全文获取类型
收费全文 | 21507篇 |
免费 | 2061篇 |
国内免费 | 7篇 |
出版年
2022年 | 228篇 |
2021年 | 368篇 |
2020年 | 254篇 |
2019年 | 339篇 |
2018年 | 492篇 |
2017年 | 409篇 |
2016年 | 668篇 |
2015年 | 1029篇 |
2014年 | 1129篇 |
2013年 | 1324篇 |
2012年 | 1638篇 |
2011年 | 1555篇 |
2010年 | 1036篇 |
2009年 | 862篇 |
2008年 | 1197篇 |
2007年 | 1155篇 |
2006年 | 1017篇 |
2005年 | 928篇 |
2004年 | 973篇 |
2003年 | 780篇 |
2002年 | 732篇 |
2001年 | 428篇 |
2000年 | 382篇 |
1999年 | 307篇 |
1998年 | 209篇 |
1997年 | 166篇 |
1996年 | 142篇 |
1995年 | 129篇 |
1994年 | 146篇 |
1993年 | 122篇 |
1992年 | 203篇 |
1991年 | 207篇 |
1990年 | 184篇 |
1989年 | 199篇 |
1988年 | 169篇 |
1987年 | 161篇 |
1986年 | 159篇 |
1985年 | 141篇 |
1984年 | 122篇 |
1983年 | 103篇 |
1982年 | 98篇 |
1981年 | 104篇 |
1980年 | 95篇 |
1979年 | 110篇 |
1978年 | 107篇 |
1976年 | 87篇 |
1975年 | 98篇 |
1974年 | 96篇 |
1973年 | 87篇 |
1972年 | 70篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
We have isolated two clones containing 19 kilobases (kb) of the human gene coding for a pro-alpha 1 (II) collagen chain from human lambda genomic DNA libraries. A 3' clone, HC2A, was selected by cross-hybridization with a cDNA clone containing sequences coding for the carboxy propeptide of chick type II procollagen. A second clone, HC2B, was obtained by screening the library with the 5' part of HC2A. The sequence analysis of exon 3 corresponding to the C propeptide reveals the presence of stretches of conserved nucleotides between the human and the chick type II procollagen genes. On Northern blots, the human collagen clone hybridizes strongly to a 5.5-kb RNA for the rat type II procollagen chain. Finally, studies of genomic DNAs from normal individuals reveal the presence of a HindIII and a BamHI polymorphic site at the 3' end of the gene. 相似文献
122.
Intact chimpanzee caput and cauda epididymal sperm, sperm cell lysates, and caput and cauda epididymal fluid were radiolabeled by enzymatic iodination with lactoperoxidase and Na125 I and were compared by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Caput epididymal sperm showed nine labeled macromolecular components of 90, 64, 56, 48, 38, 31, 20, 18 and 16 Kd and cauda epididymal sperm showed eleven macromolecular components of 90, 64, 55, 47, 42, 33, 27, 18, 17, 15 and 11 Kd. Six of the components labeled on caput sperm (90, 64, 56, 48, 18 and 16 Kd) were detected in equal amounts of cauda sperm and two (38 and 20 Kd) were detected at greatly reduced labeling intensities. In the cauda epididymidis, four new components (33, 27, 17 and 11 Kd) became prominent features of the sperm surface. Analysis of labeled caput and cauda sperm cell lysates resolved components distinct from those detected on sperm surfaces. Electrophoresis of caput epididymal fluid showed five labeled components of 66, 56, 47, 41 and 37 Kd, while electrophoresis of cauda epididymal fluid showed eight labeled components of 92, 66, 56, 48, 31, 27, 24 and 11 Kd. Three components (66, 56 and 47 Kd) were present in both caput and cauda fluid, two (41 and 37 Kd) in caput fluid only, and five (92, 31, 27, 24 and 11 Kd) in cauda fluid only. Components of 37 Kd were labeled in caput fluid and on caput sperm but not on cauda sperm, whereas components of 27 Kd and 11 Kd were labeled in cauda fluid and on cauda sperm but not on caput sperm. These data show that chimpanzee sperm undergo extensive surface modifications during epididymal maturation and that some of these modifications may be related to exogenous proteins/glycoproteins in epididymal fluids. 相似文献
123.
Virion conformational forms and the complex inactivation kinetics of echovirus by chlorine in water. 总被引:2,自引:1,他引:1
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Aberrant inactivation kinetics were observed when monodispersed echovirus type 1 (Farouk) was inactivated with chlorine. An initial 1- to 2-log10-unit decrease in titer was followed by lag period, during which the titer stayed the same or increased, and this was followed by a final decline in titer. First-order kinetics were obtained with poliovirus type 1 under the same conditions. Isoelectric focusing studies of echovirus before chlorine treatment showed that the virus distributed into two pH-dependent and interconvertible isoelectric forms. After chlorine treatment all remaining virus infectivity was associated with a third pH-independent isoelectric form. The complex inactivation kinetics appeared to be due to shifts between these conformational forms during inactivation in certain ionic environments. Under certain conditions the conformational shifts resulted in substantial resistance of monodispersed echovirus to chlorine. 相似文献
124.
