全文获取类型
收费全文 | 559篇 |
免费 | 33篇 |
学科分类
生物科学 | 592篇 |
出版年
2022年 | 4篇 |
2020年 | 7篇 |
2018年 | 6篇 |
2017年 | 14篇 |
2016年 | 16篇 |
2015年 | 17篇 |
2014年 | 22篇 |
2013年 | 34篇 |
2012年 | 40篇 |
2011年 | 33篇 |
2010年 | 17篇 |
2009年 | 19篇 |
2008年 | 26篇 |
2007年 | 17篇 |
2006年 | 17篇 |
2005年 | 7篇 |
2004年 | 11篇 |
2003年 | 12篇 |
2002年 | 17篇 |
2001年 | 17篇 |
2000年 | 7篇 |
1999年 | 13篇 |
1998年 | 9篇 |
1996年 | 7篇 |
1995年 | 7篇 |
1994年 | 4篇 |
1993年 | 8篇 |
1992年 | 7篇 |
1991年 | 9篇 |
1990年 | 5篇 |
1989年 | 8篇 |
1988年 | 4篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 11篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 11篇 |
1978年 | 6篇 |
1977年 | 11篇 |
1976年 | 7篇 |
1975年 | 5篇 |
1974年 | 9篇 |
1973年 | 5篇 |
1972年 | 12篇 |
1971年 | 7篇 |
1969年 | 6篇 |
排序方式: 共有592条查询结果,搜索用时 15 毫秒
1.
2.
High level of divergence of male-reproductive-tract proteins, between Drosophila melanogaster and its sibling species, D. simulans 总被引:1,自引:0,他引:1
We compared male-reproductive-tract polypeptides of Drosophila melanogaster
and D. simulans by using two-dimensional gel electrophoresis. Approximately
64% of male-reproductive-tract polypeptides were identical between two
randomly chosen isofemale lines from these two species, compared with 83%
identity for third-instar imaginal wing-disc polypeptides. Qualitatively
similar differences were found between reproductive tracts and imaginal
discs when D. sechellia was compared with D. melanogaster and with D.
simulans. When genic polymorphism was taken into account, approximately 10%
of male- reproductive-tract polypeptides were apparently fixed for
different alleles between D. melanogaster and D. simulans; this proportion
is the same as that found for soluble enzymes by one-dimensional gel
electrophoresis. Strikingly, approximately 20% of male-reproductive- tract
polypeptides of either D. melanogaster or D. simulans had no detectable
homologue in the other species. We propose that proteins of the Drosophila
male reproductive tract may have diverged more extensively between species
than have other types of proteins and that much of this divergence may
involve large changes in levels of polypeptide expression.
相似文献
3.
A K Singhal B I Cohen J Finver-Sadowsky C K McSherry E H Mosbach 《Journal of lipid research》1984,25(6):564-570
The effect of various dietary additions such as cholesterol, beta-sitosterol, bile acids, and bile acid analogs on gallstone formation was studied in the hamster. Gallstones were formed in 50% of the animals fed a high glucose, fat-free diet. Administration of 0.2% cholesterol or 1% beta-sitosterol had no effect on the incidence of gallstones. Ursodeoxycholic acid (0.5%) and its analog ursodeoxy-oxazoline [2-(3 alpha, 7 beta-dihydroxy-24-nor-5 beta-cholanyl)-4,4-dimethyl-2- oxazoline] were ineffective in preventing gallstones. Hyodeoxycholic acid and hyodeoxy-oxazoline [2-(3 alpha,6 alpha-dihydroxy-24-nor-5 beta-cholanyl)-4,4-dimethyl-2- oxazoline] at the same dosage effectively prevented gallstones, while the trihydroxy bile acid, hyocholic acid, was not effective. Of all the dietary regimens tested, only hyodeoxycholic acid significantly lowered serum cholesterol. The lithogenic diet produced a five-fold increase in hepatic HMG-CoA reductase activity; this activity was not affected by dietary cholesterol or beta-sitosterol. Hyodeoxycholic acid and hyocholic acid feeding increased the reductase activity by an additional 50% while the other bile acids had no effect. beta-Sitosterol doubled the cholesterol 7 alpha-hydroxylase activity whereas hyodeoxy-oxazoline lowered it. Hyodeoxycholic acid-fed animals had significantly lower cholesterol absorption than the animals on the lithogenic diet alone. Biliary cholesterol content increased dramatically in the animals fed the lithogenic diet and was increased still further by ursodeoxycholic acid, hyodeoxycholic acid, and hyodeoxy-oxazoline. These data show that hyodeoxycholic acid and hyodeoxy-oxazoline do not prevent gallstones by inhibiting hepatic cholesterol synthesis or biliary cholesterol secretion. 相似文献
4.
