全文获取类型
收费全文 | 12709篇 |
免费 | 718篇 |
国内免费 | 1991篇 |
专业分类
15418篇 |
出版年
2024年 | 24篇 |
2023年 | 211篇 |
2022年 | 327篇 |
2021年 | 403篇 |
2020年 | 338篇 |
2019年 | 442篇 |
2018年 | 385篇 |
2017年 | 341篇 |
2016年 | 337篇 |
2015年 | 398篇 |
2014年 | 673篇 |
2013年 | 730篇 |
2012年 | 523篇 |
2011年 | 726篇 |
2010年 | 628篇 |
2009年 | 712篇 |
2008年 | 755篇 |
2007年 | 733篇 |
2006年 | 666篇 |
2005年 | 616篇 |
2004年 | 569篇 |
2003年 | 550篇 |
2002年 | 412篇 |
2001年 | 353篇 |
2000年 | 329篇 |
1999年 | 286篇 |
1998年 | 276篇 |
1997年 | 241篇 |
1996年 | 212篇 |
1995年 | 225篇 |
1994年 | 238篇 |
1993年 | 208篇 |
1992年 | 210篇 |
1991年 | 139篇 |
1990年 | 138篇 |
1989年 | 122篇 |
1988年 | 93篇 |
1987年 | 95篇 |
1986年 | 81篇 |
1985年 | 99篇 |
1984年 | 72篇 |
1983年 | 66篇 |
1982年 | 88篇 |
1981年 | 52篇 |
1980年 | 58篇 |
1979年 | 60篇 |
1978年 | 64篇 |
1977年 | 25篇 |
1976年 | 28篇 |
1974年 | 22篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Summary Within the chromosome of the archaebacterium Sulfolobus sp. B12, a 7.4 kb region was identified which displayed extensive sequence similarities to the 15.5 kb genetic element SSV1 carried by the same strain both as a circular form and as a site-specifically integrated copy. DNA sequence analysis indicated that this 7.4 kb region (designated SSV1intB) represented an SSV1-like element distinguishable from the full-length integrated copy (designated SSV1intA) by extensive deletions and point mutations. The physical organization of DNA sequences of SSV1intB indicated that this element was integrated at the same attP site as previously identified for SSV1intA. A comparison of the DNA sequences at the left attachment sites of SSV1intA and SSV1intB revealed that they both represented very similar putative arginine tRNA genes followed by a 10 by inverted repeat sequence. S1 nuclease mapping experiments indicated that these tRNA genes are transcribed.
Offprint requests to: W. Zillig 相似文献
82.
It is possible to replace the CaMV (cauliflower mosaic virus) ORF (open reading frame) II with foreign sequences without interfering with virus viability. Such recom-binants can induce the synthesis of substantial amounts of a foreign protein in infected plants and confer new properties to these plants. However, so far only three genes have been successfully cloned and expressed in this way. The expression mechanism of CaMV demands precise replacement of ORF II and probably certain structural features of the viral 35S RNA, which should not be disturbed by inserted sequences. Since these features are largely unknown, it cannot at present be pre-dicted whether an insert will be tolerated. It is more likely that larger inserts will disturb the viral gene expression mechanism than smaller ones. 相似文献
83.
Linkage mapping of genes for resistance to leaf,stem and stripe rusts and ω-secalins on the short arm of rye chromosome 1R 总被引:1,自引:0,他引:1
N. K. Singh K. W. Shepherd R. A. McIntosh 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1990,80(5):609-616
Summary The genes controlling resistance to three wheat rusts, viz., leaf rust (Lr26), stem rust (Sr31) and stripe or yellow rust (Yr9), and -secalins (Sec1), located on the short arm of rye chromosome 1R, were mapped with respect to each other and the centromere. Analysis of 214 seeds (or families derived from them) from testcrosses between a 1BL.1RS/1R heterozygote and Chinese Spring ditelocentric 1BL showed no recombination between the genes for resistance to the three rusts, suggesting very tight linkage or perhaps a single complex locus conferring resistance to the three rusts. The rust resistance genes were located 5.4 ± 1.7 cM from the Sec1 locus, which in turn was located 26.1 ± 4.3 cM from the centromere; the gene order being centromere — Sec1 — Lr26/Sr31/Yr9 — telomere. In a second test-cross, using a different 1BL.1RS translocation which had only stem rust resistance (SrR), the above gene order was confirmed despite a very large proportion of aneuploids (45.8%) among the progeny. Furthermore, a map distance of 16.0 ± 4.8 cM was estimated for SrR and the telomeric heterochromatin (C-band) on 1RS. These results suggest that a very small segment of 1RS chromatin is required to maintain resistance to all three wheat rusts. It should be possible but difficult to separate the rust resistance genes from the secalin gene(s), which are thought to contribute to dough stickiness of wheat-rye translocation lines carrying 1RS. 相似文献
84.
