首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5700篇
  免费   554篇
  国内免费   2篇
  6256篇
  2022年   42篇
  2021年   71篇
  2020年   51篇
  2019年   58篇
  2018年   74篇
  2017年   65篇
  2016年   115篇
  2015年   215篇
  2014年   184篇
  2013年   299篇
  2012年   350篇
  2011年   333篇
  2010年   198篇
  2009年   185篇
  2008年   290篇
  2007年   323篇
  2006年   287篇
  2005年   294篇
  2004年   270篇
  2003年   266篇
  2002年   242篇
  2001年   53篇
  2000年   51篇
  1999年   68篇
  1998年   78篇
  1997年   58篇
  1996年   64篇
  1995年   74篇
  1994年   37篇
  1993年   56篇
  1992年   57篇
  1991年   52篇
  1990年   37篇
  1989年   44篇
  1988年   31篇
  1987年   44篇
  1986年   39篇
  1985年   48篇
  1984年   47篇
  1983年   49篇
  1982年   60篇
  1981年   57篇
  1980年   59篇
  1978年   48篇
  1977年   33篇
  1976年   39篇
  1975年   30篇
  1974年   36篇
  1972年   28篇
  1969年   30篇
排序方式: 共有6256条查询结果,搜索用时 0 毫秒
21.
The JY328 clone was identified in a human genomic library using cDNA corresponding to mRNA for HLA-B7 as a probe. The L/328 cell line was established by cotransformation of mouse Ltk cells with the herpes thymidine kinase gene and clone JY328. On Northern blots, RNA from,L/328 strongly hybridized to an HLA class I probe, and an antigen was recognized by an anti-HLA class I framework antibody on the cell surface. A DNA probe corresponding to a segment of intron 7 was developed by comparing the nucleotide sequence of clone JY328 with that of other HLA class I-type genes. Using the radiolabeled probe to screen Southern blots of DNA from families with siblings exhibiting intra-HLA recombinations, a restriction fragment length polymorphism was revealed —a 1.4 kb BstE II band not present in all individuals. A corresponding fragment was apparent in the base sequence of clone JY328. The occurrence of this band on Southern blots established that JY328 maps distinct from and centromeric to the HLA-C locus and near to the HLA-B locus. Antibody absorption studies and cytotoxicity tests indicated that the JY328 gene product was not an HLA-B antigen but that it did specifically absorb CW7-specific antibody. In sum, these results suggest a novel, polymorphic HLA class I gene which expresses a product serologically similar to HLA-Cw7 but which does not map within the corresponding locus.  相似文献   
22.
Summary The pH of fodder beet pulp was varied to see how this affected solid phase fermentation by yeast. The process is for fuel ethanol production. When pulp was adjusted to a pre-inoculation pH of 3.0–3.5, ethanol yields (78–85% of theoretical, averaging 8.9% v/v) and fermentation efficiencies (97–99%) were greatest, the fermentation time was the shortest (30–39 h) and no bacterial contamination occurred.  相似文献   
23.
The study was carried out to examine the relationship of neonatal sex, birthweight, maternal parity, hemoglobin status, gestational age at term, and duration of labor to cord- and maternal-plasma zinc, copper, and iron levels, measured by atomic absorption spectrophotometry. Significant differences were observed between maternal-and neonatal-cord plasma concentrations of the different trace elements at term. Maternal parity had no significant influence on the distribution of plasma trace elements, except in the case of maternal plasma iron levels. However, maternal hemoglobin status was observed as an important covariate of maternal zinc and iron levels at term. Neonatal birthweight was observed to be an index of maternal plasma zinc status at term. On the other hand, although gestational age at term had no significant influence on maternal-plasma trace-element concentrations, it was observed to influence neonatal-cord plasma levels. Long durations of labor (≥18 h) were associated with relatively lower, but not significant, maternal-plasma iron levels, whereas neonatal-cord-plasma iron levels seemed to show sex differences.  相似文献   
24.
A novel type of bacterium has been isolated from various geothermally heated locales on the sea floor. The organisms are strictly anaerobic, rod-shaped, fermentative, extremely thermophilic and grow between 55 and 90°C with an optimum of around 80°C. Cells show a unique sheath-like structure and monotrichous flagellation. By 16S rRNA sequencing they clearly belong to the eubacteria, although no close relationship to any known group could be detected. The majority of their lipids appear to be unique in structure among the eubacteria. Isolate MSB8 is described as Thermotoga maritima, representing the new genus Thermotoga.Dedicated to Otto Kandler on the occasion of his 65th birthday Present addresses: University of South Dakota, Vermillion, USA; University of Illinois, Urbana, USA; Universität für Bodenkultur, Wien, Austria  相似文献   
25.
A Rapid Chromosome-Mapping Method for Cloned Fragments of Yeast DNA   总被引:19,自引:2,他引:17  
A rapid and generally applicable method is described for mapping a cloned yeast DNA segment to the chromosome(s) from which it originated. The method is based upon the recent finding that the integration into a yeast chromosome of a segment of the 2 mu plasmid DNA results, in heterozygous diploids, in the specific loss of genetic information from the chromosome into which the 2 mu DNA was integrated (Falco et al. 1982). After verification of the accuracy of the method using several genes whose position was known in advance, the method was used to locate the yeast actin gene, which lies on the left arm of chromosome VI, about 50 cM distal to CDC4.  相似文献   
26.
