全文获取类型
收费全文 | 2321篇 |
免费 | 300篇 |
国内免费 | 2篇 |
出版年
2023年 | 11篇 |
2022年 | 20篇 |
2021年 | 55篇 |
2020年 | 45篇 |
2019年 | 35篇 |
2018年 | 54篇 |
2017年 | 41篇 |
2016年 | 80篇 |
2015年 | 128篇 |
2014年 | 122篇 |
2013年 | 126篇 |
2012年 | 128篇 |
2011年 | 115篇 |
2010年 | 81篇 |
2009年 | 83篇 |
2008年 | 92篇 |
2007年 | 130篇 |
2006年 | 98篇 |
2005年 | 86篇 |
2004年 | 70篇 |
2003年 | 69篇 |
2002年 | 88篇 |
2001年 | 75篇 |
2000年 | 65篇 |
1999年 | 61篇 |
1998年 | 26篇 |
1997年 | 30篇 |
1996年 | 20篇 |
1995年 | 20篇 |
1994年 | 27篇 |
1993年 | 12篇 |
1992年 | 53篇 |
1991年 | 36篇 |
1990年 | 36篇 |
1989年 | 43篇 |
1988年 | 20篇 |
1987年 | 29篇 |
1986年 | 28篇 |
1985年 | 25篇 |
1984年 | 37篇 |
1983年 | 22篇 |
1982年 | 25篇 |
1981年 | 17篇 |
1979年 | 12篇 |
1978年 | 18篇 |
1977年 | 15篇 |
1975年 | 12篇 |
1973年 | 14篇 |
1952年 | 10篇 |
1951年 | 11篇 |
排序方式: 共有2623条查询结果,搜索用时 31 毫秒
71.
Spirogyra insignis shows a long-term persistence of cell division synchrony in the absence of the synchronizing Zeitgeber, so that at least six generations are involved in the process. This tentatively suggests that a mechanism of transmission throughout generations of synchronizing information could maintain this synchrony. Apparently, a vital part of the molecular basis of this mechanism is a membrane proteoglycan complex. This complex could obtain temporal information from a synchronizing Zeitgeber and be transmitted to the progeny by distribution of plasma membrane between daughter cells. 相似文献
72.
Sequence of the Streptococcus pneumoniae bacteriophage HB-3 amidase reveals high homology with the major host autolysin. 总被引:10,自引:5,他引:5
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We have sequenced a DNA fragment containing the pneumococcal bacteriophage HB-3 hbl gene, which codes for the phage lytic amidase. A remarkable nucleotide similarity (87.1%) between the lytA gene, coding for the pneumococcal amidase, the major autolysin of Streptococcus pneumoniae, and the hbl gene was found. This similarity completely disappeared outside the open reading frames coding for both amidases. The hbl gene transformed amidase-deficient strains of S. pneumoniae to the wild-type phenotype, and Southern blotting experiments provided evidence for recombination between donor and recipient genes. A comprehensive evaluation of these and previous results on the peptidoglycan hydrolases of S. pneumoniae and its bacteriophages suggested that recombination mechanisms participate in the evolution of the genes coding for these enzymes. 相似文献
73.
Robert M. Malina John H. Himes Carol Dutton Stepick Francisca Gutierrez Lopez Peter H. Buschang 《American journal of physical anthropology》1981,54(3):327-336
Weight, stature, arm circumference, and the triceps skinfold were measured in 1,410 school children, 6 through 14 years of age, from two urban colonias in the city of Oaxaca de Juarez (n = 479), and from two rural Ladino (n = 467) and two rural Zapotec (n = 464) communities in the Valley of Oaxaca in southern Mexico. Children from rural Ladino communities and urban colonias are significantly taller, heavier, and more muscular than children in rural Zapotec communities. The differences between rural Ladino and urban colonia children favor the former, particularly for weight and stature. These observations thus suggest (1) that children in the rural, indigenous communities in the Valley of Oaxaca are relatively undernourished compared to children in Ladinoized and urban communities, and (2) that rural-to-urban migration does not necessarily result in improved growth status. 相似文献
74.
