问题 问答题

DNA热变性有何特点T m值表示什么

答案

参考答案:将DNA的稀盐溶液加热到70~100℃几分钟后,双螺旋结构即发生破坏,氢键断裂,两条链彼此分开,形成无规则线团,此过程为DNA的热变性。主要表现为:变性温度范围很窄,260 nm处的紫外吸收增加;黏度下降;生物活性丧失;比旋度下降;酸碱滴定曲线改变。T m值代表核酸的变性温度(熔解温度、熔点)。在数值上等于DNA变性时紫外吸收达到最大变化值半数时所对应的温度。

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     In our discussion with people on how education can help them succeed in life, a woman remembered the
first meeting of an introductory   1   course about 20 years ago.
     The professor   2   the lecture hall, placed upon his desk a large jar filled with dried beans (豆), and invited
the students to   3   how many beans the jar contained. After   4   shouts of wildly wrong guesses the professor
smiled a thin, dry smile, announced the   5   answer, and went on saying, "You have just   6   an important lesson
about science. That is Never   7   your own senses."
     Twenty years later, the   8   could guess what the professor had in mind. He   9   himself, perhaps, as inviting
his students to start an exciting  10  into an unknown world.
     Invisible (无形的) to the  11 , which can be discovered only through scientific  12 . But the seventeen-year-old
girl could not accept or even  13  the invitation. She was just  14  to understand the world. And she  15  that her
firsthand experience could be ture  16 . The professor, however, said that it was  17 . He was taking away her
only 18  for knowing and was providing her with no substitute. "I remember feeling small and  19 ," the women
says, "and I did the only thing I could do. I  20  the course that afternoon, and I haven't gone near science since."
( )1.A. art        
( )2.A. searched for
( )3.A. count      
( )4.A. warning    
( )5.A. ready      
( )6.A. learned    
( )7.A. lose      
( )8.A. lecturer  
( )9.A. described  
( )10.A. voyage    
( )11.A. professor  
( )12.A. model      
( )13.A. hear       
( )14.A. suggesting   
( )15.A. believed   
( )16.A. growth     
( )17.A. firm       
( )18.A. task       
( )19.A. cruel      
( )20.A. dropped    
B. history    
B. looked at  
B. guess      
B. giving    
B. possible  
B. prepared  
B. trust      
B. scientist  
B. respected  
B. movement  
B. eye        
B. senses     
B. make       
B. beginning  
B. doubted    
B. strength   
B. interesting      
B. tool       
B. proud      
B. started    
C. science    
C. got through
C. report     
C. turning away
C. correct    
C. taught    
C. sharpen    
C. speaker    
C. saw        
C. change    
C. knowledge  
C. sprint     
C. present    
C. pretending      
C. proved     
C. faith      
C. wrong      
C. success    
C. frightened      
C. passed     
D. math         
D. marched into 
D. watch         
D. listening to
D. difficult      
D. taken      
D. show         
D. woman        
D. served                         
D. rush         
D. light         
D. methods       
D. refuse        
D. waiting       
D. explained     
D. truth         
D. acceptable  
D. connection    
D. brave       
D. missed      
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