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   28053篇
  免费   2262篇
  国内免费   1358篇
  2024年   36篇
  2023年   214篇
  2022年   422篇
  2021年   943篇
  2020年   683篇
  2019年   870篇
  2018年   966篇
  2017年   726篇
  2016年   1119篇
  2015年   1722篇
  2014年   2012篇
  2013年   2126篇
  2012年   2619篇
  2011年   2481篇
  2010年   1498篇
  2009年   1390篇
  2008年   1699篇
  2007年   1537篇
  2006年   1378篇
  2005年   1192篇
  2004年   1145篇
  2003年   915篇
  2002年   749篇
  2001年   545篇
  2000年   475篇
  1999年   415篇
  1998年   241篇
  1997年   194篇
  1996年   183篇
  1995年   158篇
  1994年   138篇
  1993年   87篇
  1992年   129篇
  1991年   113篇
  1990年   104篇
  1989年   81篇
  1988年   60篇
  1987年   58篇
  1986年   40篇
  1985年   34篇
  1984年   39篇
  1983年   17篇
  1982年   15篇
  1981年   10篇
  1980年   9篇
  1979年   10篇
  1976年   8篇
  1974年   9篇
  1969年   8篇
  1967年   8篇
排序方式: 共有10000条查询结果,搜索用时 515 毫秒
81.
Heat-stable malate dehydrogenase isolated from Thermus flavus AT62 was completely inactivated by treatment with diethylpyrocarbonate. The inactivation was accompanied by the loss of 1.2 histidine residues per subunit of the enzyme. The enzyme was protected from inactivation by NADH. The enzyme was also inactivated by dye-sensitized photooxidation. Methionine residues, in addition to histidine residues, were destroyed in the inactivated enzyme. Kinetic analyses of the inactivation indicated that the pK value of the residue involved in the inactivation was 8.20 at 25.0 degrees C and 7.52 at 60.0 degrees C. From the pK values and the heat of ionization calculated from the van't Hoff plot of pKs, a histidine residue was identified to be primarily involved in the inactivation. The effect of temperature on the pK value of the essential group in this enzyme from a thermophilic organism is discussed.  相似文献   
82.
引言新银合欢(Leucaena leucocephla cv. Salvadot)又名萨尔瓦多银合欢,是一种重要的热带木本豆科植物,是大有发展前途的饲料、肥料、燃料和木料的多用途树种。据报道第一年可生长2米以上,以后每年生长1米以上。在没有冻害和立地条件好的地  相似文献   
83.
We have purified a glycoprotein from chicken sciatic nerves, sciatin, which has pronounced trophic effects on avian skeletal muscle cells in culture. Recent studies have shown that sciatin is identical to the iron-transport protein, transferrin, in terms of its physicochemical structure, immunological reactivity, and biological activity. To determine whether transferrin is synthesized and released by neuronal tissue, we incubated cultures of dissociated chicken spinal neurons in a medium free of L-leucine containing either L-3H-amino acids or L-[14C]leucine and immunoprecipitated transferrin with highly specific antibodies. The radiolabeled protein precipitated by rabbit heteroclonal, goat heteroclonal, or mouse monoclonal antitransferrin antibodies increased in specific activity in a linear manner for at least 30 min. Synthesis of this protein was abolished by the presence of puromycin (20 micrograms/ml) or cycloheximide (10(-5) M). The disappearance of the radiolabeled protein from cells was linear with a half-life (t 1/2) of 8-10 h. When immunoprecipitates were separated by SDS gel electrophoresis, a prominent band corresponding to transferrin (Mr 84,000) was visualized by staining with Coomassie Blue. However, when such gels were fluorographed, no radioactivity was apparent in the transferrin region of the gel although a prominent radioactive band was visualized at an Mr of 56,000. The protein of Mr 56,000 was not simply a degradation product of transferrin because this particular protein band was not generated by incubating radiolabeled transferrin with unlabeled neuronal homogenates. The protein of Mr 56,000 was purified from embryonic chicken brain and spinal cord by immunoabsorption chromatography on mouse monoclonal antitransferrin IgG conjugated to Sepharose 4B followed by affinity chromatography on immobilized transferrin. The purified protein bound radioiodinated transferrin and was precipitated by rabbit anti-chicken transferrin-receptor antibodies. Furthermore, this receptor protein was found to be localized on the plasma membrane of dorsal root ganglion neurons by immunocytochemistry using the peroxidase-antiperoxidase technique, and by blocking experiments, which showed that antitransferrin receptor IgG could inhibit the binding of fluorescein-conjugated transferrin at 4 degrees C to cultured neurons in vitro. From these data, we conclude that transferrin is not synthesized by cultures of chicken spinal cord neurons, but that the receptor for transferrin is synthesized by these cultures and is precipitated by antitransferrin antibodies as an antigen-receptor complex.  相似文献   
84.
Summary Two 30 g/ml BrdU-resistant sublines and two 60 g/ml BrdU-resistant sublines are induced from a Chinese hamster cell line Wg3h (HGPRT) by one-step and two-step selections, respectively. By inoculating the cells into BrdU-free medium or by adding more BrdU into the culture medium for 26–27 h, it was found that the two BrdU-resistant sublines analysed have very clear sister chromatid differential (SCD) staining patterns. This indicates that some of the nuclear DNA of the BrdU-resistant cells incorporate with BrdU to reach a kinetic balance. Frequencies of sister chromatid exchange (SCE) of the resistant cells are twice to four times as high as those of the Wg3h cells, depending on which BrdU-resistant subline is analysed. The SCE frequencies of the resistant cells also increase with the BrdU concentration in the medium. Analysis of silver-stained nucleolar organizer regions (NORs) indicates that the NOR activity of three out of the four BrdU-resistant sublines is significantly suppressed, i.e., averages of the Ag-NOR number and number of the chromosomes bearing Ag-NORs per cell decrease significantly. The degree of suppression for different BrdU-resistant sublines may be quite different. The suppressed NOR activity of the resistant cells can gradually be restored when the cells are inoculated into BrdU-free medium, but the recovery speed is far lower than that of the Wg3h cells. The suppression of the NOR activity of the BrdU-resistant sublines should be due to BrdU toxicity.  相似文献   
85.
