全文获取类型
收费全文 | 15227篇 |
免费 | 1595篇 |
国内免费 | 8篇 |
出版年
2022年 | 121篇 |
2021年 | 260篇 |
2020年 | 163篇 |
2019年 | 196篇 |
2018年 | 275篇 |
2017年 | 248篇 |
2016年 | 354篇 |
2015年 | 615篇 |
2014年 | 623篇 |
2013年 | 797篇 |
2012年 | 1036篇 |
2011年 | 946篇 |
2010年 | 635篇 |
2009年 | 516篇 |
2008年 | 777篇 |
2007年 | 728篇 |
2006年 | 703篇 |
2005年 | 629篇 |
2004年 | 635篇 |
2003年 | 580篇 |
2002年 | 516篇 |
2001年 | 326篇 |
2000年 | 259篇 |
1999年 | 226篇 |
1998年 | 164篇 |
1997年 | 155篇 |
1996年 | 147篇 |
1995年 | 133篇 |
1994年 | 123篇 |
1993年 | 123篇 |
1992年 | 190篇 |
1991年 | 178篇 |
1990年 | 167篇 |
1989年 | 186篇 |
1988年 | 159篇 |
1987年 | 158篇 |
1986年 | 130篇 |
1985年 | 194篇 |
1984年 | 171篇 |
1983年 | 129篇 |
1982年 | 145篇 |
1981年 | 175篇 |
1980年 | 149篇 |
1979年 | 134篇 |
1978年 | 111篇 |
1977年 | 112篇 |
1976年 | 125篇 |
1975年 | 98篇 |
1974年 | 106篇 |
1969年 | 82篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
The population ofP. longirostris along the Mediterranean coast of Israel spends the benthic phase of its life cycle (from body size over 15 mm) on muddy bottom deeper than 45 m. New age groups are recruited within the depth zone of 45–300 m and migrate in both inshore and offshore directions. Inshore migration is limited by unsuitable sandy ground. The limiting line for offshore migration was not found.An age group could be detected during one year within a body size range of TL = 40 mm to TL = 84.5 mm for males and to TL = 102.5 mm for females. Reproductive activity in shallow water, down to a depth of 73 m, takes place during the whole year, while in depths of 150–300 m there is an arrest of reproductive activity from June to August. 相似文献
122.
Homologous recombination between human immunodeficiency viral DNAs in cultured human cells: analysis of the factors influencing recombination 总被引:1,自引:0,他引:1
S Kalyanaraman R Jannoun-Nasr D York P A Luciw R Robinson A Srinivasan 《Biochemical and biophysical research communications》1988,157(3):1051-1060
Recombination between HIV DNAs was analyzed using DNA transfection in cell cultures and the optimal conditions for efficient recombination were determined. Recombinant plasmid DNA substrates were constructed from HIV proviral DNAs and the success of recombination was measured by the production of viable hybrid virus. The process of recombination between HIV DNAs was shown to be i) dependent on homology between the truncated HIV DNAs and ii) maximum with concentrations of the truncated DNAs 3ug and above. HIV isolates with heterogeneity in their primary sequence, thus offer an ideal system for the analysis of the requirement of homologous recombination. In addition, recombination methodology would be useful for generating hybrid HIVs for the analysis of specific viral gene functions. 相似文献
123.
The F2 progeny of a cross between a chromosome 2 multiple marker stock and an adapted cultivar of barley were analyzed for four
morphological markers and electrophoretic patterns of eight leaf isozymes. TheIdh-2 locus was linked to thePer-5 locus (27.96±5.07 cM) and to thee locus (10.26±3.13 cM). Also, thePer-5 ande loci were located on the short arm of chromosome 2. In additionIdh-2 was also located on barley chromosome 2 and was linked to thev locus (13.18±3.56 cM), which is located on the long arm of chromosome 2. Two other marker genes,li andwst,,B, were linked (26.50±5.24 cM) on chromosome 2 but segregate independently of the other loci evaluated.
This project was supported by funds from the U.S.-Spain Joint Committee for Scientific and Technological Cooperation. 相似文献
124.
