单选题 信号肽的作用是： A、保护N-端的蛋氨酸残基 B、保护蛋白质不被水解 C、维持蛋白质的空间构象 D、引导多肽链进入内质网 E、传递蛋白质之间的信息

单选题通过鸟氨酸循环生成尿素时，其分子中的两个氮原子一个直接来自游离的氨，另一个直接来源于( )(1998年)

单选题下列哪种因素不会影响真核细胞中mRNA的稳定性______ A.5'端帽子 B.siRNA C.polyA尾 D.去稳定元件 E.miRNA

单选题哪种血浆脂蛋白运输内源性甘油三酯(　　　)

单选题真核生物5S rRNA基因的启动子在转录结构基因的( )。

单选题Scientists have for the first time used cloning to create human embryos that live long enough in a laboratory dish to have their stem cells harvested. The feat could set the stage for physicians to produce cells and tissues, tailored to a patient's genetic identity that can treat a wide variety of human illnesses. The accomplishment also provides a road map for how to clone a person, an even more divisive undertaking. The new work, performed in South Korea, represents "a major advance in stem cell research. It could help spur a medical revolution as important as antibiotics and vaccines", says Robert Lanza of Advanced Cell Technology (ACT), a company in Worcester, Mass., that's also investigating the promising stem cell strategy called therapeutic cloning. "However, now that the methodology is publicly available", Lanza adds, "I think it is absolutely imperative that we pass laws worldwide to prevent the technology from being abused for reproductive-cloning purposes." While some fertility doctors and a religious cult have claimed success at creating a pregnancy via cloning, they've offered no convincing proof. In contrast, the South Korean research is being reported at the meeting of the American Association for the Advancement of Science in Seattle and will appear in an upcoming Science. "This is reality," says stem cell researcher John Gearhart of Johns Hopkins University. "He4'e is a bona fide, refereed journal saying that a human embryo has been cloned and a cell line derived from it." Although ACT has not yet published a report of a cloned human blastocyst, Lanza says that the South Korean success is "consistent with our own results." Therapeutic cloning appeals to Lanza and physicians because cells made this way could have the same DNA as a patient's cells do and thus avoid rejection after they're transplanted. Seeking a compromise that would permit this strategy to be pursued, many scientists have called for legislation that would ban cloning to produce a baby but allow the creation of cloned embryos to generate stem cells for research or therapies. "The debate has been very polarized," notes bio-ethicist Laurie Zoloth of Northwestern University in Evanston.

单选题真核生物DNA复制速度与原核生物相比： A．相同 B．略快 C．快很多 D．慢很多

单选题新生链延伸过程中tRNA的移动路径为______。

单选题类固醇激素生化信息的传递首先需通过______。

单选题磺胺类药物抑制细菌生长属于______。

单选题有关PCR描述正确的是( ) A．需要DNA限制性核酸内切酶 B．退火是为了模板复性 C．退火的温度越低越好 D．延伸的温度越高越好 E．引物浓度要适中

单选题目前在转基因小鼠中常用的基因敲除(gene knockout)是根据 ____ 的原理设计的。

单选题转运肝合成的内源性胆固醇的脂蛋白是

单选题关于真核生物DNA分子中基因排布特点的叙述中错误的是

单选题具有二硫键的二十元环多肽激素是______。

单选题有关RNA转录合成的叙述，其中错误的是?______。 A．转录过程RNA聚合酶需要引物 B．转录时只有一股DNA作为合成RNA的模板 C．RNA链的生长方向是5→3 D．所有真核生物RNA聚合酶都不能特异性地识别promoter

单选题染色体中遗传物质的主要化学成分是______ A.组蛋白 B.非组蛋白 C.DNA D.RNA E.mRNA

单选题目前基因治疗中最常用的基因载体是

单选题哪种情况会导致移码突变______ A.倒位 B.颠换 C.插入一个碱基 D.连续缺失三个碱基 E.以上都不对

单选题下列关于启动子的描述正确的是( ) A．可以表达基因产物 B．能专一地与阻遏蛋白结合 C．是RNA聚合酶的结合部位 D．是DNA聚合酶的结合部位 E．是结构基因