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   3246篇
  免费   397篇
  国内免费   5篇
  3648篇
  2021年   42篇
  2020年   27篇
  2019年   37篇
  2018年   41篇
  2017年   48篇
  2016年   51篇
  2015年   101篇
  2014年   135篇
  2013年   135篇
  2012年   164篇
  2011年   174篇
  2010年   103篇
  2009年   82篇
  2008年   134篇
  2007年   141篇
  2006年   133篇
  2005年   123篇
  2004年   122篇
  2003年   121篇
  2002年   125篇
  2001年   79篇
  2000年   88篇
  1999年   75篇
  1998年   48篇
  1997年   27篇
  1996年   35篇
  1995年   36篇
  1994年   34篇
  1992年   68篇
  1991年   57篇
  1990年   59篇
  1989年   45篇
  1988年   48篇
  1987年   56篇
  1986年   45篇
  1985年   57篇
  1984年   49篇
  1983年   39篇
  1982年   35篇
  1981年   28篇
  1980年   34篇
  1979年   49篇
  1978年   26篇
  1977年   40篇
  1976年   28篇
  1975年   29篇
  1974年   35篇
  1973年   34篇
  1968年   25篇
  1967年   31篇
排序方式: 共有3648条查询结果,搜索用时 0 毫秒
71.
72.
The binding of streptococcal competence factor (CF) was found to be specific for a strain of streptococcus which was capable of undergoing competence induction. Three other streptococcal strains which could not be induced to competence, did not bind CF. CF binding was independent of time, temperature, age of the culture, and type of growth media employed. Several observations indicated that the receptor sites for CF are located in the bacterial membrane: (i) the retention of CF by spheroplasts, (ii) the binding of CF by isolated membrane fractions, and (iii), the degradation of CF binding capacity of membranes by different chemicals and enzymes.  相似文献   
73.
Secondary structure mapping in the electron microscope was applied to ribosomal RNA and precusor ribosomal RNA molecules isolated from nucleoli and the cytoplasm of mouse L-cells. Highly reproducible loop patterns were observed in these molecules. The polarity of L-cell rRNA was determined by partial digestion with 3′-exonuclease. The 28 S region is located at the 5′-end of the 45 S rRNA precursor. Together with earlier experiments on labeling kinetics, these observations established a processing pathway for L-cell rRNA. The 45 S rRNA precursor is cleaved at the 3′-end of the 18 S RNA sequence to produce a 41 S molecule and a spacer-containing fragment (24 S RNA). The 41 S rRNA is cleaved forming mature 18 S rRNA and a 36 S molecule. The 36 S molecule is processed through a 32 S intermediate to the mature 28 S rRNA. This pathway is similar to that found in HeLa cells, except that in L-cells a 36 S molecule occurs in the major pathway and no 20 S precusor to 18 S RNA is found. The processing pathway and its intermediates in L-cells are analogous to those in Xenopus laevis, except for a considerable size difference in all rRNAs except 18 S rRNA.The arrangement of gene and transcribed spacer regions and of secondary structure loops, as well as the shape of the major loops were compared in L-cells, HeLa cell and Xenopus rRNA. The over-all arrangement of regions and loop patterns is very similar in the RNA from these three organisms. The shapes of loops in mature 28 S RNA are also highly conserved in evolution, but the shapes of loops in the transcribed spacer regions vary greatly. These observations suggest that the sequence complementarity that gives rise to this highly conserved secondary structure pattern may have some functional importance.  相似文献   
74.
Male meiosis in the grasshopper Stethophyma grossum is well known as an example of proximal chiasma localisation. An investigation of female meiosis in oocytes of this species shows that both the frequency and distribution of chiasmata are quite different from the male situation. Mean chiasma frequency per cell (14.98) in considerably higher in females than in males (11.28) which agrees with the trend established in other comparative studies of male and female meiosis. More strikingly, males and females also show not only different but quite opposite patterns of chiasma distribution. In spermatocytes of males, chiasmata are strictly localised proximally in most bivalents, but in oocytes of females very few chiasmata form in proximal regions and nearly all chiasmata form either in distal or interstitial regions. The genetical significance of these findings is considered.  相似文献   
75.
76.
Folate-activated one-carbon units are derived from serine through the activity of the pyridoxal-phosphate (PLP)-dependent isozymes of serine hydroxymethyltransferase (SHMT). The effect of vitamin B(6) availability on the activity and expression of the human mitochondrial and cytoplasmic SHMT isozymes was investigated in human MCF-7 cells. Cells were cultured for 6 months in vitamin B(6) replete (4.9 microM pyridoxine) minimal essential medium (alphaMEM) or vitamin B(6)-deficient medium containing 49, 4.9 or 0.49 nM pyridoxine. Total cellular PLP levels and SHMT activity were reduced 72% and 7%, respectively, when medium pyridoxine was decreased from 4.9 microM to 49 nM. Cells cultured in medium containing 4.9 nM pyridoxine exhibited 75%, 27% and 60% reduced levels of PLP, SHMT activity and S-adenosylmethionine, respectively, compared to cells cultured in alphaMEM. Cytoplasmic SHMT activity and protein levels, but not mRNA levels, were decreased in cells cultured in vitamin B(6) deficient medium, whereas mitochondrial SHMT activity and protein levels were less sensitive to vitamin B(6) availability. PLP bound to cytoplasmic SHMT with a K(d)=850 nM, a value two orders of magnitude lower than previously reported for the bovine cytoplasmic SHMT isozyme. Collectively, these data indicate that vitamin B(6) restriction decreases the activity and stability of SHMT, and that the cytoplasmic isozyme is more sensitive to vitamin B(6) deficiency than the mitochondrial isozyme in MCF-7 cells.  相似文献   
77.
