全文获取类型
收费全文 | 8712篇 |
免费 | 704篇 |
国内免费 | 6篇 |
出版年
2023年 | 47篇 |
2022年 | 108篇 |
2021年 | 215篇 |
2020年 | 121篇 |
2019年 | 183篇 |
2018年 | 196篇 |
2017年 | 151篇 |
2016年 | 305篇 |
2015年 | 462篇 |
2014年 | 527篇 |
2013年 | 598篇 |
2012年 | 751篇 |
2011年 | 752篇 |
2010年 | 411篇 |
2009年 | 387篇 |
2008年 | 540篇 |
2007年 | 567篇 |
2006年 | 461篇 |
2005年 | 445篇 |
2004年 | 390篇 |
2003年 | 360篇 |
2002年 | 332篇 |
2001年 | 57篇 |
2000年 | 44篇 |
1999年 | 72篇 |
1998年 | 78篇 |
1997年 | 46篇 |
1996年 | 46篇 |
1995年 | 50篇 |
1994年 | 37篇 |
1993年 | 37篇 |
1992年 | 48篇 |
1991年 | 33篇 |
1990年 | 35篇 |
1989年 | 26篇 |
1988年 | 26篇 |
1987年 | 29篇 |
1986年 | 22篇 |
1985年 | 29篇 |
1984年 | 39篇 |
1983年 | 21篇 |
1982年 | 41篇 |
1981年 | 20篇 |
1980年 | 14篇 |
1979年 | 17篇 |
1978年 | 17篇 |
1977年 | 14篇 |
1976年 | 13篇 |
1975年 | 18篇 |
1969年 | 11篇 |
排序方式: 共有9422条查询结果,搜索用时 109 毫秒
91.
The teuthids of the collection R. Stahlecker, 1929, from the Isle of Maio (Cape Verde Islands) are described.Neololigosepia stahleckeri n. gen. n. sp. andMaioteuthis morroensis n. gen. n. sp. are one of the first teuthids from the Lower Cretaceous (Barremian). 相似文献
92.
Norman R. Drinkwater Rebecca C. Corner J. Justin McCormick Veronica M. Maher 《Mutation research》1982,106(2):277-289
The sensitivity of diploid human fibroblasts to the cytotoxic effects of diphtheria toxin (DT) depended on the cell growth status. Exponentially growing cells treated with 10?3-1 lethal flocculating units (LF) of DT/ml for 4 days survived with a frequency of 4 × 10?4. However, the DT-resistant phenotype of colonies isolated under these conditions was not stable. When the growth of the cells had been arrested by confluence or deprivation of serum growth factors prior to treatment with DT (4 days, 10?3-0.6 LF/ml), the survival decreased to 2 × 10?6 and the resistance of isolated colonies was stable. An in situ assay for induced DT-resistant mutants was developed in order to avoid problems associated with the possible reduced viability of the mutants relative to that of wild-type cells. A reproducible and linear dose response was obtained for the induction of DT-resistant mutants by ethylnitrosourea. The mutants were induced with high frequency by this compound (e.g., 10?3 mutants/viable cell at a 37% survival dose); complete expression of the mutant phenotype occurred after 6 generations of growth under nonselective conditions. Isolated mutant colonies showed stable resistance to DT and were cross-resistant to Pseudomonas aeruginosa exotoxin A. 相似文献
93.
Ferdinand Bohlmann Naleen Borthakur Jasmin Jakupovic Joachim Pickard 《Phytochemistry》1982,21(6):1357-1360
An investigation of Ursinia saxatilis afforded in addition to known compounds a new eudesmanolide derived from ursialpinolide, a germacranolide rel 相似文献
94.
A new technique, the quantitative determination of total enzyme concentrations by specific immunoprecipitation with purified, radioiodinated antibodies, was used to investigate the presence and possible roles of inactive enzyme in the regulation of chalcone synthase. Dark-grown cell suspension cultures from parsley (Petroselinum hortense) contained neither catalytically active nor detectable amounts of immunoprecipitable chalcone synthase. Irradiation induced large increases and subsequent decreases of both. Significant differences in the peak positions and in the half-lives of active and total chalcone synthase indicated that induced cells contained inactive as well as active enzyme forms. The presence of inactive enzyme could be explained by two different modes of regulation, (i) simultaneous de novo synthesis of active and inactive enzyme (“Simultaneous Model”), or (ii) de novo synthesis of active enzyme only, with sequential steps of inactivation and degradation (“Sequential Model”). Both models were compatible with experimental results, as analyzed mathematically by investigating the relations between curves for rate of enzyme synthesis, enzyme activity, total enzyme, and half-lives of active and total enzyme. However, the “Simultaneous Model” postulated that de novo synthesis of inactive enzyme represented always the vast majority of total enzyme synthesis, while the Sequential Model integrated inactive enzyme with facility in a sequence of irreversible inactivation and degradation of active enzyme. Experiments with repeated induction indicated that cells containing large amounts of inactive enzyme increased enzyme activity by de novo synthesis rather than by activation of preexisting inactive enzyme. 相似文献
95.
