全文获取类型
收费全文 | 2811篇 |
免费 | 249篇 |
出版年
2022年 | 20篇 |
2021年 | 52篇 |
2020年 | 17篇 |
2019年 | 25篇 |
2018年 | 36篇 |
2017年 | 38篇 |
2016年 | 87篇 |
2015年 | 132篇 |
2014年 | 149篇 |
2013年 | 156篇 |
2012年 | 238篇 |
2011年 | 227篇 |
2010年 | 165篇 |
2009年 | 141篇 |
2008年 | 164篇 |
2007年 | 169篇 |
2006年 | 150篇 |
2005年 | 135篇 |
2004年 | 167篇 |
2003年 | 139篇 |
2002年 | 137篇 |
2001年 | 41篇 |
2000年 | 33篇 |
1999年 | 36篇 |
1998年 | 42篇 |
1997年 | 23篇 |
1996年 | 21篇 |
1995年 | 20篇 |
1994年 | 22篇 |
1993年 | 24篇 |
1992年 | 21篇 |
1991年 | 18篇 |
1990年 | 17篇 |
1989年 | 25篇 |
1988年 | 16篇 |
1987年 | 9篇 |
1986年 | 13篇 |
1985年 | 8篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 16篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 14篇 |
1978年 | 10篇 |
1977年 | 7篇 |
1975年 | 6篇 |
1973年 | 6篇 |
1968年 | 5篇 |
1961年 | 5篇 |
排序方式: 共有3060条查询结果,搜索用时 31 毫秒
1.
2.
The CMP-N-acetylneuraminic acid (CMP-NeuNAc) synthetase gene of Neisseria meningitidis group B is located on a 2.3-kb EcoRI fragment within the cps gene cluster. Nucleotide sequence determination of the gene encoding the CMP-NeuNAc synthetase revealed a 515-bp open reading frame that can encode a 18.9-kDA protein. A computer data base scan revealed a 59.4% identity to the CMP-NeuNAc synthetase gene of E. coli K1. Enzymatic activity was confirmed in vitro and in vivo. Transformation of the CMP-NeuNAc defective E. coli K1 strain EV5 with the meningococcal CMP-NeuNAc synthetase could complement the defect in E. coli. 相似文献
3.
4.
Ulrike Rosenfellner Franz Zehetner Martin H. Gerzabek 《Soil & Sediment Contamination》2009,18(6):685-687
Data from eight published studies were combined to show that the influence of traffic density on Pb contents in roadside soils increases with proximity to the road. 相似文献
5.
6.
Summary Pedigree analyses of individual yeast cells recovering from DNA damage were performed and time intervals between morphological landmark events during the cell cycle (bud emergence and cell separation), were recorded for three generations. The associated nuclear behavior was monitored with the aid of DAPI staining. The following observations were made: (1) All agents tested (X-rays, MMS, EMS, MNNG, nitrous acid) delayed the first bud emergence after treatment, which indicates inhibition of the initiation of DNA replication. (2) Cells that survived X-irradiation progressed further through the cell cycle in a similar way to control cells. (3) Progress of chemically treated cells became extremely asynchronous because surviving cells stayed undivided for periods of varying length. (4) Prolongation of the time between bud emergence and cell separation was most pronounced for cells treated with the alkylating agents MMS and EMS. This is interpreted as retardation of ongoing DNA synthesis by persisting DNA adducts. (5) Cell cycle prolongation in the second and third generation after treatment was observed only with MMS treated cells. (6) In all experiments, individual cells of uniformly treated populations exhibited highly variable responses.Abbreviations DAPI
4,6-diamidino-2-phenyl-indole
- EMS
ethyl methanesulfonate
- MMS
methyl methanesulfonate
- MNNG
N-methyl-N-nitro-N-nitrosoguanidine 相似文献
7.
Summary A new genetic polymorphism of a human serum glycoprotein, the inter--trypsin-inhibitor (ITI), has been demonstrated by population and family studies. Sera were examined after neuraminidase treatment by isoelectric focusing on agarose gels followed by immunoblotting or by immunfixation with specific ITI-antiserum. Using this method, three common ITI phenotypes 1, 1–2 and 2, as well as two further rare ITI types 1–3 and 2–3 were disclosed. Genetically, these phenotypes are controlled by three allelic genes that determine a total of six phenotypes. These alleles are designated ITI*1, ITI*2 and ITI*3. The homozygous form of the third allele ITI*3 has not been found, as yet. The frequencies of ITI were examined in two population samples from Southern Germany (n=248) and from Tyrol, Austria (n=124). The gene frequencies of the common alleles ITI*1 and ITI*2 were 0.575 and 0.417, respectively, in Southern Germany, and 0.577 and 0.423, respectively, in Tyrol, Austria. The third allele ITI*3 was found only in the sample from Southern Germany, thus far, and was calculated to be 0.008. 相似文献
8.
