全文获取类型
收费全文 | 4277篇 |
免费 | 327篇 |
国内免费 | 2篇 |
出版年
2023年 | 17篇 |
2022年 | 45篇 |
2021年 | 91篇 |
2020年 | 68篇 |
2019年 | 70篇 |
2018年 | 63篇 |
2017年 | 69篇 |
2016年 | 125篇 |
2015年 | 172篇 |
2014年 | 199篇 |
2013年 | 263篇 |
2012年 | 332篇 |
2011年 | 316篇 |
2010年 | 189篇 |
2009年 | 150篇 |
2008年 | 258篇 |
2007年 | 216篇 |
2006年 | 198篇 |
2005年 | 197篇 |
2004年 | 178篇 |
2003年 | 172篇 |
2002年 | 149篇 |
2001年 | 50篇 |
2000年 | 40篇 |
1999年 | 36篇 |
1998年 | 41篇 |
1997年 | 32篇 |
1996年 | 21篇 |
1995年 | 27篇 |
1994年 | 29篇 |
1993年 | 38篇 |
1992年 | 19篇 |
1991年 | 30篇 |
1990年 | 19篇 |
1988年 | 27篇 |
1987年 | 18篇 |
1985年 | 27篇 |
1984年 | 25篇 |
1983年 | 30篇 |
1982年 | 20篇 |
1981年 | 29篇 |
1980年 | 23篇 |
1979年 | 23篇 |
1978年 | 20篇 |
1976年 | 32篇 |
1974年 | 29篇 |
1973年 | 32篇 |
1972年 | 18篇 |
1971年 | 17篇 |
1969年 | 19篇 |
排序方式: 共有4606条查询结果,搜索用时 31 毫秒
41.
Tetrapolar fungal mating types: Sexes by the thousands 总被引:6,自引:1,他引:5
Erika Kothe 《FEMS microbiology reviews》1996,18(1):65-87
42.
TfxA is a thermostable xylanase produced by the thermophilic soil bacterium Thermomonospora fusca. The enzyme was purified to homogeneity from the culture supernatant of Streptomyces lividans transformed by plasmid pGG92, which carries the gene for TfxA, xynA. The molecular mass of TfxA by sodium dodecyl sulfate-polyacrylamide gel electrophoresis is 32 kDa. TfxA is extremely stable, retaining 96% of its activity after 18 h at 75 degrees C. It has a broad pH optimum around pH 7 and retains 80% of its maximum activity between pH 5 and 9. The native enzyme binds strongly to both cellulose and insoluble xylan even though it has no activity on cellulose. Treatment of TfxA with a T. fusca protease produced a 24-kDa catalytically active fragment that had the same N-terminal sequence as TfxA. The fragment does not bind to cellulose and binds weakly to xylan. The Vmax values for TfxA and the fragment are 600 and 540 mumol/min/mg, respectively, while the Kms are 1.1 and 2.3 mg of xylan per ml, respectively. The DNA sequence of the xynA gene was determined, and it contains an open reading frame that codes for a 42-amino-acid (42-aa) actinomycete signal peptide followed by the 32-kDa mature protein. There is a 21-aa Gly-Pro-rich region that separates the catalytic domain from an 86-aa C-terminal binding domain. The amino acid sequence of the catalytic domain of TfxA has from 40 to 72% identity with the sequence of 12 other xylanases from seven different organisms and belongs to family G.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
43.
Robinson DK Chan CP Yu Lp C Tsai PK Tung J Seamans TC Lenny AB Lee DK Irwin J Silberklang M 《Biotechnology and bioengineering》1994,44(6):727-735
Many mammalian cell fed-batch processes rely on maintaining the cells in a viable and productive state for extended periods of time in order to reach high final concentrations of secreted protein. In the work described herein, a nonamplified NSO cell line was transfected with a vector expressing a recombinant human anti-HIV gp 120 monoclonal antibody (Mab) and a selectable marker, glutamine synthetase. A fed-batch process was developed which improved product yields tenfold over the yields reached in batch culture. In this case, the clone was cultured for a period of 22 days and produced 0.85 g Mab/L. To gauge the effect of extended culture lifetime on product quality, biochemical characteristics of MAb isolated from different time points in the fed-batch culture were determined. The apparent molecular weight of the MAb was constant throughout the course of the culture. Isoelectric focusing revealed four major charged species, with a fifth more acidic species appearing later in the culture. The antigen binding kinetics were constant for MAb isolated throughout the culture period. Glycosylation analysis, on the other hand, revealed that MAb produced later in the culture contained greater percentages of truncated N-acetylglucosamine and highmannose N-glycans. Possible contributions to this underglycosylated material from either cell lysis or synthesis from noviable cells were found to be negligible. Instead, the viable cells appeared to be secreting more truncated and high mannose MAb glycoforms as the culture progressed. (c) 1994 John Wiley & Sons, Inc. 相似文献
44.
45.
Molecular characterization of ura1, a mutant allele for orotidine-5'-monophosphate decarboxylase in Schizophyllum commune 总被引:1,自引:0,他引:1
Abstract The basis of the auxotrophic ural phenotype in Schizophyllum commune has been investigated. Two point mutations causing changes in conserved amino acid positions 62 (from lysine to glutamate) and 79 (from leucine to phenylalanine) most likely are the cause for the observed phenotype, whereas the overall gene structure was unchanged. Since reversion rates in this locus are extremely low, a single point mutation could not be expected to be the cause for the mutation. Besides the two point mutations expected to be induced by UV mutagenesis, the two alleles investigated from independently isolated strains differ by approximately 7% in nucleic acid sequence and about 3% in amino acid sequence, indicating a distant relationship between the strains used. 相似文献
46.
