全文获取类型
收费全文 | 1351篇 |
免费 | 99篇 |
出版年
2023年 | 7篇 |
2022年 | 11篇 |
2021年 | 49篇 |
2020年 | 27篇 |
2019年 | 28篇 |
2018年 | 43篇 |
2017年 | 34篇 |
2016年 | 62篇 |
2015年 | 61篇 |
2014年 | 86篇 |
2013年 | 94篇 |
2012年 | 120篇 |
2011年 | 125篇 |
2010年 | 65篇 |
2009年 | 62篇 |
2008年 | 63篇 |
2007年 | 72篇 |
2006年 | 64篇 |
2005年 | 53篇 |
2004年 | 48篇 |
2003年 | 34篇 |
2002年 | 31篇 |
2001年 | 37篇 |
2000年 | 35篇 |
1999年 | 23篇 |
1998年 | 14篇 |
1997年 | 8篇 |
1996年 | 5篇 |
1995年 | 7篇 |
1994年 | 4篇 |
1993年 | 4篇 |
1992年 | 10篇 |
1991年 | 7篇 |
1990年 | 3篇 |
1989年 | 10篇 |
1988年 | 10篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 4篇 |
1981年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1968年 | 2篇 |
1967年 | 1篇 |
1959年 | 2篇 |
1957年 | 1篇 |
排序方式: 共有1450条查询结果,搜索用时 15 毫秒
1.
The reproductive variables [female size (geometric centroid size), fecundity and offspring size (longest diameter of the embryo)] of the livebearer Poecilia vivipara in the quaternary lagoons of northern Rio de Janeiro State varied significantly among sites and were partially associated within and among sites. Mean female size was directly related to salinity. Fecundity (number of offspring) was directly related to female size both within and among sites. The female size × fecundity relationship, however, was not constant among sites. Offspring size was partially and directly associated with female size and salinity. The relationship between fecundity and female size was positively related with fecundity, suggesting a non-linear relationship of these variables when the total sample was considered. The results suggest that life-history differences among sites were more related to different predation regimes than to simple effects of plasticity caused by increased growth in saltwater environments. 相似文献
2.
Fatty acid acylation of proteins in Physarum polycephalum 总被引:1,自引:0,他引:1
We have investigated the occurrence of protein-fatty acid acylation by metabolic incorporation of [3H]myristic and [3H]palmitic acids in Physarum polycephalum. We show that this organism contains fatty acylated proteins with mainly myristic acid covalently attached in alkali-stable linkages, probably amides. We find no evidence for ester-linked fatty acids, in contrast to the situation in vertebrate cells. 相似文献
3.
C. E. Scanlon N. R. Chalmers M. A. O. Monteiro da Cruz 《International journal of primatology》1989,10(2):123-136
Three wild groups of common marmoset, Callithrix jacchus jacchus,in north-east Brazil, of approximately similar size, had home ranges between 2.5 and 6.5 ha. But their core areas were similar
in size between 1.0 and 1.5 ha, with a monthly area of heavy use between 1.1 and 1.6 ha. The groups were selective in the
use of their home ranges, even though they were small: they used some areas heavily and others lightly. The core areas had
higher densities of trees that produced gum exudates than did other parts of the home ranges. Our data suggest that a group
of marmosets in this habitat may require a minimum of about 50 gum trees in its home range at a minimum density of about 50
trees/ha. In addition, the animals require suitable trees in which to sleep. We suggest that patches of forest with these
desirable properties remain relatively fixed in size and location over the years and that individual animals are constantly
in flux between them. 相似文献
4.
5.
Sequence of chicken c beta 7 tubulin. Analysis of a complete set of vertebrate beta-tubulin isotypes 总被引:6,自引:0,他引:6
In chicken, beta-tubulin is encoded by a family of seven genes. We have now isolated and sequenced overlapping cDNA clones corresponding to gene c beta 7 (previously designated c beta 4'), the only chicken beta-tubulin not previously characterized. The inferred amino acid sequence of c beta 7 tubulin is identical with the class I beta-tubulin isotype found in human, mouse and rat. Moreover, c beta 7 is highly expressed in almost all tissue and cell types in chicken, a pattern similar to those of the genes for class I beta-tubulin isotypes in other vertebrates. Comparison of the complete family of chicken beta-tubulin gene sequences reveals that the heterogeneity of beta-tubulin polypeptides encoded in a higher eukaryote is confined to six distinct beta-tubulin isotypes. Five of these are members of evolutionarily conserved isotypic classes (I to V), whereas the sixth represents a divergent erythroid-specific tubulin whose sequence has not been conserved. 相似文献
6.
Germinating seed ofDalbergia dolichopetala converted both [2H5]l-tryptophan and [2H5]indole-3-ethanol to [2H5]indole-3-acetic acid (IAA). Metabolism of [2-14C]IAA resulted in the production of indole-3-acetylaspartic acid (IAAsp), as well as several unidentified components, referred to as metabolites I, II, IV and V. Re-application of [14C]IAAsp to the germinating seed led to the accumulation of the polar, water-soluble compound, metabolite V, as the major metabolite, together with a small amount of IAA. Metabolites I, II and IV were not detected, nor were these compounds associated with the metabolism of [2-14C]IAA by shoots and excised cotyledons and roots from 26-d-oldD. dolichopetala seedlings. Both shoots and cotyledons converted IAA to IAAsp and metabolite V, while IAAsp was the only metabolite detected in extracts from excised roots. The available evidence indicates that inDalbergia, and other species, IAAsp may not act as a storage product that can be hydrolysed to provide the plant with a ready supply of IAA.Abbreviations HPLC-RC
high-performance liquid chromatography-radiocounting
- IAA
indole-3-acetic acid
- IAAsp
indole-3-acetylaspartic acid
- IAlnos
2-O-indole-3-acetyl-myo-inositol
- IEt
indole-3-ethanol 相似文献
7.