Tryptophan (Trp) fluorescence quenching of phytochrome has been studied using anionic, cationic and neutral quenchers, I-, Cs+ and acrylamide, respectively, in an effort to understand the molecular differences between the Pr and Pfr forms. The data have been analyzed using both Stern-Volmer and modified Stern-Volmer kinetic treatments. The anionic quencher, I-, was proven to be an ineffective quencher with Stern-Volmer constants, Ksv, of 0.60 and 0.63 M-1, respectively, for the Pr and Pfr forms of phytochrome. The cationic quencher, Cs+, showed about a 2-fold difference in the Ksv of Pr and Pfr, indicating a significant change in the fluorescent Trp environments during the Pr to Pfr phototransformation. However, only 25-37% of the fluorescent Trp residues were accessible to the cationic quencher. Most of the fluorescent Trp residues were accessible to acrylamide, but the quenching by acrylamide was indistinguishable for the Pr and Pfr forms. An additional quenching by acrylamide after a saturated quenching with Cs+ showed more than 40% increase in the Ksv of Pfr over Pr. These observations, along with the finding of two distinct components in the Trp fluorescence lifetime, indicate the existence of Trp residues in at least two different sets of environments in the phytochrome protein. The two components of the fluorescence had lifetimes of 1.1 ns (major) and 4.7 ns (minor) for Pr and 0.9 ns (major) and 4.6 ns (minor) for Pfr. Fluorescence quenching was found to be both static and dynamic as the Stern-Volmer constants for the steady-state fluorescence quenching were higher than for the dynamic fluorescence quenching. Based on the quenching results, in combination with the location of Trp residues in the primary structure, we conclude that the Pr to Pfr phototransformation involves a significant conformation change in the phytochrome molecule, preferentially in the 74 kDa chromophore-bearing domain. 相似文献
125.
Nucleolar organiser regions (NORs) were examined in over 90% of the species of Australian ground frogs (familiy Myobatrachidae),
including representatives from all twenty currently recognised genera and the three subfamilies. Throughout the family, location
of the NOR within the karyotype showed considerable variation yet karyotype morphology showed uniformity. The precise mechanism(s)
whereby variation in NOR location evolved while karyotype morphology was unchanged remains uncertain. Comparison of the two
major subfamilies showed that the Limnodynastinae had a greater diversity of NOR location than the Myobatrachinae. The limnodynastine
genus,Heleioporus, was the only one to show multiple NOR sites in several species. NOR location was particularly stable within most polytypic
genera. Differences in NOR location within the remaining polytypic genera (Heleioporus, Limnodynastes, Neobatrachus, Philoria andRanidella) pointed to taxonomic discriminations that were generally consistent with recent proposals based on other criteria. 相似文献
126.
Ralph W. Robinson 《Applied microbiology》1986,52(1):17-27
Methanosarcina mazei S6 and LYC were used to study the structure and differentiation of the aggregating methanogens. Cultures harvested under various conditions are described at the ultrastructural level. Cells of strain S6 are enclosed by a layer 12 nm thick in contact with the plasma membrane. In sarcinal colonies, cells are held in close association by a fibrous matrix up to 60 nm thick. Colony maturation was examined in strain S6 over a period of 1 year. Changes occurred in the shape and staining of individual cells. Also, various inclusion bodies were observed that either persist throughout colony maturation or are only found at certain growth stages. Two types of cores that are composed of double membranes in M. mazei S6 are described. One has a 90-nm diameter and contains electron-dense granules similar to those found in the cytoplasm. The other core type does not contain granules, is more numerous, and is found in older cultures. Two life cycles are described for M. mazei based on electron microscope examinations. A complex life cycle involving the release of single cells is described with two variations for strains S6 and LYC. When released cells of strain S6 are placed in fresh medium they can repeat the cycle. In addition, a limited cycle is described for both strains of M. mazei. This limited cycle contains the only sarcinal morphotypes observed in M. barkeri. When M. mazei S6 remains in the limited cycle and does not disaggregate in stationary phase, several types of possible resting forms are found. 相似文献
127.
R Jacobs J Falconer J S Robinson M E Webster 《Australian journal of biological sciences》1986,39(1):79-83
The development of secondary wool follicles in single fetal sheep subjected to hypobaric hypoxaemia was studied. One group of pregnant ewes were exposed to 57.1 kPa from 30 to 135 days gestation. Fetal weights (mean +/- s.d.) for the hypoxaemic group (3.35 +/- 0.53 kg; n = 4) were significantly lower than for the controls (4.19 +/- 0.31 kg; n = 3, P less than 0.05). At 110 days gestation, a second group had arterial and venous catheters surgically implanted into the ewe and fetus and skin samples were taken from the fetus. At 120 days gestation (10 days after surgery) these animals were subjected to hypoxia for 20 days, at a level to maintain fetal carotid pO2 between 1.47 and 1.87 kPa (mean carotid pO2 for the control fetuses was 2.84 +/- 0.28 kPa). Fetal weight at 140 days was not significantly different in the hypoxaemic and control groups. Morphometric analysis revealed that the secondary to primary follicle ratio (S:P) was less in both groups of hypoxaemic fetuses than in their respective controls. Although hypoxia for 20 days did not significantly alter fetal weight, it produced a low S:P ratio similar to the longer-term hypoxaemic animals. It is concluded that hypoxia has a marked effect in reducing the initiation of secondary follicles in the last third of gestation. 相似文献
128.
