全文获取类型
收费全文 | 19866篇 |
免费 | 1157篇 |
国内免费 | 2085篇 |
出版年
2024年 | 60篇 |
2023年 | 440篇 |
2022年 | 514篇 |
2021年 | 623篇 |
2020年 | 697篇 |
2019年 | 1022篇 |
2018年 | 766篇 |
2017年 | 784篇 |
2016年 | 769篇 |
2015年 | 612篇 |
2014年 | 1103篇 |
2013年 | 1970篇 |
2012年 | 687篇 |
2011年 | 1115篇 |
2010年 | 728篇 |
2009年 | 1087篇 |
2008年 | 1084篇 |
2007年 | 1052篇 |
2006年 | 1010篇 |
2005年 | 814篇 |
2004年 | 727篇 |
2003年 | 623篇 |
2002年 | 519篇 |
2001年 | 381篇 |
2000年 | 311篇 |
1999年 | 311篇 |
1998年 | 257篇 |
1997年 | 235篇 |
1996年 | 245篇 |
1995年 | 239篇 |
1994年 | 230篇 |
1993年 | 213篇 |
1992年 | 222篇 |
1991年 | 167篇 |
1990年 | 154篇 |
1989年 | 120篇 |
1988年 | 121篇 |
1987年 | 129篇 |
1986年 | 84篇 |
1985年 | 132篇 |
1984年 | 144篇 |
1983年 | 79篇 |
1982年 | 114篇 |
1981年 | 79篇 |
1980年 | 85篇 |
1979年 | 68篇 |
1978年 | 34篇 |
1977年 | 38篇 |
1976年 | 40篇 |
1973年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
61.
B. Friebe M. C. Cermeño F. J. Zeller 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,73(3):337-342
Summary C-banding patterns and nucleolar activity were analyzed in Dasypyrum villosum, its added chromosomes to hexaploid wheat and the hexaploid amphiploid Triticum dicoccum-D. villosum. Two different populations of the allogamous species D. villosum (2n= 14, VV) from Greece and Italy were analyzed showing a similar polymorphism for C-banding pattern. Six of the seven addition lines were identified by their characteristic C-banding pattern. No polymorphism between both members of each added alien chromosome was found. Furthermore, nucleolar activity and competition were studied by using silver staining procedure. In D. villosum only one chromosome pair, A, was found to be responsible for organizing nucleoli. The results obtained in the amphiploid and in the addition lines demonstrate that nucleolar activity is restricted to SAT-chromosomes 1B and 6B of wheat, while those of D. villosum remain inactive. 相似文献
62.
Yan Yongshan Fen Shang Liu Lianrui 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,74(1):147-153
Summary A 8.3 /ml 6-thioguanine (6TG)-resistant strain was isolated from a rat tetraploid cell line by step-by-step selection in 6TG-medium. In the 6TG-resistant cell population 51% of the cells were tetraploid and 35% of the cells were hypertetraploid, i.e., one chromosome more than a tetraploid. The 6TG-resistant strain grew very well in RPMI 1640 medium with intervals of three days between subcultures. The 6TG-resistant cells all have a homogeneously staining region (HSRs) in one of the X chromosomes which do not stain after chromosome C-banding. They also possess a higher NORs activity and much lower frequency of sister chromatid exchanges (SCE). When the 6TG-resistant RCT cells were subcultured in 6TG-free medium for three days, their SCE frequency did not change. 5-bromodeoxyuridine (BrdU) significantly suppressed the NORs activity for both 6TG-resistant cells and 6TG-sensitive cells (P<0.001).Abbreviations 6TG
6-thioguanine
- HSRs
homogeneously staining region
- NORs
nucleolar organizer region
- SCE
sister chromatid exchange
- BrdU
5-bromodeoxyuridine
- HPRT
Hypoxanthine phosphoribosyl transferase 相似文献
63.