The alpha-like globin gene cluster in rabbits contains embryonic zeta-
globin genes, an adult alpha-globin gene, and theta-globin genes of
undetermined function. The basic arrangement of genes, deduced from
analysis of cloned DNA fragments, is 5'-zeta 0-zeta 1-alpha 1-theta 1- zeta
2-zeta 3-theta 2-3'. However, the pattern of restriction fragments
containing zeta- and theta-globin genes varies among individual rabbits.
Analysis of BamHI fragments of genomic DNA from 24 New Zealand white
rabbits revealed eight different patterns of fragments containing
zeta-globin genes. The large BamHI fragments containing genes zeta 0 and
zeta 1 are polymorphic in length, whereas a 1.9-kb fragment containing the
zeta 2 gene and the 3.5-kb fragment containing the zeta 3 gene do not vary
in size. In contrast to this constancy in the size of the restriction
fragments, the copy number of the zeta 2 and zeta 3 genes does vary among
different rabbits. No length polymorphism was detected in the BamHI
fragments containing the theta-globin genes, but again the copy number
varies for restriction fragments containing the theta 2 gene. The alpha 1-
and theta 1-globin genes are located in a nonpolymorphic 7.2-kb BamHI
fragment. The combined data from hybridization with both zeta and theta
probes shows that the BamHI cleavage pattern does not vary within the
region 5'-alpha 1-theta 1- zeta 2-zeta 3-theta 2-3', but the pattern
genomic blot-hybridization patterns for the progeny of parental rabbits
with different zeta-globin gene patterns shows that the polymorphic
patterns are inherited in a Mendelian fashion. Two different haplotypes
have been mapped based on the genomic blot-hybridization data. The
variation in the alpha-like globin gene cluster in the rabbit population
results both from differences in the copy number of the duplication block
containing the zeta-zeta-theta gene set and from the presence or absence of
polymorphic BamHI sites.
相似文献
8.
R A Monteiro 《Histology and histopathology》1991,6(1):9-20
A quantitative study regarding the age-related changes occurring in the somatic organelles of the neocerebellar Purkinje cell is carried out, using female rats aged 2 to 24 months. Standard manual morphometric techniques are used to calculate the following parameters: somatic volume, volumetric fractions and absolute volumes of the intracellular structures as well as the membrane profile concentration, the membrane surface concentration and the mean surface of the rough endoplasmic reticulum cisternae per cell (RER-S). From a statistical point of view, all the cell components significantly modify their volumetric fractions (except the multivesicular bodies and nucleolus; the latter in relation to the nucleus) and their absolute volumes (except the mitochondria and the multivesicular bodies); the parameters regarding the reticulum are also modified during ageing. There is a linear trend between the age and either the somatic volume of the RER-S or the absolute volumes of the following structures: mitochondria, dense bodies, ground substance and total cytoplasm. A linear correlation is also observed between the cell volume and either the RER-S or the absolute volume of intracellular structures (the Golgi apparatus, the multivesicular bodies and nucleolus being excluded). Anatomophysiological considerations about the findings are discussed. The role of the ground substance as the major modulator of the volumetric plasticity of the Purkinje cell during ageing, is emphasized as a conclusion. 相似文献
9.
HLA antigens in Brazilian patients with paracoccidioidomycosis 总被引:1,自引:1,他引:0
L. Z. Goldani C. M. C. Monteiro E. A. Donadi R. Martinez J. C. Voltarelli 《Mycopathologia》1991,114(2):89-91
Eighty patients with paracoccidioidomycosis were typed for 43 HLA specificities from loci A, B, C and DR. A highly significant increased frequency of HLA-B40 (relative risk 29.2) and HLA-Cw 1 (relative risk 8.8) were found in patients compared to control subjects. The frequencies HLA-A2, B7 and B21 were also increased in patients and haplotypes-B40-Cw1 and -A2-B40 were positively correlated with the disease. DR antigen frequencies were not significantly altered in the patients and evidence of a protective effect was not found for any of the 43 antigens tested. These findings further support the involvement of the HLA system in the genetic susceptibility to paracoccidioidomycosis and the importance of ethnic variability in this association. 相似文献
10.
The N alpha-tubulin gene of Physarum polycephalum has an EcoRV site at codons 252/253. EcoRV digestion of physarum DNA generated two EcoRV fragments per gene copy comprising both coding and flanking sequences. Hybridisation probes which included coding sequences upstream from the central EcoRV site cross-hybridised with another alpha-tubulin gene. Probes derived from either 5'- or 3'-flanking regions were gene-specific. These probes identified two EcoRV fragments in the haploid strain CLdAXE viz 5.4 kb (5'-fragment) and 6.2 kb (3'-fragment). The same two fragments were identified in EcoRV digests of DNA of the diploid strain M3CVIII, and a second form of the gene was also identified comprising two fragments viz 5.0 kb (5'-end) and 5.5 kb (3'-end). Both forms gave rise to an identical 4.65 kb HindIII fragment as judged by restriction mapping. 相似文献