Robert M. Malina John H. Himes Carol Dutton Stepick Francisca Gutierrez Lopez Peter H. Buschang 《American journal of physical anthropology》1981,55(2):269-280
Weight, stature, arm circumference, and the triceps skinfold were measured in 1,410 school children, 6 through 14 years of age, from two urban colonias in the city of Oaxaca de Juarez (n = 479), and from two rural Ladino (n = 467) and two rural Zapotec (n = 464) communities in the Valley of Oaxaca in southern Mexico. Children from rural Ladino communities and urban colonias are significantly taller, heavier, and more muscular than children in rural Zapotec communities. The differences between rural Ladino and urban colonia children favor the former, particularly for weight and stature. These observations thus suggest (1) that children in the rural, indigenous communities in the Valley of Oaxaca are relatively undernourished compared to children in Ladinoized and urban communities, and (2) that rural-to-urban migration does not necessarily result in improved growth status. 相似文献
75.
76.
77.
Phage phi W-14 DNA (in which one-half of the thymine residues are replaced by alpha-putrescinyl thymine) was taken up by competent Bacillus subtilis cells at a rate threefold higher than the rate of homologous DNA uptake. In contrast to other types of heterologous DNA, the amount of phi W-14 DNA taken up in 15 min exceeded the amount of homologous DNA taken up by a factor of two to three, as measured in terms of acid-precipitable material. The amount of phi W-14 DNA taken up was even greater than this analysis indicated if allowance was made for the fact that phi W-14 DNA was degraded more rapidly after uptake than homologous DNA. Competition experiments showed that the affinity of phi W-14 DNA for homologous DNA receptors was lower than the affinity of homologous DNA and was similar to the affinities of other types of heterologous DNA. The more rapid and more extensive uptake of phi W-14 DNA appeared to occur via receptors other than the receptors for homologous DNA, and these receptors (like those for homologous DNA) were an intrinsic property of competent cells. Uptake of phi W-14 DNA was affected by temperature, azide, EDTA, and chloramphenicol, as was uptake of homologous DNA. This was consistent with entry of both DNAs by means of active transport. After uptake, undegraded phi W-14 [3H]DNA was found in the cells in a single-stranded form, whereas a portion of the label was associated with recipient DNA, presumably as a result of incorporation of monomers resulting from degradation. Acetylation of the amino groups of the putrescine side chains in phi W-14 DNA decreased the affinity of this DNA for its receptors without affecting its ability to compete with homologous DNA. 相似文献
78.
In order to quantitate the extent of intracellular metabolic conversions of C19 steroids in human endometrium, specimens of proliferative and secretory tissue were superfused at a constant rate with several pairs of labeled compounds at low concentrations. About 16% of dehydroepiandrosterone sulfate interacting with endometrial cells was converted to dehydroepiandrosterone and about 3% of this compound was converted to androstenedione. Androstenedione was reversibly reduced to testosterone and the extent of this conversion was shown to be several-fold higher in secretory than in proliferative tissue. About 1% of testosterone entering the cells was reduced to 5 alpha-dihydrotestosterone. These results demonstrate that the conversion of the main circulating C19 steroids in women, i.e. dehydroepiandrosterone sulfate and androstenedione, to 5 alpha-dihydrotestosterone, the compound considered to be the true intracellular androgen, is very small. In contrast, formation of testosterone from androstenedione is extensive and increases during the luteal phase under the influence of progesterone, a hormone known to stimulate the activity of 17 beta-hydroxysteroid dehydrogenase in human endometrium. 相似文献
79.
Nitrofurantoin causes the stringent response in Bacillus subtilis. After exposure of a stringent strain to this drug, the intracellular concentrations of guanosine 3'-diphosphate 5'-diphosphate (ppGpp), guanosine 3'-diphosphate 5'-triphosphate (pppGpp) and ATP increased, while that of GTP decreased. In a relaxed strain no accumulation of ppGpp or pppGpp was observed, but both GTP and ATP declined after the addition of nitrofurantoin. Protein synthesis was equally sensitive to nitrofurantoin in both the stringent and relaxed strains, but the drug inhibited RNA accumulation only in the stringent strain, not in the relaxed strain. Nitrofurantoin also caused the accumulation of ppGpp in Escherichia coli and Serratia marcescens. 相似文献
80.
F. J. Lopez Aparicio F. Zorrilla Benitez F. Santoyo Gonzalez 《Carbohydrate research》1981,90(2):315-318
The homogeneous, neutral polysaccharide isolated from the crude polysaccharide of the fruit pulp from bael (Aegle marmelos) contains arabinose, galactose, and glucose in the molar ratios of 2:3:14. The linkages among the different monosaccharide residues were established through methylation analysis and Smith-degradation studies of the polysaccharide. The anomeric configurations of the different glycosyl groups were determined by study of the chromium trioxide oxidation of the acetylated polysaccharide. Results of these experiments have been discussed in order to assess the structure of the neutral polysaccharide. 相似文献