全文获取类型
收费全文 | 3818篇 |
免费 | 286篇 |
国内免费 | 8篇 |
专业分类
4112篇 |
出版年
2022年 | 17篇 |
2021年 | 20篇 |
2020年 | 25篇 |
2019年 | 22篇 |
2018年 | 24篇 |
2017年 | 25篇 |
2016年 | 49篇 |
2015年 | 100篇 |
2014年 | 109篇 |
2013年 | 155篇 |
2012年 | 215篇 |
2011年 | 244篇 |
2010年 | 154篇 |
2009年 | 137篇 |
2008年 | 209篇 |
2007年 | 234篇 |
2006年 | 222篇 |
2005年 | 251篇 |
2004年 | 248篇 |
2003年 | 232篇 |
2002年 | 233篇 |
2001年 | 39篇 |
2000年 | 29篇 |
1999年 | 45篇 |
1998年 | 66篇 |
1997年 | 47篇 |
1996年 | 37篇 |
1995年 | 39篇 |
1994年 | 40篇 |
1993年 | 31篇 |
1992年 | 33篇 |
1991年 | 35篇 |
1990年 | 38篇 |
1989年 | 36篇 |
1988年 | 29篇 |
1987年 | 35篇 |
1986年 | 30篇 |
1985年 | 35篇 |
1984年 | 46篇 |
1983年 | 46篇 |
1982年 | 50篇 |
1981年 | 42篇 |
1980年 | 49篇 |
1979年 | 36篇 |
1978年 | 39篇 |
1977年 | 41篇 |
1976年 | 39篇 |
1975年 | 31篇 |
1974年 | 23篇 |
1968年 | 15篇 |
排序方式: 共有4112条查询结果,搜索用时 15 毫秒
81.
Ultraviolet sources with output wavelengths of 254, 300, and 366 nm were compared for detection of ethidium bromide-DNA complexes in acrylamide gels. The 254- and 300-nm sources were both much more sensitive than the 366-nm source. The 254-nm source produced a great deal of photodamage, photonicking and photodimerization, and photobleaching, while the longer wavelength sources cause little damage or bleaching. The 300-nm source is clearly the most suitable source, providing high sensitivity and a relatively low amount of photodamage and photobleaching. 相似文献
82.
For five species of hummingbirds in the laboratory, time between meals was related to energy intake on the first meal and rate of energy expenditure between meals. Field observations gave similar results. Average meal sizes were similar at one intake rate independent of food caloric density; females averaged longer bouts than males. When rate of intake was approximately halved, meal duration approximately doubled and volume intake remained similar. We postulate that feeding is initiated when crop contents reach a lower threshold and that feeding is terminated after ingestion of an optimal volume determined by the added weight of the meal. 相似文献
83.
Cellular accumulation of ascorbic acid was investigated in vitro in distal intestinal mucosa of guinea pig. With 14C-ascorbic acid present at 8 μM/L in the bathing media, tissue/media (T/M) concentration ratios of at least 5 were routinely achieved. Recently absorbed ascorbic acid appeared to be free in solution in the cellular fluid in that it diffused from tissue exposed to poisons with a disappearance half-time of approximately 10 minutes. Ascorbic acid uptake was highly dependent on the presence of sodium in the bathing media; total Tris substitution resulted in a 97% decrease in uptake. Also, metabolically depleted tissue did not accumulate ascorbic acid against a concentration gradient. Uptake of 14C-ascorbic acid from a bathing solution concentration of 8 μM/L was reduced 67% in the presence of 0.8 mM/L nonlabeled ascorbic acid. Recently absorbed 14C-ascorbic acid moved more rapidly back into the lumen when the luminal solution contained nonlabeled ascorbic acid (5 mM) than when it contained mannitol (5mM). This demonstration of counter transport substantiates a carrier mechanism in the brush border. 相似文献
84.
