全文获取类型
收费全文 | 1970篇 |
免费 | 110篇 |
专业分类
2080篇 |
出版年
2015年 | 27篇 |
2014年 | 43篇 |
2013年 | 76篇 |
2012年 | 65篇 |
2011年 | 52篇 |
2010年 | 65篇 |
2009年 | 53篇 |
2008年 | 59篇 |
2007年 | 82篇 |
2006年 | 44篇 |
2005年 | 54篇 |
2004年 | 45篇 |
2003年 | 50篇 |
2002年 | 40篇 |
2001年 | 41篇 |
2000年 | 32篇 |
1999年 | 41篇 |
1998年 | 29篇 |
1997年 | 31篇 |
1996年 | 34篇 |
1995年 | 26篇 |
1994年 | 24篇 |
1993年 | 32篇 |
1992年 | 28篇 |
1991年 | 45篇 |
1990年 | 36篇 |
1989年 | 35篇 |
1988年 | 25篇 |
1987年 | 40篇 |
1986年 | 32篇 |
1985年 | 26篇 |
1984年 | 28篇 |
1983年 | 21篇 |
1982年 | 22篇 |
1979年 | 24篇 |
1978年 | 21篇 |
1976年 | 27篇 |
1975年 | 24篇 |
1974年 | 26篇 |
1973年 | 35篇 |
1972年 | 32篇 |
1970年 | 21篇 |
1965年 | 41篇 |
1964年 | 19篇 |
1963年 | 32篇 |
1962年 | 25篇 |
1961年 | 34篇 |
1960年 | 21篇 |
1959年 | 28篇 |
1958年 | 31篇 |
排序方式: 共有2080条查询结果,搜索用时 15 毫秒
1.
2.
Kim A. Caldwell Tim Wiltshire Mary Ann Handel 《Molecular reproduction and development》1996,43(4):403-413
The goals of this work were to create germ-cell-stage-specific cDNA libraries from mouse spermatogenic cells and to employ a novel two-step genetic screen to identify gene sequences present during the critical meiotic stage of spermatogenesis. Highly enriched germ-cell fractions were prepared from adult and juvenile mouse testes, and purity of these fractions was extensively analyzed by light and electron microscopy. Standard techniques were used to prepare cDNA libraries from populations of mixed leptotene and zygotene (L/Z) spermatocytes, pachytene (P) spermatocytes, and round spermatids. These libraries were analyzed with respect to representation of sequences from ubiquitously expressed genes, and from genes expressed at specific germ-cell stages as well as from genes expressed in testicular somatic cells. For the first step of the screening procedure, testicular cDNA was prepared from mutant mice carrying the T(X;11)38H chromosomal translocation that causes spermatogenic arrest at early meiotic prophase. This mixed cDNA probe was used to screen the libraries from L/Z and P spermatocytes to detect sequences that failed to hybridize. The clones identified were characterized for ability to hybridize to various germ-cell-specific cDNAs to verify that they represented sequences present in normal spermatogenic meiotic cells. These clones were then subjected to a second screening with another mutant probe; this time the cDNA probe was from testes of sterile mice bearing the T(X;16)16H chromosomal translocation that causes spermatogenic arrest at late meiotic prophase. This screen identified 27 clones that were not represented in testicular cDNA from T38-bearing mice or from T16-bearing mice. These clones may represent sequences essential for normal completion of the genetic events of meiosis during spermatogenesis. Likewise, the secondary screen identified 19 clones that were not represented in testicular cDNA from T38-bearing mice but were represented in testicular cDNA of T16-bearing mice. These clones are thus gene sequences present in spermatogenic cells during the time from early meiotic prophase to mid-to-late prophase. This strategy represents the first use of genetic aberrations in differential screening to identify genes expressed at specific times during mammalian spermatogenesis. © 1996 Wiley-Liss, Inc. 相似文献
3.
Arnell R Johannisson R Lindholm J Fornstedt T Ersson B Ballagi A Caldwell K 《Preparative biochemistry & biotechnology》2007,37(4):309-321
The steroid 9alpha-hydroxylase gene has been cloned from Mycobacterium smegmatis into Escherichia coli BL21. Progesterone added to bioreactors was subjected to in vivo transformation into 9alpha-hydroxyprogesterone. In 7 days, 43.6 mg 9alpha-hydroxyprogesterone was formed from 53.8 mg/L progesterone. The enzyme also has shown evidence of processing 4-androstene-3,17-dione in vivo. An extensive analytical method development, including LLE, HPLC-DAD, MS, and NMR was performed to verify the product and to enable a quantitative analysis. Protocols for analytical and preparative separation have been developed, using binaphtol as internal standard. Both the growth pattern and the bioconversion rate were unaffected by the presence of binaphtol in the bioreactor. The enzyme was purified by immobilised metal affinity and ion exchange chromatography, resulting in low in vitro activity. 相似文献
4.
Activation tagging in tomato identifies a transcriptional regulator of anthocyanin biosynthesis,modification, and transport 总被引:31,自引:0,他引:31
下载免费PDF全文

Mathews H Clendennen SK Caldwell CG Liu XL Connors K Matheis N Schuster DK Menasco DJ Wagoner W Lightner J Wagner DR 《The Plant cell》2003,15(8):1689-1703
5.
