??2?????????????20?????????? ?,???????????2????????????20???????????,??(???)?????,???????????4?????????????????????

????,???????????4????????????

???,?4??????,??????????DNA???,??

??:???? ??“???????”

?? ???????????4?????

????:1????????:????????

2?????????

3????????????

4?????

5???????

??2?????????????20?????????? ?,???????????2????????????20???????????,??(???)?????,???????????4?????????????????????

????,???????????4????????????

???,?4??????,??????????DNA???,??

??:????