全文获取类型
收费全文 | 19248篇 |
免费 | 1406篇 |
国内免费 | 1276篇 |
专业分类
21930篇 |
出版年
2024年 | 42篇 |
2023年 | 264篇 |
2022年 | 544篇 |
2021年 | 946篇 |
2020年 | 574篇 |
2019年 | 826篇 |
2018年 | 812篇 |
2017年 | 570篇 |
2016年 | 845篇 |
2015年 | 1116篇 |
2014年 | 1366篇 |
2013年 | 1461篇 |
2012年 | 1711篇 |
2011年 | 1543篇 |
2010年 | 983篇 |
2009年 | 952篇 |
2008年 | 1061篇 |
2007年 | 990篇 |
2006年 | 805篇 |
2005年 | 695篇 |
2004年 | 535篇 |
2003年 | 528篇 |
2002年 | 442篇 |
2001年 | 346篇 |
2000年 | 301篇 |
1999年 | 294篇 |
1998年 | 169篇 |
1997年 | 160篇 |
1996年 | 158篇 |
1995年 | 121篇 |
1994年 | 85篇 |
1993年 | 70篇 |
1992年 | 111篇 |
1991年 | 84篇 |
1990年 | 69篇 |
1989年 | 53篇 |
1988年 | 47篇 |
1987年 | 43篇 |
1986年 | 41篇 |
1985年 | 51篇 |
1984年 | 8篇 |
1983年 | 17篇 |
1982年 | 9篇 |
1981年 | 11篇 |
1980年 | 7篇 |
1979年 | 7篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1974年 | 5篇 |
1969年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
POT1 proteins in green algae and land plants: DNA-binding properties and evidence of co-evolution with telomeric DNA 下载免费PDF全文
Eugene V. Shakirov Xiangyu Song Jessica A. Joseph Dorothy E. Shippen 《Nucleic acids research》2009,37(22):7455-7467
Telomeric DNA terminates with a single-stranded 3′ G-overhang that in vertebrates and fission yeast is bound by POT1 (Protection Of Telomeres). However, no in vitro telomeric DNA binding is associated with Arabidopsis POT1 paralogs. To further investigate POT1–DNA interaction in plants, we cloned POT1 genes from 11 plant species representing major branches of plant kingdom. Telomeric DNA binding was associated with POT1 proteins from the green alga Ostreococcus lucimarinus and two flowering plants, maize and Asparagus. Site-directed mutagenesis revealed that several residues critical for telomeric DNA recognition in vertebrates are functionally conserved in plant POT1 proteins. However, the plant proteins varied in their minimal DNA-binding sites and nucleotide recognition properties. Green alga POT1 exhibited a strong preference for the canonical plant telomere repeat sequence TTTAGGG with no detectable binding to hexanucleotide telomere repeat TTAGGG found in vertebrates and some plants, including Asparagus. In contrast, POT1 proteins from maize and Asparagus bound TTAGGG repeats with only slightly reduced affinity relative to the TTTAGGG sequence. We conclude that the nucleic acid binding site in plant POT1 proteins is evolving rapidly, and that the recent acquisition of TTAGGG telomere repeats in Asparagus appears to have co-evolved with changes in POT1 DNA sequence recognition. 相似文献
992.
993.
X. Song G. Körtner F. Geiser 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1995,165(4):291-297
Physiological mechanisms causing reduction of metabolic rate during torpor in heterothermic endotherms are controversial. The original view that metabolic rate is reduced below the basal metabolic rate because the lowered body temperature reduces tissue metabolism has been challenged by a recent hypothesis which claims that metabolic rate during torpor is actively downregulated and is a function of the differential between body temperature and ambient temperature, rather than body temperature per se. In the present study, both the steady-state metabolic rate and body temperature of torpid stripe-faced dunnarts, Sminthopsis macroura (Dasyuridae: Marsupialia), showed two clearly different phases in response to change of air temperature. At air temperatures between 14 and 30°C, metabolic rate and body temperature decreased with air temperature, and metabolic rate showed an exponential relationship with body temperature (r
2=0.74). The Q
10 for metabolic rate was between 2 and 3 over the body temperature range of 16 to 32°C. The difference between body temperature and air temperature over this temperature range did not change significantly, and the metabolic rate was not related to the difference between body temperature and air temperature (P=0.35). However, the apparent conductance decreased with air temperature. At air temperatures below 14°C, metabolic rate increased linearly with the decrease of air temperature (r
2=0.58) and body temperature was maintained above 16°C, largely independent of air temperature. Over this air temperature range, metabolic rate was positively correlated with the difference between body temperature and air temperature (r
2=0.61). Nevertheless, the Q
10 for metabolic rate between normothermic and torpid thermoregulating animals at the same air temperature was also in the range of 2–3. These results suggest that over the air temperature range in which body temperature of S. macroura was not metabolically defended, metabolic rate during daily torpor was largely a function of body temperature. At air temperatures below 14°C, at which the torpid animals showed an increase of metabolic rate to regulate body temperature, the negative relationship between metabolic rate and air temperature was a function of the differential between body temperature and air temperature as during normothermia. However, even in thermoregulating animals, the reduction of metabolic rate from normothermia to torpor at a given air temperature can also be explained by temperature effects.Abbreviations
BM
body mass
-
BMR
basal metabolic rate
-
C
apparent conductance
-
MR
metabolic rate
-
RMR
resting metabolic rate
-
RQ
respiratory quotient
-
T
a
air temperature
-
T
b
body temperature
-
T
lc
lower critical temperature
-
T
tc
critical air temperature during torpor
-
TMR
metabolic rate during torpor
-
TNZ
thermoneutral zone
- T
difference between body temperature and air temperature
- VO2
rate of oxygen consumption 相似文献
994.