The stomatopod Gonodactylus bredini is a predatory, aggressive crustacean possessing powerful raptorial appendages capable of inflicting serious injury on conspecifics. These weapons are readily employed during contests for cavities in coral rubble. In this study, we document the incidence of wounds in a population of G. bredini from Panama, determine the influence such injuries have on the behavior and fighting ability of individuals when competing for cavities, and examine associated costs of injury. A major consequence of injury in G. bredini is a severe reduction in fighting ability which in turn can alter the outcome of contests for cavities and the behaviors used by injured animals during such contests. A less immediate, but potentially costly consequence of injury due to increased chances of cannibalism and predation and lost breeding opportunities, is an acceleration in the onset of molt cycles in animals missing raptorial appendages. 相似文献
6.
Strains of enteric bacteria and pseudomonads containing plasmid R388::Tnl721 (Tpr, Tcr) or pRO101 (Hgr, Tcr) were starved for over 250 days in sterile well water to evaluate effects of starvation-survival on plasmid expression and maintenance. Viable populations dropped to between approximately 0.1 and 1% of the initial populations. Escherichia coli(pRO101) and Pseudomonas cepacia(pRO101) lost both viability and plasmid expression at a lower rate than strains containing R388::Tnl721. Three patterns of host-plasmid interaction were detected: (i) no apparent loss of plasmid expression, (ii) loss of plasmid expression on initial recovery with subsequent expression upon resuscitation, and (iii) loss of capability to produce functional plasmid resistance. 相似文献
7.
Intrusive agriculture development, searching for higher profitability, has inflicted permanent damage to agroecosystems. Rapid deterioration of structure and functional properties in agroecosystems has intensified the need for research on agroecosystem health and agroecosystem management. This paper describes the concept of agroecosystem health which plays an important conceptual role in evaluating agroecosystem and agricultural research. Firstly, the development of agroecosystem health research is reviewed, and agroecosystem health from various dimensions is provided. Then, the methods and general criterias of agroecosystem health assessment are outlined, and a model for evaluating agroecosystem health is established. Finally, pathways of agroecosystem management from a holistic dimension are proposed to promote agroecosystem health and provide a scientific basis for making science-based policy decisions and formulating new plans in agricultural development. 相似文献
8.
9.
RICHARD T. REYNOLDS RUSSELL T. GRAHAM DOUGLAS A. BOYCE JR. 《The Journal of wildlife management》2008,72(4):1047-1055
Abstract: We responded to the claim by Greenwald et al. (2005) that the management recommendations for the northern goshawk in the Southwestern United States (MRNG; Reynolds et al. 1992), a food web-based conservation plan that incorporated both northern goshawk (Accipiter gentilis) and multiple prey habitats, may be inadequate to protect goshawks. Greenwald et al. (2005) based this claim on their review of 12 telemetry studies of goshawk habitat selection and 5 nontelemetry studies of the effects of vegetation structure at the home range scale on goshawk nest occupancy and reproduction that appeared after the 1992 publication of the MRNG. Greenwald et al. (2005) summarized their review as showing that 1) goshawks were habitat specialists limited to forests with mature and old-growth structures including large trees, high canopy cover, multiple canopy layering, and abundant woody debris; 2) habitats were not selected on the basis of prey abundance and, therefore, managing for prey habitats diluted goshawk habitats; and 3) selection for openings, edges, and habitat diversity was inconclusive. Our review found that when the studies' respective authors pooled their radiotagged goshawks there were weak to strong selections for old forest structures. However, the studies also documented extensive variation in use of vegetation types and structures by individual goshawks; some avoided openings, edges, young forests, and old forests, whereas others selected for these characteristics. Additionally, by virtue of their wide geographic distribution, the studies showed that the focal populations themselves occurred in a variety of forest types, some with large structural differences. We found no evidence in Greenwald's et al. (2005) review that the MRNG are inadequate to protect goshawks. Rather, the studies reviewed by Greenwald et al. (2005), as well as many studies they missed, supported the MRNG. The suggestion of inadequacy by Greenwald et al. (2005) appeared rooted in misunderstandings of goshawk habitats described in the MRNG, a discounting of the extent of variation in vegetation structural and seral stages used by goshawks, a limited understanding of the extent to which prey limits goshawks, a failure to recognize the dynamic nature of forests, and an incomplete review of the literature. We believe the MRNG are adequate because they maximize the sustainable amount of mature and old forests in goshawk home ranges and specify the kinds and intermixtures of prey habitats within home ranges. Implementation of MRNG should reduce the likelihood that the availability of vegetation structures suited to goshawk nesting and foraging, as well as abundance and availability of prey, will limit goshawk nest occupancy and reproduction. 相似文献
10.
J E Caldwell E Ahonen U Nousiainen 《Journal of applied physiology (Bethesda, Md. : 1985)》1984,57(4):1018-1023
The physiological effects on submaximal and maximal exercise of three methods commonly used by athletes for achieving rapid weight loss were determined by measuring cardiorespiratory variables in 62 nonendurance athletes. A mean weight loss of 4.1% was achieved by those who followed either a sauna (SAU), diuretic (DIU), or exercise (ACT) protocol, compared with the average weight loss of 1.2% in the control group. At maximal exercise O2 consumption, O2 pulse, blood lactate concentration, and work load decreased in SAU and DIU groups relative to the ACT group, whereas only a few differences were observed at the aerobic threshold. Weight loss achieved over a 48-h period was less detrimental to an athlete than was a more rapid (24-h) weight reduction achieved through sauna bathing or the use of diuretics. We conclude that not only the quantity of weight loss but also the method itself may limit physical performance. 相似文献