全文获取类型
收费全文 | 4113篇 |
免费 | 335篇 |
国内免费 | 2篇 |
出版年
2023年 | 26篇 |
2022年 | 19篇 |
2021年 | 68篇 |
2020年 | 54篇 |
2019年 | 63篇 |
2018年 | 75篇 |
2017年 | 92篇 |
2016年 | 140篇 |
2015年 | 202篇 |
2014年 | 239篇 |
2013年 | 277篇 |
2012年 | 363篇 |
2011年 | 322篇 |
2010年 | 227篇 |
2009年 | 182篇 |
2008年 | 230篇 |
2007年 | 248篇 |
2006年 | 208篇 |
2005年 | 197篇 |
2004年 | 177篇 |
2003年 | 151篇 |
2002年 | 174篇 |
2001年 | 45篇 |
2000年 | 50篇 |
1999年 | 52篇 |
1998年 | 44篇 |
1997年 | 33篇 |
1996年 | 36篇 |
1995年 | 31篇 |
1994年 | 31篇 |
1993年 | 22篇 |
1992年 | 20篇 |
1991年 | 31篇 |
1990年 | 29篇 |
1989年 | 29篇 |
1988年 | 32篇 |
1987年 | 17篇 |
1986年 | 12篇 |
1985年 | 18篇 |
1984年 | 15篇 |
1983年 | 14篇 |
1982年 | 12篇 |
1981年 | 13篇 |
1980年 | 9篇 |
1979年 | 11篇 |
1978年 | 8篇 |
1977年 | 8篇 |
1976年 | 12篇 |
1973年 | 14篇 |
1972年 | 7篇 |
排序方式: 共有4450条查询结果,搜索用时 252 毫秒
41.
Marcel Weber 《Biology & philosophy》1996,11(1):67-88
Enzyme directed genetic mechanisms causing random DNA sequence alterations are ubiquitous in both eukaryotes and prokaryotes. A number of molecular geneticist have invoked adaptation through natural selection to account for this fact, however, alternative explanations have also flourished. The population geneticist G.C. Williams has dismissed the possibility of selection for mutator activity on a priori grounds. In this paper, I attempt a refutation of Williams' argument. In addition, I discuss some conceptual problems related to recent claims made by microbiologists on the adaptiveness of molecular variety generators in the evolution of prokaryotes. A distinction is proposed between selection for mutations caused by a mutator activity and selection for the mutator activity proper. The latter requires a concept of fitness different from the one commonly used in microbiology. 相似文献
42.
43.
Jiří Fajkus Aleš Kovařík Robert mKrálovics Milan Bezděk 《Molecular genetics and genomics : MGG》1995,247(5):633-638
We have examined the structure and chromatin organization of telomeres in Nicotiana tabacum. In tobacco the blocks of simple telomeric repeats (TTTAGGG)n are many times larger than in other plants, e.g., Arabidopsis thatiana or tomato. They are resolved as multiple fragments 60–160 kb in size (in most cases 90–130 kb) on pulsed-field gel electrophoresis (PFGE) of restriction endonuclease-digested DNA. The major subtelomeric repeat of the HRS60 family forms large homogeneous blocks of a basic 180 by motif having comparable lengths. Micrococcal nuclease (MNase) cleaves tobacco telomeric chromatin into subunits with a short repeat length of 157±5 bp; the subtelomeric heterochromatin characterized by tandemly repeated sequences of the HRS60 family is cut by MNase with a 180 by periodicity. The monomeric and dimeric particles of telomeric and subtelomeric chromatin differ in sensitivity to MNase treatment: the telomeric particles are readily digested, producing ladders with a periodicity of 7 bp, while the subtelomeric particles appear to be rather resistant to intranucleosomal cleavage. The results presented show apparent similarities in the organization of telomeric chromatin in higher plants and mammals. 相似文献
44.
Stephen C. Stearns Marcel Kaiser Tadeusz J. Kawecki 《Journal of evolutionary biology》1995,8(5):539-557
Canalization describes the process by which phenotypic variation is reduced by developmental mechanisms. A trait can be canalized against environmental or genetic perturbations. Stabilizing selelction should favor improved canalization, and the degree of a trait's canalization should be positively correlated with its impact on fitness. Here we report, for Drosophila melanogaster, measurements of environmental canalization for five fitness components. We compare them with measurements of genetic canalization, and we discuss the impact of inbreeding on both. In three experiments we measured the variation of fitness components within lines nested within temperature, treatment, and experiment. Lines differed in the position of a P element insert or in genetic background. Within lines flies were genetically nearly identical. We designated trait variation within lines as environmental canalization. The canalization of the traits increased with their impact on fitness, and the pattern was similar to that found for the canalization of fitness components against genetic differences, measured as the variation among lines nested within temperature, treatment, and experiment. This suggests that developmental mechanisms buffer the phenotype against both genetic and environmental disturbance. The results also suggest, less strongly, that inbreeding weakens canalization. 相似文献
45.