Multiple genes encoding 2,3-dihydroxybiphenyl 1,2-dioxygenase in the gram-positive polychlorinated biphenyl-degrading bacterium Rhodococcus erythropolis TA421, isolated from a termite ecosystem. 总被引:4,自引:3,他引:4 下载免费PDF全文
Rhodococcus erythropolis TA421 was isolated from a termite ecosystem and is able to degrade a wide range of polychlorinated biphenyl (PCB) congeners. Genetic and biochemical analyses of the PCB catabolic pathway of this organism revealed that there are four different bphC genes (bphC1, bphC2, bphC3, and bphC4) which encode 2,3-dihydroxybiphenyl dioxygenases. As determined by Southern hybridization, none of the bphC genes exhibits homology to any other bphC gene. bphC1, bphC2, and bphC4 encode enzymes that have narrow substrate specificities and cleave the first aromatic ring in the meta position. In contrast, bphC3 encodes a meta cleavage dioxygenase with broad substrate specificity. Asturias et al. have shown that the closely related organism Rhodococcus globerulus P6 contains three different bphC genes (bphC1, bphC2, and bpHC3) which encode meta cleavage dioxygenases. The data suggest that there is a diverse family of bphC genes which encode PCB meta cleavage dioxygenases in members of the genus Rhodococcus. 相似文献
995.
996.
997.
Lin Li Zhongliang Huang Wanhui Ye Honglin Cao Shiguang Wei Zhigao Wang Juyu Lian I‐Fang Sun Keping Ma Fangliang He 《Oikos》2009,118(4):495-502
The spatial dispersion of individuals in a species is an important pattern that is controlled by many mechanisms. In this study we analyzed spatial distributions of tree species in a large-scale (20 ha) stem-mapping plot in a species-rich subtropical forest of China. O-ring statistic was used to measure spatial patterns of species with abundance >10. Ω0–10 , the mean conspecific density within 10 m of a tree, was used as a measure of the intensity of aggregation of a species. Our results showed: (1) aggregated distribution was the dominant pattern in the plot. The percentage of aggregated species decreased with increased spatial scale. (2) The percentages of significantly aggregated species decreased from abundant to intermediate and to rare species. Rare species was more strongly aggregated than common species. Aggregation was weaker in larger diameter classes. (3) Seed traits determined the spatial patterns of trees. Seed dispersal mode can influence spatial patterns of species, with species dispersed by both modes being less clumped than species dispersed by animal or wind, respectively. Considering these results, we concluded that seed dispersal limitation, self-thinning and habitat heterogeneity primarily contributed to spatial patterns and species coexistence in the forest. 相似文献
998.
RG108对肺腺癌A549细胞增殖、凋亡及RASSF1A基因表达的影响 总被引:1,自引:0,他引:1
目的探讨DNA甲基转移酶抑制剂RG108对人肺腺癌细胞株A549增殖、凋亡以及对RASSF1A(Ras as-sociation domain proteinfamily1)基因启动子区域甲基化状态、表达的影响。方法用20μmol/L的RG108对A549细胞进行化学干预72h,用MTT法检测细胞生长抑制率;流式细胞术检测细胞周期以及凋亡情况;RT-PCR观察RASSF1A基因mRNA水平变化;Western blot检测RASSF1A蛋白的表达;甲基化特异性PCR(MS-PCR)检测RASSF1A基因启动子区域甲基化状态的改变。结果经RG108干预72h后,A549细胞的抑制率为17.2±0.43%,细胞周期阻滞于G0/G1期,并引起细胞凋亡。RT-PCR和Western blot结果显示在干预组细胞中分别出现RASSF1A基因的DNA条带(329bp)和蛋白质条带(39kD),而对照组中无相应条带出现。RASSF1A基因启动子区域由甲基化状态转变为非甲基化状态。结论RG108可使RASSF1A基因启动子区域去甲基化,并通过该机制诱导RASSF1A基因在人肺腺癌细胞株A549中表达。 相似文献
999.
1000.