全文获取类型
收费全文 | 16225篇 |
免费 | 1244篇 |
国内免费 | 6篇 |
出版年
2023年 | 127篇 |
2022年 | 104篇 |
2021年 | 277篇 |
2020年 | 214篇 |
2019年 | 237篇 |
2018年 | 420篇 |
2017年 | 401篇 |
2016年 | 478篇 |
2015年 | 569篇 |
2014年 | 655篇 |
2013年 | 938篇 |
2012年 | 1277篇 |
2011年 | 1306篇 |
2010年 | 774篇 |
2009年 | 619篇 |
2008年 | 1091篇 |
2007年 | 1118篇 |
2006年 | 1069篇 |
2005年 | 959篇 |
2004年 | 935篇 |
2003年 | 881篇 |
2002年 | 769篇 |
2001年 | 183篇 |
2000年 | 245篇 |
1999年 | 182篇 |
1998年 | 146篇 |
1997年 | 93篇 |
1996年 | 101篇 |
1995年 | 110篇 |
1994年 | 89篇 |
1993年 | 80篇 |
1992年 | 93篇 |
1991年 | 75篇 |
1990年 | 67篇 |
1989年 | 50篇 |
1988年 | 47篇 |
1987年 | 46篇 |
1986年 | 34篇 |
1985年 | 43篇 |
1984年 | 58篇 |
1983年 | 37篇 |
1982年 | 51篇 |
1981年 | 48篇 |
1980年 | 51篇 |
1979年 | 41篇 |
1978年 | 42篇 |
1977年 | 35篇 |
1976年 | 38篇 |
1975年 | 38篇 |
1974年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
101.
Luís C. Duarte Alexandra P. Nobre Francisco M. Gírio M. T. Amaral-Collaço 《Biotechnology Techniques》1994,8(12):859-864
Summary The kinetic parameters of the yeastDebaryomyces hansenii grown in continous cultivation on D-xylose were determined by different methods. While the values obtained for μm by the steady state and the washout methods only gave a 3% difference, the determined Ks values by the steady state and the maximal biomass output methods led a to a 305% difference. The latter method was suggested
to overestimate the Ks value. 相似文献
102.
Frédérique Tihy Nicolas Vogt Dominique Recan Bernard Malfoy France Leturcq Michelle Coquet Françoise Serville Daniel Fontan Jean-Michel Guillard Jean-Claude Kaplan Bernard Dutrillaux Nicole Lemieux 《Human genetics》1994,93(5):563-567
A girl with severe Becker muscular dystrophy and apparently normal chromosomes had a heterozygous deletion for exons 51, 52, and 53 of the dystrophin gene. This deletion was transmitted by her mother, who was unaffected. To differentiate the normal and the deleted X chromosomes, fluorescence in situ hybridization (FISH) was applied to metaphase chromosomes, using probes for both exons 51 and 52, which are only 388 and 113 base pairs long, respectively. FISH signals were observed in one or both chromatids of one chromosome, but never on both chromosomes, suggesting the lack of hybridization on the deleted X chromosome. Using 5-bromodeoxyuridine incorporation to differentiate the late (inactive) and the early replicating (active) X chromosomes, 77% of the signals were observed on the active X chromosomes in the mother. This percentage was only 18% in the daughter, suggesting that skewed inactivation of the X chromosomes was responsible for the phenotypic differences. 相似文献
103.
Jean-François Manen Alessandro Natali Friedrich Ehrendorfer 《Plant Systematics and Evolution》1994,190(3-4):195-211
A phylogenetic analysis of 25 species, representing eight genera of theRubieae tribe (Rubiaceae), has been made using the DNA sequence of the chloroplastatp B-rbc L intergene region. Six tropical genera from other tribes ofRubiaceae have been used as outgroups. Whatever the method of analysis (distance, parsimony or maximum likelihood), five groups are clearly separated and described as informal clades. Their relative relationships are not clearly resolved by the parsimony analysis, resulting in eight equally parsimonious trees, 327 steps long, with a consistency index (CI) of 0.749 (excluding uninformative sites). TheRubieae tribe appears monophyletic from the data available. Some new and partly unexpected phylogenetic relationships are suggested. The genusRubia forms a separate clade and appears to be the relatively advanced sister group of the remaining taxa. TheSherardia clade also includes the generaCrucianella andPhuopsis. Galium sect.Aparinoides appears closely attached to theAsperula sect.Glabella clade. The remaining taxa ofGalium are paraphyletic:Galium sect.Platygalium (in theCruciata clade) is linked to the advanced generaCruciata andValantia; the more apomorphic groups ofGalium form theGalium sect.Galium clade, including the perennial sectionsGalium, Leiogalium, andLeptogalium as well as the annual (and possibly polyphyletic) sect.Kolgyda. 相似文献
104.
