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   4298篇
  免费   244篇
  2022年   23篇
  2021年   46篇
  2020年   41篇
  2019年   37篇
  2018年   62篇
  2017年   58篇
  2016年   72篇
  2015年   121篇
  2014年   149篇
  2013年   219篇
  2012年   236篇
  2011年   242篇
  2010年   156篇
  2009年   167篇
  2008年   242篇
  2007年   236篇
  2006年   248篇
  2005年   246篇
  2004年   212篇
  2003年   226篇
  2002年   229篇
  2001年   87篇
  2000年   104篇
  1999年   94篇
  1998年   47篇
  1997年   42篇
  1996年   42篇
  1995年   42篇
  1994年   42篇
  1993年   41篇
  1992年   62篇
  1991年   50篇
  1990年   62篇
  1989年   55篇
  1988年   49篇
  1987年   37篇
  1986年   33篇
  1985年   32篇
  1984年   37篇
  1982年   20篇
  1981年   22篇
  1979年   19篇
  1978年   26篇
  1977年   18篇
  1976年   24篇
  1975年   20篇
  1974年   16篇
  1973年   20篇
  1972年   19篇
  1970年   25篇
排序方式: 共有4542条查询结果,搜索用时 46 毫秒
81.
82.
Published experimental data of several species of Drosophila using the population cage method were reexamined. The authors of these works have claimed that selection operates on polymorphic isozyme genes. Since selection coefficients (s) estimated by us for the above data, assuming overdominance, or for some cases, additivity between alleles, were too large (s > 0.1), we conclude that these experimental results do not reflect the effects of the single loci in question; rather, we suggest that careful experimentation and judgment are required for testing with the population cage method whether or not selection actually operates on single genes.  相似文献   
83.
A sensitive and specific radioimmunoassay for β-endorphin has been developed with an antiserum obtained in a rabbit immunized with β-endorphin contained in crude porcineACTH preparations. The minimal detectable quantity was 5 pg. The antiserum used reacted slightly with ovine β-lipotropin (5.5 %), but showed negligible cross-reactivity with other fragments of β-lipotropin, α-MSH and ACTH. Using this radioimmunoassay we have observed the presence of “big-big” β-endorphin (“big” β-lipotropin) with apparent molecular weights of 37,000 and 31,000 in human and rat pituitaries respectively, in addition to β-lipotropin and β-endorphin, by Sephadex gel-chromatography and sodium dodecyl sulfate polyacrylamide gel electrophoresis.  相似文献   
84.
Urinary excretion of prostaglandin E was measured radioimmunologically in 19 healthy persons ( 15 men and 4 women ) and in 16 patients ( 10 men and 6 women ) with essential hypertension before and after the administration of furosemide. The excretion rates were increased from 26.3±3.0 to 64.5±11.3 ng/hr in the former and from 11.9±2.7 to 26.9±85 ng/hr in the latter. There was a significant difference between them, healthy subjects showing a greater increase than patients with essential hypertension.There was an obvious sexual difference in urinary excretion of prostaglandin. In men, greater increase in the excretion rates was found than in the women. Greater increases were also obtained in healthy men than in hypertensive men and in healthy women than in hypertensive women. The present results suggest that furosemide enhances urinary excretion of prostaglandin E by mechanisms which entails either an increase in prostaglandin synthesis or a decrease in renal metabolism.  相似文献   
85.
Epichlorohydrin-triethanolamine (ECTEOLA)-cellulose films (paper and cloth) have been found to bind Saccharomyces cerevisiae cells which were able to develop metabolically active colonies on the surface of the films. Unmodified cellulose films also bound the yeast but to a lesser extent. Film fermenters were constructed by coiling a double layer of the cloth and copper screen and vertically placing the resulting cartridge into a column. These film fermenters were able to convert the sugars (14%) in the hydrolysate of a Jerusalem artichoke tuber into ethanol, with 90% of the theoretical yield after 6 h of fermentation. The bound yeast produced ethanol at a specific rate of 1.0 g ethanol per g cell per hour.  相似文献   
86.
