全文获取类型
收费全文 | 20933篇 |
免费 | 2005篇 |
国内免费 | 21篇 |
专业分类
22959篇 |
出版年
2022年 | 137篇 |
2021年 | 276篇 |
2020年 | 138篇 |
2019年 | 208篇 |
2018年 | 252篇 |
2017年 | 218篇 |
2016年 | 408篇 |
2015年 | 643篇 |
2014年 | 733篇 |
2013年 | 960篇 |
2012年 | 1243篇 |
2011年 | 1240篇 |
2010年 | 809篇 |
2009年 | 786篇 |
2008年 | 1068篇 |
2007年 | 1065篇 |
2006年 | 1033篇 |
2005年 | 1046篇 |
2004年 | 1042篇 |
2003年 | 1059篇 |
2002年 | 1098篇 |
2001年 | 400篇 |
2000年 | 288篇 |
1999年 | 348篇 |
1998年 | 327篇 |
1997年 | 250篇 |
1996年 | 189篇 |
1995年 | 196篇 |
1994年 | 184篇 |
1993年 | 226篇 |
1992年 | 250篇 |
1991年 | 231篇 |
1990年 | 215篇 |
1989年 | 199篇 |
1988年 | 203篇 |
1987年 | 195篇 |
1986年 | 190篇 |
1985年 | 219篇 |
1984年 | 170篇 |
1983年 | 186篇 |
1982年 | 200篇 |
1981年 | 194篇 |
1980年 | 181篇 |
1979年 | 138篇 |
1978年 | 146篇 |
1977年 | 142篇 |
1976年 | 125篇 |
1975年 | 125篇 |
1974年 | 166篇 |
1973年 | 142篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Detection and Identification of Translocations by Increased Specific Nondisjunction in ASPERGILLUS NIDULANS 总被引:4,自引:3,他引:1 下载免费PDF全文
A meiotic technique for visual detection of translocations has been applied to ten mitotically identified interchanges, and three new translocations were discovered using this method. Testcrosses between "standard" strains and potential translocation strains-e.g. strains with newly induced mutants or descendants from translocation crosses-are inspected for the frequency of abnormal-looking colonies. In all heterozygous translocation crosses "abnormals" are increased at least tenfold compared to the average control level of 0.15%. Most of these are disomics, and can be recognized by their characteristic phenotypes. Each translocation produces a few specific types, since nondisjunction is increased mainly in the linkage groups involved in the translocation (50-100-fold over control values). Therefore, translocations were not only detected but often tentatively assigned to linkage groups from the analysis of the disomic progeny in crosses. In addition, this technique allows reciprocal and nonreciprocal translocations to be distinguished, since only the latter produce one-third phenotypically abnormal duplication progeny. While results are clearcut in most cases, occasionally problems are encountered, e.g. when morphological mutants segregate in crosses, or when other genetic factors which increase or reduce the frequency of nondisjunction are present in certain strains. 相似文献
72.
73.
Dr. Alan R. Templeton 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1974,45(5):179-191
Summary A statistical model is presented for dealing with genotypic frequency data obtained from a single population observed over a run of consecutive generations. This model takes into account possible correlations that exist between generations by conditioning the marginal probability distribution of any one generation on the previously observed generation. Maximum likelihood estimates of the fitness parameters are derived and a hypothesis testing framework developed. The model is very general, and in this paper is applied to random-mating, selfing, parthenogenetic and mixed random-mating and selfing populations with respect to a single locus, g-allele model with constant genotypic fitness differences with all selection occurring either before or after sampling. The assumptions behind this model are contrasted with those of alternative techniques such as minimum chi-square or unconditional maximum likelihood estimation when the marginal likelihoods for any one generation are conditioned only on the initial conditions and not the previous generation. The conditional model is most appropriate when the sample size per generation is large either in an absolute sense or in relation to the total population size. Minimum chi-square and the unconditional likelihood are most appropriate when the population size is effectively infinite and the samples are small. Both models are appropriate when the samples are large and the population size is effectively infinite. Under these last conditions, the conditional model may be preferred because it has greater robustness with respect to small deviations from the underlying assumptions and has a greater simplicity of form. Furthermore, if any genetic drift occurs in the experiment, the minimum chi-square and unconditional likelihood approaches can create spurious evidence for selection while the conditional approach will not. Worked examples are presented.This study was supported in part by the U. S. Atomic Energy Commission, Contract AT (11-1) -1552 to the Department of Human Genetics (CFS), University of Michigan, and by National Science Foundation Grant BMS 74-17453 awarded to the author. 相似文献
74.
This paper describes the isolation and amino acid analysis of un-cross-linked elastin obtained by neutral salt extraction from the ligamentum nuchae of a calf fed from birth to 9 months on a diet low in copper. 相似文献
75.
76.
77.
78.
Studies on a living and freshly dead male ofM. assamensis at Y.R.P.R.C. supplemented by notes on a living pair and their male offspring observed at the Z.S.L., enable us to supplement existing data on the somatology, craniology, dental anatomy and behavioural features of the species. Collectively the new data necessitate taxonomic revision of the status of the species, viz: its removal from immediate association withM. mulatta (subgenusMaimon) and alignment within the subgenusZati. Zoogeographical discontinuity in this subgenus is compared with that of the subgenusSilenus. 相似文献
79.
Summary The method by which the results of biological assays for gibberellinlike substances in plants are presented as gibberellic acid equivalents is discussed and a technique for deriving confidence limits for such estimates is referred to. Two different assay methods used to illustrate the technique both showed the relatively wide confidence limits associated with estimates of this kind, following the necessary logrithmic transformation of the data. 相似文献
80.
Phosphotransferase system of Staphylococcus aureus: its requirement for the accumulation and metabolism of galactosides 总被引:20,自引:11,他引:9 下载免费PDF全文
The phosphotransferase system of Staphylococcus aureus was characterized. Mutants defective in enzyme I and heat-stable (HPr) protein as well as in the two components specific to lactose accumulation, factor III and enzyme II, were isolated. Colorimetric assays for each of the components are presented based on the formation of o-nitrophenyl-beta-d-galactoside-6-phosphate by the system and its hydrolysis by the staphylococcal 6-phospho-beta-galactosidase. The components were partially purified and their molecular weights were estimated: enzyme I, 100,000 +/- 15%; HPr, 10,000 +/- 15%; factor III, 30,000 +/- 15%; 6-phospho-beta-galactosidase, 45,000. Enzyme II is a membrane-bound protein. 相似文献