The cytoplasmic N-terminal domain of the human ether-a-go-go related gene (hERG) K+ channel is critical for the slow deactivation kinetics of the channel. However, the mechanism(s) by which the N-terminal domain regulates deactivation remains to be determined. Here we show that the solution NMR structure of the N-terminal 135 residues of hERG contains a previously described Per-Arnt-Sim (PAS) domain (residues 26-135) as well as an amphipathic α-helix (residues 13-23) and an initial unstructured segment (residues 2-9). Deletion of residues 2-25, only the unstructured segment (residues 2-9) or replacement of the α-helix with a flexible linker all result in enhanced rates of deactivation. Thus, both the initial flexible segment and the α-helix are required but neither is sufficient to confer slow deactivation kinetics. Alanine scanning mutagenesis identified R5 and G6 in the initial flexible segment as critical for slow deactivation. Alanine mutants in the helical region had less dramatic phenotypes. We propose that the PAS domain is bound close to the central core of the channel and that the N-terminal α-helix ensures that the flexible tail is correctly orientated for interaction with the activation gating machinery to stabilize the open state of the channel.  相似文献   
78.
Nutritional restrictions in winter may reduce the availability of protein for reproduction and survival in northern ungulates. We refined a technique that uses recently voided excreta on snow to assess protein status in wild caribou (Rangifer tarandus) in late winter. Our study was the first application of this non-invasive, isotopic approach to assess protein status of wild caribou by determining dietary and endogenous contributions of nitrogen (N) to urinary urea. We used isotopic ratios of N (δ15N) in urine and fecal samples to estimate the proportion of urea N derived from body N (p-UN) in pregnant, adult females of the Chisana Herd, a small population that ranged across the Alaska-Yukon border. We took advantage of a predator-exclosure project to examine N status of penned caribou in April 2006. Lichens were the primary forage (>40%) consumed by caribou in the pen and δ15N of fiber tracked the major forages in their diets. The δ15N of urinary urea for females in the pen was depleted relative (−1.3 ± 1.0 parts per thousand [‰], ) to the δ15N of body N (2.7 ± 0.7‰). A similar proportion of animals in the exclosure lost core body mass (excluding estimates of fetal and uterine tissues; 55%) and body protein (estimated by isotope ratios; 54%). This non-invasive technique could be applied at various spatial and temporal scales to assess trends in protein status of free-ranging populations of northern ungulates. Intra- and inter-annual estimates of protein status could help managers monitor effects of foraging conditions on nutritional constraints in ungulates, increase the efficiency and efficacy of management actions, and help prepare stakeholders for potential changes in population trends. © 2010 The Wildlife Society.  相似文献   
79.
The design of immunologic interventions to prevent postnatal transmission of human immunodeficiency virus (HIV) will require identification of protective immune responses in this setting. Simian immunodeficiency virus (SIV)-infected rhesus monkeys (RMs), a species that develops an AIDS-like illness following experimental infection, transmit the virus at a high rate during breastfeeding. In contrast, postnatal transmission of SIV occurs rarely or not at all in natural, asymptomatic primate hosts of SIV. These contrasting transmission patterns provide a unique opportunity to study mechanisms that evolved to protect suckling infants from SIV infection. We compared the virologic and immunologic properties of milk of SIV-infected and uninfected natural hosts of SIV, African green monkeys (AGMs), to that of RMs. Interestingly, despite a low number of milk CD4(+) T lymphocytes in uninfected AGMs, milk virus RNA load in SIV-infected AGMs was comparable to that of SIV-infected RMs and that in AGM plasma. This observation is in contrast to the relatively low virus load in milk compared to that in plasma of SIV-infected RMs and HIV-infected women. Milk of SIV-infected AGMs also displayed robust virus-specific cellular immune responses. Importantly, an autologous challenge virus-specific neutralization response was detected in milk of five of six SIV-infected AGMs that was comparable in magnitude to that in plasma. In contrast, autologous challenge virus neutralization was not detectable in milk of SIV-infected RMs. The autologous virus-specific adaptive immune responses in breast milk of AGMs may contribute to impedance of virus transmission in the infant oral/gastrointestinal tract and the rarity of postnatal virus transmission in natural hosts of SIV.  相似文献   
80.
Summary The respiratory surface area (SAR) per kilogram body mass (MB), the harmonic mean thickness of the air-blood barrier (htR) in the gas exchange tissue, and the anatomical diffusion factor (ADF=SAR/htR per MB) were calculated for four juvenile Nile crocodiles. The ADF of three small specimens (mean MB=3.59 kg) was 625 cm2·m–1·kg–1. The values varied considerably among individuals and were similar to that of a 5.68-kg specimen (593 cm2·m–1·kg–1). Only 9% of the ADF is located in the anterior third of the lung, which because of its conical shape makes up only 14 percent of the total lung volume. Particularly in the middle third of the lung, the proximal region near the intrapulmonary bronchus displays a greater ratio of respiratory/non-respiratory surface areas than do more distally located sampling sites. The htR is also significantly smaller proximally than distally. The cumulative ADF per unit MB is greater than that previously reported for this species on the basis of overall estimates of SAR and htR, but is still less than that of lizards and testudinids. The disposition of ADF between distal air storage region and the intrapulmonary bronchus is consistent with a bidirectional cross-current gas exchange model.Abbreviations ADF anatomical diffusion factor - %AR percent of SA included in the effective respiratory zone - M B body mass - NVP non-ventilatory period - %P percent of total lung volume containing parenchyma - S A total surface area of intrapulmonary septa - S ANR that portion ofS A lying out the effective respiratory zone - S V surface-to-volume ratio in the parenchyma - htR harmonic mean thickness of the air-blood tissue barrier within the respiratory zone - V P parenchymal volume - VP ventilatory period  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