全文获取类型
收费全文 | 202660篇 |
免费 | 13497篇 |
国内免费 | 1288篇 |
专业分类
217445篇 |
出版年
2021年 | 744篇 |
2018年 | 12736篇 |
2017年 | 11541篇 |
2016年 | 8626篇 |
2015年 | 2124篇 |
2014年 | 2211篇 |
2013年 | 2739篇 |
2012年 | 7005篇 |
2011年 | 15504篇 |
2010年 | 13777篇 |
2009年 | 9991篇 |
2008年 | 12041篇 |
2007年 | 13600篇 |
2006年 | 2576篇 |
2005年 | 2623篇 |
2004年 | 3014篇 |
2003年 | 3107篇 |
2002年 | 2838篇 |
2001年 | 9892篇 |
2000年 | 9664篇 |
1999年 | 7218篇 |
1998年 | 1621篇 |
1997年 | 1813篇 |
1996年 | 1589篇 |
1995年 | 1407篇 |
1994年 | 1312篇 |
1993年 | 1278篇 |
1992年 | 4812篇 |
1991年 | 4561篇 |
1990年 | 3999篇 |
1989年 | 4020篇 |
1988年 | 3625篇 |
1987年 | 3097篇 |
1986年 | 2776篇 |
1985年 | 2678篇 |
1984年 | 1980篇 |
1983年 | 1746篇 |
1982年 | 1232篇 |
1981年 | 983篇 |
1980年 | 914篇 |
1979年 | 1762篇 |
1978年 | 1361篇 |
1977年 | 1199篇 |
1976年 | 1028篇 |
1975年 | 1154篇 |
1974年 | 1166篇 |
1973年 | 1162篇 |
1972年 | 1279篇 |
1971年 | 1229篇 |
1970年 | 827篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Described here is a three-day protocol that directly yields DNA sequence after isolating and PCR amplifying genomic DNA from a small sample of frozen nasopharyngeal carcinoma tissue embedded in optimal cutting temperature (OCT) compound. The method is consistently successful, reproducible and will facilitate the rapid analysis of DNA sequence from very small samples. 相似文献
102.
103.
Comparisons among 16S rRNA sequences from various eubacteria reveal a natural relationship between the bacteroides (represented by the Bacteroides fragilis sequence) and a phylogenetic unit that comprises the flavobacteria, cytophagae, flexibacteria, and others (represented by the Flavobacterium heparinum sequence). Although the relationship is not a close one, it is, nevertheless, specific. rRNAs from these two organisms are not only closer to one another in overall sequence than they are to outgroup species (such as Bacillus subtilis, Escherichia coli, Desulfovibrio desulfuricans, and Agrobacterium tumefaciens), but they show common idiosyncrasies (i.e., derived characteristics) in both rRNA sequences and higher-order structures. 相似文献
104.
Daren Lee Seth Ruffins Queenie Ng Nikhil Sane Steve Anderson Arthur Toga 《BMC bioinformatics》2010,11(1):608
Background
Digital atlases provide a common semantic and spatial coordinate system that can be leveraged to compare, contrast, and correlate data from disparate sources. As the quality and amount of biological data continues to advance and grow, searching, referencing, and comparing this data with a researcher's own data is essential. However, the integration process is cumbersome and time-consuming due to misaligned data, implicitly defined associations, and incompatible data sources. This work addressing these challenges by providing a unified and adaptable environment to accelerate the workflow to gather, align, and analyze the data. 相似文献105.
James B Munro Roger B Altman Chang‐Shung Tung Kevin Y Sanbonmatsu Scott C Blanchard 《The EMBO journal》2010,29(4):770-781
A key intermediate in translocation is an ‘unlocked state’ of the pre‐translocation ribosome in which the P‐site tRNA adopts the P/E hybrid state, the L1 stalk domain closes and ribosomal subunits adopt a ratcheted configuration. Here, through two‐ and three‐colour smFRET imaging from multiple structural perspectives, EF‐G is shown to accelerate structural and kinetic pathways in the ribosome, leading to this transition. The EF‐G‐bound ribosome remains highly dynamic in nature, wherein, the unlocked state is transiently and reversibly formed. The P/E hybrid state is energetically favoured, but exchange with the classical P/P configuration persists; the L1 stalk adopts a fast dynamic mode characterized by rapid cycles of closure and opening. These data support a model in which P/E hybrid state formation, L1 stalk closure and subunit ratcheting are loosely coupled, independent processes that must converge to achieve the unlocked state. The highly dynamic nature of these motions, and their sensitivity to conformational and compositional changes in the ribosome, suggests that regulating the formation of this intermediate may present an effective avenue for translational control. 相似文献
106.
Y. Crouau 《Behavioural processes》1985,11(1):21-30
The cavernicolous crustacean Antromysis juberthiei showed unexpected behavior when presented with food extract : it avoids the area of odor inflow. This avoidance reaction is reduced by removal of various combinations of antennae and disappears almost with bilateral removal of the first antennae (the aesthetasc-bearing antennules). 相似文献
107.
In the course of studies on the oxygenation of steroids by purified P-450 cytochromes, particularly rabbit liver microsomal cytochrome P-450 form 3b, a rapid and reliable radiometric assay has been devised for progesterone 16 alpha-hydroxylation. In view of the lack of a commercially available, suitably tritiated substrate, [1,2,6,7,16,17-3H]progesterone was treated with alkali to remove the label from potential hydroxylation sites other than the 16 alpha position. The resulting [1,7,16-3H]progesterone was added to a reconstituted enzyme system containing cytochrome P-450 form 3b, NADPH-cytochrome P-450 reductase, and NADPH, and the rate of 16 alpha-hydroxylation was measured by the formation of 3H2O. This reaction was shown to be linear with respect to time and to the cytochrome P-450 concentration. An apparent tritium isotope effect of 2.1 was observed by comparison of the rates of formation of tritium oxide and 16 alpha-hydroxyprogesterone, and the magnitude of the isotope effect was confirmed by an isotope competition assay in which a mixture of [1,7,16-3H]progesterone and [4-14C]progesterone was employed. 相似文献
108.
Maria João Feio Trefor B. Reynoldson Verónica Ferreira Manuel Augusto S. Graça 《Hydrobiologia》2007,579(1):55-68
We sampled macroinvertebrates at 75 locations in the Mondego river catchment, Central Portugal, and developed a predictive
model for water quality assessment of this basin, based on the Reference Condition Approach. Sampling was done from June to
September 2001. Fifty-five sites were identified as “Reference sites” and 20 sites were used as “Test sites” to test the model.
At each site we also measured 40 habitat variables to characterize water physics and chemistry, habitat type, land use, stream
hydrology and geographic location. Macroinvertebrates were generally identified to species or genus level; a total of 207
taxa were found. By Unweighted Pair Group Method with Arithmetic mean (UPGMA) clustering and analysis of species contribution
to similarities percentage (SIMPER), two groups of reference sites were established. Using Discriminant Analysis (stepwise
forward), four variables correctly predicted 78% of the reference sites to the appropriate group: stream order, pool quality,
substrate quality and current velocity. Test sites’ environmental quality was established from their relative distance to
reference sites, in MDS ordination space, using a series of bands (BEAST methodology). The model performed well at upstream
sites, but at downstream sites it was compromised by the lack of reference sites. As with the English RIVPACS predictive model,
the Mondego model should be continually improved with the addition of new reference sites. The adaptation of the Mondego model
methodology to the Water Framework Directive is possible and would consist mainly of the integration of the WFD typology and
increasing the number of ellipses that define quality bands.
Handling editor: K. Martens 相似文献
109.
110.