Friedrich Ehrendorfer Jean-François Manen Alessandro Natali 《Plant Systematics and Evolution》1994,190(3-4):245-248
Representatives of seven genera from five tribes ofRubiaceae have been compared in respect to a non-coding intergene cpDNA region of about 1000 bp, situated between the atpB and the rbcL genes. The resulting most parsimonious PAUP cladogram corresponds very well with one based on total cpDNA restriction site data obtained byBremer & Jansen (1991). The two different molecular analyses thus corroborate each other and contribute to an improved systematic arrangement of the large family, e.g., in respect to placing the tribeHedyotideae clearly into the subfamilyRubioideae, closer toRubieae than toPsychotrieae. 相似文献
105.
An important component of the interaction between macroinvertebrates and leaf litter in streams in the extent to which consumers
can differentiate between undecomposed and decomposing leaves. The detritivores Gammarus pulex and Asellus aquaticus fed preferentially on conditioned rather on unconditioned leaf material. Growth in A. aquaticus was significantly reduced when unconditioned leaves were provided, but in G. pulex no significant effect of conditioning on growth was observed. The capacity of G. pulex to tolerate reductions in food quality seems to be a consequence of a compensatory system in which respiration rates change
to compensate for reductions in food quality. In this way a constant growth rate is maintained. Increases in ingestion rates
to compensate for low quality food were not observed. 相似文献
106.
Cobalt determinations in biological fluids are of great interest in biological or toxicological research programs. Cobalturia
is often chosen as an indicator for a biological monitoring program in occupational exposure to cobalt dusts. The method described
here derives from the IUPAC reference method for nickel determination. It enables cobaltemia and cobalturia to be measured
in small samples (1 mL). The mean usual values for cobalt in biological fluids are very low (2.7 nmol L−1 for serum and 6.7 nmol L−1 for urine), and therefore, thus require an analytical procedure with preconcentration and extraction. The sample is mineralized
by wet acid digestion. After digestion, inorganic cobalt is extracted in form of ammonium pyrrolidine dithiocarbamate complex
into isobutyl methyl ketone and measured in the organic layer by electrothermal atomic absorption spectrometry.
The analytical parameters are described in detail. The extraction output is about 99%. The detection limits are 1.93 and 1.89
nmol L−1 for serum and urine, respectively. Sensitivity (expressed as the concentration that gives a 0.044 absorbance) is 3.4 nmol
L−1 for serum and 3.3 nmol L−1 for urine.
Within-run precision ranged between 3.9 and 2.5% (coefficients of variation) for serum and 4.2 and 1.1% for urine, at 87 and
136 nmol L−1 levels, respectively. Between-run precision ranged between 4.3 and 3.3% (coefficients of variation) for serum and 4.2 and
2.3% for urine, at 87 and 136 nmol L−1 levels, respectively. At very low concentration, 5.7 nmol L−1 for serum and 2.5 nmol L−1 for urine, the between-run precision is, respectively, 19.5 and 28%.
Linearity is effective between 0 and 272 nmol L−1. Interferences and matrix effects are negligible for urine, serum, or plasma samples without hemoglobin. The method is easily
applicable for routine determinations. 相似文献
107.
Jean-Vincent Escalant Claude Teisson François Cote 《In vitro cellular & developmental biology. Plant》1994,30(4):181-186
Summary Somatic embryogenesis and plant regeneration of banana and plantain cultivars (Musa spp.) were obtained by culturing young male flowers. Multiplication and maintenance of embryogenic cultures were achieved
by culturing somatic embryos in a temporary immersion system (SIT). A multiplication rate of 40 allowed us to obtain more
than 6000 somatic embryos after 6 mo. of subculture. Plant recovery frequencies were 60 to 70%. This method was expanded to
different banana and plantain genomic groups. 相似文献
108.
Comparative tumor morphogenesis of two human colon adenocarcinoma cell clones xenografted in the immunosuppressed newborn rat 总被引:1,自引:0,他引:1
Lionel Rémy Marie-France Jacquier Noucha Daémi Jean-François Doré Jean-Claude Lissitzky 《Differentiation; research in biological diversity》1993,54(3):191-200
Abstract. Involucrin is a precursor of the keratinocyte cornified envelope that is specifically expressed in the suprabasal layers of the epidermis and other stratifying squamous epithelia. To study involucrin gene expression and the function of involucrin, we expressed a 6 kb DNA fragment of the human involucrin gene, containing approximately 2.5 kb of upstream sequence and 0.5 kb of downstream sequence, in transgenic mice. The transgene produces a 68 kDa protein that is detected by a human involucrin-specific antibody, and is expressed in a tissuespecific and differentiation-appropriate manner (i.e., expression is confined to the suprabasal layers of the epidermis, extocervix, trachea, esophagus and conjunctiva).
