首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3588篇
  免费   239篇
  3827篇
  2023年   23篇
  2022年   40篇
  2021年   77篇
  2020年   54篇
  2019年   56篇
  2018年   94篇
  2017年   76篇
  2016年   136篇
  2015年   189篇
  2014年   197篇
  2013年   258篇
  2012年   293篇
  2011年   293篇
  2010年   176篇
  2009年   178篇
  2008年   213篇
  2007年   203篇
  2006年   173篇
  2005年   183篇
  2004年   163篇
  2003年   153篇
  2002年   145篇
  2001年   27篇
  2000年   31篇
  1999年   39篇
  1998年   25篇
  1997年   36篇
  1996年   35篇
  1995年   25篇
  1994年   31篇
  1993年   26篇
  1992年   16篇
  1991年   16篇
  1990年   11篇
  1989年   9篇
  1988年   11篇
  1987年   10篇
  1986年   7篇
  1985年   9篇
  1984年   10篇
  1983年   5篇
  1982年   7篇
  1980年   9篇
  1979年   4篇
  1978年   4篇
  1977年   4篇
  1976年   5篇
  1973年   7篇
  1971年   4篇
  1946年   5篇
排序方式: 共有3827条查询结果,搜索用时 31 毫秒
101.
102.
Isolation and characterization of human heart cytochromec oxidase   总被引:2,自引:0,他引:2  
Cytochromec oxidase was isolated from human hearts and separated by SDS gel electrophoresis. The identity of polypeptide bands with known subunits was demonstrated by immunoblotting with monospecific antisera to rat liver cytochromec oxidase subunits. The polarographically determined kinetics of cytochromec oxidation were similar to those reported for the bovine heart enzyme.  相似文献   
103.
Lymphomyeloid organs of two common species of Antarctic fish, Trematomus nicolai and Chionodraco hamatus, were studied with the aim of analysing some morphological aspects of these organs in relation to adaptation to low environmental temperature. The thymuses of T. nicolai and C. hamatus were flattened, incompletely lobated, with numerous Hassall-like bodies, which were mainly located in the central part of the organ in C. hamatus. In T. nicolai, thymocytes, erythroid and reticular epithelial cells filled the organ. In C. hamatus, the thymocytes intermingled with reticular epithelial cells were often close to groups of melano-macrophages. In both species, the thymus did not show distinct compartmentalisation; however, the thymocytes had significantly different sizes in the outer and inner portions of the thymus. The head kidney of both species was completely filled by haematopoietic tissue, highly vascularised and mainly lymphopoietic in T. nicolai, while both erythropoietic and lymphopoietic in C. hamatus. The spleen appeared mainly erythropoietic in T. nicolai and mainly lymphopoietic in C. hamatus. Solitary melano-macrophages in T. nicolai were close to numerous small vascular ellipsoids where erythroid and lymphoid cells were intermingled without the formation of red and white pulp areas. In C. hamatus, large lymphoid areas were organised around the capillaries. The possible adaptation of lymphoid organs to the low temperature of polar water is discussed. Accepted: 8 November 1999  相似文献   
104.
The application of manure to fertilize arable lands is one of the major means through which veterinary sulfonamides (SAs) enter the environment. Little is known about the capacity of woody plants to phytoremediate this class of antibiotics. To this purpose we performed preliminary studies to evaluate Salix fragilis L. response to sulfadimethoxine (SDM) by investigating both its ability to absorb and tolerate doses of SDM found in fresh faeces of treated calves. Forty cuttings were exposed to either 0, 0.5, 1, or 2 mM of SDM for one month. Decreases in photosynthetic electron transport rate and net CO2 assimilation after 25 days for the higher SDM concentrations were noticed. Moreover, alterations in root morphology of treated plants were observed and further investigated through electron microscopy. However, collected data revealed high root accumulation potential. These preliminary results are promising as they demonstrate that Salix fragilis L. can both absorb and tolerate high concentrations of SAs.  相似文献   
105.
