问题 问答题 简答题

简述细胞周期中不同时期及其主要事件。

答案

参考答案:

G1期:①与DNA合成启动相关,开始合成细胞生长所需要的多种蛋白质、RNA、碳水化合物、脂类等,同时染色质去凝聚,但不合成DNA。②起始点:芽殖酵母中G1期晚期阶段的一个特定时期,通过这个特定时期,细胞可继续分裂,进入S期。在其他真核细胞中称为限制点或检验点。③起始点为G1晚期的一个基本事件,受细胞内在的一系列监控机制和外在因素影响。

S期:①合成DNA和染色体蛋白。②新合成的DNA立即与组蛋白结合,组成核小体串珠结构。

G2期:①DNA复制已完成,DNA含量倍增。②合成其他结构物质和相关的亚细胞结构,如微管蛋白、染色体凝集因子等。③通过G2期,细胞进入M期,受G2期检验点的控制。

M期:①真核细胞的细胞分裂主要包括两种方式,即有丝分裂和减数分裂。②细胞经过分裂,遗传物质和细胞内其他物质平均分配给子细胞。

阅读理解

根据短文内容,从短文后的选项中选出填入空白处的最佳选项。选项中有两项为多余选项。

     The Mayan Indians lived in Mexico for thousands of years before the Spanish arrived in the 1500s. They

had farms, beautiful palaces, and cities with many buildings. 1_____ This knowledge helped them to live a

better life than most people of that rime, because they could use it to make their life more comfortable and

rewarding.

     In ancient Mexico there were many small clearings in the forest. In each clearing was a village with fields

of corn, beans, and other crops around it. To clear the land for farms, the Maya cut down trees with stone

axes. They planted seeds by digging holes in the ground with pointed sticks. 2_____ 

     The Maya believed in many gods, including rain gods, sun gods, and corn gods. The people built large

temples to honor the Mayan gods, around which cities were built. Today, many of these ancient Mayan cities

and temples are still standing.

     Although the cities were beautiful, and the people worked hard to build them, few of them lived there.

Usually, only the priests lived in the cities. The other people lived in small villages in the forests. They lived in

small huts with no windows. The walls were made of poles covered with dried mud, and the roof was made

of grass or leaves. 3_____ 

     Measuring time was important to the Maya. 4_____ The Maya developed a system for measuring time

accurately. Mayan priests made a system to keep track of time. They made a calendar based on their study

of the sun, moon and stars. The Mayan calendar was far more accurate than the European calendars of the

time.

     Around the year 800, the Maya left their villages and beautiful cities, never to return. 5_____ They may

have died from an infectious disease. They may have left because the soil could no longer grow crops.

Scientists are still trying to find the lost secrets of the Maya. They are still one of our greatest mysteries.

A. No one knows why this happened.

B. Most Maya lived a simple life close to nature.

C. The Maya were an intelligent, culturally rich people,

D. Farmers needed to know when to plant and harvest their crops.

E. They had to fight against natural disasters as common occurrences.

F. One farmer was able to grow crops producing food for several people.

G. The Mayan people knew a lot about nature and the world around them.

问答题 简答题