全文获取类型
收费全文 | 14269篇 |
免费 | 1188篇 |
国内免费 | 12篇 |
出版年
2023年 | 51篇 |
2022年 | 151篇 |
2021年 | 239篇 |
2020年 | 195篇 |
2019年 | 249篇 |
2018年 | 366篇 |
2017年 | 297篇 |
2016年 | 491篇 |
2015年 | 706篇 |
2014年 | 864篇 |
2013年 | 982篇 |
2012年 | 1348篇 |
2011年 | 1129篇 |
2010年 | 762篇 |
2009年 | 685篇 |
2008年 | 969篇 |
2007年 | 899篇 |
2006年 | 805篇 |
2005年 | 691篇 |
2004年 | 683篇 |
2003年 | 620篇 |
2002年 | 455篇 |
2001年 | 262篇 |
2000年 | 228篇 |
1999年 | 193篇 |
1998年 | 101篇 |
1997年 | 93篇 |
1996年 | 57篇 |
1995年 | 52篇 |
1994年 | 54篇 |
1993年 | 34篇 |
1992年 | 78篇 |
1991年 | 67篇 |
1990年 | 64篇 |
1989年 | 56篇 |
1988年 | 60篇 |
1987年 | 45篇 |
1986年 | 45篇 |
1985年 | 38篇 |
1984年 | 26篇 |
1983年 | 33篇 |
1982年 | 24篇 |
1981年 | 20篇 |
1980年 | 14篇 |
1979年 | 19篇 |
1978年 | 20篇 |
1976年 | 17篇 |
1973年 | 13篇 |
1972年 | 16篇 |
1971年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Summary Deactivation ofCandida
rugosa lipase was found to be complex. Hydrophobic interaction induced by iso-octane influenced the initial phase of deactivation, and increased the turn-over rate of the intermediate in the transition phase. After urea-treatment the structure of the last phase was not further influenced by thermal treatment, whereas that of initial phase was more sensitive to temperature change. Charge interaction was important in maintaining the structure during the deactivation, and especially anion charge might be a major factor. 相似文献
22.
G Gavellas M R Disbrow K H Hwang D K Hinkle J J Bourgoignie 《Laboratory animal science》1987,37(5):657-662
Minimally invasive techniques were used to collect urine with an external catheter together with automated intermittent monitoring of arterial blood pressure in awake male baboons. Using endogenous creatinine, 24-hour creatinine clearances were measured for 2 to 3 consecutive days in four intact and in four uninephrectomized baboons. Despite large differences in urinary volume and sodium excretion, reproducibility of 24-hour creatinine clearances was within 15% in 15 of 19 studies obtained from 6 of 8 animals. Arterial blood pressure was monitored intermittently at 30 to 60 minute intervals over 24 hours with a Dinamap monitor and recorder. Mean blood pressure averaged 71 +/- 4.4 to 89 +/- 5.5 mm Hg in different animals. Blood pressure tended to be lower at night than during the day. In separate studies using 15 to 60 minute urine collection periods, inulin clearance was compared in awake and in anesthetized animals with endogenous or exogenous creatinine clearance measured simultaneously. The clearance of creatinine systematically exceeded the clearance of inulin, even in intact animals with a normal serum creatinine. The creatinine-to-inulin clearance ratio averaged 1.16 +/- 0.03 at a serum concentration of 0.7 to 0.8 mg/dl; 1.27 +/- 0.03 at a serum creatinine of 1.0 to 1.1 mg/dl and 1.56 +/- 0.04 at a serum creatinine greater than 10 mg/dl. All values exceed unity significantly (p less than 0.001). Thus, renal function, including inulin clearance, can be measured in awake baboons. Duplicate or triplicate 24-hour urine collections are needed to assess the reliability of creatinine excretion. However, creatinine clearance overestimates glomerular filtration rate, as it does in humans. 相似文献
23.
Epstein-Barr virus nuclear antigen forms a complex that binds with high concentration dependence to a single DNA-binding site. 总被引:14,自引:8,他引:6
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A bacterially synthesized 28-kilodalton carboxyl-terminal fragment (28K-EBNA of Epstein-Barr virus nuclear antigen shows highly concentration dependent binding to monomer, dimer, and trimer copies of synthetic DNA-binding site 5' GATCTAGGATAGCATATGCTACCCCGGGG 3' 3' ATCCTATCGTATACGATGGGGCCCCCTAG 5' in bacterial plasmids. The rate of the binding reaction is independent of the number of sites, but dependent upon the length of the DNA containing the sites. These data are consistent with 28K-EBNA locating its binding sites by a process of facilitated transfer or sliding along the DNA. The highly concentration dependent binding suggests that multiple 28K-EBNA monomer polypeptides form a complex before or during binding. Binding occurs equally well at 24 and 37 degrees C, but not at 0 degrees C. A 28K-EBNA complex bound to a single site has unoccupied binding sites capable of interacting with additional DNA molecules. Such interaction is confirmed by agarose gel electrophoresis of protein-DNA complexes which indicate that a 28K-EBNA complex forms bridges between two DNA molecules. A bridge between the two binding regions in the Epstein-Barr virus origin of plasmid replication (oriP) would form a loop structure which could be an important feature for the regulatory function of authentic Epstein-Barr virus nuclear antigen. 相似文献
24.
