全文获取类型
收费全文 | 2767篇 |
免费 | 280篇 |
国内免费 | 1篇 |
出版年
2023年 | 23篇 |
2022年 | 25篇 |
2021年 | 71篇 |
2020年 | 46篇 |
2019年 | 54篇 |
2018年 | 70篇 |
2017年 | 59篇 |
2016年 | 100篇 |
2015年 | 130篇 |
2014年 | 170篇 |
2013年 | 181篇 |
2012年 | 254篇 |
2011年 | 197篇 |
2010年 | 130篇 |
2009年 | 137篇 |
2008年 | 134篇 |
2007年 | 137篇 |
2006年 | 104篇 |
2005年 | 98篇 |
2004年 | 104篇 |
2003年 | 89篇 |
2002年 | 94篇 |
2001年 | 73篇 |
2000年 | 47篇 |
1999年 | 45篇 |
1998年 | 32篇 |
1997年 | 37篇 |
1996年 | 24篇 |
1995年 | 19篇 |
1994年 | 17篇 |
1993年 | 17篇 |
1992年 | 47篇 |
1991年 | 33篇 |
1990年 | 27篇 |
1989年 | 20篇 |
1988年 | 24篇 |
1987年 | 20篇 |
1986年 | 21篇 |
1985年 | 18篇 |
1984年 | 16篇 |
1983年 | 16篇 |
1982年 | 7篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1977年 | 10篇 |
1976年 | 6篇 |
1973年 | 5篇 |
1972年 | 6篇 |
1970年 | 6篇 |
排序方式: 共有3048条查询结果,搜索用时 15 毫秒
21.
Ectopic Purkinje cells in the cerebellar white matter of normal adult rodents: a Golgi study 总被引:1,自引:0,他引:1
In Golgi/Río-Hortega preparations of rat and rabbit cerebellar vermis we have occasionally found isolated ectopic Purkinje cells in the white matter. They were located beneath the bases of the folia and their dendritic branches extended within the confines of the white matter without penetrating into the overlying cortical layers. The general morphology of these ectopic cells was variable, particularly in the extension and shape of the dendritic trees, but all of them exhibited a lower density of dendritic branches than normal Purkinje cells. The less-developed ectopic neurons had multipolar dendritic trees with nonplanar branches irregularly studded with spines. The well-developed ones displayed a more extensive arborization of their processes and they usually preserved some morphological features of normal cortical Purkinje cells: distal dendritic branches studded with numerous spines, a pear-shaped soma, clearly defined morphological polarity and a tendency to display planar arrangement of the dendritic arbors. In semithin sections these neurons also showed cytological features of normal Purkinje cells, such as the Nissl substance forming a nuclear cap oriented toward the dendritic pole. We suggest that the abnormal location of the neurons results from a disorder of Purkinje cell migration which occurs naturally during the prenatal development of the cerebellum. The possible morphogenetic mechanisms involved in the migration and differentiation of these ectopic neurons are also discussed. 相似文献
22.
D R Blanco J D Radolf M A Lovett J N Miller 《Journal of immunology (Baltimore, Md. : 1950)》1986,137(6):2031-2036
Normal human serum (NHS) was shown to have complement-dependent treponemicidal activity against both Treponema pallidum and Treponema phagedenis biotype Reiter (TPR) by employing in vitro-in vivo neutralization and TPR plaque assays, respectively. The molecular basis of NHS treponemicidal activity was studied by immunoblot analysis in conjunction with treponemicidal assays. Five major T. pallidum polypeptide bands (47kDa, 35kDa, 33kDa doublet, and 30 kDa) and three major TPR polypeptide bands (47kDa and 33kDa doublet) bound IgG present in NHS. Absorption of NHS with TPR completely removed both TPR and T. pallidum treponemicidal activity; corresponding immunoblots demonstrated a significant removal of IgG antibody against all three TPR polypeptide bands as well as four T. pallidum polypeptide bands (30kDa, 33kDa doublet, and 35kDa). In contrast, T. pallidum absorption of NHS was found to remove treponemicidal activity against T. pallidum but not TPR; corresponding Western blots showed the complete removal of IgG antibody against all but one T. pallidum polypeptide band (47kDa) but no detectable loss in IgG antibody against the TPR polypeptides. These results suggest that antibody in NHS generated against nonpathogenic, indigenous treponemes is responsible for the T. pallidum treponemicidal activity. Furthermore, the treponemicidal activity against T. pallidum correlated with the presence of IgG antibody against T. pallidum polypeptides of 30kDa, 35kDa, and a 33kDa doublet. 相似文献
23.
