问题
问答题
营养缺陷型
答案
参考答案：某一野生型菌株因发生基因突变而丧失合成一种或几种生长因子、碱基或氨基酸的能力，因而无法在基本培养基上正常生长繁殖的变异类型，叫做营养缺陷型。
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营养缺陷型
参考答案：某一野生型菌株因发生基因突变而丧失合成一种或几种生长因子、碱基或氨基酸的能力，因而无法在基本培养基上正常生长繁殖的变异类型，叫做营养缺陷型。
| 阅读理解。 | ||||||||||
| Like many other animals, swallows will fly south when the days turn cold and the leaves from trees begin to fall. They travel many miles to find a warmer winter. This type of travel is called migration (迁徙). Birds are animals that like to migrate a lot when it is cold. About 600 out of 9,000 kinds of birds do it. They leave in autumn and come back during the warm spring months. Many birds don't like the cold. They need food, but plants die and insects (昆虫) hide in winter. So, they'll only come home the next year, when there will be plenty of food for them to eat. But the trip is often hard and not safe. Many birds fly in groups and make noises. Cranes (鹤) fly in groups like the letter V, or in a row. Others travel in pairs or alone. Some can travel a very long way. They fly between lands. For example, the Arctic tern (北极燕鸥) can travel 20,100 kilometres. They build a home near the North Pole (北极), go to the Antarctic (南极洲) in autumn, and then come back in spring. How do the birds find their way? They look at things like the sun, moon and stars. They never get lost. | ||||||||||
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