全文获取类型
收费全文 | 935篇 |
免费 | 178篇 |
国内免费 | 3篇 |
专业分类
1116篇 |
出版年
2021年 | 9篇 |
2018年 | 9篇 |
2017年 | 13篇 |
2016年 | 19篇 |
2015年 | 22篇 |
2014年 | 34篇 |
2013年 | 41篇 |
2012年 | 41篇 |
2011年 | 55篇 |
2010年 | 39篇 |
2009年 | 29篇 |
2008年 | 50篇 |
2007年 | 51篇 |
2006年 | 42篇 |
2005年 | 31篇 |
2004年 | 36篇 |
2003年 | 24篇 |
2002年 | 44篇 |
2001年 | 27篇 |
2000年 | 30篇 |
1999年 | 24篇 |
1998年 | 20篇 |
1997年 | 14篇 |
1996年 | 18篇 |
1995年 | 14篇 |
1993年 | 19篇 |
1992年 | 25篇 |
1991年 | 23篇 |
1990年 | 19篇 |
1989年 | 16篇 |
1988年 | 18篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 13篇 |
1984年 | 14篇 |
1983年 | 17篇 |
1982年 | 7篇 |
1980年 | 11篇 |
1979年 | 11篇 |
1978年 | 12篇 |
1977年 | 11篇 |
1976年 | 10篇 |
1975年 | 10篇 |
1973年 | 15篇 |
1972年 | 9篇 |
1971年 | 6篇 |
1970年 | 6篇 |
1969年 | 14篇 |
1968年 | 8篇 |
1967年 | 7篇 |
排序方式: 共有1116条查询结果,搜索用时 0 毫秒
11.
The biological properties (infectivity, hemagglutination, hemolysis, cell fusion, neuraminidase) of Sendai virus were dissociated on the basis of sensitivity to beta-propiolactone, by freeze-thawing, by heating at different temperatures, and by adsorption-elution with formalinized chicken erythrocytes. Possible mechanisms whereby beta-propiolactone selectively destroys viral infectivity are discussed. 相似文献
12.
Glenn T. Howe Gary Gardner Wesley P. Hackett Glenn R. Furnier 《Physiologia plantarum》1996,97(1):95-103
In trees and other woody perennial plants, short days (SDs) typically induce growth cessation, the initiation of cold acclimation, the formation of a terminal bud and bud dormancy. Phytochrome control of SD-induced bud set was investigated in two northern clones of black cottonwood (Populus trichocarpa Torr. & Gray) by using night breaks with red light (R) and far-red light (FR). For both clones (BC-1 and BC-2), SD-induced bud set was prevented when R night breaks as short as 2 min were given in the middle of the night. When night breaks with 2 min of R were immediately followed by 2 min of FR, substantial reversibility of bud set was observed for BC-1 but not for BC-2. By comparing the effects of the R night breaks on bud set and the length of specific internodes, we determined that the R night breaks influenced internode elongation in two opposing ways. First, the addition of a R night break to the SD treatment prevented the cessation of internode elongation that is associated with bud set. Those internodes that would not have elongated under SDs (and would have been found within the terminal bud) elongated in the R treatment. Second, the R night breaks decreased internode length relative to the long-day (LD) control. In contrast to the clonal differences in reversibility that we observed for bud set, the decrease in internode length (i.e. the second effect of R) was R/FR reversible in both clones. Based on these results, we conclude that internode elongation is influenced by two distinct types of phytochrome-mediated response. The first response is a typical response to photpperiod, whereas the second response is a typical “end-of-day” response to light quality. Our results demonstrate that SD-induced bud set in black cottonwood is controlled by phytochrome but that clonal differences have an important influence on the R/FR reversibility of this response. The availability of an experimental system in which SD-induced bud set is R/FR reversible will be valuable for studying the physiological genetics of photoperiodism in trees. 相似文献
13.