Genetic polymorphism in populations of Akodon rodents 总被引:1,自引:0,他引:1
By means of starch gel electrophoresis, 16-20 loci coding for enzymes and hemoglobin have been investigated in six population samples of Akodon dolores, captured in a single site of the Córdoba province (Argentina) during a 3-year period and in three samples of an Akodon azarae population. Proportion of polymorphic loci (P) ranged from 0.278 to 0.389 in A. dolores and from 0.166 to 0.300 in A. azarae. Mean heterozygosity (H) ranged from 0.138 to 0.192 in A. dolores and from 0.099 to 0.118 in A. azarae. These values are very high compared with those reported for northern hemisphere rodent populations. The high value is remarkable since the loci sample is biased towards the less variable (group I) enzymes. 相似文献
24.
L. I. Apfelbaum A. Blanco 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1985,70(5):569-572
Summary Six population samples of the South American cricetid rodent Akodon dolores were collected at the same site at six-month intervals over a three year period. Changes in density were detected. Seven out of 18 loci analyzed by means of starch gel electrophoresis were polymorphic. Only two of these loci (Est-4 and G6pdh) showed statistically significant variation in allele frequencies following a seasonal pattern. There was no correlation between allele frequencies and population density. When animals were grouped into two classes according to body weight, a clear difference in allele distribution at the Est-4 and G6pdh loci was observed between individuals 39 g or less and those heavier than 39 g. As the first group comprises predominantly younger animals, the data indicate that changes in the age-structure of population, rather than density variations, are responsible for the cyclic pattern of allele frequencies fluctuations. 相似文献
25.
Characterization of a 3''----5'' exonuclease activity in the phage phi 29-encoded DNA polymerase. 总被引:6,自引:2,他引:4
下载免费PDF全文
![点击此处可从《Nucleic acids research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Purified protein p2 of phage phi 29, characterized as a specific DNA polymerase involved in the initiation and elongation of phi 29 DNA replication, contains a 3'----5' exonuclease active on single-stranded DNA, but not on double-stranded DNA. No 5'----3' exonuclease activity was found. The 3'----5' exonuclease activity was shown to be associated with the DNA polymerase since 1) the two activities were heat-inactivated with identical kinetics and 2) both activities, present in purified protein p2, cosedimented in a glycerol gradient. 相似文献
26.
Phospholipid-Sensitive Ca2+ -Dependent Protein Kinase Preferentially Phosphorylates Serine-115 of Bovine Myelin Basic Protein 总被引:1,自引:0,他引:1
R. Scott Turner C.-H. Jen Chou† Gonzalo J. Mazzei Philip Dembure ‡ J. F. Kuo 《Journal of neurochemistry》1984,43(5):1257-1264
Phospholipid-sensitive Ca2+ -dependent protein kinase (PL-Ca-PK) and cyclic AMP-dependent protein kinase (A-PK) both preferentially phosphorylated serine residues of bovine myelin basic protein (MBP). Tryptic peptide maps of MBP phosphorylated by PL-Ca-PK or A-PK, however, revealed different phosphopeptides, suggesting a difference in the intramolecular substrate specificity for the two enzymes. Serine-115 of MBP, in the sequence (-Arg-Phe-Ser(115)-Trp-), was found to be a preferred and probably major phosphorylation site for PL-Ca-PK. Because serine-115 of bovine MBP corresponds to serine-113 of rabbit MBP, an in vivo phosphorylation site reported by Martenson et al. (1983), and PL-Ca-PK is present at a very high level in brain and myelin, it is suggested that the enzyme may be responsible for the in vivo phosphorylation of this and other sites in MBP. 相似文献
27.