A long-acting analogue of LH-RH, des-Gly10-[D-Leu6]-LH-RH-ethylamide (TAP-144), was administered once or twice on day 9 of pregnancy by subcutaneous, vaginal, rectal, and nasal routes to rats; the pregnancy-terminating effectiveness of these routes was determined on day 14. When rats were given a single dose of TAP-144, the ED50 value was 1.2 micrograms/100 g body weight by the vaginal route, the effectiveness being 752 and 1,221 times higher than those by the subcutaneous and rectal routes, respectively. Nasal administration was not effective in terminating pregnancy even at a high dose of 2,000 micrograms/100 g body weight. The addition of a surfactant (BL-9) did not increase the effectiveness by the rectal route. When TAP-144 was administered twice a day, the effectiveness was markedly increased by every route. The ED50 values were 0.32 microgram/100 g body weight/time by the subcutaneous route, which was comparable to that of the vaginal route, 14.2 micrograms of the rectal route, and 84.6 micrograms of the nasal route. The addition of the surfactant increased the effectiveness 26 times by the nasal route but not by the rectal route. The pregnancy-terminating effect of TAP-144 was reversed by treatment with progesterone.  相似文献   
87.
Twelve species of optically active metallocene derivatives having a 4-nitrophenyl group were prepared with horse liver alcohol dehydrogenase- or lipase-catalyzed optical resolution as the key step. The second harmonic generation (SHG) efficiently of these products was measured by the power method using the fundamental light from the Nd:YAG laser. (-)-1- (4-Nitrophenylthio)ethylferrocene (9), (+)-1-(4-nitrophenylthio)ethylruthenocene (16), (+)-1-(4-nitrophenylthio)ethylosmocene (19), (+) -1-(5-nitro-2-pyridylthio)ethylruthenocene (21), and (+) -1-[(4-nitrophenylhydrazono)methyl] -2-methylferrocene (12) showed SHG signals. The highest SHG efficiency was found with (+) -16, being 27 times more intense than the commonly used urea standard.  相似文献   
88.
Five hundred fifty DNA fragments 100-500 base pairs in length were cloned from total chromosomal DNA of Escherichia coli, each capable of promoting the synthesis of beta-lactamase when inserted upstream of the ampC structural gene without its own promoter in a promoter-probe plasmid. All clones in this library of putative promoters were classified based on the level of resistance to ampicillin, which ranged from 10 to more than 1,500 micrograms/ml. Most of the higher levels of drug resistance (more than 1,000 micrograms/ml) were due not only to an increase in gene expression but also to an increase in plasmid copy number. The DNA fragments which produced the highest level of drug resistance all mapped at 5.7 min on the E. coli chromosome and shared the same nucleotide sequence. In these fragments, a strong promoter was found, which carries an up stream AT-rich sequence in addition to -35 and -10 signals of the promoter consensus.  相似文献   
89.
In Japan, herpangina cases increase every summer with an incidence peak in July. During the past eight years from January, 1982 to December, 1989, a total of 3,974 viruses from herpangina cases were reported from 47 participating laboratories, and coxsackie A (CA) viruses accounted for most of them, 3,055 (76.9%). The major types associated with herpangina were, in order of frequency, CA4, 10, 5, 6, 2 and 3, representing 87.2%-80.8% of total isolations reported for each respective type in this period. Eighty-four point four percent of the total virus isolations were from children at four years of age or younger. More than 80% of total CA virus isolations were from nasopharyngeal specimens and nearly 90% of the viruses were isolated in mice.  相似文献   
90.
A column-switching high-performance liquid chromatographic method, requiring no sample preparation apart from filtration, is described for quantification of urinary orotic acid, uracil and pseudouridine. The analyses were carried out using a reversed-phase octadecylsilane-bonded column for sample clean-up and a cation-exchange column for separation; 5–20 ]sml samples of urine were directly analysed, and more than 100 samples could be analysed consecutively. Each sample required only 30 min. Detection limits of these compounds were 5 pmol. Creatinine-related urinary uracil excretion was lowest in the newborn period (17.3 ± 14.4 μmol/g of creatinine). A patient with partial ornithine transcarbamylase deficiency and his mother usually excreted a high level of uracil during the period of normal orotic acid excretion and normal serum ammonia level.  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