Relative apparent synapomorphy analysis (RASA). I: The statistical measurement of phylogenetic signal 总被引:10,自引:9,他引:1
We have developed a new approach to the measurement of phylogenetic signal
in character state matrices called relative apparent synapomorphy analysis
(RASA). RASA provides a deterministic, statistical measure of natural
cladistic hierarchy (phylogenetic signal) in character state matrices. The
method works by determining whether a measure of the rate of increase of
cladistic similarity among pairs of taxa as a function of phenetic
similarity is greater than a null equiprobable rate of increase. Our
investigation of the utility and limitations of RASA using simulated and
bacteriophage T7 data sets indicates that the method has numerous
advantages over existing measures of signal. A first advantage is
computational efficiency. A second advantage is that RASA employs known
methods of statistical inference, providing measurable sensitivity and
power. The performance of RASA is examined under various conditions of
branching evolution as the number of characters, character states per
character, and mutations per branch length are varied. RASA appears to
provide an unbiased and reliable measure of phylogenetic signal, and the
general approach promises to be useful in the development of new techniques
that should increase the rigor and reliability of phylogenetic estimates.
相似文献
14.
Kumar B. Reddy Barbara A. Hocevar Philip H. Howe 《Journal of cellular biochemistry》1994,56(3):418-425
Transforming growth factor β1 (TGFβ1) inhibits epithelial cell proliferation late in the G1 phase of the cell cycle. We examined the effect of TGFβ1 on known late G1 cell cycle regulators in an attempt to determine the molecular mechanism of growth inhibition by this physiological inhibitor. The results demonstrate the TGFβ1 inhibits the late G1 and S phase specific histone H1 kinase activity of p33cdk2. This inhibitiion is not dur to TGFβ1's effect on p33cdk2 synthesis, but rather due to its negative effect on the late G1 phosphorylation of p33cdk2. It is also shown that TGFβ1 inhibits both late G1 cyclin A and cyclin E associated histon H1 kinase activities. The inhibitor has no effects on the synthesis of cyclin E but to inhibit the synthesis of cyclin A protein in a cell cycle dependent manner. If TGFβ1 is added to cells which have progressed futher than 8 hours into G1, then it is without inhibitory effect on cyclin A synthesis. These effect on TGFβ1 on late G1 cell cycle regulators correlate well with its inhibitory effects on cellular growth and suggest that these G1 cyclin dependent kinases might serve as targets for TGFβ1-mediated growth arrest. 相似文献
15.
Induction of the cell cycle in baby rat kidney cells by adenovirus type 5 E1A in the absence of E1B and a possible influence of p53. 总被引:9,自引:6,他引:3 下载免费PDF全文
From previous studies on the induction of DNA synthesis in quiescent primary baby rat kidney cells by adenovirus type 5 (Ad5) E1A deletion mutants, we concluded that induction is prevented only when cellular proteins p300 and pRb are both uncomplexed with E1A (J.A. Howe, J.S. Mymryk, C. Egan, P.E. Branton, and S.T. Bayley, Proc. Natl. Acad. Sci. USA 87:5883-5887, 1990). We have now examined induction by these same mutants in virus lacking the E1B region, so that cellular p53 was no longer complexed to the E1B 55-kDa protein. E1A mutants that fail to bind pRb induced DNA synthesis at a significantly lower level in Ad5 lacking E1B than in Ad5 containing E1B. Apparently, therefore, uncomplexed p53 can partially replace p300 in cooperating with pRb to suppress DNA synthesis in baby rat kidney cells. 相似文献
16.
Identifying the drivers of community structure and dynamics is a major pursuit in ecology. Emphasis is typically placed on the importance of local scale interactions when attempting to explain these fundamental ecological patterns. However, regional scale phenomena are also important predictors. The importance of regional scale context should be more evident in assemblages where multiple species are close to their range margins. Here, we test the importance of regional scale context using data from a temperate forest plot that contains two species groups – one near its northern range limit and one near its southern range limit. We show the proximity of species to their southern or northern range margins is linked to local scale co-occurrence, similarity in gene expression responses to a key environmental driver, demographic performance and inter-specific variation in conspecific negative density dependence. In sum, many of the key local scale patterns and processes of interest to community ecologists are linked to biogeographic context that is frequently ignored. 相似文献
17.