Relationship between melanin content and superoxide dismutase (SOD) activity in the liver of various species of animals 总被引:4,自引:0,他引:4
The scavenger effect of melanin and of superoxide dismutase (SOD) activity on superoxide anion has been shown. In this work we show the relationship between melanin content and SOD activity in livers containing different quantities of melanin which were taken from various species of animals. The mitochondrial SOD activity disappears when the melanin content in the liver is very high; moreover it increases, in the liver of various species of animals examined, proportionally to the decrease of melanin content. No significant variation of the SOD activity localized in the soluble fraction has been detected when related to the melanin content. We think that in the pigmented liver the antioxidant activity of the melanin could mimic part of the function of SOD. The loss of Mn SOD activity could be mediated by a low intracellular level of superoxide anion due to the scavenger effect of melanin on superoxide anion; in fact, it is well known that the biosynthesis of Mn SOD is induced by intracellular levels of superoxide anion. 相似文献
64.
Arthur L. Weber 《Journal of molecular evolution》1987,25(1):7-11
Summary Reaction ofDl-serine and adenosine-5-phosphorimidazolide in the presence of adenosine-5-(O-methylphosphate) and imidazole resulted in the stereoselective synthesis of the aminoacyl nucleotide ester 2(3)-O-seryl-adenosine-5-(O-methylphosphate). The enantiomeric excess ofd-serine incorporated into 2(3)-O-seryl-adenosine-5-(O-methylphosphate) was about 9%. Adenylyl-(5N)-serine and an unknown product also incorporated an excess ofd-serine; however, serylserine showed an excess ofl-serine. The relationship of these results to the origin of the biological pairing ofl-amino acids and nucleotides containingd-ribose is discussed. 相似文献
65.
66.
The relationship between nitrification potential and nitrogen accumulation was studied in an early successional sere on Mt.
Fuji. Soil organic nitrogen accumulated with the invasion ofPolygonum cuspidatum and successively withMiscanthus oligostachyus and other species. Laboratory incubation experiments showed a higher nitrification potential at theM. oligostachyus state. The numbers of nitrifying bacteria increased with the progress of succession. No significant difference in nitrate
reductase activity was found between pioneer and succeeding species. The soil solution at theM. oligostachyus stage contained a lower level of nitrate than rainwater, while that of the bare ground and theP. cuspidatum stage contained a higher nitrate level than rainwater. It was concluded that the high nitrate levels in the soil solution
of the bare ground and theP. cuspidatum stage were due to lower nitrate-absorbing activity, leading to loss of nitrogen with precipitation, while the lower nitrate
levels at theM. oligostachyus stage when higher nitrification activity occurred were due to higher nitrate-absorbing activity, preventing net loss of nitrogen
from the ecosystem. 相似文献
67.
The distribution of dens and den use by the red fox was studied in Kumamoto Prefecture between 1968 and 1982. A total of 80
dens were classified into natal, residential, and temporarily-visited dens. Natal dens comprised 12.4% of all dens. Relative
proportions were quite stable for a long period. This indicates a constant number of reproducing females in the area for a
considerable length of time. Seasonally the utilization of den showed three peaks in February, May and October. The peaks
coincided well with the peaks of reproductive activity of female foxes. The results of multiple regression analysis of distribution
of dens with respect to selected environmental parameters around densites indicated that the fox selectively utilize open
land, rather than densely vegetated areas. The persistence of dens, the selective utilization by foxes of open land and the
sizes of dens were all related to the breeding activities of female foxes inhabiting the area. Reproductive females usually
used plural dens (“den group”) in one reproductive season. The numbers of those females occuring in the area were maintained
at a steady level for fairy long periods. These indicate that the existence of a stable social relationship between females
mediated through occupancy of “den groups.” 相似文献
68.
