全文获取类型
收费全文 | 5477篇 |
免费 | 456篇 |
国内免费 | 3篇 |
出版年
2023年 | 33篇 |
2022年 | 54篇 |
2021年 | 108篇 |
2020年 | 76篇 |
2019年 | 89篇 |
2018年 | 108篇 |
2017年 | 98篇 |
2016年 | 148篇 |
2015年 | 237篇 |
2014年 | 249篇 |
2013年 | 318篇 |
2012年 | 412篇 |
2011年 | 378篇 |
2010年 | 299篇 |
2009年 | 237篇 |
2008年 | 343篇 |
2007年 | 320篇 |
2006年 | 281篇 |
2005年 | 305篇 |
2004年 | 250篇 |
2003年 | 227篇 |
2002年 | 236篇 |
2001年 | 77篇 |
2000年 | 50篇 |
1999年 | 63篇 |
1998年 | 80篇 |
1997年 | 48篇 |
1996年 | 66篇 |
1995年 | 47篇 |
1994年 | 45篇 |
1993年 | 47篇 |
1992年 | 44篇 |
1991年 | 43篇 |
1990年 | 29篇 |
1989年 | 26篇 |
1988年 | 31篇 |
1987年 | 32篇 |
1986年 | 19篇 |
1985年 | 38篇 |
1984年 | 27篇 |
1983年 | 24篇 |
1982年 | 29篇 |
1981年 | 21篇 |
1980年 | 28篇 |
1979年 | 18篇 |
1978年 | 16篇 |
1977年 | 19篇 |
1976年 | 22篇 |
1974年 | 16篇 |
1973年 | 21篇 |
排序方式: 共有5936条查询结果,搜索用时 15 毫秒
21.
Pedro J. N. Silva Richard K. Koehn Walter J. Diehl III Robin P. Ertl Elaine B. Winshell Mauro Santos 《Biochemical genetics》1989,27(7-8):451-467
Four samples of the musselMytilus edulis were taken between 1984 and 1987 from Stony Brook, New York, and used to study the glucose-6-phosphate isomerase (GPI) polymorphism
in this species.In vitro specific activity andin vivo flux measured in the same animals were found to be significantly correlated. A significant effect of GPI genotype on flux
was observed in one of the samples; overall, significant evidence of effect of genotype on enzyme activity was also obtained.
GPI activities of common genotypes tend to deviate less from the population mean than those of rare (frequency less than 5%)
genotypes. This suggests the possibility that rare GPI genotypes are rare as a consequence of having biochemical properties
that deviate from an optimum level and, therefore, having a lower fitness. In support of this hypothesis, we found in one
of our samples that shell length is a concave function of GPI activity with an intermediate optimum activity level.
The financial support provided to P.J.N.S. by the Luso-American Educational Commission (Fulbright Program), the Instituto
Nacional de Investigacao Científica (Portugal), and the Faculdade de Ciências da Universidade de Lisboa during several stages
of this research is gratefully acknowledged. Financial support from the Ministerio de Educatión y Ciencia (Spain) in the form
of a postdoctoral Fulbright/MEC fellowship to M.S. is also gratefully acknowledged. Research was supported by National Science
Foundation Grant BSR-8415060 to R.K.K. This is contribution No. 736 from the Program in Ecology and Evolution, State University
of New York at Stony Brook.
On leave from Departamento de Biologia Vegetal, Faculdade de Ciências, Universidade de Lisboa, Campo Grande C2, Lisboa, Portugal. 相似文献
22.
