下列有关mRNA的叙述，正确的是
A．为线状单链结构，5'端有多聚腺苷酸帽子结构
B．可作为蛋白质合成的模板
C．链的局部不可形成双链结构
D．3'末端特殊结构与mRNA的稳定无关
E．三个相连核苷酸组成一个反密码子
参考答案：B
解析：mRNA与其他RNA一样，都是由单链核糖核苷酸链组成，但局部可形成双螺旋结构，mRNA5'端有一个m7Gppp帽结构，3'端有多聚A尾结构，可增强mRNA稳定性。在帽、尾结构之间为由三核苷酸组成的密码子，可作为蛋白质合成时的模板。在试题所给出的几种关于mRNA的叙述中，备选答案A叙说“5'端有多聚腺苷酸帽子结构”，显然混淆了与3'端尾结构的差别；答案C关于“局部双链”的描述恰好与事实相反；D、E所描述的有关结构、功能错误，只有B答案叙述是正确的。
| 阅读理解。 | ||||||||||
| Italian Lakes and Greek Islands(12Days) Prices starting from $1,999 Your tour begins in Milan, Italy, and moves on to the pretty Italian Lake District and the attractive resort (胜地) of Stresa, your home for two nights. Collette Vacations has carefully chosen the Costa Victoria as your home away from home for your 7-night journey along the waterways of the Mediterranean. The cruise ship is filled with the warmth and culture of Italy and is richly designed with entertainment (娱乐) areas and very good living conditions. It will take you to the places of your dreams. You'll spend 4 days touring Greek cities you've always heard about. In Katakolon, you will have the only unguided tour to nearby Olympia on the whole journey. Then with a local guide you will visit the Greek islands of Santorini, which is often related to the story of the lost city of Atlantis, and Mykonos, a wonderful island with beautiful beaches. Your journey ends in Verona, home of the love story Romeo and Juliet, with a fun-filled farewell dinner-a perfect ending to a pleasant journey. 12Day,25Meals:10Breakfasts,6 Lunches,9Dinners Day 1 Overnight flight to Italy Day 2-3 Regina Palace,stresa, Italy Day 4-10 Costa Victoria(Costa cruises) Day 11 Hotel Leopardi,Verona, Italy Day 12 Leave for home Please Note:
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| 1. How is the journey planned? | ||||||||||
| A. It starts and ends in Italy. B. It starts and ends in Greece. C. It starts in Italy and ends in Greece. D. It starts in Greece and ends in Italy. | ||||||||||
| 2. What can be inferred from the travel plan? | ||||||||||
| A. The price may get lower than those in the plan. B. The prices include three meals a day. C. The price is the highest in summer. D. The prices include entertainment service. | ||||||||||
| 3. What does the underlined part "the Costa Victoria" most probably refer to? | ||||||||||
| A. Afamous hotel. B. A beautiful resort. C. A comfortable ship. D. A long-distance bus. | ||||||||||
| 4. Tourists will travel on their own in ____. | ||||||||||
| A. Stresa B. Olympia C. Mykonos D. Verona |