全文获取类型
收费全文 | 7246篇 |
免费 | 853篇 |
国内免费 | 3篇 |
出版年
2021年 | 101篇 |
2020年 | 72篇 |
2019年 | 78篇 |
2018年 | 93篇 |
2017年 | 77篇 |
2016年 | 153篇 |
2015年 | 201篇 |
2014年 | 240篇 |
2013年 | 298篇 |
2012年 | 382篇 |
2011年 | 313篇 |
2010年 | 215篇 |
2009年 | 200篇 |
2008年 | 307篇 |
2007年 | 293篇 |
2006年 | 294篇 |
2005年 | 276篇 |
2004年 | 253篇 |
2003年 | 234篇 |
2002年 | 238篇 |
2001年 | 219篇 |
2000年 | 198篇 |
1999年 | 173篇 |
1998年 | 89篇 |
1997年 | 115篇 |
1996年 | 115篇 |
1995年 | 93篇 |
1994年 | 93篇 |
1993年 | 92篇 |
1992年 | 121篇 |
1991年 | 114篇 |
1990年 | 150篇 |
1989年 | 125篇 |
1988年 | 107篇 |
1987年 | 115篇 |
1986年 | 105篇 |
1985年 | 126篇 |
1984年 | 94篇 |
1983年 | 72篇 |
1982年 | 80篇 |
1981年 | 72篇 |
1980年 | 76篇 |
1979年 | 85篇 |
1978年 | 74篇 |
1977年 | 64篇 |
1976年 | 76篇 |
1975年 | 70篇 |
1974年 | 73篇 |
1973年 | 74篇 |
1972年 | 75篇 |
排序方式: 共有8102条查询结果,搜索用时 62 毫秒
1.
Defective transducing phages carrying aroG, the structural gene for phenylalanine (phe)-inhibitable phospho-2-keto-heptonate aldolase (EC 4.1.2.15; previously known as 3-deoxy-D-arabinoheptulosonate-7-phosphate synthetase[phe]), have been isolated, and DNA from two of these phages has been used to construct a restriction map of the region from att lambda to aroG. A 7.6-kb PstI-HindIII fragment from one of these phages was cloned into pBR322 and shown to contain aroG. The location of aroG within the 7.6 kb was established by subcloning and Tn3 transpositional mutagenesis. A fragment carrying the aroG promoter and operator has been cloned into a high copy number promoter-cloning vector (pMC489), and the resulting aroGpo-LacZ' (alpha) fusion subcloned in a low copy number vector. Strains with this fusion on the low copy number vector exhibit negative regulation of beta-galactosidase expression by both phenylalanine and tryptophan and positive regulation by tyrosine in a tyrR+ background. 相似文献
2.
3.
4.
5.
6.
Stairway climbing provides a ubiquitous and inconspicuous method of burning calories. While typically two strategies are employed for climbing stairs, climbing one stair step per stride or two steps per stride, research to date has not clarified if there are any differences in energy expenditure between them. Fourteen participants took part in two stair climbing trials whereby measures of heart rate were used to estimate energy expenditure during stairway ascent at speeds chosen by the participants. The relationship between rate of oxygen consumption () and heart rate was calibrated for each participant using an inclined treadmill. The trials involved climbing up and down a 14.05 m high stairway, either ascending one step per stride or ascending two stair steps per stride. Single-step climbing used 8.5±0.1 kcal min−1, whereas double step climbing used 9.2±0.1 kcal min−1. These estimations are similar to equivalent measures in all previous studies, which have all directly measured The present study findings indicate that (1) treadmill-calibrated heart rate recordings can be used as a valid alternative to respirometry to ascertain rate of energy expenditure during stair climbing; (2) two step climbing invokes a higher rate of energy expenditure; however, one step climbing is energetically more expensive in total over the entirety of a stairway. Therefore to expend the maximum number of calories when climbing a set of stairs the single-step strategy is better. 相似文献
7.
8.
Abstract: Slices cut from five frozen human brains were dissected into 2-mm cubes and assayed for choline acetyltransferase (ChAT) activity and protein content. A pattern of enrichment of ChAT activity was found ventral to the anterior commissure; this finding is consistent with the location of the enzyme in the cells of the nucleus basalis of Meynert. The region beneath the anterior commissure was the only place a discrete enrichment of activity could be found, and the precise topography of the enrichment was somewhat variable from brain to brain. The results are discussed in the light of recent knowledge concerning the source of the cortical cholinergic innervation. 相似文献
9.
10.
Riccardo E. Marioni Lars Penke Gail Davies Jennifer E. Huffman Caroline Hayward Ian J. Deary 《Proceedings. Biological sciences / The Royal Society》2014,281(1781)
Human cognitive ability shows consistent, positive associations with fitness components across the life-course. Underlying genetic variation should therefore be depleted by selection, which is not observed. Genetic variation in general cognitive ability (intelligence) could be maintained by a mutation–selection balance, with rare variants contributing to its genetic architecture. This study examines the association between the total number of rare stop-gain/loss, splice and missense exonic variants and cognitive ability in childhood and old age in the same individuals. Exome array data were obtained in the Lothian Birth Cohorts of 1921 and 1936 (combined N = 1596). General cognitive ability was assessed at age 11 years and in late life (79 and 70 years, respectively) and was modelled against the total number of stop-gain/loss, splice, and missense exonic variants, with minor allele frequency less than or equal to 0.01, using linear regression adjusted for age and sex. In both cohorts and in both the childhood and late-life models, there were no significant associations between rare variant burden in the exome and cognitive ability that survived correction for multiple testing. Contrary to our a priori hypothesis, we observed no evidence for an association between the total number of rare exonic variants and either childhood cognitive ability or late-life cognitive ability. 相似文献