问题 单项选择题

下列DNA中,一般不用作克隆载体的是

A.质粒DNA

B.大肠杆菌DNA

C.病毒DNA

D.噬菌体DNA

E.酵母人工染色体

答案

参考答案:B

解析:克隆载体又称基因载体,是指为“携带”感兴趣的外源DNA,实现外源DNA的无性繁殖或表达有意义的蛋白质所采用的一些DNA分子。可充当克隆载体的DNA分子有质粒DNA、噬菌体DNA和病毒DNA,它们经适当改造后仍具有自我复制能力,或兼有表达外源基因的能力。为增加克隆载体插入外源基因的容量,还设计有柯斯质粒载体和酵母人工染色体载体。为适应真核细胞重组DNA技术需要,特别是为了满足真核基因表达或基因治疗的需要,发展了一些用动物病毒DNA改造的载体,如腺病毒载体、逆转录病毒载体以及用于昆虫细胞表达的杆状病毒载体等。大肠杆菌DNA不用作克隆载体(B)

单项选择题
单项选择题

A small group of Internet security specialists gathered in Singapore to start up a global system to make e-mail and e-commerce more secure, end the rapid growth of passwords and raise the bar significantly for Internet fraud, spies and troublemakers.

The Singapore event included an elaborate technical ceremony to create and then securely store numerical keys that will be kept in three hardened data centers there, in Zurich and in San Jose, Calif. The keys and data centers are working parts of a technology known as Secure DNS, or DNSSEC. DNS refers to the Domain Name System, which is a directory that connects names to numerical Internet addresses. Preliminary work on the security system had been going on for more than a year, but this was the first time the system went into operation, even though it is not quite complete.

The three centers are fortresses made up of five layers of physical, electronic and cryptographic security, making it virtually impossible to damage the system. Four layers are active now. The fifth, a physical barrier, is being built inside the data center.

The technology is viewed by many computer security specialists as a ray of hope amid the recent cascade of data thefts, attacks, disruptions and scandals, including break-ins at Citibank, Sony, Lockheed Martin, RSA Security and elsewhere. It allows users to communicate via the Internet with high confidence that the identity of the person or organization they are communicating with is not being tricked or forged.

Internet engineers like Dan Kaminsky, an independent network security researcher who is one of the engineers involved in the project, want to counteract three major deficiencies in today’s Internet. There is no mechanism for ensuring trust, the quality of software is uneven, and it is difficult to track down bad actors.

One reason for these flaws is that from the 1960s through the 1980s the engineers who designed the network’s underlying technology were concerned about reliable, rather than secure, communications. That is starting to change with the introduction of Secure DNS by governments and other organizations.

The event in Singapore capped a process that began more than a year ago and is expected to be complete after 300 so-called top-level domains have been digitally signed. Before the Singapore event, 70 countries had adopted the technology, and 14 more were added as part of the event. While large countries are generally doing the technical work to include their own domains in the system, the association of Internet security specialists is helping smaller countries and organizations with the process.

What would be the best title for the text()

A. An Independent Net Security System Is Used

B. Secure DNS Has Grown Mature Nowadays

C. Secure DNS Will Become A Popular Technology

D. A Stronger Net Security System Is Under Way