Sunlight is a key environmental factor in almost all ecosystems, and it is necessary for many physiological functions. Many vertebrates require ultraviolet (UV) radiation to perform different physiological processes. Artificial light is used to supplement UV in captive animals, through appropriate photoperiods and UV wavelengths. Previous studies reported that repeated exposure to artificial UV radiation may cause damage to the immune system. Taking into account the importance of UV effects and the serum complement system, the relationship between them was investigated. The study lasted 90 days and was carried out in plastic chambers. Ninety six broad‐snouted caiman (C. latirostris) were assigned to four treatment groups with two replicates each: total darkness (TD), 8 hr per day (8 hr) and 16 hr per day (16 hr) of artificial UV/visible light exposure, and normal photoperiod of natural light (NP). Snout–vent length was measured to determine animal growth. Hemolytic assays were performed to evaluate the effects of artificial UV/visible light, TD, and NP on the serum complement system. Results showed that animals grew more in the NP group. The capacity of C. latirostris serum to hemolyze sheep red blood cells was higher in the NP group than when they are maintained in constant light–dark cycles (8 and 16 hr) or in TD. These data demonstrate that artificial UV should be considered as a potential hazard for captive crocodilians if it is not properly managed, and this should be taken into account in the general design of facilities for reptilian husbandry. Zoo Biol 31:523‐533, 2012. © 2011 Wiley Periodicals, Inc.  相似文献   
106.
Scabies is endemic in Fiji and is a significant cause of morbidity. Little is known about the sociocultural beliefs and practices that affect the occurrence of scabies and impetigo, or community attitudes towards the strategy of mass drug administration that is emerging as a public health option for scabies and impetigo control in Fiji and other countries. Data were collected during semi-structured interviews with 33 community members in four locations in Fiji’s Northern Division. Thematic analysis examined participants’ lived experiences of scabies and impetigo; community knowledge and perceptions about scabies and impetigo aetiology and transmission; community-based treatment and prevention measures; and attitudes towards mass drug administration. Many indigenous Fijian (iTaukei) participants noted extensive and ongoing experience of scabies and impetigo among children in their families and communities, but only one participant of Indian descent (Indo-Fijian) identified personal childhood experience of scabies. Scabies and impetigo were perceived as diseases affecting children, impacting on school attendance and families’ quality of sleep. Awareness of scabies and impetigo was considerable, but there were major misconceptions around disease causation and transmission. Traditional remedies were preferred for scabies treatment, followed by biomedicines provided by local health centres and hospitals. Treatment of close household contacts was not prioritised. Attitudes towards mass drug administration to control scabies were mostly positive, although some concerns were noted about adverse effects and hesitation to participate in the planned scabies elimination programme. Findings from this first study to document perspectives and experiences related to scabies and impetigo and their management in the Asia Pacific region illustrate that a community-centred approach to scabies and impetigo is needed for the success of control efforts in Fiji, and most likely in other affected countries. This includes community-based health promotion messaging on the social dynamics of scabies transmission, and a campaign of education and community engagement prior to mass drug administration.  相似文献   
107.
Summary— Using two-dimensional polyacrylamide gels stained with Coomassie blue we have studied the protein composition of the nuclear matrix obtained from mouse erythroleukemic nuclei kept at O°C throughout the isolation procedure to prepare the high ionic strength resistant fraction (control matrix) or stabilized in vitro or in vivo by different procedures prior to subfractionation (ie 37°C incubation of isolated nuclei; sodium tetrathionate exposure of purified nuclei; heat shock of intact cells). When the matrix obtained from 37°C incubated nuclei was compared with the control matrix, striking differences in the polypeptide pattern were seen if the protein was obtained in both cases from an equivalent number of nuclei. On the other hand, if the same amount of protein for both the samples was applied to the gels the differences were less evident. Sodium tetrathionate stabilization of isolated nuclei and heat shock of intact cells produced a matrix protein pattern that was very similar and differed from that of the in vitro heat-exposed matrix. Using specific polyclonal antisera, we demonstrate that nucleolar proteins B23/numatrin and C23/nucleolin were very abundant in the matrix obtained from chemically-treated nuclei or in vivo heat-stabilized nuclei but were recovered in very small amounts (B23) or completely absent (C23) in the matrix prepared from nuclei heated to 37°C in vitro. Differences were seen also in the recovery of nuclear lamins, and especially lamin B, that was poorly represented in the sodium tetrathionate-stabilized matrix. The results demonstrate that in mouse erythroleukemia cells the increased recovery of nuclear matrix protein that is seen after in vitro heating of isolated nuclei is predominantly due to an additional recovery of the same types of polypeptides that are detected also in the absence of such a treatment. The data also indicate that in vivo heat shock of intact cells produces a nuclear matrix protein pattern that is more similar to the pattern seen after stabilization of purified nuclei with sodium tetrathionate and differs significantly from that obtained by exposing nuclei to 37°C in vitro, unlike to that what previous reports have indicated.  相似文献   
108.
