问题 问答题

何谓造血干细胞造血干细胞必须具备哪些特征

答案

参考答案:答:造血干细胞是一种具有自身复制能力的细胞,必须具备:①有分裂能力:一个干细胞在经过一个细胞周期活动后可生成两个与分裂前性质相同的肝细胞,这就是干细胞自身复制的特性。由于它有自身复制能力,所以能保持本细胞库数的恒定。②有分化为更成熟细胞的能力:即具有向骨髓多系细胞分化的能力。造血干细胞既能自我复制,经常维持一定的数量,又能分化使多系细胞相对稳定。概括上述,血细胞来源于骨髓的造血多能干细胞。多能干细胞能分化为多系统的定向干细胞,然后经过原始、幼稚各阶段,而后成熟为多种血细胞。

阅读理解

Governments all over the world make public reports about the condition of their economies. Most countries, including the United States, have used a measure called the gross national product (国民生产总值)or GNP. It includes all goods and services produced by citizens of the country anywhere in the world.

Recently the American Commerce Department has started using a new method to measure production. It is known as the gross domestic product(国内生产总值)or GDP. It counts only goods and services that have been produced within the nation’s borders. Money earned by foreign companies operating in the United States is included in the GDP, but money earned by American companies operating in other countries is not.

Economic experts generally approve of the change. They say that the gross domestic product provides a truer measure of the economy. They also note that most other industrial countries use this method. Therefore it will be easier to study the economies of different countries. Some officials also hope the new system will help them make better economic policy decisions. It will provide them with a clearer understanding of economic activity in the U.S. . The new measure is less likely to be affected by sudden changes in foreign oil prices or in the value of the American dollar in other countries.

Economic experts believe that the change from GNP to GDP will immediately reduce the value of American production by at least 40,000 million dollars a year. But that is really a very small change in the American economy – less than 1% . The Commerce Department reports the unofficial gross domestic product once every three months. The government also continues to report GNP as it has four times a year since 1941.

小题1:To report the American GNP, the government should not consider goods and services produced _______.

A.by American companies at home

B.by American companies in China

C.by American companies in Singapore

D.by Japanese companies in the United States小题2:To report the American GDP, the Commerce Department should count money earned ________.

A.by both American and foreign companies in America

B.by American companies within its own borders

C.by foreign companies in the United States

D.by American companies in its foreign markets小题3:GDP is considered better than GNP by some economists because ________.

A.it is something new

B.it is used by most countries

C.it is easier to calculate

D.it is less likely to be affected by the foreign market小题4:What will happen to the value of American economy if GDP is used to replace GNP?

A.It will be seriously affected.

B.It will not be affected.

C.It will come down a little.

D.It will slightly go up.小题5:How often are the reports of GDP issued by the American Commerce Department?

A.once every four months

B.once every three months

C.once every six months

D.once a year

单项选择题