Jean-Pierre Berry Marcel Chaintreau Danile Chassoux Robert Dennebouy Franoise Escaig Pierre Galle Jean-Michel Rossignol George Slodzian Sylvie Tlouzeau 《Biology of the cell / under the auspices of the European Cell Biology Organization》1994,81(1):65-72
Summary— The intra-cellular distribution of eight halogen glucocorticoids was investigated by ion microscopy in two cellular varieties of cultured non-cancer cells (fibroblast 3T3) and cancer cells (human breast tumor cells MCF-7). Two types of ion microscopy helped to determine this distribution, a direct imaging ion microscope (SMI 300) with low spatial resolution, and a scanning ion microscope (IMS4F), featuring high resolution, serving to obtain maps representing the intra-cellular distribution of the fluorine elements and drugs present in these monolayer cultured cells. The fluorine images representative of the drugs containing fluorine showed that these drugs are essentially concentrated in the cell nuclei. In these nuclei, the distribution of these drugs is different from that of heterochromatin and of the nucleolus. 相似文献
46.
In a controlled animal experiment the effects of dietary subacute Zn deficiency on growth, Zn concentration, and tissue 42-K
distribution were studied. Growth retardation caused lower body weight because both skeletal and heart muscle showed a reduction
in cell mass. Zn concentrations were reduced in most tissues, however, they remained unaltered in heart muscle. 42-K activity
increased in skeletal muscle and pancreas. We hypothesize the latter reflects the organs rate of metabolism, inducing the
exocrine pancreas to increase Zn absorption; in skeletal muscle it may induce also alterations in cell potentiation, causing
restless behavior. As suggested by the calculated specific K activity (Bq/mol), the K uptake was highest in liver and bone,
high in pancreas and skeletal muscle and low in heart muscle. The latter suggests K retention in heart muscle. Specific activity
in plasma and jejunum remained unaltered: K status and absorption seem unaffected. Zn deficiency causes different 42-K activities
in the various tissues, that respond by alterations in K metabolism without the induction of K deficiency. 相似文献
47.
48.
Alain Gerbi Marcel Debray Jean-Michel Maixent† Claude Chanez Jean-Marie Bourre 《Journal of neurochemistry》1993,60(1):246-252
Abstract: The Na+ sensitivity of whole brain membrane Na+ ,K+ -ATPase isoenzymes was studied using the differential inhibitory effect of ouabain (α1, low affinity for ouabain; α2, high affinity; and α3, very high affinity). At 100 m M Na+ , we found that the proportion of isoforms with low, high, and very high ouabain affinity was 21, 38, and 41%, respectively. Using two ouabain concentrations (10−5 and 10−7 M ), we were able to discriminate Na+ sensitivity of Na+ , K+ -ATPase isoenzymes using nonlinear regression. The ouabain low-affinity isoform, α1, exhibited high Na+ sensitivity [ K a of 3.88 ± 0.25 m M Na+ and a Hill coefficient ( n ) of 1.98 ± 0.13]; the ouabain high-affinity isoform, α2, had two Na+ sensitivities, a high ( K a of 4.98 ± 0.2 m M Na+ and n of 1.34 ± 0.10) and a low ( K a of 28 ± 0.5 m M Na+ and an n of 1.92 ± 0.18) Na+ sensitivity activated above a thresh old (22 ± 0.3 m M Na+ ); and the ouabain very-high-affinity isoform, α3, was resolved by two processes and appears to have two Na+ sensitivities (apparent K a values of 3.5 and 20 m M Na+ ). We show that Na+ dependence in the absence of ouabain is the result of at least of five Na+ reactivities. This molecular functional characteristic of isoenzymes in membranes could explain the diversity of physiological roles attributed to isoenzymes. 相似文献
49.
Abstract. An inventory was made of the vegetation and soils of a watershed area of 480 ha in the tropical rain forest of Guyana. 252 plots of 0.05 ha were sampled. In total 111 tree species > 20 cm were recorded. A TWINSPAN analysis resulted in seven groupings. Correspondence Analysis revealed that the major environmental differentiation underlying the floristic variation is according to soil type. Finally, five main forest communities were described. Most sites in the watershed area are characterized by the dominance of one or a few species. Species distribution patterns are strongly determined by soil type and drainage class. 相似文献
50.
Thelma Panaïotis Marcel Babin Tristan Biard François Carlotti Laurent Coppola Lionel Guidi Helena Hauss Lee Karp-Boss Rainer Kiko Fabien Lombard Andrew M. P. McDonnell Marc Picheral Andreas Rogge Anya M. Waite Lars Stemmann Jean-Olivier Irisson 《Global Ecology and Biogeography》2023,32(11):1991-2005