Mary Anne Nelson Mark Ericson Larry Gold John F. Pulitzer 《Molecular & general genetics : MGG》1982,188(1):60-68
Summary We have isolated mutants of Escherichia coli B (called TabR) that restrict the growth of bacteriophage T4 rII mutants at high temperature. TabR strains lysed very rapidly after infection with rII mutants, and no progeny phage were produced. T4+-infected TabR cells also lysed quickly, but the cells remained intact long enough to give a small burst. We have selected pseudorevertants of rII deletion mutants that grow on TabR at high temperature; tk (thymidine kinase) is a component of one class of these pseudorevertants.T4 strains harboring mutations in genes 12, 16, 25, 34, 36, 45 and 63 were also specifically restricted on TabR strains at high temperature. Bacteriophages T2, T4, T5, T6, and T7 grew normally on TabR, while , 80, and P1 failed to grow at any temperature. The most restrictive TabR strains were auxotrophic for methionine at high temperature, and most spontaneous Met+ revertants had also lost the ability to restrict rII mutants, suggesting that the TabR phenotype and methionine auxotrophy result from the same mutation.Although the mechanism by which TabR strains exert their restriction has not been determined, one model is described. The potential uses of these and similar strains is discussed. 相似文献
85.
Rotation of cytochrome P-450. II. Specific interactions of cytochrome P-450 with NADPH-cytochrome P-450 reductase in phospholipid vesicles 总被引:2,自引:0,他引:2
J Gut C Richter R J Cherry K H Winterhalter S Kawato 《The Journal of biological chemistry》1982,257(12):7030-7036
Purified rat liver microsomal cytochrome P-450 and NADPH-cytochrome P-450 reductase were co-reconstituted in phosphatidylcholine-phosphatidylethanolamine-phosphatidylserine vesicles using a cholate dialysis technique. The co-reconstitution of the enzymes was demonstrated in proteoliposomes fractionated by centrifugation in a glycerol gradient. The proteoliposomes catalyzed the N-demethylation of a variety of substrates. Rotational diffusion of cytochrome P-450 was measured by detecting the decay of absorption anisotropy r(t), after photolysis of the heme.CO complex by a vertically polarized laser flash. The rotational mobility of cytochrome P-450, when reconstituted alone, was found to be dependent on the lipid to protein ratio by weight (L/P450) (Kawato, S., Gut, J., Cherry, R. J., Winterhalter, K. H., and Richter, C. (1982) J. Biol. Chem. 257, 7023-7029). About 35% of cytochrome P-450 was immobilized and the rest was rotating with a mean rotational relaxation time phi 1 of about 95 mus in L/P450 = 1 vesicle. In L/P450 = 10 vesicles, about 10% of P-450 was immobile and the rest was rotating with phi 1 congruent to 55 mus. Co-reconstitution of equimolar amounts of NADPH-cytochrome P-450 reductase into the above vesicles results in completely mobile cytochrome P-450 with a phi 1 congruent to 40 mus. Only a small decrease in the immobile fraction of cytochrome P-450 is observed when the molar ratio of cytochrome P-450 to the reductase is 5. The results suggest the formation of a monomolecular 1:1 complex between cytochrome P-450 and NADPH-cytochrome P-450 reductase in the liposomes. 相似文献
86.
Summary We have developed a miniature silver-silver chloride electrode. The outer diameter of the electrodes averaged 22 m and the input resistance 8.8 k. Since the core of the electrode is a glass fiber, the problem of the extreme malleability of a small diameter silver fiber is circumvented. The properties of the electrode permit us to insert it into short (600 m) fragments of the amphibian collecting duct while they are being perfusedin vitro. The passage of currents in the range of 0 to 6×10–8 amperes allowed us to voltage clamp the nephron fragment between +20 and –20 mV. The current-voltage plots are linear over this range. Two lines of evidence suggest that the voltage clamp is homogeneous. First, the voltage measured at the perfusion end during a voltage-clamp experiment of the tubule is not significantly different from that measured at the collecting end. Secondly, the specific resistance of collecting ducts estimated from the core conductor analysis is 3.3±0.8×104 cm, a value not significantly different from that computed from the current-voltage plots as determined with the Ag–AgCl electrode, 3.0±0.5×104 cm. This method permits precise control of both the ionic and electrical gradients across fragments of the amphibian collecting duct. 相似文献
87.
88.
The intravenous administration of d-lysergic acid diethylamide (LSD) to pregnant female rabbits induced hyperthermia as well as disaggregation of polysomes in fetal organs and maternal brain. The LSD-induced polysome shift in maternal brain and fetal organs was not due to an activation of ribonuclease or associated with alterations in the levels of free amino acids. Pretreatment with the receptor blocking agents haloperidol and pizotyline blocked both LSD-induced polysome shift in maternal brain and fetal organs and LSD-induced hyperthermia. Fine dissection of adult rabbit brain showed that the extent of LSD-induced disaggregation of polysomes does not exhibit any marked regional variation. 相似文献
89.