The intrinsic fluorescence of lauryl maltoside solubilized bovine heart cytochrome c oxidase has been determined to arise from tryptophan residues of the oxidase complex. The magnitude of the fluorescence is approximately 34% of that from n-acetyltryptophanamide (NATA). This level of fluorescence is consistent with an average heme to tryptophan distance of 30 A. The majority of the fluorescent tryptophan residues are in a hydrophobic environment as indicated by the fluorescence emission maximum at 328 nm and the differing effectiveness of the quenching agents: Cs+, I-, and acrylamide. Cesium was ineffective up to a concentration of 0.7 M, whereas quenching by the other surface quenching agent, iodide, was complex. Below 0.2 M, KI was ineffective whereas between 0.2 and 0.7 M 15% of the tryptophan fluorescence was found to be accessible to iodide. This pattern indicates that protein structural changes were induced by iodide and may be related to the chaotropic character of KI. Acrylamide was moderately effective as a quenching agent of the oxidase fluorescence with a Stern-Volmer constant of 2 M-1 compared with acrylamide quenching of NATA and the water-soluble enzyme aldolase having Stern-Volmer constants of 12 M-1 and 0.3 M-1, respectively. There was no effect of cytochrome c on the tryptophan emission intensity from cytochrome c oxidase under conditions where the two proteins form a tight, 1:1 complex, implying that the tryptophan residues near the cytochrome c binding site are already quenched by energy transfer to the homes of the oxidase. The lauryl maltoside concentration used to solubilize the enzyme did not affect the fluorescence of NATA.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
129.
Summary Conditions were used where the action of porcine pancreatic phospholipase A2 on phospholipids can be followed in the absence of added calcium and the catalytic activity is supported by the calcium brought with the nanomolar enzyme. Therefore, alterations in the enzyme velocity resulting from the presence of spermine or spermidine could be specifically studied using 1-palmitoyl-2-(pyren-1-yl)hexanoyl-sn-glycero-3-phosphocholine (PPHPC) and 1-palmitoyl-2-(pyren-1-yl)hexanoyl-sn-glycero-3-phosphoglycerol (PPHPG) as substrates. Both spermine and spermidine activated the hydrolysis of PPHPG fourfold at polyamine/phospholipid molar ratios of approximately 11 and 121, respectively. Double-reciprocal plots of enzyme activityvs. PPHPG concentration revealed the enhancement to be due to increased apparentV
max while the apparentK
m
was slightly increased. In the presence of 4mm CaCl2 inhibition by polyamines of PPHPG hydrolysis by phospholipase A2 was observed. Using synthetic diamines we could further demonstrate that two primary amino groups are required for the activation. In the absence of exogenous CaCl2 polyamines inhibited the hydrolysis of PPHPC by phospholipase A2. The presence of 4mm CaCl2 reversed this inhibition and a twofold activation was observed at 10 m spermine. The results obtained indicate that the activation of PLA2 by spermine and spermidine is produced at the level of the substrate, PPHPG. This implies the formation of complexes of phosphatidylglycerol and polyamines with defined stoichiometries. 相似文献
130.
γ-Aminobutyric Acid Receptor Complex of Insect CNS: Characterization of a Benzodiazepine Binding Site 总被引:2,自引:2,他引:0
Timothy Robinson David MacAllan George Lunt Martin Battersby 《Journal of neurochemistry》1986,47(6):1955-1962
The specific binding of [N-methyl-3H]flunitrazepam ([3H]FNZP) to a membrane fraction from the supraoesophageal ganglion of the locust (Schistocerca gregaria) has been measured. The ligand binds reversibly with a KD of 47 nM. The binding is Ca2+-dependent, a property not found for the equivalent binding site in vertebrate brain. The pharmacological characteristics of the locust binding site show similarities to both central and peripheral benzodiazepine receptors in mammals. Thus binding is enhanced by gamma-aminobutyric acid (GABA), a feature of mammalian central receptors, whereas the ligand Ro 5-4864 was more effective in displacing [3H]FNZP than was clonazepam, which is the pattern seen in mammalian peripheral receptors. The locust benzodiazepine binding site was photoaffinity-labelled by [3H]FNZP, and two major proteins of Mr 45K and 59K were specifically labelled. In parallel experiments with rat brain membranes a single major protein of Mr 49K was labelled, a finding in keeping with many reports in the literature. We suggest that the FNZP binding site described here is part of the GABA receptor complex of locust ganglia. The insect receptor appears to have the same general organization as its mammalian counterpart but differs significantly in its detailed properties. 相似文献