全文获取类型
收费全文 | 5747篇 |
免费 | 553篇 |
国内免费 | 3篇 |
出版年
2022年 | 50篇 |
2021年 | 132篇 |
2020年 | 54篇 |
2019年 | 89篇 |
2018年 | 87篇 |
2017年 | 72篇 |
2016年 | 139篇 |
2015年 | 202篇 |
2014年 | 229篇 |
2013年 | 321篇 |
2012年 | 379篇 |
2011年 | 358篇 |
2010年 | 265篇 |
2009年 | 198篇 |
2008年 | 289篇 |
2007年 | 254篇 |
2006年 | 254篇 |
2005年 | 236篇 |
2004年 | 237篇 |
2003年 | 198篇 |
2002年 | 217篇 |
2001年 | 97篇 |
2000年 | 98篇 |
1999年 | 106篇 |
1998年 | 57篇 |
1997年 | 56篇 |
1996年 | 53篇 |
1995年 | 54篇 |
1994年 | 44篇 |
1993年 | 62篇 |
1992年 | 58篇 |
1991年 | 63篇 |
1990年 | 66篇 |
1989年 | 65篇 |
1988年 | 52篇 |
1987年 | 58篇 |
1986年 | 68篇 |
1985年 | 73篇 |
1984年 | 49篇 |
1983年 | 43篇 |
1982年 | 37篇 |
1980年 | 50篇 |
1979年 | 55篇 |
1978年 | 38篇 |
1977年 | 31篇 |
1976年 | 41篇 |
1975年 | 41篇 |
1974年 | 36篇 |
1973年 | 44篇 |
1972年 | 37篇 |
排序方式: 共有6303条查询结果,搜索用时 15 毫秒
51.
Effects of recombinant plasmid content on growth properties and cloned gene product formation in Escherichia coli 总被引:1,自引:0,他引:1
Plasmid-host cell interactions have been investigated experimentally using Escherichia coli HB101, plasmid RSF1050 which contains the origin of replication of pMB1, and four other closely related copy number mutant plasmids. Growth characteristics of these recombinant strains and beta-lactamase activity expressed from a plasmid gene were investigated in Luria broth (LB) and in minimal medium (M9) containing in some cases casamino acids or different concentrations of alpha-methylglucoside, a competitive inhibitor of glucose transport. Maximum specific growth rates in LB and minimal media were reduced for increasing plasmid content per cell. Plasmid copy number increased when specific growth rate was reduced by changing medium composition. Growth rates of high copy number strains were less sensitive to alpha-methylglucoside than lower copy number strains and the plasmidfree host. The overall efficiency of plasmid gene expression, measured as the ratio of beta-lactamase specific activity to plasmid content, decreased significantly with increasing plasmid content in LB medium. 相似文献
52.
Nutrient deprivation was used to synchronize an immobilized live cell culture of Acetobacter suboxydans. The substrate supply was increased by a step change in the dilution rate to the reactor. Oscillations in cell, substrate, and product concentrations were observed. A population balance model was developed to explain the observed reactor dynamics. Simulation results based on the model were used to substantiate the premise that cell synchrony is the likely phenomenon responsible for the observed oscillations. The implications of cell synchrony in immobilized cell systems are discussed briefly. 相似文献
53.
Plasmid propagation in populations of unstable, binary fission recombinant organisms has been studied using a segregated, population balance mathematical model. Segregated models have the advantage of direct incorporation of basic information on mechanisms and kinetics of plasmid replication and segregation at the single-cell level. The distribution of cellular plasmid content and specific rates of plasmid gene expression have been obtained for several single-cell models of plasmid replication, partition, and gene expression. Plasmid replication kinetics during cell growth significantly influence the plasmid content distribution. In the case of transient growth of plasmid-containing and plasmid-free cells in partially selective medium, the degree of selection required for stable maintenance of plasmid-containing cells has been determined. Guidelines are presented for applicability of simpler, nonsegregated models and for evaluation of the parameters in these models based on single-cell mechanisms and associated parameters. 相似文献
54.
55.
56.
