全文获取类型
收费全文 | 391篇 |
免费 | 35篇 |
出版年
2017年 | 5篇 |
2016年 | 7篇 |
2015年 | 10篇 |
2014年 | 8篇 |
2013年 | 5篇 |
2012年 | 11篇 |
2011年 | 17篇 |
2010年 | 8篇 |
2009年 | 7篇 |
2008年 | 5篇 |
2007年 | 13篇 |
2006年 | 11篇 |
2005年 | 11篇 |
2004年 | 19篇 |
2002年 | 7篇 |
2001年 | 11篇 |
2000年 | 9篇 |
1999年 | 13篇 |
1998年 | 16篇 |
1997年 | 11篇 |
1996年 | 11篇 |
1995年 | 12篇 |
1994年 | 6篇 |
1993年 | 6篇 |
1992年 | 14篇 |
1991年 | 8篇 |
1990年 | 7篇 |
1989年 | 8篇 |
1988年 | 5篇 |
1987年 | 10篇 |
1986年 | 6篇 |
1985年 | 14篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1982年 | 6篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1977年 | 10篇 |
1976年 | 8篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1973年 | 6篇 |
1972年 | 5篇 |
1971年 | 4篇 |
1970年 | 7篇 |
1969年 | 4篇 |
1968年 | 3篇 |
1967年 | 4篇 |
1966年 | 3篇 |
排序方式: 共有426条查询结果,搜索用时 15 毫秒
401.
Avise JC; Shapira JF; Daniel SW; Aquadro CF; Lansman RA 《Molecular biology and evolution》1983,1(1):38-56
We address the problem of the possible significance of biological
speciation to the magnitude and pattern of divergence of asexually
transmitted characters in bisexual species. The empirical data for this
report consist of restriction endonuclease site variability in maternally
transmitted mitochondrial DNA (mtDNA) isolated from 82 samples of
Peromyscus polionotus and P. leucopus collected from major portions of the
respective species' ranges. Data are analyzed together with previously
published information on P. maniculatus, a sibling species to polionotus.
Maps of restriction sites indicate that all of the variation observed can
be reasonably attributed to base substitutions leading to loss or gain of
particular recognition sites. Magnitude of mtDNA sequence divergence within
polionotus (maximum approximately equal to 2%) is roughly comparable to
that observed within any of five previously identified mtDNA assemblages in
maniculatus. Sequence divergence within leucopus (maximum approximately
equal to 4%) is somewhat greater than that within polionotus. Consideration
of probable evolutionary links among mtDNA restriction site maps allowed
estimation of matriarchal phylogenies within polionotus and leucopus.
Clustering algorithms and qualitative Wagner procedures were used to
generate phenograms and parsimony networks, respectively, for the
between-species comparisons. Three simple graphical models are presented to
illustrate some conceivable relationships of mtDNA differentiation to
speciation. In theoretical case I, each of two reproductively defined
species (A and B) is monophyletic in matriarchal genealogy; the common
female ancestor of either species can either predate or postdate the
speciation. In case II, neither species is monophyletic in matriarchal
genotype. In case III, species B is monophyletic but forms a subclade
within A which is thus paraphyletic with respect to B. The empirical
results for mtDNA in maniculatus and polionotus appear to conform closely
to case III. These theoretical and empirical considerations raise a number
of questions about the general relationship of the speciation process to
the evolution of uniparentally transmitted traits. Some of these
considerations are presented, and it is suggested that the distribution
patterns of mtDNA sequence variation within and among extant species should
be of considerable relevance to the particular demographies of speciation.
相似文献
402.
403.
A.L. Boskey A.S. Posner J.M. Lane M.R. Goldberg D.M. Cordella 《Archives of biochemistry and biophysics》1980,199(2):305-311
Calcium-acidic phospholipid-phosphate complexes, known to induce in vitro hydroxyapatite formation from metastable calcium phosphate sotutions, have been isolated from the morphologically defined zones of the bovine epiphyseal growth plate. The changes in zonal distribution of these complexes in epiphyseal cartilage correlate directly with other biochemical changes which occur prior to cartilage calcification. The concentration of calcium-acidic phospholipid-phosphate complexes increases going from the morphologically defined reserve zone to the proliferative zone, peaking in the hypertrophic zone, where mineralization is initiated, and decreasing in primary spongiosa and diaphyseal bone. Expressed as milligrams of calcium-phospholipid-phosphate complex per milligram hydroxyproline the concentration ranged from 19 (articular cartilage) to 535 (hypertrophic cell zone) decreasing to 43 (diaphyseal bone) with parallel changes being seen when the concentration was expressed per gram of demineralized dry tissue, per total lipid, per DNA, or, per 5′-AMPase activity. 相似文献
404.
405.
Summary A number of fractions have been obtained from a soil humic acid by salting-out and by leaching with ammonium sulphate. The fractions obtained are characterised by infra-red and ultra-violet spectroscopy and osmometry. The fractions removed at relatively low saturation with ammonium sulphate are less highly charged and contain more aliphatic hydrocarbon groups per unit weight than the fractions precipitated at high concentrations of salt. In addition, the number average molecular weight of successive fractions precipitated with ammonium sulphate decreases significantly. 相似文献
406.
Accessing genetic diversity for crop improvement 总被引:1,自引:0,他引:1
407.
408.
Lactogen binding and prolactin content were measured in hepatic subcellular fractions from tumor-bearing rats (TBR; MtT/F4, MtT/W5, MtT/W10) with elevated prolactin and growth hormone levels and from control animals. Specific binding of 125I-oPRL to Golgi fractions from tumor-bearing animals was 2.5 to 7 fold greater than that from controls. Binding to plasmalemma was 6-fold greater in tumor-bearing rats. The specific binding of 125I-labelled bGH and insulin showed less marked differences between TBR and controls. Subcellular fractions were extracted with HCl to determine hormonal content. The content of prolactin and growth hormone in Golgi fractions from TBR was at least 20-fold that in fractions from controls. Rat prolactin extracted from Golgi heavy elements was 50% as effective as native material in binding to lactogen receptors as judged by radioreceptor assay. These studies demonstrate that the chronic elevation of prolactin was associated with an increase of receptors not only in the intracellular compartment but on the cell surface as well. Furthermore, they demonstrate that native prolactin is internalized and accumulated in rat liver Golgi fractions. 相似文献
409.
Developing taste buds in the anterior mandibular floor of perihatching
chicks were studied by high voltage electron microscopic autoradiography in
order to identify proliferating gemmal cell types. Montaged profiles of 29
taste buds in five cases euthanized between embryonic day 21 and
posthatching day 2 were analyzed after a single [3H]thymidine injection
administered on embryonic day 16, 17 or 18. Results showed that dark cells
comprised 55% of identified (n = 900 cells) and 62% of labeled (n = 568
cells) gemmal cells as compared with light, intermediate, basal or
perigemmal bud cells. Dark cells had both a greater (P < 0.05) number of
labeled cells and a greater amount of label (grains/nucleus) than the other
four bud cell types, irrespective of injection day. The nuclear area
(micron 2) of dark cells was not significantly larger (P > 0.05) than
that of the other gemmal cell types and therefore cannot account for the
greater amount for label in the dark cells. Interestingly, only dark cells
showed a positive correlation (P < 0.003) between amount of label and
nuclear area. Results suggest that, during the perihatching period of
robust cell proliferation, dividing dark cells may give rise primarily, but
not exclusively, to dark cell progeny.
相似文献
410.