Using resistance to the base analog 8-azaguanine as a genetic marker, we showed that adenovirus type 2, but not adenovirus type 12, is mutagenic at the hypoxanthine phosphoribosyltransferase locus of cloned diploid rat liver epithelial cells. Adenovirus type 2 increased the frequency of 8-azaguanine-resistant colonies by up to ninefold over the spontaneous frequency, depending on expression time and virus dose.  相似文献   
27.
The mutagenicity of diethylstilbestrol (DES) in V79 Chinese hamster cells was examined under a variety of conditions. DES over a concentration range 0.01–10 μg/ml failed to induce any increase above the spontaneous frequency of 6-thioguanine-resistant V79 cells. The effect of varying the expression time after treatment in the mutation assay from 3 to 9 days was studied and DES was nonmutagenic at all time points, while N-methyl-N′-nitro-N-nitrosoguanidine was highly mutagenic with a peak response after a 5–7 day expression time. The mutagenicity of benzo[a]pyrene and DES, both of which induce morphological and neoplastic transformation of Syrian hamster embryo (SHE) cells, was tested by cocultivating V79 cells with SHE cells for possible metabolic activation of the chemicals. Neither compound was mutagenic to V79 cells in the absence of SHE cells. Benzo[a]pyrene, but not DES, was mutagenic to V79 cells cocultivated with SHE cells. These results support the observation that DES can induce cell transformation under conditions that do not result in any measurable gene mutations. Moreover, the ability of DES to enhance the recovery of 6-thioguanine-resistant mutations was studied by determining the ability of DES to inhibit metabolic cooperation of V79 cells. Unlike the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate, DES was a weak or inactive inhibitor of metabolic cooperation.  相似文献   
28.
An examination of the essential oil of myrrh from Commiphora molmol has permitted the identification of 1(10)Z,4Z-furanodiene-6-one, 2-meth  相似文献   
29.
Tunicamycin was found to specifically inhibit the incorporation of a number of sugars into L1210 leukemia cell glycoproteins. This inhibition of glyco-protein biosynthesis led to a cessation of cell growth which was reversible in a dose-dependent and time-dependent manner. After removal of the antibiotic from L1210 cell cultures resumption of sugar incorporation preceded that of thymidine incorporation and the recovery of cell growth. The treatment of cells with tunicamycin resulted in a significant increase in the intracellular pool of UDP-N-acetylglucosamine which occurred concurrently with alterations in cell ultrastructure including distentions of the endoplasmic reticulum and nuclear membranes. Similar ultrastructural changes and increases in the intracellular pools of UDP-sugars were observed in L1210 cells exposed to 5 mM D-glucosamine, which suggested that the antiproliferative effects of tunicamycin may be related to the accumulation in the endoplasmic reticulum of one or more nucleotide sugar precursors of asparagine-linked glycoprotein biosynthesis. However, the biological effects of tunicamycin could be distinguished from those caused by D-glucosamine. Exposure of L1210 cells to tunicamycin resulted in specific alterations in the biochemical composition of the plasma membrane and in the inhibition of cellular agglutination by wheat germ agglutinin which were not apparent following exposure to equitoxic concentrations of the aminosugar. These studies, together with those which demonstrated that recovery of the cellular capacity to synthesize glycoproteins was obligatory for the recovery of cellular proliferation in tunicamycin-treated cells, suggested that inhibition of the synthesis of glycoproteins was the major factor limiting L1210 leukemic cell proliferation.  相似文献   
30.
When the 42,000-dalton major outer membrane protein of Escherichia coli O111 is examined on alkaline polyacrylamide gels containing sodium dodecyl sulfate, it is resolved into three distinct bands designated as proteins 1, 2, and 3. Band 3 consists of two distinct polypeptides, proteins 3a and 3b. E. coli K-12 does not make any protein 2, but makes proteins similar to 1, 3a, and 3b as indicated by comparison of cyanogen bromide peptide patterns. Several Shigella species and most other strains of E. coli resemble E. coli K-12 in that they lack protein 2, whereas Salmonella typhimurium is more similar to E. coli O111. In addition to these species and strain differences, cultural differences resulted in differences in the outer membrane protein profiles. Under conditions of catabolite repression, the level of protein 2 in E. coli O111 decreased while the level of protein 1 increased. An enterotoxin-producing strain similar to E. coli O111 produced no protein 1 and an elevated level of protein 2 under conditions of low catabolite repression. The levels of proteins 1 and 3 are also different in different phases of the growth curve, with protein 1 being the major species in the exponential-phase cells and protein 3 being the major species in stationary-phase cells. A multiply phage-resistant mutant of E. coli K-12 with no obvious cell wall defects produced no protein 1 or 2, but made increased amounts of protein 3. Thus, the major outer membrane proteins of E. coli and related species may vary considerably without affecting outer membrane integrity.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号