Soluble involucrin levels are two to four times higher in transgenic epidermal keratinocytes compared to human foreskin keratinocytes. Newborn heterozygous animals have a normal birth weight and a normal appearing epidermis and hair growth begins at 4 to 5 days of age (i.e., the same time as hair growth in non-transgenic animals). In a subpopulation of the newborn homozygous animals birth weight is reduced, the epidermis is scaly and hair growth begins late, at around 9 to 10 days of age. In addition, the hair tends to stand erect on both heterozygous and homozygous adult animals giving the appearance of diffuse alopecia.
Immunofluorescent and electron microscopy localize involucrin in the hair follicle and cornified envelope, respectively. These results suggest that overexpression of involucrin may cause abnormalities in hair follicle structure/function and cornified envelope structure. These animals provide a new model for the study of cornified envelope structure and function. 相似文献
Soluble involucrin levels are two to four times higher in transgenic epidermal keratinocytes compared to human foreskin keratinocytes. Newborn heterozygous animals have a normal birth weight and a normal appearing epidermis and hair growth begins at 4 to 5 days of age (i.e., the same time as hair growth in non-transgenic animals). In a subpopulation of the newborn homozygous animals birth weight is reduced, the epidermis is scaly and hair growth begins late, at around 9 to 10 days of age. In addition, the hair tends to stand erect on both heterozygous and homozygous adult animals giving the appearance of diffuse alopecia.
Immunofluorescent and electron microscopy localize involucrin in the hair follicle and cornified envelope, respectively. These results suggest that overexpression of involucrin may cause abnormalities in hair follicle structure/function and cornified envelope structure. These animals provide a new model for the study of cornified envelope structure and function. 相似文献
109.
Dr M. T. S. Peraçoli M. T. Rezkallah-Iwasso N. G. S. Mota M. R. Montenegro 《Mycopathologia》1993,121(3):149-156
The effect of dialysable leukocyte extracts (DLE) obtained from hamsters immunized withParacoccidioides brasiliensis (immune DLE) and from non-immunized hamsters (non-immune DLE) was studied in hamsters inoculated withP. brasiliensis by the intratesticular route. Treatment with immune or non-immune DLE was started during the third week of infection and was repeated at 7, 11, 15 and 19 weeks. A group of untreated infected animals was used as control. Animals were submitted to the delayed hypersensitivity skin test toP. brasiliensis antigen (PbAg) in vivo and assayed in vitro by the macrophage migration inhibition test in the presence of Phytohemagglutinin (PHA) and PbAg and by immunodiffusion for specific antibody. The animals were sacrificed at 4, 8, 12, 16 and 20 weeks. The morphology and extension of the lesions were studied at the inoculation site, and in lymph nodes, lungs, liver, spleen and kidneys. In contrast to the controls, animals treated with both DLEs maintained a positive cell-mediated immune response throughout the experiment and developed less extensive infection with a significantly lower number of fungi in the lesions. The results suggest that immune and non-immune DLE preparations modified the evolution of experimental paracoccidioidomycosis with equal efficiency. This similarity may be explained by the immunoregulatory activities of both extracts. 相似文献
110.
Prospects for the genetics of human longevity 总被引:8,自引:0,他引:8
Longevity varies between and within species. The existence of species-specific limit to human life-span and its partial heritability indicate the existence of genetic factors that influence the ageing process. Insight into the nature of these genetic factors is provided by evolutionary studies, notably the disposable soma theory, which suggests a central role of energy metabolism in determining life-span. Energy is important in two ways. First, the disposable soma theory indicates that the optimum energy investment in cell maintenance and repair processes will be tuned through natural selection to provide adequate, but not excessive, protection against random molecular damages (e.g. to DNA, proteins). All that is required is that the organism remains in a sound condition through its natural expectation of life in the wild environment, where accidents are the predominant cause of mortality. Secondly, energy is implicated because of the intrinsic vulnerability of mitochondria to damage that may interfere with the normal supply of energy to the cell via the oxidative phosphorylation pathways. Oxidative phosphorylation produces ATP, and as a by-product also produces highly reactive oxygen radicals that can damage many cell structures, including the mitochondria themselves. Several lines of evidence link, on the one hand, oxidative damage to cell ageing, and on the other hand, energy-dependent antioxidant defences to the preservation of cellular homeostasis, and hence, longevity. Models of cellular ageing in vitro allow direct investigation of mechanisms, such as oxidative damage, that contribute to limiting human life-span. The genetic substratum of inter-individual differences in longevity may be unraveled by a two-pronged reverse genetics approach: sibling pair analysis applied to nonagenarian and centenarian siblings, combined with association studies of centenarians, may lead to the identification of genetic influences upon human longevity. These studies have become practicable thanks to recent progress in human genome mapping, especially to the development of microsatellite markers and the integration of genetic and physical maps. 相似文献