Summary The distribution of nematophagous fungi in soil collected from a deciduous woodland is compared to various biotic and abiotic soil factors. The microfungi are isolated at all depths down to a maximum of 35 cm. Predators forming constricting rings, adhesive branches and adhesive knobs are restricted to the upper litter and humus layers. The net forming predators and endoparasites are isolated at all depths, although they are significantly more abundant in the lower mineral rich soils. A much greater species diversity of nematophagous fungi is recorded in the upper organic zones.Preliminary soil analysis indicates thatCephalosporium balanoides is independent of all soil variables, while predators able to form traps spontaneously are restricted to the organic soils which are rich in nematodes. Non-spontaneous trap forming predators, which are excellent saprophytes, are isolated from the deeper soils which are low in nutrients. The ecological significance of these results is discussed. 相似文献
57.
Experiments were conducted using twin-well calorimetry to determine the averaged whole-body specific absorption rate (SAR) for rat carcasses exposed to 360, 700, 915, and 2,450 MHz CW radiation in an anechoic chamber. All exposures were done with the long axis of the rat in an E-polarization. Additional experiments were conducted using a fiber optical temperature probe to determine local SAR in the brain, esophagus, colon, rectum, and tail during microwave exposure. The whole-body averaged SAR for the radiation frequencies examined follows a nonmonotonic function with 700 MHz as the resonant frequency. This result agrees with previous analytical estimates. Local SARs within the body and tail are nonuniform with significant frequency-specific hotspots in the colon, rectum, and tail. 相似文献
58.
Evidence for glucose-mediated covalent cross-linking of collagen after glycosylation in vitro. 总被引:1,自引:0,他引:1
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Rabbit forelimb tendons incubated for 15 or 21 days at 35 degrees C in the presence of 8 or 24 mg of glucose/ml were shown to change their chemical, biochemical and mechanical characteristics. The tendons treated with glucose contained up to three times as much hexosyl-lysine and hexosylhydroxylysine as did control tendons as judged by assay of NaB3H4-reduced samples. Measurement of the force generated on thermal contraction showed significant increases in glycosylated tendons compared with controls, indicating the formation of new covalent stabilizing bonds. This conclusion was supported by the decreased solubility of intact tendons and re-formed fibres glycosylated in vitro, and by the evidence from peptide maps of CNBr-digested glucose-incubated tendons. The latter, when compared with peptide maps of control tendons, revealed the presence of additional high-Mr peptide material. These peptides appear to be cross-linked by a new type of covalent bond stable to mild thermal and chemical treatment. This system in vitro provides a readily controlled model for the study of the chemistry of changes brought about in collagen by non-enzymic glycosylation in diabetes. 相似文献
59.
Effect of ARS1 mutations on chromosome stability in Saccharomyces cerevisiae. 总被引:30,自引:12,他引:18
下载免费PDF全文
![点击此处可从《Molecular and cellular biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We have used a set of deletion mutations in the ARS1 element of Saccharomyces cerevisiae to measure their effect on chromosome stability. This work establishes the previously proposed existence of three domains in ARS1. Domain C, which we have previously inferred, but not proved, to be a part of ARS1, is now established. In addition, we show that increasingly large deletions of the domain have increasingly large effects, which was not realized before. Furthermore, we have provided the first positive evidence for the central importance of a 14-base-pair core sequence containing the ARS consensus element by showing that it has the ability to act as a replicator on a plasmid containing no other ARS1 flanking sequence. The method of analyzing plasmid stability used in our study employs a novel and sensitive flow cytometry assay for beta-galactosidase. We discuss ways in which flow cytometry, based on this assay, could be generalized beyond its particular application in this work to studying other aspects of the cell biology of yeast and higher cells. The actual flow cytometry method will be described in detail elsewhere. 相似文献
60.
It is sometimes necessary to identify eitherH. bulbosum orH. murinum on the basis of the inflorescence or seeds alone. The majority of taxonomic keys use the presence of swollen basal culms for the former against the annual habit for the latter. Confusion is due to similarities in inflorescences and spikelet morphology. Lodicules which always persist and are present beside the fruit in a mature caryopsis, and other characters such as the awns of the lemmas of the lateral spikelets enable conclusive distinction. 相似文献