全文获取类型
收费全文 | 71346篇 |
免费 | 5062篇 |
国内免费 | 3243篇 |
专业分类
79651篇 |
出版年
2024年 | 168篇 |
2023年 | 1116篇 |
2022年 | 1826篇 |
2021年 | 2342篇 |
2020年 | 2192篇 |
2019年 | 2464篇 |
2018年 | 2460篇 |
2017年 | 1767篇 |
2016年 | 1746篇 |
2015年 | 2254篇 |
2014年 | 4248篇 |
2013年 | 5294篇 |
2012年 | 3189篇 |
2011年 | 4306篇 |
2010年 | 3309篇 |
2009年 | 3680篇 |
2008年 | 3754篇 |
2007年 | 3820篇 |
2006年 | 3399篇 |
2005年 | 3040篇 |
2004年 | 2700篇 |
2003年 | 2269篇 |
2002年 | 2026篇 |
2001年 | 1395篇 |
2000年 | 1179篇 |
1999年 | 1220篇 |
1998年 | 1123篇 |
1997年 | 979篇 |
1996年 | 923篇 |
1995年 | 846篇 |
1994年 | 768篇 |
1993年 | 708篇 |
1992年 | 611篇 |
1991年 | 574篇 |
1990年 | 446篇 |
1989年 | 411篇 |
1988年 | 375篇 |
1987年 | 353篇 |
1986年 | 315篇 |
1985年 | 435篇 |
1984年 | 620篇 |
1983年 | 477篇 |
1982年 | 517篇 |
1981年 | 360篇 |
1980年 | 361篇 |
1979年 | 301篇 |
1978年 | 221篇 |
1977年 | 172篇 |
1976年 | 143篇 |
1975年 | 131篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
171.
V M McGowan T E Reed M S Schanfield S Goliah T Poon-King 《American journal of physical anthropology》1990,81(4):555-562
The sera of a sample of 204 Creoles from Trinidad were tested for the presence of polymorphic gene complexes occurring on immunoglobulin light- and heavy-chain molecules including the allotypic markers IGKC 1, IGHA2 1 and 2, IGHG1 A, X, F, and Z, and IGHG3 G, G5, B0, B1, B3, B4, B5, C3, C5, S, and T. Nine IGHG (GM) haplotypes occur in polymorphic frequencies (greater than .01) in this population, including known African, Asian, Caucasian, and Amerindian marker haplotypes. Significant differences (P less than .01) were found in the frequency distributions of three IGHG (GM) haplotypes and the frequency of IGKC*1 in these data and data from Creole populations of Belize and St. Vincent. The Creoles of Trinidad and St. Vincent are more similar in IGHG (GM) haplotype distributions than are Trinidad and Belize populations. Previous testing has revealed no significant differences between St. Vincent and Belize Creoles at the Ig allotypic loci. Analysis of migration patterns in the Caribbean suggests that different rates of Asian migration have maintained regional diversity at these loci, while continuous gene flow from the eastern Caribbean to Trinidad has had a relative homogenizing effect on the gene pools of this area. 相似文献
172.
Sorbitol dehydrogenase genetics in the mouse: A ';null' mutant in a ';European' C57BL strain 总被引:1,自引:0,他引:1
A ';null' activity variant phenotype for sorbitol dehydrogenase (SDH) was observed in C57BL/LiA mice and used to examine the genetics of this enzyme. Linkage studies of the locus ( Sdh-1 ) with non-agouti (a) and a biochemical Iocus encoding liver L-α-hydroxyacid oxidase ( Hao-1 ) demonstrated that it is coincident with or closely linked to the structural locus, previously localized on chromosome 2. Alcohol dehydrogenase (ADH) isozymes were also examined, since the liver A2 isozyme exhibited some activity as a sorbitol dehydrogenase on cellulose acetate zymograms. It is apparent that SDH activity is not ';essential' in this mouse strain. 相似文献
173.
The nucleotide sequence of a segment of mtDNA from Rattus norvegiens (rat) which contains the genes for tRNAile, tRNAgl and tRNAf-met has been determined. A detailed comparison has been made between this sequence and the corresponding sequences of mouse, human and bovine mtDNAs with regard to the primary and secondary structure of the tRNA genes, the regions connecting the tRNA genes, and the regions flanking the tRNA genes which code for the carboxyl terminus of URF-1 and the amino terminus of URF-2. No differences were found in the nucleotide sequences of the genes for tRNAile, tRNAgln and tRNAf-met in mtDNAs from three different female lines of rats (SASCO-1, SASCO-2 and Wild-UT) that differ by substitutions of 0.8% to 1.8% of their total nucleotides. 相似文献
174.
