全文获取类型
收费全文 | 3946篇 |
免费 | 426篇 |
出版年
2022年 | 30篇 |
2021年 | 51篇 |
2020年 | 37篇 |
2019年 | 48篇 |
2018年 | 47篇 |
2017年 | 51篇 |
2016年 | 75篇 |
2015年 | 120篇 |
2014年 | 131篇 |
2013年 | 171篇 |
2012年 | 199篇 |
2011年 | 200篇 |
2010年 | 142篇 |
2009年 | 114篇 |
2008年 | 164篇 |
2007年 | 210篇 |
2006年 | 167篇 |
2005年 | 119篇 |
2004年 | 148篇 |
2003年 | 141篇 |
2002年 | 131篇 |
2001年 | 117篇 |
2000年 | 133篇 |
1999年 | 104篇 |
1998年 | 54篇 |
1996年 | 51篇 |
1995年 | 55篇 |
1994年 | 28篇 |
1993年 | 40篇 |
1992年 | 69篇 |
1991年 | 64篇 |
1990年 | 70篇 |
1989年 | 67篇 |
1988年 | 61篇 |
1987年 | 65篇 |
1986年 | 49篇 |
1985年 | 45篇 |
1984年 | 44篇 |
1983年 | 51篇 |
1982年 | 33篇 |
1981年 | 35篇 |
1979年 | 47篇 |
1978年 | 32篇 |
1977年 | 36篇 |
1976年 | 37篇 |
1975年 | 44篇 |
1974年 | 29篇 |
1973年 | 37篇 |
1971年 | 36篇 |
1970年 | 34篇 |
排序方式: 共有4372条查询结果,搜索用时 156 毫秒
181.
A longitudinal study of the mental and physical development of 200 children with normal haemoglobin and 21 with the sickle-cell trait was carried out in a small rural community in Jamaica. At about 2 and 10 years of age heights and weights showed no significant differences. At about 10 years of age classroom behaviour, sociability, and educational achievement were similar. The results suggest that the sickle-cell trait does not affect growth and mental development. 相似文献
182.
183.
J L Gellerman W H Anderson D G Richardson H Schlenk 《Biochimica et biophysica acta》1975,388(2):277-290
Lipid classes from four species of mosses, Mnium cuspidatum, and Mnium medium from Minnesota, and Hylocomium splendens and Pleurozium schreberi from Alaska, were analyzed. The total lipids of all species contained 30-40% arachidonic and eicosapentaenoic acids. However, the lipids from the Alaskan mosses contained about 75% neutral lipids (triacylglycerols, steryl esters and wax esters) whereas the lipids of the other species contained only 20% or less of these neutral lipids. Consistently, monogalactosyldiacylglycerols and phosphatidylethanol-amines were enriched in arachidonic acid and the galactolipids in eicosapentaenoic acid. The distribution of these acids in the phospholipids shows some preference for position 2. Together, the highly unsaturated C20 acids represented 80% of acyl groups in steryl esters. In triacylglycerols they were at average levels, while they were much less in sulfolipids and phosphatidylglycerols. Wax esters contained very little of the highly unsaturated acids but appreciable amounts of phytol and phytenic acid were found as wax constituents. 相似文献
184.
Deoxyribonucleic acid polymerase III of Escherichia coli. Characterization of associated exonuclease activities. 总被引:1,自引:0,他引:1
Purified DNA polymerase III has two distinct exonuclease activities: one initiates hydrolsis at the 3 termini, and the other at the 5 termini of single-stranded DNA. Both exonucleases have the same relative mobility on polyacrylamide gels as the polymerase activity. Molecular identity of the three activities is further indicated by their comparative rates of thermal inactivation and their sensitivity to ionic strength. The 3-5 exonuclease activity hydrolyzes only single-standed DNA. The rate of hydrolysis is twice the optimal rate of polymerization. The products are 5-mononucleotides, but the 3-5 activity is unable to cleave free dinucleotides or the 5-terminal dinucleotide of a polydeoxynucleotide chain. The 3-5 activity will not degrade 3-phosphoryl-terminated oligonucleotides such as d(pTpTpTp). The 5-3 activity catalyzes the hydrolysis of single-stranded DNA at 1/15 the rate of the 3-5 exonuclease. The 5-3 exonuclease requires the presence of a 5 single-stranded terminus in order to initiate hydrolysis, but will thereafter proceed into a double-stranded region. Although the limit products found during hydrolysis of substrates designed to assay specifically the 5-3 activity are predominantly mono- and dinucleotides, these products probably arise from the subsequent hydrolysis of oligonucleotides by the 3-5 hydrolytic activity. This interpretation is supported by (a) the relatively greater activity of the 3-5 exonuclease, (b) the inability of the enzyme to degrade d(pTpTpTp), and (c) the release of the 5 terminus of a single-stranded DNA molecule as an oligonucleotide. The 5-3 exonuclease attacks ultraviolet-irradiated duplex DNA which has first been incised by the Micrococcus luteus endonuclease specific for thymine dimers in DNA. 相似文献
185.
