全文获取类型
收费全文 | 19335篇 |
免费 | 1291篇 |
国内免费 | 5篇 |
出版年
2023年 | 94篇 |
2022年 | 224篇 |
2021年 | 415篇 |
2020年 | 277篇 |
2019年 | 381篇 |
2018年 | 454篇 |
2017年 | 408篇 |
2016年 | 652篇 |
2015年 | 997篇 |
2014年 | 1074篇 |
2013年 | 1336篇 |
2012年 | 1674篇 |
2011年 | 1556篇 |
2010年 | 982篇 |
2009年 | 905篇 |
2008年 | 1172篇 |
2007年 | 1115篇 |
2006年 | 1058篇 |
2005年 | 947篇 |
2004年 | 881篇 |
2003年 | 828篇 |
2002年 | 748篇 |
2001年 | 155篇 |
2000年 | 116篇 |
1999年 | 146篇 |
1998年 | 176篇 |
1997年 | 162篇 |
1996年 | 160篇 |
1995年 | 119篇 |
1994年 | 132篇 |
1993年 | 114篇 |
1992年 | 98篇 |
1991年 | 91篇 |
1990年 | 68篇 |
1989年 | 80篇 |
1988年 | 54篇 |
1987年 | 52篇 |
1986年 | 50篇 |
1985年 | 63篇 |
1984年 | 63篇 |
1983年 | 51篇 |
1982年 | 55篇 |
1981年 | 62篇 |
1980年 | 48篇 |
1979年 | 43篇 |
1978年 | 39篇 |
1977年 | 35篇 |
1975年 | 32篇 |
1974年 | 25篇 |
1973年 | 34篇 |
排序方式: 共有10000条查询结果,搜索用时 375 毫秒
81.
Pere Santamaria Michael T. Boyce-Jacino Alan L. Lindstrom Jose J. Barbosa Anthony J. Faras Stephen S. Rich 《Immunogenetics》1991,33(5-6):374-387
Nucleic acid sequences of the second exons of HLA-DRB1, –DRB3/4/5, –DQB1, and –DQA1 genes were determined from 43 homozygous cell lines, representing each of the known class II haplotypes, and from 30 unrelated Caucasian subjects, comprising 60 haplotypes. This systematic sequence analysis was undertaken in order to a) determine the existence of sequence microheterogeneity among cell lines which type as identical by methods other than sequencing; b) determine whether direct sequencing of class II genes will identify the presence of more extensive sequence polymorphism at the population level than that identified with other typing methods; c) accurately determine the molecular composition of the known class II haplotypes; and d) study their evolutionary relatedness by maximum parsimony analysis. The identification of seven previously unidentified haplotypes carrying five new allelic amino acid sequences suggests that sequence microheterogeneity at the population level may be more frequent than previously thought. Maximum parsimony analysis of these haplotypes allowed their evolutionary classification and indicates that the higher mutation rate at DRB1 compared to DQB1 loci in most haplotypic groups is inversed in specific haplotype lineages. Furthermore, the extent and localization of gene conversions and point mutations at class II loci in the evolution of these haplotypes is significantly different at each locus. Identification of additional HLA class II molecular microheterogeneity suggests that direct sequence analysis of class II HLA genes can uncover new allelic sequences in the population and may represent a useful alternative to current typing methodologies to study the effects of sequence allelism in organ transplantation.The nucleotide sequence data reported in this paper have been submitted to the GenBank nucleotide sequence database and have been assigned the accession numbers M35890 through M35953. 相似文献
82.
Summary Previous studies examining the regulation of the synthesis of G6PDH and 6PGDH in rat liver and adipose tissue have focused on the induction of these enzymes by different diets and some hormones. In rat liver these enzymatic activities seem to be regulated by a mechanism involving changes in the NADPH requirements. In this paper we have studied the effect of changes in the flux through different NADPH-consuming pathways on G6PDH and 6PGDH levels in adipose tissue and on the NADPH/NADP ratio. The results show that: I) an increase in the consumption of NADPH, caused by the activation of either fatty acid synthesis or detoxification systems which consume NADPH, is paralleled by an increase in the levels of these enzymes; II) when the increase in consumption of NADPH is prevented, the G6PDH and 6PGDH levels do not change.Abbreviations G6PDH
Glucose-6-Phosphate Dehydrogenase
- 6PGDH
6-Phosphogluconate Dehydrogenase
- GR
Glutathione Reductase
- ME
Malic Enzyme
- tBHP
t-Butyl Hydroperoxide
- NF
Nitrofurantoin
- CumOOH
Cumene Hydroperoxide 相似文献
83.
Rafael Balaña Fouce Maria I. Escribano Jose M. Alunda 《Molecular and cellular biochemistry》1991,107(2):127-133
Summary The putrescine uptake/efflux regulation and their regulatory role on intracellular polyamine pools have been studied in the parasitic protozoa Leishmania infantum. Putrescine uptake was age-dependent with maximal values in logarithmic phase promastigotes and minimal in stationary phase. Moreover, putrescine uptake was activated in response to depletion of intracellular polyamines by alpha-difluoromethylornithine (DFMO) — a well known irreversible enzyme-activated inhibitor of ornithine decarboxylase. Kinetic studies of putrescine uptake induction showed a notable rise in Vmax without Km changes, suggesting a de novo synthesis of putrescine carriers. Putrescine uptake was able to replenish polyamine content and also to recover the proliferative rate in cells treated during 24 hours with DFMO. 相似文献
84.