5-Azacytidine treatment of a murine cytotoxic T cell line alters interferon-gamma gene induction by interleukin 2 总被引:1,自引:0,他引:1
W L Farrar F W Ruscetti H A Young 《Journal of immunology (Baltimore, Md. : 1950)》1985,135(3):1551-1554
Genetic susceptibility to multiple sclerosis (MS) in Caucasians was previously shown to be correlated to the presence of given alleles at the HLA-DR and Gm loci. We now demonstrate that the humoral immune response in MS central nervous system (CNS) is modulated by both loci: the levels of IgG1 subclass and IgG1 allotypes in cerebrospinal fluid of MS patients depend on both their Gm genotype and their HLA-DR2 or HLA-DR7 phenotype. That HLA-DR molecules may either participate in a preferential recruitment of IgG1 allotype-producing B cells in MS CNS or act after such a selective homing is discussed. These results demonstrate that both HLA and Gm loci are synergistically involved in the modulation of the humoral immune response. 相似文献
125.
R. M. Case D. I. Cook M. Hunter M. C. Steward J. A. Young 《The Journal of membrane biology》1985,84(3):239-248
Summary The characteristics of nonelectrolyte secretion by the rabbit mandibular salivary gland have been investigated in anin vitro perfused preparation. The concentrations of14C-labeled nonelectrolytes were measured in saliva samples collected over a range of flow rates during the secretory response of the gland to continuous acetylcholine infusion. Of the nine nonelectrolytes studied, the two particularly lipid-soluble molecules, ethanol and antipyrine, appeared in the saliva at approximately the same concentration as in the perfusate, regardless of the secretory flow rate. The more polar molecules (urea, ethanediol, thiourea, glycerol, erythritol, mannitol and sucrose) appeared at saliva/perfusate concentration ratios () which showed a strong dependence on flow. With the exception of thiourea, this could be attributed to the combined contributions of diffusion and solvent drag.For the polar nonelectrolytes, estimates have been obtained of both the permeability coefficients of the gland (P) and the solvent-drag filtration coefficients (1–). The relation between 1– and molecular radius suggests that small polar nonelectrolytes and the bulk of the secreted water cross the epithelium via aqueous channels that are approximately 0.8 nm in width. The location of the channels remains uncertain because tissue space measurements indicate that the nonelectrolytes most affected by solvent drag have access to both transcellular and paracellular pathways. 相似文献
126.
Hepatocellular carcinoma, polycystic renal disease and pneumonia are reported in an aged woodchuck, and a metastatic fibrosarcoma is reported in a relatively young animal born and raised in the laboratory. 相似文献
127.
Random association of Epstein-Barr virus genomes with host cell metaphase chromosomes in Burkitt''s lymphoma-derived cell lines. 总被引:12,自引:8,他引:4
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)
The random association of Epstein-Barr virus DNA with host cell metaphase chromosomes of all sizes in Burkitt's lymphoma-derived cell lines was demonstrated by two substantially different techniques, namely fluorescence-activated chromosome sorting and in situ hybridization. The nature and potential importance of this association are discussed. 相似文献
128.
Subcellular localization of lethal lysis proteins of bacteriophages lambda and phiX174. 总被引:14,自引:1,他引:13
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)
The gene products of the lethal lysis genes S and E of the bacteriophages lambda and phiX174, respectively, were shown to be associated primarily with inner membrane material by isopycnic sucrose gradient centrifugation of lysates of infected cells. A small amount of each polypeptide appeared to be in the outer membrane fraction. 相似文献
129.
Autointoxication in residues ofAsparagus officinalis L. 总被引:1,自引:1,他引:0
Summary In a greenhouse experiment the growth of asparagus seedlings was retarded by the residue treatments in both vermiculite and sand cultures. In general, the retardation of asparagus root by residues was slightly greater than the retardation of shoot in both cultures. The retardation of the growth of asparagus seedlings by root and stem treatments was usually higher than that by old root litter. Root and stem extracts strongly inhibited the development of asparagus seedlings in the seed bioassay. The inhibition of extracts to the growth of shoot was greater than that to the growth of root. The quantities in the total phenolics and catachol type phenolics from root, stem and old root litter extracts corresponded to the autotoxicity in the seed bioassay. The soil extracts obtained from using acetone, methanol, and XAD-4 extractions strongly inhibited the shoot and root development of asparagus seedlings in the bioassay. The efficiency of phenolics extraction by the XAD-4 method was significantly higher than that by acetone and methanol extractions. The results obtained in the greenhouse experiment and bioassay revealed that phytotoxic substances present in the residues and the soil of asparagus and may be partially responsible for the asparagus replanting problems. 相似文献
130.
Resistance of citrus fruit to mass transport of water vapor and other gases 总被引:4,自引:0,他引:4
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The resistance of oranges (Citrus sinensis L. Osbeck) and grapefruit (Citrus paradisi Macf.) to ethylene, O2, CO2, and H2O mass transport was investigated anatomically with scanning electron microscope and physiologically by gas exchange measurements at steady state. The resistance of untreated fruit to water vapor is far less than to ethylene, CO2 and O2. Waxing partially or completely plugs stomatal pores and forms an intermittent cracked layer over the surface of fruit, restricting transport of ethylene, O2, and CO2, but not of water; whereas individual sealing of fruit with high density polyethylene films reduces water transport by 90% without substantially inhibiting gas exchange.
Stomata of harvested citrus fruits are essentially closed. However, ethylene, O2 and CO2 still diffuse mainly through the residual stomatal opening where the relative transport resistance (approximately 6,000 seconds per centimeter) depends on the relative diffusivity of each gas in air. Water moves preferentially by a different pathway, probably through a liquid aqueous phase in the cuticle where water conductance is 60-fold greater. Other gases are constrained from using this pathway because their diffusivity in liquid water is 104-fold less than in air.
相似文献