I explore the specialization versus generalization paradigm in frugivory and seed dispersal. This view predicts that some tropical trees produce nutritious fruits adapted for use by a small coterie of specialized frugivores that provide reliable seed dissemination. Other tree species are expected to offer superabundant fruits of lower nutritional reward, relying instead on common opportunistic frugivores that are individually less reliable, but collectively disperse seeds effectively. Though widely referenced, many aspects of the paradigm are untested with tropical trees and avian frugivores, primarily because plant ecologists rarely determine whether specialist or generalist foragers are responsible for different patterns of seed distribution, while students of foraging behavior rarely determine the effects seed dispersal by different animals for survival of seeds or seedlings of specialist or generalist trees.Ecological paradigms provide alternative hypotheses, without evolutionary arguments. Keystone species have ecological effects disproportionate to their abundance; it is important for management considerations to know whether fruiting trees or frugivores serve as keystone mutualists in tropical forests. Alternatively, the extent to which vertebrate seed dispersers influence density-dependent seed, seedling, sapling, or adult mortality may have important consequences for spatial dispersion and population dynamics of tree species in tropical forests. 相似文献
18.
mtDNA diversity in rhesus monkeys reveals overestimates of divergence time and paraphyly with neighboring species 总被引:4,自引:0,他引:4
Reconstructions of the human-African great ape phylogeny by using
mitochondrial DNA (mtDNA) have been subject to considerable debate. One
confounding factor may be the lack of data on intraspecific variation. To
test this hypothesis, we examined the effect of intraspecific mtDNA
diversity on the phylogenetic reconstruction of another Plio- Pleistocene
radiation of higher primates, the fascicularis group of macaque (Macaca)
monkey species. Fifteen endonucleases were used to identify 10 haplotypes
of 40-47 restriction sites in M. mulatta, which were compared with similar
data for the other members of this species group. Interpopulational,
intraspecific mtDNA diversity was large (0.5%- 4.5%), and estimates of
divergence time and branching order incorporating this variation were
substantially different from those based on single representatives of each
species. We conclude that intraspecific mtDNA diversity is substantial in
at least some primate species. Consequently, without prior information on
the extent of genetic diversity within a particular species, intraspecific
variation must be assessed and accounted for when reconstructing primate
phylogenies. Further, we question the reliability of hominoid mtDNA
phylogenies, based as they are on one or a few representatives of each
species, in an already depauperate superfamily of primates.
相似文献
19.
Cis-Effects of Heterochromatin on Heterochromatic and Euchromatic Gene Activity in Drosophila Melanogaster 总被引:4,自引:2,他引:2
Chromosomal rearrangements that juxtapose heterochromatin and euchromatin can result in mosaic inactivation of heterochromatic and euchromatic genes. This phenomenon, position effect variegation (PEV), suggests that heterochromatic and euchromatic genes differ in their regulatory requirements. This report describes a novel method for mapping regions required for heterochromatic genes, and those that induce PEV of a euchromatic gene. P transposase mutagenesis was used to generate derivatives of a translocation that variegated for the light(+) (lt(+)) gene and carried the euchromatic white(+) (w(+)) gene on a transposon near the heterochromatin-euchromatin junction. Cytogenetic and genetic analyses of the derivatives showed that P mutagenesis resulted in deletions of several megabases of heterochromatin. Genetic and molecular studies showed that the derivatives shared a euchromatic breakpoint but differed in their heterochromatic breakpoint and their effects on seven heterochromatic genes and the w(+) gene. Heterochromatic genes differed in their response to deletions. The lt(+) gene was sensitive to the amount of heterochromatin at the breakpoint but the heterochromatic 40Fa gene was not. The severity of variegated w(+) phenotype did not depend on the amount of heterochromatin in cis, but varied with local heterochromatic environment. These data are relevant for considering mechanisms of PEV of both heterochromatic and euchromatic genes. 相似文献
20.
Specific activities of eight enzymes involved in glycerol metabolism were determined in crude extracts of three strains ofNeurospora crassa after growth on six different carbon sources. One of the strains was wild type, which grew poorly on glycerol as sole carbon source; the other two were mutant strains which were efficient glycerol utilizers. A possible basis for this greater effeciency of glycerol utilization was catabolite repression of glyceraldehyde kinase by glycerol in wild type, and two-fold higher glycerate kinase activity in the mutant strains after growth on glycerol, thus apparently allowing two routes for glyceraldehyde to enter the glycolytic pathway in the mutant strains but only one in wild type. The preferential entry of glyceraldehyde to the glycolytic pathway through glycerate was suggested by the lack of glyceraldehyde kinase in all three strains after growth on one or more of the carbon sources and the generally higher levels of aldehyde dehydrogenase and of glycerate kinase than of glyceraldehyde kinase. 相似文献