than 2,4,5-TMA. Two methylene-bis-aniline derivatives were also tested with the above methods: 4,4'-methylene-bis-(2-chloroaniline) (MOCA) was moderately genotoxic in all 3 test systems whereas 4,4'-methylene-bis-(2-ethyl-6-methylaniline) (MMEA) showed no genotoxicity at all. DNA binding studies in rats, however, revealed that both MOCA and MMEA produced DNA adducts in the liver at levels typically found for moderately strong genotoxic carcinogens. These results indicate that the predictive value of the in vitro test systems and particularly the Salmonella/microsome assay is inadequate to detect genotoxicity in aromatic amines. Genotoxicity seems to be a general property of aniline derivatives and does not seem to be greatly influenced by substitution at both ortho positions. 相似文献
9.
The effect of temperature on the development times of eggs,naupliar and copepodite stages of five species of cyclopoid copepods 总被引:2,自引:1,他引:1
Gerhard Maier 《Hydrobiologia》1989,184(1-2):79-88
The duration times of eggs, combined naupliar instars and of the different copepodite stages of five species of cyclopoid
copepods — Acanthocyclops robustus, Cyclops vicinus, Diacyclops bicuspidatus, Mesocyclops leuckarti, and Thermocyclops crassus — were investigated at five different temperatures. The five species can be divided in two groups: two species, C. vicinus and D. bicuspidatus, adapted to cold water conditions and three species, A. robustus, M. leuckarti and T. crassus adapted to warm water conditions. The cold water species showed a faster egg development than M. leuckarti and T. crassus at 5–15 °C. The eggs and instars of the warm water species M. leuckarti tend to develop faster than those of the former two species at higher temperatures. A. robustus showed the shortest egg and instar development at 10–25 °C. The warm water species T. crassus produced no eggs at 10 °C and temperatures below. At higher temperatures (20, 25 °C) the egg and instar duration times were
similar or longer than those of the other species. When cultured in total darkness a great part of the CIV respectively CV copepodites of the summer forms entered arrest and the percentage of copepodites that showed an arrest of development was
highest at lowest temperatures. The present results are compared with data from literature and differences are discussed. 相似文献
10.
Curtis C. Maier Bianca Marchetti Robert D. LeBoeuf J. Edwin Blalock 《Cellular and molecular neurobiology》1992,12(5):447-454
1. A luteinizing hormone-releasing hormone (LHRH)-like molecule produced by thymocytes is similar to hypothalamic LHRH in both bioactivity and antigenicity. 2. We determined whether this thymic LHRH is identical to or only homologous with hypothalamic LHRH by synthesizing and sequencing the cDNA of rat thymus LHRH. 3. The thymocyte and hypothalamic LHRH cDNAs are identical, indicating, that the amino acid sequences of LHRH produced in the hypothalamus and the immune system are also identical. 4. This is the first report showing conclusively that cell of the immune system transcribe the authentic mRNA for a hypothalamic releasing factor, LHRH. 相似文献