首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   112805篇
  免费   2213篇
  国内免费   918篇
  115936篇
  2023年   102篇
  2022年   219篇
  2021年   465篇
  2020年   209篇
  2019年   314篇
  2018年   12124篇
  2017年   10900篇
  2016年   7958篇
  2015年   1562篇
  2014年   1349篇
  2013年   1554篇
  2012年   5882篇
  2011年   14357篇
  2010年   12996篇
  2009年   9051篇
  2008年   11056篇
  2007年   12585篇
  2006年   1462篇
  2005年   1682篇
  2004年   2103篇
  2003年   2084篇
  2002年   1787篇
  2001年   422篇
  2000年   254篇
  1999年   225篇
  1998年   262篇
  1997年   177篇
  1996年   145篇
  1995年   123篇
  1994年   120篇
  1993年   141篇
  1992年   112篇
  1991年   129篇
  1990年   87篇
  1989年   72篇
  1988年   70篇
  1987年   72篇
  1986年   47篇
  1985年   65篇
  1984年   72篇
  1983年   86篇
  1982年   79篇
  1981年   74篇
  1980年   75篇
  1978年   65篇
  1977年   43篇
  1975年   42篇
  1972年   258篇
  1971年   280篇
  1962年   43篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Due to a long-term transgression since the Early Cambrian, an extensive shallow-water carbonate platform was developed in the entire Tarim Basin (NW China). During the deposition of the Yingshan Formation (Early-Middle Ordovician), a carbonate ramp system was formed in the intrashelf basin in the Bachu-Keping area of the western basin. Four well-exposed outcrop sections were selected to investigate their depositional facies, cycles, and sequences, as well as the depositional evolution. Detailed facies analyses permit the recognition of three depositional facies associations, including peritidal, semi-restricted subtidal, and open-marine subtidal facies, and eleven types of lithofacies. These are vertically arranged into meter-scale, shallowing-upward peritidal, semi-restricted subtidal, and open-marine subtidal cycles, in the span of Milankovitch frequency bands, suggesting a dominant control of Earth’s orbital forcing on the cyclic sedimentation on the platform. On the basis of vertical facies (or lithofacies) and cycle stacking patterns, as well as accommodation changes illustrated graphically by Fischer plots at all studied sections, six third-order depositional sequences are recognized and consist of lower transgressive and upper regressive parts. In shallow depositional settings, the transgressive packages are dominated by thicker-than-average, shallow subtidal cycles, whereas the regressive parts are mainly represented by thinner-than-average, relatively shallow subtidal to peritidal cycles. In relatively deep environments, however, the transgressive and regressive successions display the opposite trends of cycle stacking patterns, i.e., thinner-than-average subtidal cycles of transgressive packages. Sequence boundaries are mainly characterized by laterally traceable, transitional zones without apparent subaerial exposure features. Good correlation of the long-term changes in accommodation space inferred from vertical facies and cycle stacking patterns with sea-level fluctuations elsewhere around the world suggests an overriding eustatic control on cycle origination, platform building-up and evolution during the Early-Middle Ordovician, although with localized influences of syndepositional faulting and depositional settings.  相似文献   
952.
The effect of algae on the production of musty-smelling compounds by actinomycetes was studied. Streptomyces spp., causing intensive musty odor, were isolated from hypertrophic Lake Kasumigaura and cultured in association with algae from the same lake. Isolate E and I effectively utilized the cyanobacteria, Microcystis aeruginosa and Anabaena spiroides, and the diatom, Synedra acus, as a carbon source and produced a musty-smelling 2-methylisoborneol in the shaken sediment cultures. High populations of algae and actinomycetes, and aerobic condition in the sediment seem responsible for the occurrence of musty odor in Lake Kasumigaura.  相似文献   
953.
Proteus mirabilis and Proteus vulgaris expressed a combination of superoxide dismutase (Sod) activities, which was assigned to FeSod1, FeSod2 and MnSod for P. mirabilis, and FeSod, MnSod and CuZnSod for P. vulgaris. Production of the Sod proteins was dependent on the availability of iron, whether cells were grown under anaerobiosis or aerobiosis and growth phase. Nalidixic acid and chloramphenicol inhibited cell growth and the iron- and dioxygen-dependent production of Sod. These results support the involvement of metal ions and redox status in the production of Proteus Sods.  相似文献   
954.
