题干

下列粒子结构示意图中,表示阴离子的是(  )

A:

B:

C:

D:

上一题 下一题 0.0难度 选择题 更新时间:2012-05-10 10:46:15

答案(点此获取答案解析)

A

同类题1

科普阅读题

干细胞是具有分裂能力的一类细胞,在一定条件下可以分化成多种功能细胞,而普通体细胞,如口腔上皮细胞、神经细胞、成纤维细胞(一种结缔组织细胞)等,不具有分裂能力。干细胞有多种类型,如胚胎干细胞、造血干细胞,其中胚胎干细胞是来源于形成胚胎的原始(未分化)细胞,它们具有分化成为多种类型细胞的潜能。鉴于这种特性,可以通过将干细胞移植入患者体内,替换损伤细胞从而治愈疾病。

然而胚胎干细胞研究一直是一个颇具争议的领域。支持者认为这项研究有助于根治很多疑难杂症,因为胚胎干细胞可以分化成多种功能的细胞,被认为是一种挽救生命的慈善行为,是科学进步的表现;而反对者则认为,进行胚胎干细胞研究就必须破坏胚胎,而胚胎是人尚未成形时在子宫的生命形式。在这样的伦理问题面前,科学家猜想:能不能把体细胞培养成类似胚胎干细胞的具有多种分化能力的干细胞呢?

2007年,日本学者将四个基因Oct4、Sox2、c-Myc和Klf4转入小鼠成纤维细胞(一种体细胞),使之成为具有胚胎干细胞某些特性的多能性干细胞—“诱导多能性干细胞”(简称iPS细胞)。

中国科学院动物研究所和上海交通大学医学院的研究组合作完成的工作表明,利用iPS细胞能够培养得到小鼠胚胎,植入母鼠体内,并能最终生长发育成具有繁殖能力的小鼠,从而在世界上第一次证明了iPS细胞与胚胎干细胞具有相似的多能性。截至目前,iPS细胞已经成功诱导分化形成成纤维细胞、角质细胞、造血干细胞、脂肪细胞等。

利用iPS技术可以使用病人自己的体细胞制备干细胞,应用于临床医学,如在猪等动物体内培养病人的器官,移植给患者,为器官移植提供了选择,避免了免疫排斥的问题。与经典的胚胎干细胞技术和体细胞核移植技术不同,iPS技术不使用胚胎细胞或卵细胞,只需对体细胞进行操作及适当的培养即可,因此没有伦理学的问题。同时,将iPS技术与转基因动物技术相结合,打破物种界限,克服种间繁殖障碍,从而获得用传统方法无法得到的新性状,为新品种培育等转基因动物研究提供了试验基础。

正是由于iPS细胞的意义重大,其研究成果成为干细胞探索过程中的一个里程碑。2012年,日本学者和英国学者因其在该领域的杰出贡献,获得诺贝尔生理学或医学奖。

同类题4

根据短文理解,选择正确答案。

    At one time, computers were expected largely to remove the need for paper copies of documents(文件) because they could be stored electronically. But for all the texts that are written, stored and sent electronically, a lot of them are still ending up on paper.

    It is difficult to measure the quantity of paper used as a result of use of Internet-connected computers, although just about anyone who works in an office can tell you that when e-mail is introduced, the printers start working overtime. “I feel in my bones this revolution is causing more trees to be cut down,” says Ted Smith of the Earth Village Organization.

    Perhaps the best sign of how computer and Internet use pushes up demand for paper comes from the high-tech industry itself, which sees printing as one of its most promising new markets. Several Internet companies have been set up to help small businesses print quality documents from a computer. Earlier this week Hewlett-Packard Co. announced a plan to develop new technologies that will enable people to print even more so they can get a hard copy of a business document, a medical record or just a on-line e-mail, even if they are nowhere near a computer. As the company sees it, the more use of the Internet the greater demand for printers.

    Does all this mean environmental concerns (环境问题) have been forgotten? Some activists suggest people have been led to believe that a lot of dangers to the environment have gone away.

    “I guess people believe that the problem is taken care of, because of recycling(回收利用),” said Kelly Quirke, director of the Rainforest Action Network in San Francisco. Yet Quirke is hopeful that high-tech may also prove helpful. He says printers that print on both sides are growing in popularity.

    The action group has also found acceptable paper made from materials other than wood, such as agricultural waste.