Establishment of a C3Hf Mammary Tumor Cell Line Expressing Endogenous Mouse Mammary Tumor Virus: Antigenic and Genetic Relationships of This Virus with Highly Oncogenic Mouse Mammary Tumor Viruses
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 single-cell clone of C3Hf mammary tumor cells (clone 14) was developed into a continuous cell line expressing high levels of endogenous mouse mammary tumor virus (MMTV) with less than 0.1% murine leukemia virus expression. Comparison of the C3Hf MMTV protein profile on sodium dodecyl sulfatepolyacrylamide gel electrophoresis with that of C3H MMTV revealed that the protein content of the two viruses was quite similar. However, oligonucleotide fingerprints obtained of MMTV 70S RNA revealed that approximately 20% of the large oligonucleotides examined were unique to each virus. The oligonucleotide fingerprint indicated that although the viruses were similar, they differed in their genetic content. The differences in the two viruses extended to immunological differences in the major envelope glycoprotein, gp52. C3Hf MMTV competed only partially in a homologous radioimmunoassay for gp52 of C3H MMTV, whereas C3H MMTV gave complete competition, indicating that gp52 of C3H MMTV contained type-specific determinants not present on gp52 of C3Hf MMTV. Comparison of C3Hf MMTV with highly oncogenic C3H, GR, and RIII MMTVs in a homologous C3H MMTV gp52 assay gave two patterns of reactivity: complete competition by GR and C3H MMTV and incomplete competition by C3Hf and RIII MMTV. Absorption of anti-C3H MMTV serum by either C3Hf MMTV or RIII MMTV removed all antibodies against both viruses but not against GR and C3H MMTVs. These results indicate that C3H and GR MMTVs are more closely related to each other than to RIII and C3Hf MMTVs. 相似文献
90.
Mice with Spontaneous Mammary Tumors Develop Type-Specific Neutralizing and Cytotoxic Antibodies Against the Mouse Mammary Tumor Virus Envelope Protein gp52
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)
Gerald Schochetman Larry O. Arthur Cedric W. Long Richard J. Massey 《Journal of virology》1979,32(1):131-139
Sera from C3H mammary tumor-bearing mice contain cytotoxic antibodies for mouse mammary tumor virus (MMTV)-producing cells, based on (51)Cr release in a complement-dependent serum cytotoxicity assay. The cytotoxic antibodies could be absorbed by purified C3H MMTV gp52 and C3H MMTV-infected cat cells (C3H [MMTV] CrFK) containing cell surface MMTV gp52. However, purified MMTV p27 and uninfected CrFK cat cells were negative. Absorption of the sera with GR (MMTV) CrFK cells also removed all of the cytotoxicity, whereas absorption with RIII (MMTV) CrFK cells was negative, even though all three infected cat cells contained equivalent amounts of gp52. The same C3H cytotoxic sera also neutralized the focus-forming capacity of a C3H MMTV pseudotype of Kirsten sarcoma virus containing MMTV gp52. In contrast, sera from mammary tumor-bearing GR and RIII mice did not neutralize the pseudotype. Furthermore, neutralization could be achieved only by anti-gp52 but not by anti-gp36, -p27, -p14, or -p10 C3H MMTV sera. The gp52's of C3H, GR, and RIII MMTV could also be distinguished by using a type-specific competition radioimmunoassay employing (125)I-gp52 of C3H MMTV and a hyperimmune rabbit anti-C3H MMTV serum. To demonstrate these differences directly, we studied the primary structure of gp52 on the surface of the C3H, GR, and RIII (MMTV) CrFK cells. Two-dimensional tryptic peptide maps of the cell surface lactoper-oxidase-catalyzed iodinated gp52's revealed a greater number of peptides common to the gp52's of C3H and GR MMTVs than to RIII MMTV gp52. These results demonstrate that gp52 is a major target antigen for both cytotoxic and neutralizing antibodies, that the cell surface and virion-associated gp52's of C3H, GR, and RIII MMTV contain both group- and type-specific determinants, and that C3H and GR MMTV gp52's are antigenically more related to each other than to RIII MMTV gp52. Furthermore, C3H mammary tumor-bearing mice develop type-specific antibodies capable of recognizing unique gp52 determinants and, therefore, are able to distinguish the gp52 of C3H MMTV from the gp52's of GR and RIII MMTV. 相似文献