The physical-chemical and carbohydrate binding specificity ofGriffonia simplicifolia I (GS I) isolectins, one of the 4 lectins isolated fromGriffonia simplicifolia seeds, are described.Association constants for the binding of methyl α- and β-D-galactopyranoside and methyl 2-acetamido-2-deoxy-α-D-galactopyranoside to the A4, A2 B2 and B4 isolectins are reported.Precipitation reactions of theGriffonia simplicifolia isolectins with guaran and type B blood group substance are described.The hypothesis that subunit B is a precursor of subunit A, a process involving proteolytic cleavage of the B subunit, was tested by conducting structural studies on the 2 subunits. The results indicated that the A and B subunits are probably products of 2 separate but closely related, possibly contiguous genes.  相似文献   
955.
956.
Distal hereditary motor neuropathies predominantly affect the motor neurons of the peripheral nervous system leading to chronic disability. Using whole genome sequencing (WGS) we have identified a novel structural variation (SV) within the distal hereditary motor neuropathy locus on chromosome 7q34–q36.2 (DHMN1). The SV involves the insertion of a 1.35 Mb DNA fragment into the DHMN1 disease locus. The source of the inserted sequence is 2.3 Mb distal to the disease locus at chromosome 7q36.3. The insertion involves the duplication of five genes (LOC389602, RNF32, LMBR1, NOM1, MNX1) and partial duplication of UBE3C. The genomic structure of genes within the DHMN1 locus are not disrupted by the insertion and no disease causing point mutations within the locus were identified. This suggests the novel SV is the most likely DNA mutation disrupting the DHMN1 locus. Due to the size and position of the DNA insertion, the gene(s) directly affected by the genomic re-arrangement remains elusive. Our finding represents a new genetic cause for hereditary motor neuropathies and highlights the growing importance of interrogating the non-coding genome for SV mutations in families which have been excluded for genome wide coding mutations.  相似文献   
957.
958.
959.
The hooded crow Corvus cornix is a west Palaearctic, solitary nesting, monogamous corvid. In the breeding season, populations are characterized by a social organization wherein breeding pairs are territorial and non-breeding individuals, called floaters, live in flocks. During a study of the breeding ecology of the hooded crow, conducted in a protected flooded area, we monitored nests with video cameras. We recorded two separate incidents when intruders attacked a female at the nest. We believe that she remained in the nest in order to prevent the strangers cannibalizing the nestlings by mantling over the brood. The spatio-temporal occurrence of these attacks suggests that the observed behaviour is intraspecific sexual aggression wherein non-breeding males mounted an immobilized female.  相似文献   
960.
The gene integration method is an important tool to stably express desirable genes in bacteria. To avoid heavy workload and cost, we constructed a rapid and efficient method for genome modification. This method depended on a mobilizable plasmid, which contains a P tac promoter, an introduced multiple cloning site (iMCS), and rrnBT1T2 terminator. Briefly, the mobilizable plasmid pK18-MBPMT with the P tac-iMCS-rrnBT1T2 cartridge derived from pK18mobsacB was prepared to directly integrate hetero-/homologous DNA into the Corynebacterium glutamicum genome. Like our previous method, this method was based on insertional inactivation and double-crossover homologous recombination, which simultaneously achieved gene overexpression and inactivation in the genome without the use of genetic markers. Compared to the previous method, this protocol omitted the construction of a recombinant expression plasmid and clone of the target gene(s) cassette, which significantly decreased the workload, cost, and operational time. Using this method, the heterologous gene amy and the homologous gene lysC T311I were successfully integrated into the C. glutamicum genome at alaT and avtA loci, respectively. Moreover, the operation time of this method was shorter than that of the previous method, especially for repeated integration. This method, which is based on the mobilizable plasmid pK18-MBPMT, thus represents a potentially attractive protocol for the integration of genes in the course of genetic modification of C. glutamicum.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号