Menstrual-cycle phase and sexual behavior in semi-free-ranging stumptail macaques (Macaca arctoides)
The sexual behavior and female reproductive cycles of a group of island-dwelling stumptail macaques (Macaca arctoides)were monitored over a 6-month period, yielding 530 observation hr and 268 copulations. Compared to nondominant males, the
dominant male copulated at a relatively high rate throughout the cycle, but largely with one high-ranking female. The non-dominant
males copulated most frequently at midcycle. Female presenting was highest at midcycle, but only to the dominant male. Cross-study
discrepancies may be due to different observation methods and restricted environmental conditions that mask female-initiated
sexual behavior. The more naturalistic setting of this study allowed for a fuller expression of proceptivity. Contrary to
some previous conclusions, present findings suggest that both hormonal and socioenvironmental factors influence the patterns
of sexual behavior found in stumptail macaque colonies. 相似文献
23.
24.
Jeanine Brederoo Piet de Wildt Corrie Popp-Snijders Robin F. Irvine Alan Musgrave Herman van den Ende 《Planta》1991,184(2):175-181
InChlamydomonas eugametos gametes, phosphatidylinositol 4-phosphate (PtdInsP) and phosphatidylinositol 4,5-bisphosphate (PtdInsP2) comprised 0.4 and 0.3% of the whole-cell phospholipids. They were concentrated in the plasma membrane around the cell body and were present in low concentrations in the flagellar membrane. When gametes were fed32PO
4
-
, the label was rapidly incorporated into PtdInsP and PtdInsP2 and only slowly incorporated into structural lipids such as phosphatidylethanolamine and phosphatidylglycerol. Similarly, when a pulse of32PO
4
-
was chased with PO
4
-
, the label was rapidly lost from the polyphosphoinositol lipids but not from the structural lipids. The major fatty acids in the polyphosphoinositides were C-22 carbon polyenoic acids (70%). The significance of these results in relationship to intracellular signalling via inositol phosphates and Ca2+ is discussed.Abbreviations InsP3
inositol 1,4,5-trisphosphate
- mt–/mt+
mating-type plus or minus
- PtdA
phosphatidic acid
- PtdEtn
phosphatidylethanolamine
- PtdGro
phosphatidylglycerol
- PtdIns
phosphatidylinositol
- PtdInsP
phosphatidylinositol 4-phosphate;
- PtdInsP2
phosphatidylinositol 4,5-bisphosphate
- TCA
trichloroacetic acid
We thank Frank Schuring for Fig. 5A and Susan Kenter, Hans Kruisselbrink, Saskia Bijvank and Nelleke Corbett for their enthousiastic assistance. 相似文献
25.
Intramitochondrial factors controlling hepatic fatty acid oxidation at weaning in the rat 总被引:3,自引:0,他引:3
Fatty acid oxidation was studied in isolated liver mitochondria of rats during the suckling-weaning transition. The oxidation rate of oleyl-CoA and palmitoylcarnitine was reduced 2.5-fold in rats weaned on a high-carbohydrate diet compared to suckling rats, when acetyl-CoA produced by beta-oxidation was directed towards ketone-body synthesis. Weaning on a high-fat diet minimized this change. Channeling of acetyl-CoA towards citrate synthesis doubled the oxidation rate of both substrates in HC-weaned rats. Thus, in addition to changes in carnitine palmitoyltransferase I activity, the beta-hydroxymethylglutaryl-CoA synthase pathway is also involved in the decreased fatty acid oxidation at weaning. This was confirmed by measurement of beta-hydroxymethylglutaryl-CoA synthase pathway activity. 相似文献
26.
Estimation of Allele Frequencies at Isoloci 总被引:3,自引:0,他引:3
R. S. Waples 《Genetics》1988,118(2):371-384
In some polyploid animals and plants, pairs of duplicated loci occur that share alleles encoding proteins with identical electrophoretic mobilities. Except in cases where these ``isoloci' are known to be inherited tetrasomically, individual genotypes cannot be determined unambiguously, and there is no direct way to assign observed variation to a particular locus of the pair. For a pair of diallelic isoloci, nine genotypes are possible but only five phenotypes can be identified, corresponding to individuals with 0-4 doses of the variant allele. A maximum likelihood (ML) approach is used here to identify the set of allele frequencies (p, q) at the individual gene loci with the highest probability of producing the observed phenotypic distribution. A likelihood ratio test is used to generate the asymmetrical confidence intervals around ML estimates. Simulations indicate that the standard error of p is typically about twice the binomial sampling error associated with single locus allele frequency estimates. ML estimates can be used in standard indices of genetic diversity and differentiation and in goodness-of-fit tests of genetic hypotheses. The noncentral χ(2) distribution is used to evaluate the power of a test of apparent heterozygote deficiency that results from attributing all variation to one locus when both loci are polymorphic. 相似文献
27.