186.
187.
Evolution of Hawaiian Drosophilidae. II. Patterns and Rates of Chromosome Evolution in an Antopocerus Phylogeny 下载免费PDF全文
The phylogenetic relationships of seven species of the genus Antopocerus (Family Drosophilidae) have been determined by means of a study of the metaphase configurations and polytene chromosomes. Based on biogeographical, behavioral and cytogenetic information, A. longiseta from Molokai is tentatively identified as the primitive species of the genus. The metaphase karyotypes of all Antopocerus species are either five pairs of rod chromosomes and a pair of dots (5R1D), or six rods (6R). Heterochromatin additions converted the dots to rods. Chromosome breakpoints for inversions also are clustered at heterochromatic loci. The chromosome segments between heterochromatic loci may represent sets of functionally related loci, evolving as a unit. The rate of chromosomal inversion substitution is estimated in the origin of the taxon (probably a subgenus of Drosophila rather than a separate genus). It averages no greater than one substitution per 1,000 years, or one per 5,000 generations. The average genetic death rate per generation of one individual per hundred is required to achieve this substitution rate. The rate of inversion substitution during radiation of this taxon may be only 4.4 x 10-3 times as fast as that present in forming the taxon. Alternatively, radiation may have required only 250,000 years if rates of substitution are the same as in the origination of the taxon. Average rates of substitution reflect genetic accidents, selection pressures and rates of adaptation to new niches, as well as the rate of encountering new niches. Rate of adaptation probably is much greater in this instance than rate of encountering new niches. Therefore, the average rate of evolution reflects more nearly biogeographic and ecological factors than genetic factors. 相似文献
188.
J E Levasseur H A Kontos D W Richardson J L Patterson 《Journal of applied physiology》1976,40(4):549-558
Five chronically instrumented healthy dogs were exposed to a 5-day period of breathing 10% oxygen in a chamber. The response to hypoxia was found to be time dependent. During the first 24 h of hypoxia the circulatory response was characterized by increases in cardiac output, heart rate, pulmonary and systemic arterial blood pressures, and pulmonary vascular resistance. Systemic vascular resistance increased; left atrial pressure decreased. During the early part of hypoxia the animals became hypocapnic; the arterial blood pH rose significantly. During the rest of the hypoxic period cardiac output, heart rate, and arterial blood pH returned to the control values; pulmonary and systemic arterial pressures and pulmonary vascular resistance remained significantly elevated. Systemic vascular resistance rose; left atrial pressure remained below control. This response to hypoxia was not substantially modified when the experiment was repeated during the administration of the antihistamine promethazine, an H1-receptor blocking agent, in a dose which blocked the pulmonary vasoconstrictor response to small doses of exogenous histamine. The circulatory response to acute hypoxia in five anesthetized dogs was not modified by intravenous administration of metiamide, an H2-receptor blocking agent. 相似文献
189.
Abstract— Plasmalogenase was assayed by measuring the disappearance of the plasmalogen by two-dimensional thin-layer chromatography. The enzyme was present in a glycerol-bicarbonate extract of an acetone-dried powder from bovine brain. With ethanolamine plasmalogens as the substrate, the Km was 180 μM. Diacyl glycerophosphorylcholines, diacyl glycerophosphorylethanolamines and choline plasmalogens were competitive inhibitors. With choline plasmalogens as the substrate, the Km was 208 μM and competitive inhibition was observed with diacyl glycerophosphorylcholines and ethanolamine plasmalogens. The same enzyme may be responsible for the hydrolysis of the alk-1-enyl moiety from both plasmalogens. Plasmalogenase activity was 5.1 μmol/h/g of dog brain, 3.9 μmol/h/g of rat brain and 3.4 μmol/h/g of gerbil brain. A lysophospholipase was also found in the glycerol-bicarbonate extract from the acetone-dried powder. The lysophospholipase was more active in hydrolysing acyl groups from 2-acyl-sn-glycero-3-phosphorylethanolamines than the plasmalogenase was active in hydrolyzing alk-1-enyl groups from 1-alk-1′-enyl-2-acyl-sn-glycero-3-phosphorylethanolamines. 相似文献
190.
Anopheles b. balabacensis mosquitoes were infected with Plasmodium fragile when fed upon splenectomized Macaca mulatta monkeys. Highest level mosquito infections were obtained when feedings were made from 2 to 4 days prior to the peak in the parasitemia. Transmission to M. mulatta monkeys was obtained via mosquito bite on 2 occasions and via intrahepatic and intravenous inoculation of dissected infected salivary glands on 9 occasions. The prepatent periods ranged from 12 to 17 days with a median of 15 days. 相似文献