The present study was undertaken to investigate the role of calcium ions (Ca2+) in the induction and secretion of the dengue type 2 virus induced cytotoxic factor and the cytotoxin. This was done by using
calcium channel blocking drugs such as verapamil, nifedipine or diltiazem hydrochloride. The production of cytotoxic factor
was significantly reduced by treatment of dengue type 2 virus infected mice with verapamil. Similarly, a dosedependent inhibition
of the secretion of cytotoxic factor was observed, when spleen cells of the virus-primed mice were treatedin vitro with the 3 calcium channel blockers. The production of cytotoxin by macrophages was abrogated by pretreatment with calcium
channel blockers but had little effect on its secretion as shown by treatment of macrophages with verapamil at 1 h after the
induction to later periods up to 18 h. The findings thus show that in the induction of both the cytokines Ca2+ plays a critical role; on the other hand it is required for the secretion of the cytotoxic factor but not for that of the
cytotoxin. 相似文献
85.
86.
87.
88.
Shlomo Nir Nejat Düzgünes Maria C. Pedroso De Lima Dick Hoekstra 《Cell biochemistry and biophysics》1990,17(2):181-201
The fusion of viruses with cells and liposomes is reviewed with focus on the analysis of the final extents and kinetics of
fusion.Influenza virus andSendai virus exhibit 100% of fusion capacity with cells at pH 5 and pH 7.5, respectively. On the other hand, there may be in certain
cases, a limit on the number of virions that can fuse with a single cell, that is significantly below the limit on binding.
It still remains to be resolved whether this limit reflects a limited number of possible fusion sites, or a saturation limit
on the amount of viral glycoproteins that can be incorporated in the cellular membrane, like the case of virus fusion with
pure phospholipid vesicles, in which the fusion products were shown to consist of a single virus and several liposomes. Both
viruses demonstrate incomplete fusion activity towards liposomes of a variety of compositions. In the case ofSendai virus, fusion inactive virions bind essentially irreversibly to liposomes. Yet, preliminary results revealed that such bound,
unfused virions can be released by sucrose gradient centrifugation. The separated unfused virions subsequently fuse when incubated
with a “fresh” batch of liposomes. We conclude, therefore, that the fraction of initially bound unfused virions does not consist
of dective particles, but rather of particles bound to liposomes via “inactive” sites.
Details of the low pH inactivation of fusion capacity ofinfluenza virus towards cells and liposomes are presented. This inactivation is caused by protonation and exposure of the hydrophobic
segment of HA2, and affects primarily the fusion rate constants. Some degree of inactivation also occurs when virions are bound to cellular
membranes. 相似文献
89.
Yasuhisa Kunimi James R. Fuxa Bruce D. Hammock 《Entomologia Experimentalis et Applicata》1996,81(3):251-257
Virus replication and polyhedra production of two polyhedron-positive recombinant nuclear polyhedrosis viruses of Autographa californica, AcJHE.KK and AcAaIT which encode juvenile hormone esterase and scorpion toxin, respectively, were compared with those of a plaque purified wild-type nuclear polyhedrosis virus, AcMNPV-C6, in Trichoplusia ni larvae. Though average times required to kill the T. ni larvae increased with the age of the larvae, killing time by either recombinant virus was significantly shorter than that by wild-type virus. Killing time was reduced ca. 30% for AcAaIT-infected larvae and 5 to 8% for AcJHE.KK-infected larvae as compared to that for AcMNPV-C6-infected larvae. The average weight of larvae infected with AcAaIT was significantly lower than that of larvae infected with AcJHE.KK and AcMNPV-C6. The mean numbers of polyhedra produced in each larva inoculated with AcAaIT and AcJHE.KK were ca. 20% and 60%, respectively, of those for AcMNPV-C6. Total virus titers in AcMNPV-C6-infected larvae were significantly higher than those in AcJHE.KK- and AcAaIT-infected larvae until 2 days post infection. 相似文献
90.
I. S. Novella J. J. Holland S. F. Elena A. Moya E. Domingo 《Molecular & general genetics : MGG》1996,252(6):733-738
The population dynamics of RNA viruses have an important influence on fitness variation and, in consequence, on the adaptative potential and virulence of this ubiquitous group of pathogens. Earlier work with vesicular stomatitis virus showed that large population transfers were reproducibly associated with fitness increases, whereas repeated transfers from plaque to plaque (genetic bottlenecks) lead to losses in fitness. We demonstrate here that repeated five-plaque to five-plaque passage series yield long-term fitness stability, except for occasional stochastic fitness jumps. Repeated five-plaque passages regularly alternating with two consecutive large population transmissions did not cause fitness losses, but did limit the size of fitness gains that would otherwise have occurred. These results underscore the profound effects of bottleneck transmissions in virus evolution. 相似文献