45Ca2+ incorporated in response to glucose was selectively mobilized from the beta-cell-rich pancreatic islets of ob/ob-mice after raising the intracellular Na+ by removal of K+ or addition of ouabain or veratridine. Also studies of insulin release indicated opposite effects of glucose and Na+ on the intracellular sequestration of calcium. The fact that glucose inhibits insulin release induced by raised intracellular Na+ indicates that this sugar can lower the cytoplasmic [Ca2+]. The concept of a dual action of glucose on the cytoplasmic [Ca2+]. The concept of a dual action of glucose on the cytoplasmic [Ca2+] might well explain previous observations of an inhibitory component in the glucose action on the 45Ca2+ efflux. 相似文献
175.
An immobilized cyclic multi-step enzyme system -- the urea cycle 总被引:1,自引:0,他引:1
176.
Monoclonal antibodies against human low density lipoprotein. Stoichiometric binding studies using Fab fragments 总被引:1,自引:0,他引:1
Because of its physical properties, apolipoprotein B (apo B) has remained poorly characterized. In an attempt to elucidate apo B structure, the Fab fragments of 3 different monoclonal anti-human apo B antibodies were tested in a quantitative assay for their binding to human low density lipoprotein (LDL). In each case the assay gave a linear Scatchard plot with a maximum of 1 Fab fragment bound to a single LDL particle. This result favors an LDL model containing a single large Mr apo B protein, which is not composed of multiple, identical, small Mr subunits. 相似文献
177.
178.
Mutants at 2 new loci which control mutagen-sensitivity are described. Mutants at both loci are female-sterile and are hypersensitive to killing by MMS; neither increases the frequency of sex-linked recessive lethals. A screen of previously described female-sterile and meotic mutants has revealed that a number of these are also sensitive to mutagens. In addition, several new mutants have been identified on the basis of sensitivity to either HN2 or MMS. An anlysis of complementation data suggests that all of the X-linked genes controlling sensitivity to MMS may now have been identified. Among the new mei-41 alleles are mutants which show verly little meiotic nondisjunction or loss. Cytogenetic mapping of previously known mutants is also described. The mutants mus(1)104D1 and mei-41D5 are located in th eregion 14B13±?14D1,2 on the polytene chromosome map, and they map very close to each other genetically. Cytogenetically mus(1)101D1 is between salivary chromosome bands 12A6,7 and 12D3, mus(1)103D1 is between bands 12A1,2 and 12A6,7, and mus(1)-109A1 is in section 8F3-9A2. 相似文献
179.
Summary The relative toxicity of nitrapyrin 2-chloro-6-(trichloromethyl) pyridine and ATC (4-amino-1, 2, 4-triazole) on the growth
of chick peas (Cicer arietinum L.) cow peas (Vigna sinensis L.), green beans (Phaseolus vulgaris L.), green peas (Pisum sativum L.) and mung beans (Phaseolus aureus Roxb.) and their effectiveness as nitrification inhibitor were studied under greenhouse conditions. ATC produced no toxicity
symptoms in green peas, whereas resulted in leaf chlorosis in cow peas, chick peas and green beans. However, nitrapyrin toxicity
appeared as leaf chlorosis in cow peas, and interveinal chlorosis in chick peas. Moreover, nitrapyrin-treated green beans
and peas developed leaf curling and cupping. Although ATC had no significant effect on growth, a suppression in plant growth
was associated with nitrapyrin application. Furthermore, green beans was the most resistant and chick peas the most sensitive
to nitrapyrin. Nitrapyrin was more effective nitrification inhibitor than ACT, especially at the lower rates. 相似文献
180.
The response of photosynthesis to irradiance and temperature during growth was investigated in two soybean genotypes. Soybean is a species that can modify its structure and metabolism so as to adapt to differing light conditions; its responses to rapid changes in irradiance are characterized by their flexibility. However, the temperature during growth can change the response to irradiance: moreover, there may be a marked interaction with genotype.The response of photosynthesis to irradiance consists of changes in leaf thickness, which bring about variations in the mesophyll resistance to CO2 transfer. The increase in net photosynthesis per unit of leaf area is due to the increase in the amount of assimilating material beneath unit of area, as corroborated by the stability of the net photosynthesis per unit volume. Moreover, the response of photosynthesis to temperature is due to the mesophyll diffusion constant which decreases with the growth temperature. 相似文献