全文获取类型
收费全文 | 5004篇 |
免费 | 347篇 |
国内免费 | 1篇 |
专业分类
5352篇 |
出版年
2023年 | 34篇 |
2022年 | 72篇 |
2021年 | 121篇 |
2020年 | 88篇 |
2019年 | 103篇 |
2018年 | 118篇 |
2017年 | 101篇 |
2016年 | 171篇 |
2015年 | 265篇 |
2014年 | 291篇 |
2013年 | 376篇 |
2012年 | 459篇 |
2011年 | 383篇 |
2010年 | 269篇 |
2009年 | 236篇 |
2008年 | 318篇 |
2007年 | 305篇 |
2006年 | 274篇 |
2005年 | 233篇 |
2004年 | 194篇 |
2003年 | 202篇 |
2002年 | 162篇 |
2001年 | 44篇 |
2000年 | 47篇 |
1999年 | 49篇 |
1998年 | 30篇 |
1997年 | 33篇 |
1996年 | 37篇 |
1995年 | 30篇 |
1994年 | 23篇 |
1993年 | 25篇 |
1992年 | 31篇 |
1991年 | 27篇 |
1990年 | 20篇 |
1989年 | 20篇 |
1988年 | 17篇 |
1987年 | 12篇 |
1986年 | 15篇 |
1985年 | 15篇 |
1984年 | 10篇 |
1983年 | 6篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 9篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 7篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1973年 | 8篇 |
排序方式: 共有5352条查询结果,搜索用时 0 毫秒
11.
12.
Summary pSE211 fromSaccharopolyspora erythraea integrates site-specifically into the chromosome through conservative recombination betweenattP andattB, the plasmid and chromosomal attachment sites. Integration depends on the presence ofint, an open reading frame (ORF) that lies adjacent toattP and encodes the putative integrase. Immediately upstream ofint liesxis (formerly calledorf2) which encodes a basic protein that is thought to exhibit DNA binding.xis andint were cloned in various combinations in pUC18 and expressed constitutively inEscherichia coli from thelac promoter.attP andattB were cloned inStreptomyces orE. coli plasmids containing kanamycin resistance (KmR) or chloramphenicol resistance (CmR) markers. Stable KmR CmR cointegrates formed byattP ×attB orattP ×attP recombination (integration) were obtained inE. coli hosts that expressedint. Co-integrates were not found in hosts expressingint+xis. Excision (intraplasmidatt site recombination) was examined by constructing plasmids carryingattL andattR or twoattP sites separating CmR from KmR and by following segregation of the markers in various hosts. BothattL ×attR andattP ×attP excision depended on bothxis andint inE. coli. pSE211att site integration and excision were not affected by a deletion inhimA, the gene encoding a subunit of integration host factor. 相似文献
13.
B Corman A di Stefano M Wittner G Geelen 《Comptes rendus des séances de la Société de biologie et de ses filiales》1991,185(3):143-155
In the course of aging, the renal concentrating ability is markedly reduced. This defect may result from an inappropriate synthesis of antidiuretic hormone in the central nervous system or may be due to an impaired renal response to vasopressin. The two hypotheses have been studied in vivo in rats and in vitro in mice. The results of these studies indicated that: 1) dehydration induces a comparable release of vasopressin along the hypothalamo-hypophysial axis in 10, 20 and 30 month-old rats; 2) there is no change with age of the number of nephrons, single nephron filtration rate or transport capacity of Henle's loop of cortical nephrons which could account for an impaired renal response to vasopressin; 3) the reduced concentrating ability of the kidney appears to be linked to a decreased response of the medullary thick ascending limb of Henle's loop which in part is responsible for the cortico-papillary gradient of solutes within the kidney. 相似文献
14.
Nucleid acid metabolism in the LAM of the rat, both before and after castration, and during testosterone treatment, was investigated. RNA synthesis was increased by testosterone treatment, to varying degree, in adult and in prepubertal castrated rats, and was not merely dependent on the degree of hypertrophy of the LAM. The DNA content and the incorporation rate of formate-14C into DNA showed a characteristic profile under the same conditions: the atrophy of LAM following castration and its subsequent restoration appeared to be accompanied by variations in nuclei number. The possible role of testosterone in DNA duplication and in cell mitosis is hypothesized here; further investigation must be integrated by careful morphometric observation. 相似文献
15.