Robin L. Clark 《Hydrobiologia》1986,143(1):63-69
Pollen analysis is widely used to reconstruct vegetation and land use histories, but can also provide sedimentological information. At Burrinjuck Reservoir, in south-eastern Australia, annual grass pollen peaks are used to distinguish each year's sediment, even when there are no visible laminations. In conjunction with other dating methods, this allows the determination of year by year influxes of all sediment components. Pollen grains in the Burrinjuck sediments are shown to be predominantly waterborne so that they can be used to trace sediment to its source in particular vegetation stands. Pollen concentration and the proportion of damaged pollen might also distinguish sediment eroded from topsoils and that from subsoils. Pollen analysis can thus be used to locate specific erosion events in both time and space. 相似文献
28.
Summary The leaf and root nitrate reductase activities were measured in 7 day-old barley seedlings by anoxic nitrite accumulation in darkness, during 48h after the transfer from a N-starved medium to a 1.5 mM K15NO3 medium. Thisin situ nitrate reduction was compared with the15N incorporation in the reduced N fraction of the whole seedlings.The nitrate reduction integrated fromin situ measurements was lower than the reduced15N accumulation. The rootin situ nitrate reductase activity seemed to account for only the third of the real root nitrate reduction, which may have been responsible for the overall underestimation. This discrepancy was partly explained by the ability of the root to reduce nitrite in an anoxic environment.These results suggest that, after correction of thein situ estimation of the nitrate reduction. the roots contribute to about 50% of the total assimilation. 相似文献
29.
John J. Langone Chandra Das Robin Mainwaring William T. Shearer 《Molecular and cellular biochemistry》1985,65(2):159-170
Summary Protein A of Staphylococcus aureus is an Fc receptor for IgG that has been used as a therapeutic reagent to treat cancer in humans and experimental animals. We used ultracentrifugation combined with analysis of isolated fractions by radioimmunoprecipitation and competitive radioimmunoassay with chicken antibodies that bind free protein A or protein A in complexes but do bind free immunoglobulin reagents to localize and characterize the types of complexes formed with different molar ratios of 125I-protein A and human 131I-IgG alone or in serum, and 1311-Fc fragments. This approach offers a distinct advantage over direct counting of radioactivity in the fractions because resolution of complexes and free reagents is much improved. With excess 131I-IgG or 1311-Fc, all the 125I-protein A is present only in complexes that contained 4 molecules of immunoglobulin reagent and 2 molecules of protein A (4:2 complexes), whereas with excess 125I-protein A the stoichiometry of the complexes was 1:1. We have also shown the preformed 4:2 and 1:1 complexes will interconvert in the presence of added excess protein A or IgG, respectively, and that fresh IgG will exchange with IgG or Fc in preformed complexes. Because protein A has been found to elute from an immobilized reagent used in serotherapy of human cancer and is present in a large excess of IgG, the 4:2 complexes may play an active role in the tumoricidal or toxic reactions observed.Abbreviations SpA
protein A of Staphyloccus aureus
- VBS
EDTA gel, 0.0055 M veronal buffered saline containing 0.01 M EDTA and 0.1% gelatin, pH 7.4
- PBS
0.01 M phosphate buffered saline, pH 7.4 相似文献
30.
Eugene D Robin 《BMJ (Clinical research ed.)》1985,290(6477):1259-1260