Inadequate prenatal care is associated with poor birth outcomes. Recognizing barriers to care is necessary to improve results. Postpartum in-hospital interviews were conducted with women admitted through emergency departments with no physician of record (n = 69) in 8 Sacramento hospitals during April and May 1991. A focus group of local obstetrician-gynecologists was used to determine physicians'' attitudes about caring for low-income women. We undertook the study in response to an increased number of "no doc" births. The inability to find a physician willing to accept them was reported by the women as the single largest barrier to obtaining care, cited by 64% of women overall and 96% of those who tried but were unable to obtain care. Transportation difficulties were a problem regardless of women''s success in obtaining care and were ranked as the top barrier by women who never tried to obtain care. Physicians cited administrative difficulties and reimbursement levels of Medi-Cal plus extra care requirements and resource dependency of low-income patients as barriers to caring for this population. The value ascribed to prenatal care by women and physicians'' perceptions of women''s attitudes about care contrasted sharply. The link between poor women and physicians providing obstetric services can be fragile. The difficulty finding physicians willing to take them indicates that these women need special support services to ensure adequate care during pregnancy. 相似文献
47.
Elasticity theory of naturally curved rods is employed to study the effects of intrinsic curvature on the properties of the equilibrium conformations of supercoiled DNA. The results stand in sharp contrast to those obtained when the molecule is viewed as being straight in its relaxed form. Starting from very fundamental principles of the theory, we show that the torsion of an open segment with a curved duplex axis can vary when the temperature, and along with it, the intrinsic twist is changed. Conversely, an imposed helicity, such as might be associated with binding to a histone, can change the intrinsic twist. It is also shown that another consequence of the presence of naturally curved sequences is that the twist density will, in general, vary with position along the chain in all equilibrium states. Then portions of the molecule will be more or less susceptible to interaction with other agents sensitive to such a variation. Finally, some closed equilibrium global structures uniquely associated with intrinsic curvature are discussed. © 1993 John Wiley & Sons, Inc. 相似文献
48.
David W. Kikuchi William L. Allen Kevin Arbuckle Thomas G. Aubier Emmanuelle S. Briolat Emily R. Burdfield-Steel Karen L. Cheney Klára Daňková Marianne Elias Liisa Hämäläinen Marie E. Herberstein Thomas J. Hossie Mathieu Joron Krushnamegh Kunte Brian C. Leavell Carita Lindstedt Ugo Lorioux-Chevalier Melanie McClure Callum F. McLellan Iliana Medina Viraj Nawge Erika Páez Arka Pal Stano Pekár Olivier Penacchio Jan Raška Tom Reader Bibiana Rojas Katja H. Rönkä Daniela C. Rößler Candy Rowe Hannah M. Rowland Arlety Roy Kaitlin A. Schaal Thomas N. Sherratt John Skelhorn Hannah R. Smart Ted Stankowich Amanda M. Stefan Kyle Summers Christopher H. Taylor Rose Thorogood Kate Umbers Anne E. Winters Justin Yeager Alice Exnerová 《Journal of evolutionary biology》2023,36(7):975-991
Prey seldom rely on a single type of antipredator defence, often using multiple defences to avoid predation. In many cases, selection in different contexts may favour the evolution of multiple defences in a prey. However, a prey may use multiple defences to protect itself during a single predator encounter. Such “defence portfolios” that defend prey against a single instance of predation are distributed across and within successive stages of the predation sequence (encounter, detection, identification, approach (attack), subjugation and consumption). We contend that at present, our understanding of defence portfolio evolution is incomplete, and seen from the fragmentary perspective of specific sensory systems (e.g., visual) or specific types of defences (especially aposematism). In this review, we aim to build a comprehensive framework for conceptualizing the evolution of multiple prey defences, beginning with hypotheses for the evolution of multiple defences in general, and defence portfolios in particular. We then examine idealized models of resource trade-offs and functional interactions between traits, along with evidence supporting them. We find that defence portfolios are constrained by resource allocation to other aspects of life history, as well as functional incompatibilities between different defences. We also find that selection is likely to favour combinations of defences that have synergistic effects on predator behaviour and prey survival. Next, we examine specific aspects of prey ecology, genetics and development, and predator cognition that modify the predictions of current hypotheses or introduce competing hypotheses. We outline schema for gathering data on the distribution of prey defences across species and geography, determining how multiple defences are produced, and testing the proximate mechanisms by which multiple prey defences impact predator behaviour. Adopting these approaches will strengthen our understanding of multiple defensive strategies. 相似文献
49.
50.
The composition of the seed bank was compared with that of the above-ground vegetation in two British salt marshes. Vegetation and seed banks were sampled in autumn and spring. There was a relatively low correlation between seed bank and vegetation in most perennial-dominated zones. Percentage annual plant cover was significantly positively correlated to percentage similarity value between vegetation and seed bank. Cluster analysis indicated that the zones dominated by annuals and by Suaeda vera on the ungrazed Nod Marsh in Norfolk formed groups between seed bank and vegetation, whereas on the Llanrhidian Marsh in Wales the seed bank of four of the six zones formed a group. 相似文献