问题 单项选择题

DNA复性过程可发生

A.碱基堆积力的形成

B.磷酸二酯键的形成

C.糖苷键的形成

D.酯键的形成

答案

参考答案:A

解析:DNA变性是指在某些理化因素作用下,DNA双链的互补碱基对之间的氢键断裂,使DNA双螺旋结构松散,成为单链的现象。变性的DNA在适当条件下,两条互补链可重新配对,恢复天然的双螺旋构象,这一现象称为复性。本题内容教科书上没有讲到,但只要真正理解本题的要求,也不难作答。本题解题的关键,就是理解“复性是变性的可逆反应”。变性时,是DNA双链→单链,复性时,DNA单链→双链。核酸是由多个核苷酸以3',5'-磷酸二酯键结合而成,而核苷酸由核苷和磷酸以酯键结合而成,核苷由核糖和碱基以糖苷键结合而成,因此3',5'-磷酸二酯键、酯键和糖苷键都是组成DNA一级结构的化学键。而变性和复性都不涉及DNA一级结构的改变。因此,复性时,不会重新生成这些化学键。碱基堆积力是DNA双螺旋结构(二级结构)纵向结构的维系力,因此在DNA变性和复性过程中都会发生改变。

阅读理解

  Art is for everyone. Parents can help kids appreciate art by making it available for them. Unfortunately, many adults do not enjoy art simply because they do not understand the stories or context (背景). Actually, children can appreciate art as well as any adult. Here are some simple ways to encourage children to appreciate art.

Find art that will allow children to easily make a connection with their lives. Choose the material according to their ages. Pictures and sculptures of children, for example, or dogs and cats are easily understood, and it is easy to start a conversation about how the themes in the art are connected with familiar people and animals in kids’ lives. Music and dance are very important to children and should also be used.

Explore the stories behind famous, and not so famous, art. Encourage children to make up their own stories about paintings and then research the real story behind the paintings.

Create art with children. Teach them that art is not something that they must only observe but something that they can create. Discuss art with children. Encourage them to express their views on art. Let them know that it is okay to dislike something, and listen to their preferences when planning events.

Visit museums and performances. Explore the artwork in local museums that will interest them. Most cities have children’s theaters and music groups, and often free performances are offered. Many art museums have special tours for children.

Through a variety of artistic experiences, kids will learn how to appreciate art and will enjoy it.

64. When choosing art for children, parents should________.      

A. take their children’s ages and experiences into consideration

B. pay close attention to their children’s ability to understand

C. ask professional artists for some useful advice

D. try to find some art beyond their imagination

65. Which of the following should not be down in creating art with children?

A. Allowing children to speak out their own opinions on art. 

B. Allowing children to have their own likes and dislikes.

C. Making children interested in everything about art.

D. Talking with children about something on art.

66. We can learn from the text that helping children appreciate art_________.

A. is highly necessary for their growth 

B. will improve their school performance

C. can help them make more friends at school 

D. needs parents’ encouragement and instruction

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