Study of the hydrolysis and ionization constants of Schiff base from pyridoxal 5''-phosphate and n-hexylamine in partially aqueous solvents. An application to phosphorylase b.
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
J Donoso F Mu?oz A García Del Vado G Echevarría F García Blanco 《The Biochemical journal》1986,238(1):137-144
Formation and hydrolysis rate constants as well as equilibrium constants of the Schiff base derived from pyridoxal 5'-phosphate and n-hexylamine were determined between pH 3.5 and 7.5 in ethanol/water mixtures (3:17, v/v, and 49:1, v/v). The results indicate that solvent polarity scarcely alters the values of these constants but that they are dependent on the pH. Spectrophotometric titration of this Schiff base was also carried out. We found that a pKa value of 6.1, attributed in high-polarity media to protonation of the pyridine nitrogen atom, is independent of solvent polarity, whereas the pKa of the monoprotonated form of the imine falls from 12.5 in ethanol/water (3:17) to 11.3 in ethanol/water (49:1). Fitting of the experimental results for the hydrolysis to a theoretical model indicates the existence of a group with a pKa value of 6.1 that is crucial in the variation of kinetic constant of hydrolysis with pH. Studies of the reactivity of the coenzyme (pyridoxal 5'-phosphate) of glycogen phosphorylase b with hydroxylamine show that this reaction only occurs when the pH value of solution is below 6.5 and the hydrolysis of imine bond has started. We propose that the decrease in activity of phosphorylase b when the pH value is less than 6.2 must be caused by the cleavage of enzyme-coenzyme binding and that this may be related with protonation of the pyridine nitrogen atom of pyridoxal 5'-phosphate. 相似文献
28.
29.
In this paper two points are considered: the methods of evaluating the helical content θ and the calculation of the parameters of the transition from experimental data and its interpretation. The parameter ΔH obtained is in good agreement with the calorimetric one and v is found to be independent of temperature and solvent and in agreement with the ordinarily accepted value for poly(γ-benzyl-L -glutamate). The different methods of estimating θ are discussed for both polypeptides. 相似文献
30.
Serotypes and antibiotic resistance of Verotoxigenic (VTEC) and Necrotizing (NTEC) Escherichia coli strains isolated from calves with diarrhoea 总被引:2,自引:0,他引:2
Serotypes and antibiotic resistance of 51 Verotoxigenic (VTEC) and 33 Necrotizing (NTEC) bovine Escherichia coli strains were determined and compared with those shown by 205 non-VTEC non-NTEC strains isolated from the same batch of calves. E. coli untypable for O-antigen represented 47% of the VTEC, 12% of the NTEC and 8.8% of the non-VTEC non-NTEC. Typable VTEC belonged to serotypes 02:K?, 0103:K-, 0104:K?, 0128:K?, 0153:K- and O157:K-:H7, whereas typable NTEC were of serotypes 08:K87, 015:K14, 015:K-, 054:K?, 076:K-, 078:K(80), 088:K?, 0123:K-, 0139:K- and 0153:K-. Non-VTEC non-NTEC showed a wide variety of serotypes which were generally unrelated to those found in VTEC and NTEC. VTEC were resistant to antibiotics at higher rates than NTEC and non-VTEC non-NTEC, and showed also the highest multidrug-resistant pattern. Our results show that bovine VTEC strains belonged to O-groups usually found in human VTEC causing sporadic diarrhoea, haemorrhagic colitis and/or haemolytic uraemic syndrome, such as 02, 0103, 0104, 0153 and especially 0128 and O157. In contrast, bovine NTEC strains belonged to serotypes different from those previously found in necrotizing E. coli strains of human origin. 相似文献