全文获取类型
收费全文 | 11281篇 |
免费 | 594篇 |
国内免费 | 535篇 |
专业分类
12410篇 |
出版年
2024年 | 21篇 |
2023年 | 114篇 |
2022年 | 180篇 |
2021年 | 194篇 |
2020年 | 225篇 |
2019年 | 263篇 |
2018年 | 305篇 |
2017年 | 221篇 |
2016年 | 275篇 |
2015年 | 264篇 |
2014年 | 483篇 |
2013年 | 595篇 |
2012年 | 326篇 |
2011年 | 461篇 |
2010年 | 404篇 |
2009年 | 567篇 |
2008年 | 591篇 |
2007年 | 627篇 |
2006年 | 575篇 |
2005年 | 547篇 |
2004年 | 496篇 |
2003年 | 409篇 |
2002年 | 440篇 |
2001年 | 285篇 |
2000年 | 208篇 |
1999年 | 263篇 |
1998年 | 299篇 |
1997年 | 235篇 |
1996年 | 279篇 |
1995年 | 216篇 |
1994年 | 226篇 |
1993年 | 159篇 |
1992年 | 172篇 |
1991年 | 164篇 |
1990年 | 125篇 |
1989年 | 133篇 |
1988年 | 111篇 |
1987年 | 101篇 |
1986年 | 92篇 |
1985年 | 111篇 |
1984年 | 120篇 |
1983年 | 78篇 |
1982年 | 96篇 |
1981年 | 81篇 |
1980年 | 77篇 |
1979年 | 52篇 |
1978年 | 33篇 |
1977年 | 24篇 |
1976年 | 28篇 |
1975年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
《Bioscience, biotechnology, and biochemistry》2013,77(9):2472-2475
Conjugational transfer of pLS20 in Bacillus subtilis Marburg 168 is restricted by the BsuM restriction-modification system. Restriction efficiency was measured using pLS20 derivatives possessing various numbers of XhoI sites, which are known to be recognized by BsuM. An increase in XhoI sites clearly reduced the conjugational efficiency of pLS20 as compared with that of pUB110 plasmid lacking XhoI. 相似文献
2.
Sandrine Poncet Armelle Delecluse Guido Anello ré Klier Georges Rapoport 《FEMS microbiology letters》1994,117(1):91-95
Abstract The genes encoding the CryIVB and CryIVD crystal polypeptides of B. thuringiensis subsp. israelensis were cloned indepently on a stable shuttle vector, and transfered into B. sphaericus 2297. Recombinant cells expressed the B. thuringiensis toxins during sporulation and were shown to be toxic to Aedes aegypti fourth instar larvae, whereas the parental strain was not. 相似文献
3.
Burt V. Bronk Joe D. Patton David N. Mellard 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1982,697(3):278-285
Chick embryo fibroblasts were treated with the monofunctional alkylating agent methylmethane sulfonate at various concentrations for 1 h at 42°C, rinsed and then incubated post-treatment at various temperatures at which the kinetics of alkali-labile bond disappearance was followed. Growth experiments showed that these cells grew similarly at temperatures of either 37°C or 42°C. Repair as assessed by removal of alkali-labile bond was also similar for postincubation in the temperature range 37–42°C for damage due to methylmethane sulfonate treatment at concentrations less than 1.5 mM. When the postincubation temperature was raised higher than 42.5–43°C, this type of repair was stopped. The normal internal body temperature of adult chickens is about 41.6°C. Hence the present finding indicates that chick cells are much more severely restricted in DNA repair at temperatures above normal than are mammalian cells, which can function in this respect for several deg. C above 37°C. 相似文献
4.
5.
Thermoanaerobacter brockii 《Anaerobe》1997,3(6)
Thermoanaerobacter brockiifermented serine to acetate and ethanol. It oxidized leucine to isovalerate, isoleucine to 2-methylbutyrate, and valine to isobutyrate only in the presence of thiosulfate, or when co-cultured withMethanobacteriumsp. This oxidative deamination was rendered thermodynamically possible by the ability ofT. brockiito reduce thiosulfate to sulfide or the transfer of reducing equivalents to the hydrogenotrophic methanogen. The results suggest thatT. brockiimay be of ecological significance in thermal environments in the turnover of amino acids, especially with thiosulfate or H2-utilizing methanogens are present. 相似文献
6.
Copper is an essential yet toxic metal ion. To satisfy cellular requirements, while, at the same time, minimizing toxicity,
complex systems of copper trafficking have evolved in all cell types. The best conserved and most widely distributed of these
involve Atx1-like chaperones and P1B-type ATPase transporters. Here, we discuss current understanding of how these chaperones bind Cu(I) and transfer it to the
Atx1-like N-terminal domains of their cognate transporter. 相似文献
7.