Molecular cloning and differential expression of somatic and testis-specific H2B histone genes during rat spermatogenesis 总被引:10,自引:0,他引:10
We have cloned cDNA of a testis-specific histone, TH2B (a variant of H2B), and rat somatic H2B gene to investigate regulation of testis-specific histone genes during rat spermatogenesis. The amino acid sequences deduced from DNA sequences show extensive sequence divergence in the N-terminal third of the two histones. The rest is highly conserved. One cysteine residue was found in TH2B. No cysteine is present in somatic histones except in H3 histone. We investigated the expression of TH2B and H2B genes using the regions of sequence divergence as hybridization probes. The TH2B gene is expressed only in the testis, and the expression of this gene is detected 14 days after birth, reaching a maximum at Day 20. The level of H2B mRNA shows a reciprocal pattern. This contrasting pattern can be explained by the gradually changing proportion of spermatogonia and spermatocytes with testicular maturation. In situ cytohybridization studies show that H2B gene is expressed primarily in proliferating spermatogonia and preleptotene spermatocytes, whereas TH2B gene is expressed exclusively in pachytene spermatocytes which first appear in testis about 14 days after birth. H2B and TH2B genes appear to be ideal markers for the study of proliferation and differentiation events in spermatogenesis and their regulatory mechanisms. 相似文献
25.
Semiquantitative calculations of catalytic free energies in genetically modified enzymes 总被引:3,自引:0,他引:3
The catalytic free energy and binding free energies of the native and the Asn-155----Thr, Asn-155----Leu, and Asn-155----Ala mutants of subtilisin are calculated by the empirical valence bond method and a free energy perturbation method. Two simple procedures are used; one "mutates" the substrate, and the other "mutates" the enzyme. The calculated changes in free energies (delta delta G not equal to cat and delta delta Gbind) between the mutant and native enzymes are within 1 kcal/mol of the corresponding observed values. This indicates that we are approaching a quantitative structure-function correlation. The calculated changes in catalytic free energies are almost entirely due to the electrostatic interaction between the enzyme-water system and the charges of the reacting system. This supports the idea that the electrostatic free energy associated with the changes of charges of the reacting system is the key factor in enzyme catalysis. 相似文献
26.
The phasic velocity field in the vicinity of the venous anastomosis in a hemodialysis angioaccess arteriovenous fistula loop graft (AVLG) is investigated employing a laser Doppler anemometer (LDA) system. Detailed LDA velocity profiles are obtained by sectional survey performed in a transparent, elastic flow model which was fabricated to represent the geometry of the AVLG system under physiological pressure and flow waveforms. The geometry of the flow model was based on a silicone rubber cast obtained from an experimental dog model. In the present study, detailed distribution of velocity profiles is obtained. The distribution of wall shear stress in the model is computed from the slope of the local velocity profiles near the wall. The relationship between the results obtained by flow visualization and the LDA measurement is discussed. 相似文献
27.
28.
Pung Pung Hwang 《Cell and tissue research》1990,260(1):203-205
Summary Using an antiserum to highly purified chum salmon prolactin, prolactin cells were identified in the putative rostral pars distalis of newly hatched tilapia larvae (Oreochromis mossambicus) by the immunogold method for the electron microscope. In the putative rostral pars distalis, some cells had another kind of secretory granule which was much less numerous, much smaller in size, and without immunoreactivity to salmon prolactin antiserum. Controls incubated with salmon prolactin-preabsorbed antiserum or normal serum showed no immunoreactive cells, confirming the specificity of the antiserum. The possible role of prolactin in the osmoregulation of tilapia larvae is discussed. 相似文献
29.
Roger V. Lebo Lee A. Anderson Salvatore DiMauro Eric Lynch Peter Hwang Robert Fletterick 《Human genetics》1990,86(1):17-24
Summary When probes throughout the McArdle disease (myophosphorylase) gene region were used to search for DNA polymorphisms, only an MspI polymorphism was found in 94 enzyme-probe combinations. Along with an insertion/deletion polymorphism more 3 to the gene locus, these polymorphisms will be informative in 75% of at-risk patients. These results contrast strikingly to the six polymorphic sites detected in 15 enzyme-probe combinations in the homologous Her's disease (liver phosphorylase) gene region. This single MspI polymorphic site includes a CpG sequence of known increased mutability suggesting that DNA regions with rare polymorphisms will have most polymorphic sites at sequences with enhanced mutability. Fluorescence in situ hybridization sublocalized this gene to proximal band 11q13, establishing a point of cross-reference between the physical and genetic maps. 相似文献
30.
In order to determine the profile of arachidonic acid (AA) metabolites synthesized by bovine embryos during early developmental stages, embryos collected from superovulated beef cattle (days 6 through 17) were incubated with AA and its metabolites were analyzed by high performance liquid chromatography and radioimmunoassay (RIA). Embryos harvested and cultured before day 12 of the estrous cycle metabolized AA primarily to prostaglandin E2 (PGE2), whereas, those harvested on day 13 of the cycle metabolized AA to both PGE2 and PGF2 alpha. Furthermore, embryos collected after day 15 of the cycle metabolized AA to PGI2 in addition to PGE2 and PGF2 alpha. In view of the luteotropic properties that have been attributed to PGE2 and the vasodilatory effect of PGI2, this transitional change in prostaglandin synthesis during early stages of embryonic development may be a part of the mechanism by which the embryo exerts a luteotropic effect leading to maternal recognition of pregnancy and by which the conceptus begins preparing for subsequent implantation. 相似文献