全文获取类型
收费全文 | 3455篇 |
免费 | 343篇 |
国内免费 | 5篇 |
出版年
2023年 | 20篇 |
2022年 | 69篇 |
2021年 | 90篇 |
2020年 | 56篇 |
2019年 | 66篇 |
2018年 | 63篇 |
2017年 | 55篇 |
2016年 | 97篇 |
2015年 | 150篇 |
2014年 | 164篇 |
2013年 | 179篇 |
2012年 | 225篇 |
2011年 | 253篇 |
2010年 | 126篇 |
2009年 | 110篇 |
2008年 | 172篇 |
2007年 | 165篇 |
2006年 | 137篇 |
2005年 | 128篇 |
2004年 | 98篇 |
2003年 | 104篇 |
2002年 | 108篇 |
2001年 | 76篇 |
2000年 | 69篇 |
1999年 | 76篇 |
1998年 | 32篇 |
1997年 | 28篇 |
1996年 | 29篇 |
1995年 | 21篇 |
1994年 | 25篇 |
1992年 | 47篇 |
1991年 | 35篇 |
1990年 | 37篇 |
1989年 | 37篇 |
1988年 | 32篇 |
1987年 | 26篇 |
1986年 | 29篇 |
1985年 | 39篇 |
1984年 | 21篇 |
1983年 | 30篇 |
1982年 | 30篇 |
1981年 | 20篇 |
1980年 | 21篇 |
1979年 | 27篇 |
1978年 | 20篇 |
1977年 | 21篇 |
1973年 | 24篇 |
1972年 | 25篇 |
1971年 | 19篇 |
1970年 | 21篇 |
排序方式: 共有3803条查询结果,搜索用时 31 毫秒
71.
The 135-nucleotide-long “5 + S” RNA molecule found in Drosophila tissue culture cells after labelling at 37 °C has been identified as a precursor to 5 S RNA by pulse-chase experiments. The structure of the 15-nucleotide-long 3′-terminal sequence which differentiates this molecule from mature 5 S RNA has been determined. This ends in a stretch of U residues, suggestive of a polymerase termination signal. 相似文献
72.
Summary Fox and Woese (1975a) have shown that a model of 5S RNA secondary structure similar to the one originally derived forChlorella 5S RNA can be generalized with relatively minor variations to all sequenced 5S RNA molecules, i.e. that corresponding base paired regions can be formed at approximately the same positions. We present experimental data in favour of this hypothesis and show that the points at which ribonucleases T1, T2 and pancreatic ribonuclease cleave six different 5S RNA molecules under mild conditions (high ionic strength, low temperature, low RNAase concentration) nearly always fall in the proposed single-stranded regions. We conclude that this model is a good approximation to the conformation of 5S RNA in solution. 相似文献
73.
Summary Intermolecular transposition of Tn2660 into pCR1 was measured at 30°C in recA
– and recA
+ hosts as between 2.6 and 5.5x10–3, a similar value to that previously found for Tn3. No cointegrate structures were found under conditions where 104 transposition events occurred. Immunity to intermolecular transposition of Tn2660, similar to that found for Tn3 was demonstrated by showing that the above transposition frequency was reduced by a factor of between 10–3 and 10–4 when a mutant Tn2660 (resulting in the synthesis of a temperaturesensitive -lactamase) was present in the recipient plasmid. Intramolecular transposition of Tn3 was found to occur under the same conditions as previously demonstrated for Tn2660 giving rise to similar end products, in which the newly introduced Tn3 is oriented inversely to the resident Tn3 and the DNA sequence between the two transposons has been inverted. Thus, in all respects functional identity of the transposition activities of Tn3 and Tn2660 is shown, thereby identifying characteristics of intramolecular transposition that are not readily accommodated by current models of transposition. 相似文献
74.
Summary A simple container is described whereby small seedlings may be grown at controlled levels of water stress. The water stress was induced in the soil by an osmoticum which is separated from the soil by a semi-permeable membrane. The mortality ofBetula pendula seedlings was markedly increased at a matric potential of –1.6 bars whereas the mortality ofBuddleia davidii was only affected below –2.8 bars. This difference in tolerance to water stress at the seedling stage might not be reflected in the distribution of the species in the colonisation of chalk and sand pits in England unless there is a dry spring. 相似文献
75.