Twelve indole alkaloids belonging to the Ajmaline-, Sarpagine-, Yohimbine-, and Heteroyohimbine-type have been isolated and identified from cell suspension cultures of Rauwolfia serpentina. Ten of the alkaloids were found for the first time in cultured R. serpentina cells. The yield of the main alkaloid vomilenine was 51 times more than that of differentiated plants. Crude enzymes isolated from this cell suspension culture completely metabolize the biogenetic precursor strictosidine under formation of several alkaloidal compounds. 相似文献
96.
Ann E. Aust Michael R. Antczak Veronica M. Maher J. Justin McCormick 《Journal of cellular biochemistry》1981,16(3):269-279
Human cells that appear capable of metabolizing various classes of carcinogens have been identified using one of two methods: metabolism of tritiated benzo(a)pyrene to aqueous-acetone soluble forms or inhibition of cellular DNA synthesis. Each of the assay systems was optimized and the results on 15 human epithelial cell lines were compared. One or more cell lines were found to activate each of four classes of carcinogens examined: polycyclic hydrocarbons, aromatic amines, heterocyclic hydrocarbons, and nitrosamines. Cells that appeared capable of metabolizing polycyclic hydrocarbons or aromatic amines by these methods were also found to produce metabolites which were cytotoxic to cocultivated human xeroderma pigmentosum fibroblasts after a 48-hr exposure to the carcinogen. 相似文献
97.
Summary Addition of heterochromatin suppresses while subtraction enhances position effect variegation. The heterochromatin-sensitive period has been determined in white/white-apricot variegated eyes of Y
S
w
a
/w
a
; Dp (1;3) w
265-58 flies. When such larvae, carrying a Y-short (Y
S
) arm at the distal end of one X chromosome, are X-rayed, mitotic recombination leads to one daughter cell with two Y
S
arms and an adjacent daughter cell with no Y
S
arm. When induced after clonal initiation, the frequency of dark clones developing from daughter cells with two Y
S
arms is significantly higher than the frequency of dark clones in the rest of the eye; and this frequency is. even higher when induced before clonal initiation. The modifying action of the Y-heterochromatin is exerted, therefore, during and after clonal initiation. Surprisingly, the frequency of dark clones developing from cells with no Y
S
arm is not lower than the frequency of dark clones in the rest of the eye. 相似文献
98.
Veronica M. Maher J. Justin McCormick Phillip L. Grover Peter Sims 《Mutation research》1977,43(1):117-137
The cytotoxicity of the “K-region” epoxides as well as several other reactive metabolites or chemical derivatives of polycyclic hydrocarbons was compared in normally-repairing human diploid skin fibroblasts and in fibroblasts from a classical xeroderma pigmentosum (XP) patient (XP2BE) whose cells have been shown to carry out excision repair of damage induced in DNA by ultraviolet (UV) radiation at a rate approx. 20% that of normal cells. Each compound tested exhibited a 2- to 3-fold greater cytotoxicity in this XP strain than in the normal strain. To determine whether this difference in survival reflected a difference in the capacity of the strains to repair DNA damage caused by such hydrocarbon derivatives, we compared the cytotoxic effect of several “K-region” epoxides in two additional XP strains, each with a different capacity for repair of UV damage. The ration of the slopes of the survival curves for each of the XP strains to that of the normal strain, following exposure to each epoxide, was very similar to that which we had previously determined for their respective UV curves, suggesting that human cells repair damage induced in DNA by exposure to hydrocarbon derivatives with the same system used for UV-induced lesions.To determine whether the deficiency in rate of excision repair in this classical XP strain (XP2BE) causes such cells to be abnormally susceptible to mutations induced by “K-region” epoxides of polycyclic hydrocarbons, we compared them with normal cells for the frequency of induced mutations to 8-azaguanine resistance. The XP cells were two to three times more susceptible to mutations induced by the “K-region” epoxide of benzo(a)pyrene (BP), 7,12-dimethylbenz(a)anthracene (DMBA), and dibenz(a,h)anthracene (DBA). Evidence also was obtained that cells from an XP variant patient are abnormally susceptible to mutations induced by hydrocarbon epoxides and, as is the case following exposure to UV, are abnormally slow in converting low molecular weight DNA, synthesized from a template following exposure to hydrocarbon epoxides, into large-size DNA. 相似文献
99.
Summary Iodination of proteins and lipoproteins is a widely used in vitro labelling procedure in metabolic, autoradiographic and various other studies. However, all available iodination techniques have involved the possible damage to the proteins by self-irradiation, oxidizing agents, the alkaline milieu or by the introduction of iodine into the molecular structure itself. To evaluate the integrity of iodinated lipoproteins, we observed the electron microscopic appearance of normal and iodinated rabbit very low density lipoproteins (VLDL) by negative staining with phosphotungstic acid. Iodination up to a molar iodine/protein ratio of 2.89 did not result in any change of shape, size or aggregating tendency of the particles. No stacks or disk-like particles like those of various hyperlipoproteinemic states were found. We conclude that electron microscopy is a valuable tool in assessing the morphological appearance of lipoprotein iodination, but it should be complemented by other techniques. 相似文献
100.