Summary This paper describes the ovarian pathologies observed when 108 different heteroallelic combinations were made involving 17 independent mutations at the ovarian tumor (otu) locus. Most of the mutant phenotypes can be explained as graded responses by individual germ cells to different levels of functionally active otu gene product (OGP) synthesized by the mutant cells themselves. The lowest and highest levels of OGP appear to be produced by otu
10 and otu
14, respectively. In most heteroallelic ovaries the alleles have additive effects, and hybrid germ cells reach a developmental stage more advanced than the weaker homozygote but less advanced than the stronger homozygote. However, examples of both positive and negative complementation also have been found, and these suggest that the products encoded by different mutant alleles can combine to form dimers or multimers which may be superior or inferior to the homodimers. In flies homozygous for otu
11 most ovarioles contain tumors, but some germ cells are able to develop further than those in otu
14 homozygotes. This suggests that, while otu
11 produces intermediate levels of OGP, it also produces a second product (which otu
14 cannot make) that is utilized at the period in oogenesis when development in cells homozygous for otu
14 is blocked. When otu
11 is combined with any one of eight specific alleles, it allows oocyte/nurse cell syncytia to differentiate that can complete development and undergo embryogenesis, if fertilized. The endopolyploid nurse cells of these hybrids have giant polytene chromosomes, and the presence of GPCs in functionally active, germ-line derived cells provides an interesting new system for experimental study. Analysis of the characteristic ovarian pathologies produced by flies of different genotypes leads to the conclusion that the products of the otu
+ gene are utilized during at least six different periods in Drosophila oogenesis. 相似文献
69.
Mohammad A. Hafeez Horst -W. Korf Professor Andreas Oksche 《Cell and tissue research》1987,250(3):571-578
Summary Lacertilian species display a remarkable diversity in the organization of the neural apparatus of their pineal organ (epiphysis cerebri). The occurrence of immunoreactive S-antigen and opsin was investigated in the retina and pineal organ of adult lizards, Uromastix hardwicki. In this species, numerous retinal photoreceptors displayed S-antigen-like immunoreactivity, whereas only very few pinealocytes were labeled. Immunoreactive opsin was found neither in retinal photoreceptors nor in pinealocytes. Electron microscopy showed that all pinealocytes of Uromastix hardwicki resemble modified pineal photoreceptors. A peculiar observation is the existence of a previously undescribed membrane system in the inner segments of these cells. It is evidently derived from the rough endoplasmic reticulum but consists of smooth membranes. The modified pineal photoreceptor cells of Uromastix hardwicki were never seen to establish synaptic contacts with somata or dendrites of intrapineal neurons, which are extremely rare. Vesiclecrowned ribbons are prominent in the basal processes of the receptor cells, facing the basal lamina or establishing receptor-receptor and receptor-interstitial type synaptoid contacts. Dense-core granules (60–250 nm in diameter) speak in favor of a secretory activity of the pinealocytes. Attention is drawn to the existence of receptor-receptor and receptor-interstitial cell contacts indicating intramural cellular relationships that deserve further study.Supported by the Deutsche Forschungsgemeinschaft (Ko 758/31) and the Deutscher Akademischer Austauschdienst (Senior DAAD Research Fellowship to M.A.H.) 相似文献
70.
Calvin E. Olson 《Chronobiology international》1987,4(1):19-29
Several models of erosive peptic disease have used drug-induced lesions to examine protective mechanisms of the gastric mucosa. Physiological processes such as acid secretion, motility, or epithelial cell turnover have circadian rhythms which may modulate the susceptibility of the gastric mucosa to injury. In this review are described recent studies which demonstrated that susceptibility to gastric mucosal injury by acidified aspirin and absolute ethanol varied with the phases of the light-dark cycle. Acidified aspirin caused significantly more gastric mucosal lesions when administered early in the light phase compared to administration early in the dark phase. The differences in susceptibility were not altered by pretreatment conditions such as immobilization or length of the fasting period. Absolute ethanol also caused significantly greater gastric mucosal injury when administered in the light than in the dark phase, but this difference was only evident in rats immobilized during the pretreatment fasting period. Further studies are needed to correlate circadian susceptibility to drug-induced gastric mucosal injury with physiological defense mechanisms. Careful attention to circadian timekeeping may allow us to refine therapy to optimize physiological defense mechanisms in the stomach. 相似文献