C. Jordan W. Caskey G. Escalante R. Herrera F. Montagnini R. Todd C. Uhl 《Plant and Soil》1982,67(1-3):325-332
Standing stocks and fluxes of nitrogen, including nitrogen fixation and denitrification, were measured in a tropical rainforest on Oxisol in the Amazon Territory of Venezuela. The standing stock of nitrogen was comparable to that of temperate forests, but was higher than that in an adjacent forest on Spodosol. Fluxes were higher than in forests in the temperate zone, but lower than in another tropical forest on more fertile soil. Even though nitrogen was abundant, this does not mean that nitrogen could not be limiting to agriculture if the forest is cleared and the land cultivated. The nitrogen fixing and nitrogen conserving mechanisms are dependent upon the structure of the undisturbed forest, and destruction of the forest would eventually decrease the input of nitrogen to the soil. 相似文献
76.
TERESA MARYAKA HALSZKA OSMÓLSKA 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1979,12(3):265-273
Supraorbital bones in Saurolophus angustirostris are described, and their presence in all hadrosaurs is suggested. Frontal-nasal and premaxillar-nasal fontanellae are distinguished in hadrosaurs; their presence is explained as connected with growth and considered to he responsible for the variability of crest structures. New data indicating the presence of a cartilaginous diverticulum nasi within the circumnarial depression in Saurobphus ongustirostris are presented. A physiological (respiratory and/or thermoregulatory) function of the nasal diverticulum is proposed. 相似文献
77.
Movement of Kinetin and Gibberellic Acid in Leaf Petioles during Water Stress-induced Abscission in Cotton 下载免费PDF全文
Movement of [14C]kinetin and [14C]gibberellic acid was examined in cotton (Gossypium hirsutum L.) cotyledonary petiole sections independent of label uptake or exit from the tissue. Sections 20 millimeters in length were taken from well watered, stressed, and poststressed plants. Transport capacity was determined using a pulse-chase technique. Movement of both kinetin and gibberellic acid was found to be nonpolar with a velocity of 1 millimeter per hour or less, suggesting passive diffusion. Neither water stress nor anaerobic conditions during transport of labeled material affected the transport capacity of the petioles. 相似文献
78.
Control of porphyrin biosynthesis in Rhodopseudomonas spheroides and Propionibacterium shermanii. A direct 13C nuclear-magnetic-resonance spectroscopy study. 下载免费PDF全文
G Burton P M Jordan N E MacKenzie P E Fagerness A I Scott 《The Biochemical journal》1981,194(2):627-631
The facultative anaerobes Rhodopseudomonas spheroides and Propionibacterium shermanii were grown under anaerobic and aerobic conditions. The effect of light was studied with the photosynthetic R. spheroides, and the adaptation of both species to dark anaerobic life was monitored by direct observation of 5-amino[5-13C]laevulinic acid metabolism by using 13C nuclear-magnetic-resonance spectroscopy. 相似文献
79.
A comparative study was carried out in order to determine which of the most commonly used alkalies for protein hydrolysis in tryptophan determination gave the best results. Hydrolyses were performed with 2.5 and 4 n Ba (OH)2, 4 and 10 n NaOH, 5 n NaOH containing 5% SnCl2, and with 4 n LiOH, not previously reported for use. The effect of temperature and hydrolysis time on the measured tryptophan content was also determined. Based on results obtained with lysozyme and with seven high protein preparations 4 n LiOH gave the best results. A temperature of 145°C was selected as the most convenient temperature since maximum tryptophan values were obtained with 4–8 h. The hydrolysis time required was inversely related to the protein content of the preparation. Lysozyme, casein, bovine plasma protein, and dehydrated whole egg gave maximum tryptophan content after 4 h hydrolysis while skimmed milk powder, rice flour, wheat flour, and wild legume flour required 8 h hydrolysis. 相似文献
80.
On integrating experimental data published previously, the following picture of the mitochondrial adenine nucleotide (AdN) translocation system is being presented: 1. The AdN translocation system serves not only to transport ATP synthesized within mitochondria into the cytosol but also to transport cytosolic ATP into the mitochondria when oxidative phosphorylation is not functioning. 2. The AdN translocator is coded for by nuclear genes and the mitochondrial protein synthesis is not involved in its formation. 3. The AdN translocation system must be preserved and functioning even in cells which could dispense with oxidative phosphorylation. It assures appropriate concentrations of intramitochondrial ATP. 4. The intramitochondrial ATP is required for normal replication of mitochondrial DNA. Tis supports the view that the mitochondrion is a self-replicating semi-autonomous organelle. 5. The appropriate concentration of ATP must be present in mitochondria to make possible cell growth or multiplication. This points to a direct or indirect role of mitochondria in the control of cell proliferation. 相似文献