In 300 Sienese skulls of known age and sex (146 male and 154 female) 6 modes of expression of bipartition of the hypoglossal canal were studied on the basis of a new scheme of notation that takes into account gradually increasing intensity. The analysis of data, including also the traditional method of notation confirms the criticisms expressed by various authors on the loss of information when the variability of this trait is neglected, especially with reference to age-dependent changes (hyperostotic effects). 相似文献
16.
Summary Restriction endonucleases have been recently proved to be active on fixed chromosomes, thus they are useful in chromatin structure studies. Within this class of enzymes, Alu I is able to detect the presence and localization of highly repetitive DNA sequences in human and in other mammalian and dipteran species. In this paper the pattern obtained on fixed metaphase chromosomes of orangutan (Pongo pygmaeus) by Alu I digestion and Giemsa staining is shown. The results are discussed in the light of the distribution, in this species, of the I–IV human satellite DNAs. It is also suggested that in Pongo some highly repetitive sequences, different from the major human satellites, are present. 相似文献
17.
Chick embryo chorioallantoic membrane, infected with the Blacksburg strain of Newcastle disease virus, was examined with an electron microscope to investigate the sequence of viral-induced host cell alterations. These were evident mostly in the endodermal epithelial cells lining the allantoic sac and were divided arbitrarily into three stages. Stage 1 was characterized by commencement of cell hypertrophy and hyperplasia and presence of fewer cytoplasmic inclusion bodies normally found in the cells; in stage 2, juxtanuclear nucleocapsid-glycogen aggregates appeared, and there were increased numbers of microvilli; stage 3 was characterized by increased cytoplasmic density and evidence of viral assembly and release. The morphological features of viral assembly and the virion are also described. 相似文献
18.
A Tabucchi L Terzuoli A Di Stefano M Pizzichini R Leoncini E Dispensa E Marinello 《Bollettino della Società italiana di biologia sperimentale》1990,66(4):349-355
Adenylic acid (AMP) deaminase is a "catabolic enzyme" involved in nucleotide degradation, transforming AMP into inosinic acid (IMP). We present a simple method for the determination of the enzyme activity, which combines high sensitivity with requirement of low quantities of lymphocytes. Human lymphocytes were isolated with a Lymphocyte Separation Medium from FLOW and sonicated. After centrifugation at 2,000 rpm x 10 min and treatment with Norit A, the cells were incubated at 37 degrees C with ATP 0.8 mM and 14C-AMP 0.1 mM (specific activity 12 microCi/mumole) in potassium phosphate 100 mM (pH 7.4). 14C-IMP and 14C-AMP were separated through HPLC by an isocratic elution, with 20 mM KH2PO4 (pH 5.5) at a 1.5 ml/min flow rate. Identification of the nucleotides was carried out through retention time, coelution with internal standards: their evaluation by determining the radioactivity of the collected peaks. The enzyme activity is decreased in patients affected by CLL: the decrease is evident only when data are referred to the single cells and not when they are referred to the protein. 相似文献
19.
Genetic structure of the population of Sicily. 总被引:1,自引:0,他引:1
O Rickards G Biondi G F De Stefano F Vecchi H Walter 《American journal of physical anthropology》1992,87(4):395-406
Genetic heterogeneity within Sicily was investigated on the basis of ACP1, ADA, ESD, GLO1, PGD, PGM1, PGM2, SODA, ABO, and MN gene frequencies, and compared to those of other regions of Italy for which these same loci have been examined. Correspondence analysis revealed no differences within the island, at least at the provincial level, but showed genetic differentiation among Italian regions, distinctly clustering northern, central, and southern populations, respectively. These data indicate a close relationship between Sicily and southern Italy. In addition, the contribution of Middle Eastern populations to the gene pool of Sicily was evident. 相似文献
20.
Synopsis Morphological changes in the sea bass, Dicentrarchus labrax (Perciformes: Moronidae), were investigated after an experimental acclimation trial to freshwater. The sea bass is an euryhaline species occurring in the Mediterranean and west Atlantic from 30° N to 55° N. Part of the offspring of a pool of breeders was acclimated to freshwater at 9 months of age while maintaining the original stock in marine water. The effect of acclimation to freshwater over the entire form of the fish was studied through geometric morphometrics (shape coordinates). Changes in the form were shown graphically as landmark displacements though age classes 7,12, 15, 19 and 24 months and were discussed in the light of integrated growth. A significantly faster growth in freshwater was detected. Shape coordinates analyzed through multivariate statistics show that significant differences in shape arise after acclimation to freshwater. Moreover, the uniform component of shape change reveals a major effect of stretching or compression perpendicular to the body axis which is common and conservative in both trials. Differences are discussed in terms of selection and ecophenotypism. 相似文献