Comparison of Beclomethasone Dipropionate Aerosol and Prednisolone in Reversible Airways Obstruction
S. Lal D. M. Harris K. K. Bhalla S. N. Singhal A. G. Butler 《BMJ (Clinical research ed.)》1972,3(5822):314-317
This paper describes a double-bond crossover trial of prednisolone and beclomethasone dipropionate aerosol in 38 steroid-dependent patients with reversible diffuse airways obstruction. Altogether there was no difference in the patient''s preference of the two treatment groups or in the number of times they used their bronchodilator aerosol, or in the forced expiratory volume in one second, vital capacity, or peak expiratory flow rate in the two treatment groups. The plasma cortisol levels when the patients were on the aerosol were much higher than when they were on prednisolone. The use of inhaled aerosol steroids seems to be preferable as it eliminates the usual complications of oral steroid therapy. 相似文献
5.
6.
7.
8.
The effect of Ca2+ and Mg2+ on relative fluidity of phosphatidylcholine liposomes was studied by measuring the degree of chlorophyll fluorescence polarization.
An increase in the degree of fluorescence polarization was observed on incubation of liposomes with different concentrations
of Ca2+ or Mg2+. The results have been interpreted on the basis of increase in the size of liposomes which could be brought about by calcium
or magnesium induced fusion of small unilamellar liposomes to form larger vesicles. Fusion of liposomes has also been confirmed
by the experiments on efficiency of energy transfer from chlorophyll b to chlorophyll a, and transmission electron microscopy
of liposomes before and after incubation with Ca2+ and Mg2+. 相似文献
9.
Boronate forms an anionic complex with the cis-2′,3′ hydroxyls of unsubstituted ribonucleosides and the 3′-terminal adenosine of unacylated tRNAs, but not with ribosesubstituted nucleosides such as 2′-O-methylnucleosides and aminoacyl-tRNAs. We have synthesized phenyl boronates with hydrophobic side chains of about 1-nm-long and coated inert 10-μm solid beads of polychlorotrifluoroethylene with this material. This matrix complexes easily with compounds containing free cis-hydroxyls, but not with their O-alkyl or O-acyl derivatives. This permits the separation of mammalian and bacterial amino-acyl-tRNAs from uncharged tRNAs and O-methyl nucleosides from ribose-unsubstituted nucleosides in one chromatographic step, as the substituted members of each group do not undergo boronate complex formation and are thus not as much retarded in passing through the column. Complex formation between ribofuranoses and the boronate matrix appears to be enhanced by the hydrophobic “tail” of the boronate compound, by the high ionic environment of the solvent, and by the hydrophobic nature of the inert support. This method of one-step purification of tRNAs on reversed-phase boronate columns has been tested for several tRNAs specific for amino acids of different hydrophobicity and ionic character. The results indicate that each tRNA tested can be purified with appreciable purity (70–95%) and high yield (80%). However, recovery of the queuine base containing aminoacyl-tRNAs is only about 6% of the applied material. Several other boronate matrices have also been synthesized using cellulose, agarose. Sepharose, or porous glass beads as the inert support with different lengths of the spacer arm. Cellulose with a 1-nm-long spacer arm is satisfactory not only for the separation of aminoacyl-tRNAs and O-methylribose nucleosides, but also for the separation as a group of tRNAs containing the base of Q, queuine. However, other inert supports are unsatisfactory because of a non-specific binding of the tRNAs. 相似文献
10.
The lymphoproliferative responses of rat peripheral blood lymphocytes to phytohemagglutinin (PHA) were studied following treatment with single or multiple doses of cyclophosphamide. A dose-dependent lymphocytopenia was observed with both regimes. The remaining lymphocytes had decreased responses to PHA. Serum collected 24 hr after a single injection of cyclophosphamide and used at a concentration of 5% enhanced the response of cells from normal or cyclophosphamide-treated rats. Serum collected after a course of treatment did not have this effect, but it lacked the marked suppressive activity, at a concentration of 20%, which was shown by normal rat serum. The enhancing activity was not dialysable. Doses of cyclophosphamide adequate to abolish primary antibody production to sheep erythrocytes did not totally abrogate responsiveness to PHA. Thus, the pattern of immunological defects in cyclophosphamide-treated rats consisted of decreased primary antibody production, lymphocytopenia with a decreased response of the remaining lymphocytes to PHA, and diminution of serum suppressive activity. 相似文献