全文获取类型
收费全文 | 1255篇 |
免费 | 137篇 |
出版年
2021年 | 14篇 |
2019年 | 11篇 |
2018年 | 13篇 |
2017年 | 12篇 |
2016年 | 27篇 |
2015年 | 43篇 |
2014年 | 37篇 |
2013年 | 55篇 |
2012年 | 78篇 |
2011年 | 58篇 |
2010年 | 68篇 |
2009年 | 58篇 |
2008年 | 56篇 |
2007年 | 50篇 |
2006年 | 45篇 |
2005年 | 52篇 |
2004年 | 46篇 |
2003年 | 25篇 |
2002年 | 35篇 |
2001年 | 44篇 |
2000年 | 32篇 |
1999年 | 18篇 |
1998年 | 14篇 |
1997年 | 21篇 |
1996年 | 17篇 |
1995年 | 17篇 |
1994年 | 16篇 |
1993年 | 18篇 |
1992年 | 19篇 |
1991年 | 17篇 |
1990年 | 13篇 |
1989年 | 21篇 |
1988年 | 17篇 |
1987年 | 15篇 |
1986年 | 18篇 |
1985年 | 15篇 |
1984年 | 17篇 |
1983年 | 14篇 |
1982年 | 24篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 13篇 |
1978年 | 13篇 |
1977年 | 25篇 |
1976年 | 9篇 |
1974年 | 8篇 |
1973年 | 10篇 |
1972年 | 10篇 |
1970年 | 15篇 |
1968年 | 8篇 |
排序方式: 共有1392条查询结果,搜索用时 500 毫秒
981.
Size homoplasy and mutational processes of interrupted microsatellites in two bee species, Apis mellifera and Bombus terrestris (Apidae) 总被引:6,自引:0,他引:6
Similar microsatellite electromorphs (PCR products of the same size) can
arise from independent mutational events. Such alleles are not identical by
descent. This phenomenon, termed size homoplasy, was studied by sequencing
electromorphs of two microsatellite loci in which the stretch of basic
repeats is interrupted by different short (1-2 bp) DNA motifs. The number
and position of these interruptions were established for electromorphs from
closely and distantly related populations of honeybees and bumblebees. No
sequence difference was found when electromorphs came from the same
subspecies or from closely related subspecies, suggesting that they were
probably identical by descent. In contrast, sequence differences were often
detected in distantly related subspecies, showing that size homoplasy
frequently occurs at this level of population differentiation. Size
homoplasy is increased by limits to free length variation of alleles, a
phenomenon that seems to act on interrupted microsatellites when comparing
distantly related taxa, that is, honeybee subspecies from different
evolutionary lineages. Electromorph sequences suggest that, within the
scope of these limits, large mutation events have occurred frequently at
both interrupted loci studied. In good agreement with the molecular data,
computations based on the observed heterozygosity and number of
electromorphs and simulation studies showed that neither locus fits the
one-step stepwise mutant model (SMM). We speculate that interrupted
microsatellites in general could be characterized by a higher variance in
repeat number and consequently a lower homoplasy rate than pure ones.
Hence, interrupted microsatellites should be most appropriate for
investigating population differentiation and evolutionary relationship
between relatively distant populations.
相似文献
982.
Solution studies on heme proteins. Circular dichroism and optical rotation of Glycera dibranchiata hemoglobins 总被引:1,自引:0,他引:1
Circular dichroism (CD) and optical rotatory dispersion (ORD) spectra of several liganded derivatives of the monomer and polymer hemoglobin components of the marine annelid, Glycera dibranchiata were measured over the wavelength range 650--195 nm. The differences observed between the monomer and polymer components for the heme dichroic bands in the visible, Soret and ultraviolet wavelength regions seem to result from changes in the heme environment, geometry and coordination state of the central heme iron in these proteins. Within the Soret region, the liganded derivatives of the monomer hemoglobin exhibit predominantly negative circular dichroic bands. The heme band at 260 nm is also absent for the monomer hemoglobin. The ORD and CD spectra in the far-ultraviolet, peptide absorbing region suggest also differences in the alpha-helix content of the monomer and polymer hemoglobins. The values for the single-chain G. dibranchiata hemoglobin are in the expected range (about 70% alpha-helix) as predicted by the X-ray structure of this protein. The lower estimates of the alpha-helix content for the polymer hemoglobin (approx. 50%), may reflect the differences in amino acid composition, primary structure and polypeptide chain foldings. Changes in oxidation state and ligand binding appears to have no pronounced effect on the helicity of either the monomer or polymer hemoglobins. The removal of the heme moiety from the monomer hemoglobin did result in a major decrease in its helix content similar to the loss of heme from myoglobin. 相似文献
983.
984.
Rodney E. Harrington 《Biopolymers》1968,6(1):105-116
The streaming birefringence and extinction angles of DNA from T2 bacteriophage in neutral aqueous buffers have been determined over a range of concentrations from 5 to 44 μg./ml. and of velocity gradients from 5 to 50 sec.?1. Although the extinction angles are relatively small even at the lowest velocity gradients and concentrations studied, a lower limit estimate of the limiting extinction angle to shear rate ratio can be made, and this value is interpreted in terms of available dynamical theory. The statistical segment length has been estimated from the birefringence data as ~ 1100 A. This figure is in satisfactory agreement with the results of independent calculations by others. 相似文献
985.
986.
The inner histone complex, extracted from chicken erythrocyte chromatin in 2 M NaCL AT pH 7.4, has been characterized by sedimentation equilibrium and sedimentation velocity. High speed sedimentation equilibrium studies indicate that in 2 M NaCl the inner histones are a weakly associating system with contributions from species ranging in molecular weight from dimer to octamer. The appearance of a single boundary (3.8S at 2 M NaCl) in sedimentation velocity studies conducted over a wide range of protein concentrations and ionic conditions indicates that the various histone oligomers present are in rapid equilibrium with one another. At higher salts the equilibrium is shifted to favor higher molecular weight species; in 4 M NaCl essentially all of the histone is octameric at protein concentrations above 0.2 mg/ml. The facile interconversion of histone oligomers suggests that small alterations in histone-histone interactions may be responsible for changes in nucleosome conformations during various biological processes. 相似文献
987.
988.
989.
Mature nucleated amphibian erythrocytes can be stimulated to undergo morphological changes resembling de-differentiation when exposed to small amounts of electric current. Puromycin and cyclohexamide will inhibit these morphological changes, and cytochemical staining indicates that RNA synthesis begins in concert with the changes in morphology. Autoradiographic studies show that the erythrocytes exposed to electric current synthesize RNA and protein whereas the erythrocytes not exposed to current do not make appreciable amounts of macromolecules. Electrophoretic separation of proteins from stimulated cells exhibit a band pattern different from that of unstimulated erythrocytes, and scintillation counting shows that some of the new proteins have been synthesized after electrical stimulation. 相似文献
990.
Crosslinking of myosin and myosin filaments 总被引:1,自引:0,他引:1
E Reisler M Burke R Josephs W F Harrington 《Journal of mechanochemistry & cell motility》1973,2(3):163-179