全文获取类型
收费全文 | 7910篇 |
免费 | 770篇 |
国内免费 | 6篇 |
出版年
2022年 | 45篇 |
2021年 | 78篇 |
2020年 | 55篇 |
2019年 | 82篇 |
2018年 | 98篇 |
2017年 | 99篇 |
2016年 | 153篇 |
2015年 | 283篇 |
2014年 | 288篇 |
2013年 | 402篇 |
2012年 | 541篇 |
2011年 | 508篇 |
2010年 | 321篇 |
2009年 | 317篇 |
2008年 | 503篇 |
2007年 | 514篇 |
2006年 | 460篇 |
2005年 | 475篇 |
2004年 | 455篇 |
2003年 | 433篇 |
2002年 | 354篇 |
2001年 | 84篇 |
2000年 | 72篇 |
1999年 | 87篇 |
1998年 | 108篇 |
1997年 | 88篇 |
1996年 | 86篇 |
1995年 | 76篇 |
1994年 | 57篇 |
1993年 | 69篇 |
1992年 | 75篇 |
1991年 | 58篇 |
1990年 | 62篇 |
1989年 | 67篇 |
1988年 | 64篇 |
1987年 | 55篇 |
1986年 | 47篇 |
1985年 | 57篇 |
1984年 | 62篇 |
1983年 | 59篇 |
1982年 | 68篇 |
1981年 | 57篇 |
1980年 | 62篇 |
1979年 | 55篇 |
1978年 | 43篇 |
1977年 | 37篇 |
1976年 | 41篇 |
1975年 | 37篇 |
1974年 | 57篇 |
1973年 | 43篇 |
排序方式: 共有8686条查询结果,搜索用时 15 毫秒
21.
Recessive Lethal Mutations and the Maintenance of Duplication-Bearing Strains of Dictyostelium discoideum 总被引:1,自引:0,他引:1 下载免费PDF全文
Recessive lethal mutations have been isolated and used to maintain n + 1 aneuploid strains of Dictyostelium discoideum carrying a duplication of part or all of linkage group VII. The recessive lethal mutations, relA351 and relB352, arose spontaneously in diploids; no mutagenic treatment was used in the isolation of these mutations. The probable gene order on linkage group VII is: centromere, relB couA, bsgB, cobA, relA. Maintenance of aneuploids disomic for linkage group VII was made possible by complementation of a rel mutation on each linkage group VII homologue by the corresponding wild-type allele on the other linkage group VII homologue. The duplication-bearing disomic strains were slow-growing and produced faster-growing sectors on the colony edge. Haploid sectors probably arise by a combination of mitotic recombination and subsequent loss of one homologue, diploid sectors may be formed by chromosome doubling to 2n + 2, followed by chromosome loss to return to 2n, and aneuploid sectors may arise by deletion or new mutation. 相似文献
22.
Attached bacterial populations cultured without an exogenous carbon source or grown in conjunction with attached diatoms incorporated [3H]thymidine at a rate between four and five times lower than that of replicate bacterial populations which were dispersed before being assayed. 相似文献
23.
Two expressed brewer's yeast tRNAsPhe, a major and a minor one, have been purified and sequenced. The major tRNAPhe corresponds to the already known tRNAPhe, whereas the minor one differs from the former in the substitution of T6-A67 by C6-G67 base pair in the "acceptor stem". The minor tRNAPhe contaminates all preparations of yeast tRNAPhe except those prepared by polyacrylamide gel electrophoresis. 相似文献
24.
Plasmid-free Enterococcus faecalis excrete peptides (sex pheromones) which specifically induce a mating response in strains harboring certain conjugative plasmids. The response is characterized by the synthesis of a “fuzzy” surface material, visible by electron microscopy, which is believed to facilitate the aggregation of donors and recipients. Transconjugants which receive a specific plasmid shut down the production of endogenous pheromone; however, they continue to produce pheromones specific for donors harboring different classes of plasmids. In this review, we summarize what is known about the biochemistry and genetics of this phenomenon. Some emphasis is given to the hemolysin plasmid pAD1 and the regulation of its conjugal transfer. 相似文献
25.
Shobha Thangada Keith Alvares Mario Mangino Mohammed I. Usman M.Sambasiva Rao Janardan K. Reddy 《FEBS letters》1989,250(2):205-210
Using the normal adult rat hepatocytes, plated on rat tail collagen-coated dishes and fed a chemically defined medium, we demonstrate here that ciprofibrate at 0.1 mM concentration, increases significantly the mRNA levels of fatty acyl-CoA oxidase, enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase bifunctional protein, and thiolase (the three enzymes of the β-oxidation system), and causes peroxisome proliferation. Increase in mRNA levels of these genes was evident within 1 h and was maximal 24 h after the addition of ciprofibrate. In hepatocytes cultured in the absence of ciprofibrate, the basal levels of these enzymes were low and further declined with time. Concomitant treatment of hepatocytes with cycloheximide did not inhibit or superinduce the mRNA levels, indicating that this induction may represent a primary (direct) effect of this compound on the expression of these genes and does not apparently involve short-lived repressor protein(s). 相似文献
26.