确定了在pH 5.2到8.7和温度22°到45℃时钼铁蛋白和铁蛋白的稳定性。按logKm和logV分别对pH作图,得到下列参数。于30℃在酸性侧,乙炔络合常数PK_b(C_2H_2)=6.43—6.50,乙炔催化常数pK_b′(C_2H_2)=4.78,在碱性侧乙炔络合常数pK_a(C_2H_2)=7.42—7.55,乙炔催化常数pK_a′(C_2H_2)=7.88—8.29,氮络合常数pK_a(N_2)=7.75—7.9及氮催化常数pK_a′(N_2)=8.08—8.68。既然pK_a与pK_b或pK_a′与pK_b′二者间隔小于3.5个单位,又按V/Km及V′对pH作图,其结果为pKb(C_2H_2)=6.28pK_a(C_2H_2)=7.68和pK_b′(C_2H_2)=5.53和pK_a′(C_2H_2)=8.93。将不同温度下的pK_b或pK_a分别对各自温度的倒数作图,其热力学数据为由pH 5.7到7.3范围内,质子移变基团的解离热⊿H(ioh)=7742卡/克分子,相应的pK_b(C_2H_2)=6.16—6.36;由pH 7.3到8.4范围内⊿H_(ion)=6983卡/克分子,相应的pK_a=8.24—8.44。这些数据指出固氮酶钼铁蛋白络合乙炔的功能基团可能具有咪唑基·巯基和氨基相类似的结构,这种推测正在用化学修饰的方法加以验证。  相似文献   
86.
亮叶杨桐(石芽茶)中黄酮类成分的研究   总被引:8,自引:0,他引:8  
金静兰  文永新  成桂仁   《广西植物》1985,(3):297-300
从广西产亮叶杨桐(Adinandra nitida Merr.ex H.L.Li)中,分得三种黄酮类成分:Ⅰ、Ⅱ和Ⅲ。经IR、UV(位移诊断)、熔点、混熔点、薄层层析鉴定:Ⅰ为芹菜素(Apigenin);Ⅱ为芹菜素-5-0-α-L-吡喃鼠李糖基(1→4-6~(?)-乙酰基-β-D-吡喃葡萄糖吡;Ⅲ为芹菜素-5-0-α-L-吡喃鼠李糖基(1→4)-β-D-吡喃葡萄糖甙。  相似文献   
87.
应用Southern blot杂交试验检测HBsAg及HBeAg均阳性母亲流产的9例胎儿肝细胞中HBV DNA的存在状态,并与其HBV血清学、免疫电镜及肝脏免疫组织化学的结果相比较。结果在3例胎肝高分子DNA中检出了整合的HBV DNA顺序,且此3例HBV DNA整合到胎肝细胞基因组并无特定部位,提示为随机整合。3例中2例的血清及肝匀浆都检出HBsAg颗粒,其胎肝细胞胞浆HBsAg也阳性;另1例受HBV感染的唯一标志是在胎肝细胞中存在着整合的HBVDNA。此外,另1例则仅胎肝细胞中HBsAg阳性而无整合的HBV DNA。在胎肝细胞中检出整合的HBV DNA进一步证实HBV子宫内传播途径的存在。  相似文献   
88.
<正> 本文报道以pAT153质粒为载体克隆的adr亚型乙型肝炎病毒(HBV)全基因的限制性内切酶图谱。重组质粒已命名为pHBV-NCl。重组质粒的提取和酶解采用常规方法。限制性内切酶为Bio-Labs公司产品。用Sepharcry S-1000纯化得到的质粒,经电泳鉴定都是完整的超螺旋DNA。经过鉴定其BamHⅠ、XhoⅠ、XbaⅠ、SstⅡ、SphⅠ、BglⅠ、BglⅡ、BstEⅡ、AceⅠ、AvaⅠ、HincⅡ、HpaⅠ等12种酶的21个切口已被定位。其中XhoⅠ、XbaⅠ、SstⅡ、  相似文献   
89.
本文进一步研究了我国不同民族的正常个体以及β地中海贫血患者θ珠蛋白基因5′侧序列中的多态性HincⅡ位点及其遗传性质。在广西壮族正常个体和β地中海贫血纯合子中,该多态性位点的发生频率均为75%,与正常汉族人测得值相近。家系分析资料表明,该多态性位点完全按照孟德尔规律进行遗传。  相似文献   
90.
On the Theory of Speciation Induced by Transposable Elements   总被引:5,自引:3,他引:2       下载免费PDF全文
A simple methematical model describes the invasion of panmictic, sexually reproducing populations by a newly introduced transposon. The model places important constraints on the properties that transposons must have to successfully invade a population and describes the kinetics with which such an invasion will occur. Invasibility conditions serve as a basis for new, detailed scenarios whereby transposon-mediated depression in fitness produces reproductive isolation of populations. These scenarios, in turn, lead to several speculations concerning the role of transposons in evolution.  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