全文获取类型
收费全文 | 335283篇 |
免费 | 38782篇 |
国内免费 | 138篇 |
出版年
2018年 | 3025篇 |
2016年 | 4101篇 |
2015年 | 5766篇 |
2014年 | 6768篇 |
2013年 | 9588篇 |
2012年 | 10907篇 |
2011年 | 11041篇 |
2010年 | 7394篇 |
2009年 | 6675篇 |
2008年 | 9644篇 |
2007年 | 10070篇 |
2006年 | 9377篇 |
2005年 | 9102篇 |
2004年 | 9066篇 |
2003年 | 8741篇 |
2002年 | 8606篇 |
2001年 | 14464篇 |
2000年 | 14450篇 |
1999年 | 11650篇 |
1998年 | 4276篇 |
1997年 | 4280篇 |
1996年 | 4063篇 |
1995年 | 3936篇 |
1994年 | 3913篇 |
1993年 | 3797篇 |
1992年 | 9883篇 |
1991年 | 9463篇 |
1990年 | 9295篇 |
1989年 | 9022篇 |
1988年 | 8420篇 |
1987年 | 8040篇 |
1986年 | 7281篇 |
1985年 | 7390篇 |
1984年 | 6133篇 |
1983年 | 5366篇 |
1982年 | 4249篇 |
1981年 | 3778篇 |
1980年 | 3589篇 |
1979年 | 6149篇 |
1978年 | 4612篇 |
1977年 | 4323篇 |
1976年 | 4004篇 |
1975年 | 4324篇 |
1974年 | 4705篇 |
1973年 | 4748篇 |
1972年 | 4375篇 |
1971年 | 4088篇 |
1970年 | 3456篇 |
1969年 | 3353篇 |
1968年 | 2977篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Phylogenetic revision of Agapophytinae subf.n. (Diptera: Therevidae) based on molecular and morphological evidence 总被引:1,自引:0,他引:1
Shaun L. Winterton† Longlong Yang† Brian M. Wiegmann† David K. Yeates . 《Systematic Entomology》2001,26(2):173-211
Agapophytinae subf.n. is a highly diverse lineage of Australasian Therevidae, comprising eight described and two new genera: Agapophytus Guérin‐Méneville, Acupalpa Kröber, Acraspisa Kröber, Belonalys Kröber, Bonjeania Irwin & Lyneborg, Parapsilocephala Kröber, Acatopygia Kröber, Laxotela Winterton & Irwin, Pipinnipons gen.n. and Patanothrix gen.n. A genus‐level cladistic analysis of the subfamily was undertaken using sixty‐eight adult morphological characters and c. 1000 base pairs of the elongation factor‐1α (EF‐1α) protein coding gene. The morphological data partition produced three most parsimonious cladograms, whereas the molecular data partition gave a single most parsimonious cladogram, which did not match any of the cladograms found in the morphological analysis. The level of congruence between the data partitions was determined using the partition homogeneity test (HTF) and Wilcoxon signed ranks test. Despite being significantly incongruent in at least one of the incongruence tests, the partitions were combined in a simultaneous analysis. The combined data yielded a single cladogram that was better supported than that of the individual partitions analysed separately. The relative contributions of the data partitions to support for individual nodes on the combined cladogram were investigated using Partitioned Bremer Support. The level of support for many nodes on the combined cladogram was non‐additive and often greater than the sum of support for the respective nodes on individual partitions. This synergistic interaction between incongruent data partitions indicates a common phylogenetic signal in both partitions. It also suggests that criteria for partition combination based solely on incongruence may be misleading. The phylogenetic relationships of the genera are discussed using the combined data. A key to genera of Agapophytinae is presented, with genera diagnosed and figured. Two new genera are described: Patanothrix with a new species (Pat. skevingtoni) and Pat. wilsoni (Mann) transferred from Parapsilocephala, and Pipinnipons with a new species (Pip. kroeberi). Pipinnipons fascipennis (Kröber) is transferred from Squamopygia Kröber and Pip. imitans (Mann) is transferred from Agapophytus. Agapophytus bicolor (Kröber) is transferred from Parapsilocephala. Agapophytus varipennis Mann is synonymised with Aga. queenslandi Kröber and Aga. flavicornis Mann is synonymised with Aga. pallidicornis (Kröber). 相似文献
52.
53.
B Daneholt 《Journal of molecular biology》1970,49(2):381-391
54.
55.
56.
57.
58.
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
59.
60.
A simple new procedure was described for producing a sequential series of overlapping clones for use in DNA sequencing. The technique used single-stranded M13 DNA and complementary DNA oligomers to form specific cleavage and ligation substrates. It was, therefore, independent of the sequence of the DNA cloned into the vector. Deletions of varying sizes were generated from one end of the insert through the 3' to 5' exonuclease activity of T4 DNA polymerase. The approximate size of the deletion and therefore the starting point for DNA sequencing could be estimated by electrophoresis of the subcloned phage DNA on a agarose gel. This greatly reduced the number of templates that must be sequenced to obtain a complete sequence. The entire procedure could be carried out in one tube in less than a day. The procedure was used to subclone and sequence the maize mitochondrial 18 S rDNA and 5' flanking region (2622 bases) in less than a week. Other applications of oligomers and single-stranded DNA in the construction of insertions, deletions, and cDNAs are discussed. 相似文献