问题 单项选择题

DNA理化性质中Tm值所表达的含义是()

A.复制时的温度

B.复性时的温度

C.50%双链被打开的温度

D.由B型转变为A型的温度

E.DNA开始变性时的温度

答案

参考答案:C

解析:考点:Tm。DNA变性从开始解链到完全解链,是在一个相当窄的温度范围内完成的。在解链过程中,紫外吸光度的变化达到最大变化值的一半时所对应的温度称为DNA的解链温度,或称融解温度,用Tm表示。在此温度时,50%的DNA双链被打开。

问答题
单项选择题

"It’s no use, Mum," said Johnny. "I’m just no good at dancing."

"You’ve got to keep trying. Tonight will be (1) , dear. Try a turn with that pretty Lisette. "

Johnny (2) . Every Saturday night used to be the best of the week. He and his parents went to the (3) at the Club, where his hero, Alcide, played the accordion with the band. But lately everything had changed. Now that Johnny was older, he was (4) to dance with a girl!

(5) Johnny and his parents arrived at the Club, music had already started. Johnny got up his (6) to approach Lisette. "May I have this dance" Johnny asked. "That’s all right."said Lisette. Johnny struggled to keep up with Lisette’s (7) steps, but he was always one beat behind her. Then Johnny heard his friend Pierre say, "Look! Johnny has two left feet!" (8) burst from the crowd. Johnny (9) and ran outside, determined never to go to another dance.

The next Saturday, Alcide (10) to Johnny’s house for some potatoes. He happened to hear Johnny playing the accordion. Alcide’s eyes (11) . "Bring that accordion and play some songs tonight," Alcide said. Then he drove off, leaving Johnny staring open mouthed (12) him.

At the Club, Johnny scanned the crowd for Lisette and (13) her. The band played for a long time before Alcide said, "Dear friends, I got a (14) for you tonight. Young Johnny is going to join us!" (15) , Johnny stepped up on the platform, his eyes on the floor. He began to play, and the band (16) behind him. When the song ended, he heard cheers. Johnny kept playing until the dance was (17) . "You did a fine job tonight. Play with us again next Saturday night, " Alcide said. "Yes, sir! " said Johnny. (18) he went outside, Johnny saw Lisette and her friends near the door. Lisette stepped (19) , smiling. "You played really good tonight!" she said."Thank you," Johnny blushed. As he walked on, Pierre (20) moved out of the way for him to pass.Johnny patted his accordion. Come to think of it, in his whole life, he had never once seen Alcide out on the dance floor.

3().

A.platform

B.appointment

C.meeting

D.dance