Important classes of antibiotics acting on bacterial cell wall biosynthesis, such as beta-lactams and glycopeptides, are used extensively in therapy and are now faced with a challenge because of the progressive spread of resistant pathogens. A discovery program was devised to target novel peptidoglycan biosynthesis inhibitors capable of overcoming these resistance mechanisms. The microbial products were first screened according to their differential activity against Staphylococcus aureus and its L-form. Then, activities insensitive to the addition of a beta-lactamase cocktail or d-alanyl-d-alanine affinity resin were selected. Thirty-five lantibiotics were identified from a library of broth extracts produced by 40,000 uncommon actinomycetes. Five of them showed structural characteristics that did not match with any known microbial metabolite. In this study, we report on the production, structure determination, and biological activity of one of these novel lantibiotics, namely, planosporicin, which is produced by the uncommon actinomycete Planomonospora sp. Planosporicin is a 2194 Da polypeptide originating from 24 proteinogenic amino acids. It contains lanthionine and methyllanthionine amino acids generating five intramolecular thioether bridges. Planosporicin selectively blocks peptidoglycan biosynthesis and causes accumulation of UDP-linked peptidoglycan precursors in growing bacterial cells. On the basis of its mode of action and globular structure, planosporicin can be assigned to the mersacidin (20 amino acids, 1825 Da) and the actagardine (19 amino acids, 1890 Da) subgroup of type B lantibiotics. Considering its spectrum of activity against Gram-positive pathogens of medical importance, including multi-resistant clinical isolates, and its efficacy in vivo, planosporicin represents a potentially new antibiotic to treat emerging pathogens.  相似文献   
109.
Eukaryotic chromosomes are duplicated during S phase and transmitted to progeny during mitosis with high fidelity. Chromosome duplication is controlled at the level of replication initiation, which occurs at cis-acting replicator sequences that are spaced at intervals of approximately 40 kb along the chromosomes of the budding yeast Saccharomyces cerevisiae. Surprisingly, we found that derivatives of yeast chromosome III that lack known replicators were replicated and segregated properly in at least 96% of cell divisions. To gain insight into the mechanisms that maintain these "originless" chromosome fragments, we screened for mutants defective in the maintenance of an "originless" chromosome fragment, but proficient in the maintenance of the same fragment that carries its normal complement of replicators (originless fragment maintenance mutants, or ofm). We show that three of these Ofm mutations appear to disrupt different processes involved in chromosome transmission. The OFM1-1 mutant seems to disrupt an alternative initiation mechanism, and the ofm6 mutant appears to be defective in replication fork progression. ofm14 is an allele of RAD9, which is required for the activation of the DNA damage checkpoint, suggesting that this checkpoint plays a key role in the maintenance of the "originless" fragment.  相似文献   
110.
Skin, lymph node (popliteal), and bone marrow samples were collected from 50 red foxes (Vulpes vulpes) from May 2004 to May 2005 in southern Italy. Samples were tested for Leishmania infantum by polymerase chain reaction (PCR). The parasite was detected by PCR from 20 of 50 (40%) fox carcasses. All 20 positive cases were PCR-positive from lymph node and bone marrow samples, whereas 17 of 20 positive cases were PCR-positive from skin samples. Infection status was not related to age or sex. This is the first report of leishmaniasis in red foxes in Italy based on PCR results, and these results reinforce the assumption that this wild canid can serve as a reservoir for Leishmania.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号