85.
Fiorella D'Anna Michele De Luca Ranieri Cancedda Antonio Zicca Adriano T. Franzi 《The Histochemical journal》1988,20(12):674-678
Summary Human epidermal keratinocytes grown in culture and at different stages of differentiation are shown to be viably separated by elutriation. A specific fraction enriched in melanocytes was obtained. Elutriation of cells obtained fromin vitro cultured epithlium could prove useful in studies concerning the biochemistry and molecular markers of cells isolated from normal epithelium and from different pathologies. 相似文献
86.
Maria Jose Gil Quilez 《Hydrobiologia》1986,143(1):209-212
Siliceous remains from Miocene lacustrine sediments near Libros (Teruel, Spain) are studied. Most of them are sponge spicules and may be assigned to Ephydatia fluviatilis. Some chrysophycean cysts and several diatom genera (Melosira, Cyclotella, Fragilaria, Navicula, Pinnularia, and Cymbella) have also been found. 相似文献
87.
Jose M. Martinez-Zapater Mark A. Estelle Chris R. Somerville 《Molecular & general genetics : MGG》1986,204(3):417-423
Summary Three members of a family of highly repeated DNA sequences from Arabidopsis thaliana have been cloned and characterized. The repeat unit has an average length of 180 bp and is tandemly repeated in arrays longer than 50 kb. This family represents more than one percent of the Arabidopsis genome. Sequence comparisons with tandemly repeated DNA sequences from other Cruciferae species show several regions of homology and a similar length of the repeat unit. Homologies are also found to highly repeated sequences from other plant species. When the sequence CCGG occurs in the repeated DNA, the inner cytosine is generally methylated. 相似文献
88.
Antonio Fernández Tiburcio Ravindar Kaur-Sawhney Arthur W. Galston 《Plant physiology》1986,82(2):375-378
We have attempted to improve the viability of cereal mesophyll protoplasts by pretreatment of leaves with dl-α-difluoromethylarginine (DFMA), a specific `suicide' inhibitor of the enzyme (arginine decarboxylase) responsible for their osmotically induced putrescine accumulation. Leaf pretreatment with DFMA before a 6 hour osmotic shock caused a 45% decrease of putrescine and a 2-fold increase of spermine titer. After 136 hours of osmotic stress, putrescine titer in DFMA-pretreated leaves increased by only 50%, but spermidine and spermine titers increased dramatically by 3.2- and 6-fold, respectively. These increases in higher polyamines could account for the reduced chlorophyll loss and enhanced ability of pretreated leaves to incorporate tritiated thymidine, uridine, and leucine into macromolecules. Pretreatment with DFMA significantly improved the overall viability of the protoplasts isolated from these leaves. The results support the view that the osmotically induced rise in putrescine and blockage of its conversion to higher polyamines may contribute to the lack of sustained cell division in cereal mesophyll protoplasts, although other undefined factors must also play a major role. 相似文献
89.
Abstract cAMP-dependent phosphoprotein changes were determined using 1-dimensional SDS-gel electrophoresis in a cAMP-requiring yeast mutant ( Saccharomyces cerevisiae AM18). During cAMP starvation, the yeast cells accumulated 3 32 P-labeled bands with M r / 72000, 54000, and 37000. The M r / 72000 protein was the most prominent phosphorylated protein. After the readdition of cAMP, these phosphoproteins lost their 32 P-label while phosphoproteins with M r / 76000, 65000, 56000 and 30000 were accumulated. Similar phosphoprotein changes were also detected in cdc35 at the nonpermissive temperature, but not in wildtype (A363A) or cdc7 strains of S. cerevisiae . 相似文献
90.
Cartesio Favalli Teresa Jezzi Antonio Mastino Cristina Rinaldi-Garaci Carlo Riccardi Enrico Garaci 《Cancer immunology, immunotherapy : CII》1985,20(3):189-192
Summary A single injection of -interferon (-IFN) (30 000 units/mouse), a major biological modifier of natural killer (NK) cytolytic activity, strongly stimulated NK activity in normal mice, as expected, while the same treatment did not statistically alter the NK response in cyclophosphamide (CY)-suppressed animals.We investigated the possibility of thymosin 1 cooperating with -IFN in boosting NK activity in CY-suppressed animals.The results show that treatment with thymosin 1 (200 g/kg) for 4 days, followed by a single injection of -IFN 24 h before testing, strongly restored NK activity in CY-suppressed mice. Thymosin 1 was, moreover, able to accelerate the recovery rate of NK activity in bone marrow reconstituted murine chimeras.Taken together the data support the concept that the synergic effect between thymosin 1 and -IFN could be the result of effects on differentiation of the NK lineage at different levels. 相似文献