Frank Kempken 《Molecular & general genetics : MGG》1995,248(1):89-94
Direct evidence for horizontal transfer of a mitochodnrial plasmid from the discomyceteAscobolus immersus to the pyrenomycetePodospora anserina is presented. Southern blot hybridisation analysis, polymerase chain reaction (PCR) amplification, and DNA sequencing demonstrate
transmission of a linear plasmid upon hyphal contact. DNA extraction from isolated organelles indicates a mitochondrial localisation
for the plasmid inP. anserina. This is the first report of horizontal gene transfer among unrelated fungi. These results have important evolutionary implications
for plasmid propagation in fungi. 相似文献
8.
Metabolic rate in diapause and nondiapause brown locust eggs correlated with embryonic development 总被引:1,自引:0,他引:1
INNOCENT N. KAMBULE SHIRLEY A. HANRAHAN FRANCES D. DUNCAN 《Physiological Entomology》2011,36(4):299-308
Insects use dormancy to survive adverse conditions. Brown locust Locustana pardalina (Walk.) eggs offer a convenient model to study dormancy (diapause and quiescence), which contributes to their survival under arid conditions. The metabolic rates of developing nondiapause, diapause and quiescent eggs are compared in the present study using closed‐system respirometry. The embryo becomes committed to continue development and hatch or to enter diapause 6 days after the eggs are placed on moist soil. The metabolic rate of nondiapause eggs increases exponentially until hatching, whereas that of diapause eggs is low and stable. The metabolic rate of diapause laboratory eggs (1.9 ± 0.6 µL CO2 mg?1 h?1) is significantly higher than that of field eggs (0.5 ± 0.3 µL CO2 mg?1 h?1), although the ranges of metabolic rate overlap and the embryos are all in late anatrepsis. The metabolic rate of quiescent eggs is similar to that of diapause eggs but decreases with time. Low metabolic rates during arrested development allow eggs to persist over long periods before hatching. 相似文献
9.
G. R. Bauchan 《Plant Cell, Tissue and Organ Culture》1987,10(1):21-29
Resistance to the alfalfa weevil (Hypera postica (Gyllenhal)) and the potato leafhopper (Empoasco fabae (Harris)) is lacking in cultivated alfalfa. However, a closely related annual Medicago, Medicago scutellata, possesses dense glandular stem and leaf hairs which provides a mechanism for resistance. Several attempts have been made at transfering the glandular haired traint from M. scutellata to perennial alfalfa with limited success. Earlier studies have shown that one reason for the lack of success is embryo abortion. Therefore, this study was initiated to observe zygotic embryo-genesis and to develop an embryo rescue technique for M. scutellata and M. sativa. Observations of zygotic embryogenesis showed that the two species are similar in morphology and can be described from youngest to oldest as globular, heart, torpedo, and hook shaped embryos. M. sativa embryos are smaller than M. scutellata embryos and develop three to four days later. Self pollinated M. scutellata (PI 307446) and sib mated M. sativa (Saranac AR) embryos were cultivated on Murashige and (2,4-D), indolacetic acid (IAA), 6-benzylaminopurine (BAP), and kinetic (KIN). Embryos from both species were also cultured on Schenk and Hildebrandt's (SH) basal medium with the addition of L-glutamine and L-proline. The experimental design was a completely randomized factorial for each experiment. Heart and torpedo shaped embryos from M. scutellata grew best (27.5% plantlet recovery) when cultured on MS medium with 0.05 mgl-1 of both IAA and BAP. After 15 to 30 days on this medium, the embryos had only developed shoots. Therefore, it was necessary to transfer the shoots to MS basal medium without phytohormones for rooting. Rooting occurred in 15 to 30 days and the plantlets could be acclimatized to soil within 2 to 4 weeks. M. sativa embryos grew best (31% plantlet recovery) on SH medium with 50 mM L-glutamine. M. sativa embryos developed both shoots and roots on this medium. This information may now be applied to the development of an embryo culture method for recovering insect resistant hybrids between M. scutellata and M. sativa.
Disclaimer statement: Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the USDA, and does not imply its approval to the exclusion of other products that may also be suitable. 相似文献
10.
Summary The intercellular contacts of the migrating edge of chick and quail blastoderms during gastrulation were studied by transmission electron microscopy of thin sections and of freeze-fracture replicas. Tight junctions and gap junctions as well as desmosomes were found. Tight junctions were organized as single junctional strands or as a complex of numerous junctional strands interposed between the lamellae and the bodies of the cells building up the margin of overgrowth. The function of these intercellular junctions is considered in relation to the locomotion of the margin of overgrowth cells. 相似文献