Physiological Characterization of Dicarboxylate-Induced Pleomorphic Forms of Bradyrhizobium japonicum 总被引:1,自引:0,他引:1 下载免费PDF全文
When Bradyrhizobium japonicum I-110 was transferred into medium containing 40 mM succinate or 40 mM fumarate, over 90% of the bacteria acquired a swollen, pleomorphic form similar to that of bacteroids. The induction of pleomorphism was dependent on the carbon substrate and concentration but was independent of the hydrogen ion and sodium ion concentration. Cell extracts of rod-shaped and pleomorphic cells contained enzymes required for sugar catabolism and gluconeogenesis. Variations in these enzyme profiles were correlated with the carbon source used and not with the conversion to the bacteroid-like morphology. Rod-shaped cells cultured on glucose or 10 mM succinate transported glucose and succinate; however, the pleomorphic cells behaved similarly to symbiotic bacteroids in that they lacked the ability to transport glucose and transported succinate at lower rates than did rod-shaped cells. 相似文献
27.
The relationship between actual and perceived profitability was investigated by quantifying choice behavior of foraging honey bees (Apis mellifera).Rate of net energy gain was used as a measure of profitability. Individual bees repeatedly chose between a yellow and a blue tubular artificial flower which had different associated profitabilities. Handling costs were manipulated by using flowers of different depth;energy gains were manipulated by using different volumes of a 20% (w/w) sucrose solution. Bees' discrimination between the two flowers increased as the absolute difference in profitabilities increased and discrimination decreased as profitabilities of both flowers increased. Therefore, discrimination depended on the relative difference in profitability between flowers, and therefore, perception was a nonlinear function of actual rates of net energy gain. The results also suggested that motivation to choose between flowers depended on the level of profitability. We suggest that nonlinear perceptual scales and motivation should be incorporated into models of food choice as constraints and should be considered in the design of experiments used to test predictions of models and in the interpretation of experimental results. 相似文献
28.
Marijo G. Kent Robert N. Shoffner Allan Hunter Keith O. Elliston Wesley Schroder Elizabeth Tolley Stephen S. Wachtel 《Human genetics》1988,79(4):321-328
Summary An inherited genetic disorder causes XY embryos of the horse to develop as mares. On the basis of our study of 38 such mares, we have identified four grades or classes of XY sex reversal according to this scheme: class I, nearly normal female, of which some are fertile; class II, female with gonadal dysgenesis, normal mullerian development; calss III, intersex mare with gonadal dysgenesis, abnormal mullerian development, enlarged clitoris; class IV, virilized intersex characterized by high levels of testosterone. In general, class I and calss II mares were typed H-Y antigen-negative whereas class III and class IV mares were typed H-Y antigen-positive. 相似文献
29.
A catalogue of Clostridium thermocellum endoglucanase, β-glucosidase and xylanase genes cloned in Escherichia coli 总被引:3,自引:0,他引:3
Geoffrey P. Hazlewood Marek P.M. Romaniec Keith Davidson Olivier Grépinet Pierre Béguin Jacqueline Millet Odette Raynaud Jean-Paul Aubert 《FEMS microbiology letters》1988,51(2-3):231-236
Abstract Two independent collections of clones containing Clostridium thermocellum genes involved in cellulose have been previously obtained at IAPGR, Cambridge, and at the Pasteur Institute, Paris. The two collections were compared for cross-hybridization, restriction maps and enzyme phenotypes. Truly distinct genes were one β-glucosidase gene, two xylanase genes, and fifteen endogluconase genes. Two of the cloned fragments contained extraneous DNA which was absent from their respective counterparts isolated in the other collection. The dicrepancies resulted from in vivo rearrangements which had occurred in either of the C. thermocellum NCIB 10682 stocks used to generate the two gene banks. 相似文献
30.
The relationship among egg size,density and food level on larval development in the wood frog (Rana sylvatica) 总被引:1,自引:0,他引:1
Summary Although inter- and intraspecific variation in egg size among amphibians has been well documented, the relationship between egg size and fitness remains unclear. Recent attempts to correlate egg size intraspecifically with larval developmental patterns have been equivocal. In this study the development of larvae derived from large eggs and small eggs, from a single population in Maryland were compared under a range of food levels and larval population densities. Both food level and density had significant effects on the length of the larval period and size at metamorphosis. However, the response among larvae derived from different egg sizes was not additive. At low densities and high food levels, larvae from small eggs had longer larval periods and a larger size at metamorphosis than larvae derived from large eggs. In contrast, at high densities larvae from small eggs had longer developmental periods but were smaller at metamorphosis than larvae from large eggs. In addition, larvae from small eggs were more sensitive to density irrespective of food level. These results suggest that optimal egg size is correlated with environmental factors, which may explain the maintenance of both geographic and within population variation in egg size commonly observed in amphibians. 相似文献