Dr. Heribert Gras Ulrike Spörhase-Eichmann Michael Hörner Franz Killmann 《Cell and tissue research》1988,251(1):71-79
Summary The anatomical organization of the two dorsal giant fiber systems of the earthworm Lumbricus terrestris is demonstrated in whole mounts and serial-section reconstructions based on backfillings of the ventral nerve cord with cobalt chloride. Both the medial and lateral fiber systems can be labeled selectively over more than ten body segments. They show a characteristic segmental pattern of collaterals with some modification in tail segments and of dorsal plasma protrusions in the unpaired medial giant fiber presumably representing openings in the myelin sheath. We found no multisegmental cobalt transport in other large neurons of the nerve cord. Cobalt passes through the segmentai septa between consecutive axonal elements of the metameric giant fibers and presumably also through commissural contacts between specific collaterals of the lateral giant fibers. Since these sites of contact are known to represent electrical synapses, cobalt coupling may, in L. terrestris, correlate with functional electrotonic coupling.Abbreviations
CL
collateral of lateral giant fiber
-
CM
collateral of medial giant fiber
-
GIN
giant interneuron
-
LGF
lateral giant fiber
-
MGF
medial giant fiber
-
SN
segmental nerve 相似文献
9.
Ekkehard Schnbohm Ulrich Stute Peter Thienhaus Ulrike Werner 《Physiologia plantarum》1988,72(3):541-546
Schönbohm, E., Stute, U., Thienhaus, P. and Werner, U. 1988. The stimulating effect of a cold, dark pretreatment on the etioplast/chloroplast transformation of angiosperms I. The stimulating effect of cold predarkening on different stages of greening under white light. - Physiol. Plant. 72: 541–546.
The etioplast/chloroplast transformation in angiosperms is controlled by light; most of the processes are mediated by phytochrome. We have shown that in the primary leaves of etiolated seedlings of wheat ( Triticum aestivum L. cv. Kolibri), fire-bean ( Phaseolus multiflorus L. cv. Preisgewinner) and in the cotyledons of etiolated sun flower seedlings ( Helianthus annuus L. cv. macrocarpa) the chlorophyll accumulation in the phase after the end of the lag phase can be greatly stimulated by a cold predarkening period. This effect is not necessarily coupled with a red preirradiation. Furthermore the lag phase can be dramatically shortened by the cold, dark pretreatment, whereas the amount of photoconvertible protochlorophyll(ide) in the darkness remains unaffected by the cold, dark pretreatment. The stimulating effect of a cold, predarkening period on greening is fully reversible by a warm, dark phase that is placed between the cold period and the onset of the continuous white light phase. These findings cannot be generalized: We could demonstrate that in the tropical plant Momordica charantia greening under white light was not affected by different temperature pretreatments during predarkening. The stimulating effect of a cold, predarkening period on greening is assumed to have ecological relevance. 相似文献
The etioplast/chloroplast transformation in angiosperms is controlled by light; most of the processes are mediated by phytochrome. We have shown that in the primary leaves of etiolated seedlings of wheat ( Triticum aestivum L. cv. Kolibri), fire-bean ( Phaseolus multiflorus L. cv. Preisgewinner) and in the cotyledons of etiolated sun flower seedlings ( Helianthus annuus L. cv. macrocarpa) the chlorophyll accumulation in the phase after the end of the lag phase can be greatly stimulated by a cold predarkening period. This effect is not necessarily coupled with a red preirradiation. Furthermore the lag phase can be dramatically shortened by the cold, dark pretreatment, whereas the amount of photoconvertible protochlorophyll(ide) in the darkness remains unaffected by the cold, dark pretreatment. The stimulating effect of a cold, predarkening period on greening is fully reversible by a warm, dark phase that is placed between the cold period and the onset of the continuous white light phase. These findings cannot be generalized: We could demonstrate that in the tropical plant Momordica charantia greening under white light was not affected by different temperature pretreatments during predarkening. The stimulating effect of a cold, predarkening period on greening is assumed to have ecological relevance. 相似文献
10.
Abstract. The position-dependent mitotic index before, and 1, 2 and 3 h after vincristine was scored. the accumulation of cells in mitosis leads to an increase in the mitotic index from 0.06 to 0.34 at crypt positions 8-12. Surprisingly, the leading edge of the position-related mitotic index distribution moves to higher crypt positions although cell division was stopped. In addition, the vertical clustering of mitotic figures in sections was recorded. the data were examined using a previously described computer crypt model. We conclude: the average mitotic phase duration is about 0.7 h (40 min) and varies little with cell position; the geometrical correction factor for overscoring mitoses in crypt sections is about 0.6-0.7 and adjacent cell columns can merge. Lateral cell displacement after mitosis, as predicted in a previous model analysis, would be a mechanism to counteract other forces that tend to reduce the crypt circumference. In the normal steady state merging and expansion processes would just balance each other. This would not follow if one mechanism was blocked. Thus we propose a new concept in which the crypt geometry would be dynamically determined by cell proliferative activity in connection with lateral positioning of new cells on one hand and contracting forces on the other hand. 相似文献