题干

下列有关基因工程中运载体的说法正确的是

A:在进行基因工程操作中,被用作运载体的质粒都是天然质粒

B:所有的质粒都可以作为基因工程中的运载体

C:质粒是一种独立于细菌染色体外的链状DNA分子

D:作为载体的质粒DNA分子上应有对重组DNA进行鉴定和选择的标记基因

上一题 下一题 0.0难度 选择题 更新时间:2013-12-09 05:23:36

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D

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    Many Years ago three soldiers,hungry and tired of battle,came upon a small village.The villagers,suffering a bad harvest and the many years of war,quickly hid what little they had to eat and met the three in the village square, shaking their heads and sighing,"Well,we are starving."The soldiers communicated with each other quietly and then the first soldier turned to the villagers.“Your poor fields have left you nothing to share,so we will share what little we have: the secret of making soup from stones.”

    Naturally the villagers were intrigued and soon a fire was built to the village's greatest pot,and then the soldiers dropped in three smooth stones.

“Now this will be a fine soup,”said the second soldier,"but a handful of salt and some parsley(香芹)would make it wonderful!"Up jumped a villager,crying “What a piece of luck!I've just remembered where some has been left!”

    And off she ran,returning with parsley.As the pot was boiling,the memory of the villagers improved: soon carrots,beef and cream had found their way into the great pot.They ate and danced and sang well into the night, thrilled about the feast and their new-found friends.In the morning the three soldiers awoke,finding all the villagers standing before them.At their feet lay a bag of the village's best bread and cheese.“You have given us the greatest gift,”said an elder,“and we shall never forget.”The third soldier turned to the crowd,and said,“There is no secret,but this is certain: people working together can accomplish great things.”At last the light dawned on villagers.Off the soldiers wandered,down the road.